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Title: Coral Reefs
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CORAL REEFS
by CHARLES DARWIN
EDITORIAL NOTE.
Although in some respects more technical in their subjects and style than
Darwin's "Journal," the books here reprinted will never lose their value
and interest for the originality of the observations they contain. Many
parts of them are admirably adapted for giving an insight into problems
regarding the structure and changes of the earth's surface, and in fact
they form a charming introduction to physical geology and physiography in
their application to special domains. The books themselves cannot be
obtained for many times the price of the present volume, and both the
general reader, who desires to know more of Darwin's work, and the student
of geology, who naturally wishes to know how a master mind reasoned on most
important geological subjects, will be glad of the opportunity of
possessing them in a convenient and cheap form.
The three introductions, which my friend Professor Judd has kindly
furnished, give critical and historical information which makes this
edition of special value.
G.T.B.
CORAL REEFS.
TABLE OF CONTENTS.
CRITICAL INTRODUCTION.
INTRODUCTION.
CHAPTER I.--ATOLLS OR LAGOON-ISLANDS.
SECTION 1.I.--DESCRIPTION OF KEELING ATOLL.
Corals on the outer margin.--Zone of Nulliporae.--Exterior reef.--Islets.--
Coral-conglomerate.--Lagoon.--Calcareous sediment.--Scari and Holuthuriae
subsisting on corals.--Changes in the condition of the reefs and islets.--
Probable subsidence of the atoll.--Future state of the lagoon.
SECTION 1.II.--GENERAL DESCRIPTION OF ATOLLS.
General form and size of atolls, their reefs and islets.--External slope.--
Zone of Nulliporae.--Conglomerate.--Depth of lagoons.--Sediment.--Reefs
submerged wholly or in part.--Breaches in the reef.--Ledge-formed shores
round certain lagoons.--Conversion of lagoons into land.
SECTION 1.III.--ATOLLS OF THE MALDIVA ARCHIPELAGO--GREAT CHAGOS BANK.
Maldiva Archipelago.--Ring-formed reefs, marginal and central.--Great
depths in the lagoons of the southern atolls.--Reefs in the lagoons all
rising to the surface.--Position of islets and breaches in the reefs, with
respect to the prevalent winds and action of the waves.--Destruction of
islets.--Connection in the position and submarine foundation of distinct
atolls.--The apparent disseverment of large atolls.--The Great Chagos
Bank.--Its submerged condition and extraordinary structure.
CHAPTER II.--BARRIER REEFS.
Closely resemble in general form and structure atoll-reefs.--Width and
depth of the lagoon-channels.--Breaches through the reef in front of
valleys, and generally on the leeward side.--Checks to the filling up of
the lagoon-channels.--Size and constitution of the encircled islands.--
Number of islands within the same reef.--Barrier-reefs of New Caledonia and
Australia.--Position of the reef relative to the slope of the adjoining
land.--Probable great thickness of barrier-reefs.
CHAPTER III.--FRINGING OR SHORE-REEFS.
Reefs of Mauritius.--Shallow channel within the reef.--Its slow filling
up.--Currents of water formed within it.--Upraised reefs.--Narrow
fringing-reefs in deep seas.--Reefs on the coast of E. Africa and of
Brazil.--Fringing-reefs in very shallow seas, round banks of sediment and
on worn-down islands.--Fringing-reefs affected by currents of the sea.
--Coral coating the bottom of the sea, but not forming reefs.
CHAPTER IV.--ON THE DISTRIBUTION AND GROWTH OF CORAL-REEFS.
SECTION 4.I.--ON THE DISTRIBUTION OF CORAL-REEFS, AND ON THE CONDITIONS
FAVOURABLE TO THEIR INCREASE.
SECTION 4.II.--ON THE RATE OF GROWTH OF CORAL-REEFS.
SECTION 4.III.--ON THE DEPTHS AT WHICH REEF-BUILDING POLYPIFERS CAN LIVE.
CHAPTER V.--THEORY OF THE FORMATION OF THE DIFFERENT CLASSES OF
CORAL-REEFS.
The atolls of the larger archipelagoes are not formed on submerged craters,
or on banks of sediment.--Immense areas interspersed with atolls.--Recent
changes in their state.--The origin of barrier-reefs and of atolls.--Their
relative forms.--The step-formed ledges and walls round the shores of some
lagoons.--The ring-formed reefs of the Maldiva atolls.--The submerged
condition of parts or of the whole of some annular reefs.--The disseverment
of large atolls.--The union of atolls by linear reefs.--The Great Chagos
Bank.--Objections, from the area and amount of subsidence required by the
theory, considered.--The probable composition of the lower parts of atolls.
CHAPTER VI.--ON THE DISTRIBUTION OF CORAL-REEFS WITH REFERENCE TO THE
THEORY OF THEIR FORMATION.
Description of the coloured map.--Proximity of atolls and barrier-reefs.--
Relation in form and position of atolls with ordinary islands.--Direct
evidence of subsidence difficult to be detected.--Proofs of recent
elevation where fringing-reefs occur.--Oscillations of level.--Absence of
active volcanoes in the areas of subsidence.--Immensity of the areas which
have been elevated and have subsided.--Their relation to the present
distribution of the land.--Areas of subsidence elongated, their
intersection and alternation with those of elevation.--Amount and slow rate
of the subsidence.--Recapitulation.
APPENDIX.
Containing a detailed description of the reefs and islands in Plate III.
INDEX.
THE STRUCTURE AND DISTRIBUTION OF CORAL REEFS.
CRITICAL INTRODUCTION.
A scientific discovery is the outcome of an interesting process of
evolution in the mind of its author. When we are able to detect the germs
of thought in which such a discovery has originated, and to trace the
successive stages of the reasoning by which the crude idea has developed
into an epoch-making book, we have the materials for reconstructing an
important chapter of scientific history. Such a contribution to the story
of the "making of science" may be furnished in respect to Darwin's famous
theory of coral-reefs, and the clearly reasoned treatise in which it was
first fully set forth.
The subject of corals and coral-reefs is one concerning which much popular
misconception has always prevailed. The misleading comparison of coral-rock
with the combs of bees and the nests of wasps is perhaps responsible
for much of this misunderstanding; one writer has indeed described a
coral-reef as being "built by fishes by means of their teeth." Scarcely
less misleading, however, are the references we so frequently meet with,
both in prose and verse, to the "skill," "industry," and "perseverance" of
the "coral-insect" in "building" his "home." As well might we praise men
for their cleverness in making their own skeletons, and laud their assiduity
in filling churchyards with the same. The polyps and other organisms, whose
remains accumulate to form a coral-reef, simply live and perform their
natural functions, and then die, leaving behind them, in the natural course
of events, the hard calcareous portions of their structures to add to the
growing reef.
While the forms of coral-reefs and coral-islands are sometimes very
remarkable and worthy of attentive study, there is no ground, it need
scarcely be added, for the suggestion that they afford proofs of design on
the part of the living builders, or that, in the words of Flinders, they
constitute breastworks, defending the workshops from whence "infant
colonies might be safely sent forth."
It was not till the beginning of the present century that travellers like
Beechey, Chamisso, Quoy and Gaimard, Moresby, Nelson, and others, began to
collect accurate details concerning the forms and structure of coral-masses,
and to make such observations on the habits of reef-forming polyps,
as might serve as a basis for safe reasoning concerning the origin of
coral-reefs and islands. In the second volume of Lyell's "Principles of
Geology," published in 1832, the final chapter gives an admirable summary
of all that was then known on the subject. At that time, the ring-form of
the atolls was almost universally regarded as a proof that they had grown
up on submerged volcanic craters; and Lyell gave his powerful support to
that theory.
Charles Darwin was never tired of acknowledging his indebtedness to Lyell.
In dedicating to his friend the second edition of his "Naturalist's Voyage
Round the World," Darwin writes that he does so "with grateful pleasure, as
an acknowledgment that the chief part of whatever scientific merit this
journal and the other works of the author may possess, has been derived
from studying the well-known and admirable 'Principles of Geology.'"
The second volume of Lyell's "Principles" appeared after Darwin had left
England; but it was doubtless sent on to him without delay by his faithful
friend and correspondent, Professor Henslow. It appears to have reached
Darwin at a most opportune moment, while, in fact, he was studying the
striking evidences of slow and long-continued, but often interrupted
movement on the west coast of South America. Darwin's acute mind could not
fail to detect the weakness of the then prevalent theory concerning the
origin of the ring-shaped atolls--and the difficulty which he found in
accepting the volcanic theory, as an explanation of the phenomena of
coral-reefs, is well set forth in his book.
In an interesting fragment of autobiography, Darwin has given us a very
clear account of the way in which the leading idea of the theory of
coral-reefs originated in his mind; he writes, "No other work of mine was
begun in so deductive a spirit as this, for the whole theory was thought
out on the west coast of South America, before I had seen a true
coral-reef. I had therefore only to verify and extend my views by a
careful examination of living reefs. But it should be observed that I had
during the two previous years been incessantly attending to the effects on
the shores of South America of the intermittent elevation of the land,
together with the denudation and deposition of sediment. This necessarily
led me to reflect much on the effects of subsidence, and it was easy to
replace in imagination the continued deposition of sediment by the upward
growth of corals. To do this was to form my theory of the formation of
barrier-reefs and atolls."
On her homeward voyage, the "Beagle" visited Tahiti, Australia, and some of
the coral-islands in the Indian Ocean, and Darwin had an opportunity of
testing and verifying the conclusion at which he had arrived by studying
the statements of other observers.
I well recollect a remarkable conversation I had with Darwin, shortly after
the death of Lyell. With characteristic modesty, he told me that he never
fully realised the importance of his theory of coral-reefs till he had an
opportunity of discussing it with Lyell, shortly after the return of the
"Beagle". Lyell, on receiving from the lips of its author a sketch of the
new theory, was so overcome with delight that he danced about and threw
himself into the wildest contortions, as was his manner when excessively
pleased. He wrote shortly afterwards to Darwin as follows:--"I could think
of nothing for days after your lesson on coral-reefs, but of the tops of
submerged continents. It is all true, but do not flatter yourself that you
will be believed till you are growing bald like me, with hard work and
vexation at the incredulity of the world." On May 24th, 1837, Lyell wrote
to Sir John Herschel as follows:--"I am very full of Darwin's new theory of
coral-islands, and have urged Whewell to make him read it at our next
meeting. I must give up my volcanic crater forever, though it cost me a
pang at first, for it accounted for so much." Dr. Whewell was president of
the Geological Society at the time, and on May 31st, 1837, Darwin read a
paper entitled "On Certain Areas of Elevation and Subsidence in the Pacific
and Indian oceans, as deduced from the Study of Coral Formations," an
abstract of which appeared in the second volume of the Society's
proceedings.
It was about this time that Darwin, having settled himself in lodgings at
Great Marlborough Street, commenced the writing of his book on "Coral-Reefs."
Many delays from ill-health and the interruption of other work,
caused the progress to be slow, and his journal speaks of "recommencing"
the subject in February 1839, shortly after his marriage, and again in
October of the same year. In July 1841, he states that he began once more
"after more than thirteen month's interval," and the last proof-sheet of
the book was not corrected till May 6th, 1842. Darwin writes in his
autobiography, "This book, though a small one, cost me twenty months of
hard work, as I had to read every work on the islands of the Pacific, and
to consult many charts." The task of elaborating and writing out his books
was, with Darwin, always a very slow and laborious one; but it is clear
that in accomplishing the work now under consideration, there was a long
and constant struggle with the lethargy and weakness resulting from the sad
condition of his health at that time.
Lyell's anticipation that the theory of coral-reefs would be slow in
meeting with general acceptance was certainly not justified by the actual
facts. On the contrary the new book was at once received with general
assent among both geologists and zoologists, and even attracted a
considerable amount of attention from the general public.
It was not long before the coral-reef theory of Darwin found an able
exponent and sturdy champion in the person of the great American
naturalist, Professor James D. Dana. Two years after the return of the
"Beagle" to England, the ships of the United States Exploring Expedition
set sail upon their four years' cruise, under the command of Captain
Wilkes, and Dana was a member of the scientific staff. When, in 1839, the
expedition arrived at Sydney, a newspaper paragraph was found which gave
the American naturalist the first intimation of Darwin's new theory of the
origin of atolls and barrier-reefs. Writing in 1872, Dana describes the
effect produced on his mind by reading this passage:--"The paragraph threw
a flood of light over the subject, and called forth feelings of peculiar
satisfaction, and of gratefulness to Mr. Darwin, which still come up afresh
whenever the subject of coral islands is mentioned. The Gambier Islands in
the Paumotus, which gave him the key to the theory, I had not seen; but on
reaching the Feejees, six months later, in 1840, I found there similar
facts on a still grander scale and of a more diversified character, so that
I was afterward enabled to speak of his theory as established with more
positiveness than he himself, in his philosophic caution, had been ready to
adopt. His work on coral-reefs appeared in 1842, when my report on the
subject was already in manuscript. It showed that the conclusions on other
points, which we had independently reached, were for the most part the
same. The principal points of difference relate to the reason for the
absence of corals from some coasts, and the evidence therefrom as to
changes of level, and the distribution of the oceanic regions of elevation
and subsidence--topics which a wide range of travel over the Pacific
brought directly and constantly to my attention."
Among the Reports of the United States Exploring Expedition, two important
works from the pen of Professor Dana made their appearance;--one on
"Zoophytes," which treats at length on "Corals and Coral-Animals," and the
other on "Coral-Reefs and Islands." In 1872, Dana prepared a work of a
more popular character in which some of the chief results of his studies
are described; it bore the title of "Corals and Coral-Islands." Of this
work, new and enlarged editions appeared in 1874 and 1890 in America, while
two editions were published in this country in 1872 and 1875. In all these
works their author, while maintaining an independent judgment on certain
matters of detail, warmly defends the views of Darwin on all points
essential to the theory.
Another able exponent and illustrator of the theory of coral-reefs was
found in Professor J.B. Jukes, who accompanied H.M.S. "Fly", as naturalist,
during the survey of the Great Barrier-Reef--in the years 1842 to 1846.
Jukes, who was a man of great acuteness as well as independence of mind,
concludes his account of the great Australian reefs with the following
words:--"After seeing much of the Great Barrier-Reefs, and reflecting much
upon them, and trying if it were possible by any means to evade the
conclusions to which Mr. Darwin has come, I cannot help adding that his
hypothesis is perfectly satisfactory to my mind, and rises beyond a mere
hypothesis into the true theory of coral-reefs."
As the result of the clear exposition of the subject by Darwin, Lyell,
Dana, and Jukes, the theory of coral-reefs had, by the middle of the
present century, commanded the almost universal assent of both biologists
and geologists. In 1859 Baron von Richthofen brought forward new facts in
its support, by showing that the existence of the thick masses of dolomitic
limestone in the Tyrol could be best accounted for if they were regarded as
of coralline origin and as being formed during a period of long continued
subsidence. The same views were maintained by Professor Mojsisovics in his
"Dolomit-riffe von Sudtirol und Venetien," which appeared in 1879.
The first serious note of dissent to the generally accepted theory was
heard in 1863, when a distinguished German naturalist, Dr. Karl Semper,
declared that his study of the Pelew Islands showed that uninterrupted
subsidence could not have been going on in that region. Dr. Semper's
objections were very carefully considered by Mr. Darwin, and a reply to
them appeared in the second and revised edition of his "Coral-Reefs," which
was published in 1874. With characteristic frankness and freedom from
prejudice, Darwin admitted that the facts brought forward by Dr. Semper
proved that in certain specified cases, subsidence could not have played
the chief part in originating the peculiar forms of the coral-islands. But
while making this admission, he firmly maintained that exceptional cases,
like those described in the Pelew Islands, were not sufficient to
invalidate the theory of subsidence as applied to the widely spread atolls,
encircling reefs, and barrier-reefs of the Pacific and Indian Oceans. It
is worthy of note that to the end of his life Darwin maintained a friendly
correspondence with Semper concerning the points on which they were at
issue.
After the appearance of Semper's work, Dr. J.J. Rein published an account
of the Bermudas, in which he opposed the interpretation of the structure of
the islands given by Nelson and other authors, and maintained that the
facts observed in them are opposed to the views of Darwin. Although, so
far as I am aware, Darwin had no opportunity of studying and considering
these particular objections, it may be mentioned that two American
geologists have since carefully re-examined the district--Professor W.N.
Rice in 1884 and Professor A. Heilprin in 1889--and they have independently
arrived at the conclusion that Dr. Rein's objections cannot be maintained.
The most serious opposition to Darwin's coral-reef theory, however, was
that which developed itself after the return of H.M.S. "Challenger" from
her famous voyage. Mr. John Murray, one of the staff of naturalists on
board that vessel, propounded a new theory of coral-reefs, and maintained
that the view that they were formed by subsidence was one that was no
longer tenable; these objections have been supported by Professor Alexander
Agassiz in the United States, and by Dr. A. Geikie, and Dr. H.B. Guppy in
this country.
Although Mr. Darwin did not live to bring out a third edition of his
"Coral-Reefs," I know from several conversations with him that he had given
the most patient and thoughtful consideration to Mr. Murray's paper on the
subject. He admitted to me that had he known, when he wrote his work, of
the abundant deposition of the remains of calcareous organisms on the sea
floor, he might have regarded this cause as sufficient in a few cases to
raise the summits of submerged volcanoes or other mountains to a level at
which reef-forming corals can commence to flourish. But he did not think
that the admission that under certain favourable conditions, atolls might
be thus formed without subsidence, necessitated an abandonment of his
theory in the case of the innumerable examples of the kind which stud the
Indian and Pacific Oceans.
A letter written by Darwin to Professor Alexander Agassiz in May 1881 shows
exactly the attitude which careful consideration of the subject led him to
maintain towards the theory propounded by Mr. Murray:--"You will have
seen," he writes, "Mr. Murray's views on the formation of atolls and
barrier-reefs. Before publishing my book, I thought long over the same
view, but only as far as ordinary marine organisms are concerned, for at
that time little was known of the multitude of minute oceanic organisms. I
rejected this view, as from the few dredgings made in the "Beagle", in the
south temperate regions, I concluded that shells, the smaller corals, etc.,
decayed and were dissolved when not protected by the deposition of
sediment, and sediment could not accumulate in the open ocean. Certainly,
shells, etc., were in several cases completely rotten, and crumbled into
mud between my fingers; but you will know whether this is in any degree
common. I have expressly said that a bank at the proper depth would give
rise to an atoll, which could not be distinguished from one formed during
subsidence. I can, however, hardly believe in the existence of as many
banks (there having been no subsidence) as there are atolls in the great
oceans, within a reasonable depth, on which minute oceanic organisms could
have accumulated to the depth of many hundred feet."
Darwin's concluding words in the same letter written within a year of his
death, are a striking proof of the candour and openness of mind which he
preserved so well to the end, in this as in other controversies.
"If I am wrong, the sooner I am knocked on the head and annihilated so much
the better. It still seems to me a marvellous thing that there should not
have been much, and long-continued, subsidence in the beds of the great
oceans. I wish some doubly rich millionaire would take it into his head to
have borings made in some of the Pacific and Indian atolls, and bring home
cores for slicing from a depth of 500 or 600 feet."
It is noteworthy that the objections to Darwin's theory have for the most
part proceeded from zoologists, while those who have fully appreciated the
geological aspect of the question, have been the staunchest supporters of
the theory of subsidence. The desirability of such boring operations in
atolls has been insisted upon by several geologists, and it may be hoped
that before many years have passed away, Darwin's hopes may be realised,
either with or without the intervention of the "doubly rich millionaire."
Three years after the death of Darwin, the veteran Professor Dana
re-entered the lists and contributed a powerful defence of the theory of
subsidence in the form of a reply to an essay written by the ablest
exponent of the anti-Darwinian views on this subject, Dr. A. Geikie. While
pointing out that the Darwinian position had been to a great extent
misunderstood by its opponents, he showed that the rival theory presented
even greater difficulties than those which it professed to remove.
During the last five years, the whole question of the origin of coral-reefs
and islands has been re-opened, and a controversy has arisen, into which,
unfortunately, acrimonious elements have been very unnecessarily
introduced. Those who desire it, will find clear and impartial statements
of the varied and often mutually destructive views put forward by different
authors, in three works which have made their appearance within the last
year,--"The Bermuda Islands," by Professor Angelo Heilprin; "Corals and
Coral-Islands," new edition by Professor J.D. Dana; and the third edition
of Darwin's "Coral-Reefs," with Notes and Appendix by Professor T.G.
Bonney.
Most readers will, I think, rise from the perusal of these works with the
conviction that, while on certain points of detail it is clear that,
through the want of knowledge concerning the action of marine organisms in
the open ocean, Darwin was betrayed into some grave errors, yet the main
foundations of his argument have not been seriously impaired by the new
facts observed in the deep-sea researches, or by the severe criticism to
which his theory has been subjected during the last ten years. On the
other hand, I think it will appear that much misapprehension has been
exhibited by some of Darwin's critics, as to what his views and arguments
really were; so that the reprint and wide circulation of the book in its
original form is greatly to be desired, and cannot but be attended with
advantage to all those who will have the fairness to acquaint themselves
with Darwin's views at first hand, before attempting to reply to them.
JOHN W. JUDD.
CORAL-REEFS.
INTRODUCTION.
The object of this volume is to describe from my own observation and the
works of others, the principal kinds of coral-reefs, more especially those
occurring in the open ocean, and to explain the origin of their peculiar
forms. I do not here treat of the polypifers, which construct these vast
works, except so far as relates to their distribution, and to the
conditions favourable to their vigorous growth. Without any distinct
intention to classify coral-reefs, most voyagers have spoken of them under
the following heads: "lagoon-islands," or "atolls," "barrier" or
"encircling reefs," and "fringing" or "shore-reefs." The lagoon-islands
have received much the most attention; and it is not surprising, for every
one must be struck with astonishment, when he first beholds one of these
vast rings of coral-rock, often many leagues in diameter, here and there
surmounted by a low verdant island with dazzling white shores, bathed on
the outside by the foaming breakers of the ocean, and on the inside
surrounding a calm expanse of water, which from reflection, is of a bright
but pale green colour. The naturalist will feel this astonishment more
deeply after having examined the soft and almost gelatinous bodies of these
apparently insignificant creatures, and when he knows that the solid reef
increases only on the outer edge, which day and night is lashed by the
breakers of an ocean never at rest. Well did Francois Pyrard de Laval, in
the year 1605, exclaim, "C'est une merueille de voir chacun de ces
atollons, enuironne d'un grand banc de pierre tout autour, n'y ayant point
d'artifice humain." The accompanying sketch of Whitsunday island, in the
South Pacific, taken from Captain Beechey's admirable "Voyage," although
excellent of its kind, gives but a faint idea of the singular aspect of one
of these lagoon-islands.
(PLATE: UNTITLED WOODCUT, WHITSUNDAY ATOLL.)
Whitsunday Island is of small size, and the whole circle has been converted
into land, which is a comparatively rare circumstance. As the reef of a
lagoon-island generally supports many separate small islands, the word
"island," applied to the whole, is often the cause of confusion; hence I
have invariably used in this volume the term "atoll," which is the name
given to these circular groups of coral-islets by their inhabitants in the
Indian Ocean, and is synonymous with "lagoon-island."
(PLATE: UNTITLED WOODCUT, REEF AT BOLABOLA ISLAND.)
Barrier-reefs, when encircling small islands, have been comparatively
little noticed by voyagers; but they well deserve attention. In their
structure they are little less marvellous than atolls, and they give a
singular and most picturesque character to the scenery of the islands they
surround. In the accompanying sketch, taken from the "Voyage of the
'Coquille'," the reef is seen from within, from one of the high peaks of
the island of Bolabola. (I have taken the liberty of simplifying the
foreground, and leaving out a mountainous island in the far distance.)
Here, as in Whitsunday Island, the whole of that part of the reef which is
visible is converted into land. This is a circumstance of rare occurrence;
more usually a snow-white line of great breakers, with here and there an
islet crowned by cocoa-nut trees, separates the smooth waters of the
lagoon-like channel from the waves of the open sea. The barrier-reefs of
Australia and of New Caledonia, owing to their enormous dimensions, have
excited much attention: in structure and form they resemble those
encircling many of the smaller islands in the Pacific Ocean.
With respect to fringing, or shore-reefs, there is little in their
structure which needs explanation; and their name expresses their
comparatively small extension. They differ from barrier-reefs in not lying
so far from the shore, and in not having within a broad channel of deep
water. Reefs also occur around submerged banks of sediment and of worn-down
rock; and others are scattered quite irregularly where the sea is very
shallow; these in most respects are allied to those of the fringing class,
but they are of comparatively little interest.
I have given a separate chapter to each of the above classes, and have
described some one reef or island, on which I possessed most information,
as typical; and have afterwards compared it with others of a like kind.
Although this classification is useful from being obvious, and from
including most of the coral-reefs existing in the open sea, it admits of a
more fundamental division into barrier and atoll-formed reefs on the one
hand, where there is a great apparent difficulty with respect to the
foundation on which they must first have grown; and into fringing-reefs on
the other, where, owing to the nature of the slope of the adjoining land,
there is no such difficulty. The two blue tints and the red colour
(replaced by numbers in this edition.) on the map (Plate III.), represent
this main division, as explained in the beginning of the last chapter. In
the Appendix, every existing coral-reef, except some on the coast of Brazil
not included in the map, is briefly described in geographical order, as far
as I possessed information; and any particular spot may be found by
consulting the Index.
Several theories have been advanced to explain the origin of atolls or
lagoon-islands, but scarcely one to account for barrier-reefs. From the
limited depths at which reef-building polypifers can flourish, taken into
consideration with certain other circumstances, we are compelled to
conclude, as it will be seen, that both in atolls and barrier-reefs, the
foundation on which the coral was primarily attached, has subsided; and
that during this downward movement, the reefs have grown upwards. This
conclusion, it will be further seen, explains most satisfactorily the
outline and general form of atolls and barrier-reefs, and likewise certain
peculiarities in their structure. The distribution, also, of the different
kinds of coral-reefs, and their position with relation to the areas of
recent elevation, and to the points subject to volcanic eruptions, fully
accord with this theory of their origin. (A brief account of my views on
coral formations, now published in my Journal of Researches, was read May
31st, 1837, before the Geological Society, and an abstract has appeared in
the Proceedings.)
(DESCRIPTION OF THE PLATES.
PLATE I.--MAP SHOWING THE RESEMBLANCE IN FORM BETWEEN BARRIER CORAL-REEFS
SURROUNDING MOUNTAINOUS ISLANDS, AND ATOLLS OR LAGOON ISLANDS.)
In the several original surveys, from which the small plans on this plate
have been reduced, the coral-reefs are engraved in very different styles.
For the sake of uniformity, I have adopted the style used in the charts of
the Chagos Archipelago, published by the East Indian Company, from the
survey by Captain Moresby and Lieutenant Powell. The surface of the reef,
which dries at low water, is represented by a surface with small crosses:
the coral-islets on the reef are marked by small linear spaces, on which a
few cocoa-nut trees, out of all proportion too large, have been introduced
for the sake of clearness. The entire ANNULAR REEF, which when surrounding
an open expanse of water, forms an "atoll," and when surrounding one or
more high islands, forms an encircling "barrier-reef," has a nearly uniform
structure. The reefs in some of the original surveys are represented
merely by a single line with crosses, so that their breadth is not given; I
have had such reefs engraved of the width usually attained by coral-reefs.
I have not thought it worth while to introduce all those small and very
numerous reefs, which occur within the lagoons of most atolls and within
the lagoon-channels of most barrier-reefs, and which stand either isolated,
or are attached to the shores of the reef or land. At Peros Banhos none of
the lagoon-reefs rise to the surface of the water; a few of them have
been introduced, and are marked by plain dotted circles. A few of the
deepest soundings are laid down within each reef; they are in fathoms, of
six English feet.
Figure 1.--VANIKORO, situated in the western part of the South Pacific;
taken from the survey by Captain D'Urville in the "Astrolabe;" the
soundings on the southern side of the island, namely, from thirty to forty
fathoms, are given from the voyage of the Chev. Dillon; the other soundings
are laid down from the survey by D'Urville; height of the summit of the
island is 3,032 feet. The principal small detached reefs within the
lagoon-channel have in this instance been represented. The southern shore
of the island is narrowly fringed by a reef: if the engraver had carried
this reef entirely round both islands, this figure would have served (by
leaving out in imagination the barrier-reef) as a good specimen of an
abruptly-sided island, surrounded by a reef of the fringing class.
Figure 2.--HOGOLEU, or ROUG, in the Caroline Archipelago; taken from the
"Atlas of the Voyage of the 'Astrolabe,'" compiled from the surveys of
Captains Duperrey and D'Urville; the depth of the immense lagoon-like space
within the reef is not known.
Figure 3.--RAIATEA, in the Society Archipelago; from the map given in the
quarto edition of "Cook's First Voyage;" it is probably not accurate.
Figure 4.--BOW, or HEYOU ATOLL (or lagoon-island), in the Low Archipelago,
from the survey by Captain Beechey, R.N.; the lagoon is choked up with
reefs, but the average greatest depth of about twenty fathoms, is given
from the published account of the voyage.
Figure 5.--BOLABOLA, in the Society Archipelago, from the survey of Captain
Duperrey in the "Coquille:" the soundings in this and the following figures
have been altered from French feet to English fathoms; height of highest
point of the island 4,026 feet.
Figure 6.--MAURUA, in the Society Archipelago; from the survey by Captain
Duperrey in the "Coquille:" height of land about eight hundred feet.
Figure 7.--POUYNIPETE, or SENIAVINE, in the Caroline Archipelago; from the
survey by Admiral Lutke.
Figure 8.--GAMBIER ISLANDS, in the southern part of the Low Archipelago;
from the survey by Captain Beechey; height of highest island, 1,246 feet;
the islands are surrounded by extensive and irregular reefs; the reef on
the southern side is submerged.
Figure 9.--PEROS BANHOS ATOLL (or lagoon-island), in the Chagos group in
the Indian Ocean; from the survey by Captain Moresby and Lieutenant Powell;
not nearly all the small submerged reefs in the lagoon are represented; the
annular reef on the southern side is submerged.
Figure 10.--KEELING, or COCOS ATOLL (or lagoon-island), in the Indian
Ocean; from the survey by Captain Fitzroy; the lagoon south of the dotted
line is very shallow, and is left almost bare at low water; the part north
of the line is choked up with irregular reefs. The annular reef on the
north-west side is broken, and blends into a shoal sandbank, on which the
sea breaks.
CHAPTER I.--ATOLLS OR LAGOON-ISLANDS.
SECTION 1.I.--KEELING ATOLL.
Corals on the outer margin.--Zone of Nulliporae.--Exterior reef.--Islets.--
Coral-conglomerate.--Lagoon.--Calcareous sediment.--Scari and Holuthuriae
subsisting on corals.--Changes in the condition of the reefs and islets.--
Probable subsidence of the atoll.--Future state of the lagoon.
(PLATE: UNTITLED WOODCUT, VERTICAL SECTION THROUGH KEELING ATOLL.)
A.--Level of the sea at low water: where the letter A is placed, the depth
is twenty-five fathoms, and the distance rather more than one hundred and
fifty yards from the edge of the reef.
B.--Outer edge of that flat part of the reef, which dries at low water:
the edge either consists of a convex mound, as represented, or of rugged
points, like those a little farther seaward, beneath the water.
C.--A flat of coral-rock, covered at high water.
D.--A low projecting ledge of brecciated coral-rock washed by the waves at
high water.
E.--A slope of loose fragments, reached by the sea only during gales: the
upper part, which is from six to twelve feet high, is clothed with
vegetation. The surface of the islet gently slopes to the lagoon.
F.--Level of the lagoon at low water.
KEELING or COCOS atoll is situated in the Indian Ocean, in 12 deg 5' S.,
and longitude 90 deg 55' E.: a reduced chart of it was made from the
survey of Captain Fitzroy and the Officers of H.M.S. "Beagle," is given in
Plate I., Figure 10. The greatest width of this atoll is nine miles and a
half. Its structure is in most respects characteristic of the class to
which it belongs, with the exception of the shallowness of the lagoon. The
accompanying woodcut represents a vertical section, supposed to be drawn at
low water from the outer coast across one of the low islets (one being
taken of average dimensions) to within the lagoon.
The section is true to the scale in a horizontal line, but it could not be
made so in a vertical one, as the average greatest height of the land is
only between six and twelve feet above high-water mark.
I will describe the section, commencing with the outer margin. I must
first observe that the reef-building polypifers, not being tidal animals,
require to be constantly submerged or washed by the breakers. I was
assured by Mr. Liesk, a very intelligent resident on these islands, as well
as by some chiefs at Tahiti (Otaheite), that an exposure to the rays of the
sun for a very short time invariably causes their destruction. Hence it is
possible only under the most favourable circumstances, afforded by an
unusually low tide and smooth water, to reach the outer margin, where the
coral is alive. I succeeded only twice in gaining this part, and found it
almost entirely composed of a living Porites, which forms great irregularly
rounded masses (like those of an Astraea, but larger) from four to eight
feet broad, and little less in thickness. These mounds are separated from
each other by narrow crooked channels, about six feet deep, most of which
intersect the line of reef at right angles. On the furthest mound, which I
was able to reach by the aid of a leaping-pole, and over which the sea
broke with some violence, although the day was quite calm and the tide low,
the polypifers in the uppermost cells were all dead, but between three and
four inches lower down on its side they were living, and formed a
projecting border round the upper and dead surface. The coral being thus
checked in its upward growth, extends laterally, and hence most of the
masses, especially those a little further inwards, had broad flat dead
summits. On the other hand I could see, during the recoil of the breakers,
that a few yards further seaward, the whole convex surface of the Porites
was alive; so that the point where we were standing was almost on the exact
upward and shoreward limit of existence of those corals which form the
outer margin of the reef. We shall presently see that there are other
organic productions, fitted to bear a somewhat longer exposure to the air
and sun.
Next, but much inferior in importance to the Porites, is the Millepora
complanata. (This Millepora (Palmipora of Blainville), as well as the M.
alcicornis, possesses the singular property of stinging the skin where it
is delicate, as on the face and arm.)
It grows in thick vertical plates, intersecting each other at various
angles, and forms an exceedingly strong honeycombed mass, which generally
affects a circular form, the marginal plates alone being alive. Between
these plates and in the protected crevices on the reef, a multitude of
branching zoophytes and other productions flourish, but the Porites and
Millepora alone seem able to resist the fury of the breakers on its upper
and outer edge: at the depth of a few fathoms other kinds of stony corals
live. Mr. Liesk, who was intimately acquainted with every part of this
reef, and likewise with that of North Keeling atoll, assured me that these
corals invariably compose the outer margin. The lagoon is inhabited by
quite a distinct set of corals, generally brittle and thinly branched; but
a Porites, apparently of the same species with that on the outside, is
found there, although it does not seem to thrive, and certainly does not
attain the thousandth part in bulk of the masses opposed to the breakers.
The woodcut shows the form of the bottom off the reef: the water deepens
for a space between one and two hundred yards wide, very gradually to
twenty-five fathoms (A in section), beyond which the sides plunge into the
unfathomable ocean at an angle of 45 deg. (The soundings from which this
section is laid down were taken with great care by Captain Fitzroy himself.
He used a bell-shaped lead, having a diameter of four inches, and the
armings each time were cut off and brought on board for me to examine. The
arming is a preparation of tallow, placed in the concavity at the bottom of
the lead. Sand, and even small fragments of rock, will adhere to it; and
if the bottom be of rock it brings up an exact impression of its surface.)
To the depth of ten or twelve fathoms the bottom is exceedingly rugged, and
seems formed of great masses of living coral, similar to those on the
margin. The arming of the lead here invariably came up quite clean, but
deeply indented, and chains and anchors which were lowered, in the hopes of
tearing up the coral, were broken. Many small fragments, however, of
Millepora alcicornis were brought up; and on the arming from an eight-fathom
cast, there was a perfect impression of an Astraea, apparently
alive. I examined the rolled fragments cast on the beach during gales, in
order further to ascertain what corals grew outside the reef. The
fragments consisted of many kinds, of which the Porites already mentioned
and a Madrepora, apparently the M. corymbosa, were the most abundant. As I
searched in vain in the hollows on the reef and in the lagoon, for a living
specimen of this Madrepore, I conclude that it is confined to a zone
outside, and beneath the surface, where it must be very abundant.
Fragments of the Millepora alcicornis and of an Astraea were also numerous;
the former is found, but not in proportionate numbers, in the hollows on
the reef; but the Astraea I did not see living. Hence we may infer, that
these are the kinds of coral which form the rugged sloping surface
(represented in the woodcut by an uneven line), round and beneath the
external margin. Between twelve and twenty fathoms the arming came up an
equal number of times smoothed with sand, and indented with coral: an
anchor and lead were lost at the respective depths of thirteen and sixteen
fathoms. Out of twenty-five soundings taken at a greater depth than twenty
fathoms, every one showed that the bottom was covered with sand; whereas,
at a less depth than twelve fathoms, every sounding showed that it was
exceedingly rugged, and free from all extraneous particles. Two soundings
were obtained at the depth of 360 fathoms, and several between two hundred
and three hundred fathoms. The sand brought up from these depths consisted
of finely triturated fragments of stony zoophytes, but not, as far as I
could distinguish, of a particle of any lamelliform genus: fragments of
shells were rare.
At a distance of 2,200 yards from the breakers, Captain Fitzroy found no
bottom with a line of 7,200 feet in length; hence the submarine slope of
this coral formation is steeper than that of any volcanic cone. Off the
mouth of the lagoon, and likewise off the northern point of the atoll,
where the currents act violently, the inclination, owing to the
accumulation of sediment, is less. As the arming of the lead from all the
greater depths showed a smooth sandy bottom, I at first concluded that the
whole consisted of a vast conical pile of calcareous sand, but the sudden
increase of depth at some points, and the circumstance of the line having
been cut, as if rubbed, when between five hundred and six hundred fathoms
were out, indicate the probable existence of submarine cliffs.
On the margin of the reef, close within the line where the upper surface of
the Porites and of the Millepora is dead, three species of Nullipora
flourish. One grows in thin sheets, like a lichen on old trees; the second
in stony knobs, as thick as a man's finger, radiating from a common centre;
and the third, which is less common, in a moss-like reticulation of thin,
but perfectly rigid branches. (This last species is of a beautiful bright
peach-blossom colour. Its branches are about as thick as crow-quills; they
are slightly flattened and knobbed at the extremities. The extremities
only are alive and brightly coloured. The two other species are of a dirty
purplish-white. The second species is extremely hard; its short knob-like
branches are cylindrical, and do not grow thicker at their extremities.)
The three species occur either separately or mingled together; and they
form by their successive growth a layer two or three feet in thickness,
which in some cases is hard, but where formed of the lichen-like kind,
readily yields an impression to the hammer: the surface is of a reddish
colour. These Nulliporae, although able to exist above the limit of true
corals, seem to require to be bathed during the greater part of each tide
by breaking water, for they are not found in any abundance in the protected
hollows on the back part of the reef, where they might be immersed either
during the whole or an equal proportional time of each tide. It is
remarkable that organic productions of such extreme simplicity, for the
Nulliporae undoubtedly belong to one of the lowest classes of the vegetable
kingdom, should be limited to a zone so peculiarly circumstanced. Hence
the layer composed by their growth merely fringes the reef for a space of
about twenty yards in width, either under the form of separate mammillated
projections, where the outer masses of coral are separate, or, more
commonly, where the corals are united into a solid margin, as a continuous
smooth convex mound (B in woodcut), like an artificial breakwater. Both
the mound and mammillated projections stand about three feet higher than
any other part of the reef, by which term I do not include the islets,
formed by the accumulation of rolled fragments. We shall hereafter see
that other coral reefs are protected by a similar thick growth of
Nulliporae on the outer margin, the part most exposed to the breakers, and
this must effectually aid in preserving it from being worn down.
The woodcut represents a section across one of the islets on the reef, but
if all that part which is above the level of C were removed, the section
would be that of a simple reef, as it occurs where no islet has been
formed. It is this reef which essentially forms the atoll. It is a ring,
enclosing the lagoon on all sides except at the northern end, where there
are two open spaces, through one of which ships can enter. The reef varies
in width from two hundred and fifty to five hundred yards, its surface is
level, or very slightly inclined towards the lagoon, and at high tide the
sea breaks entirely over it: the water at low tide thrown by the breakers
on the reef, is carried by the many narrow and shoal gullies or channels on
its surface, into the lagoon: a return stream sets out of the lagoon
through the main entrance. The most frequent coral in the hollows on the
reef is Pocillopora verrucosa, which grows in short sinuous plates, or
branches, and when alive is of a beautiful pale lake-red: a Madrepora,
closely allied or identical with M. pocillifera, is also common. As soon
as an islet is formed, and the waves are prevented breaking entirely over
the reef, the channels and hollows in it become filled up with cemented
fragments, and its surface is converted into a hard smooth floor (C of
woodcut), like an artificial one of freestone. This flat surface varies in
width from one hundred to two hundred, or even three hundred yards, and is
strewed with a few large fragments of coral torn up during gales: it is
uncovered only at low water. I could with difficulty, and only by the aid
of a chisel, procure chips of rock from its surface, and therefore could
not ascertain how much of it is formed by the aggregation of detritus, and
how much by the outward growth of mounds of corals, similar to those now
living on the margin. Nothing can be more singular than the appearance at
low tide of this "flat" of naked stone, especially where it is externally
bounded by the smooth convex mound of Nulliporae, appearing like a
breakwater built to resist the waves, which are constantly throwing over it
sheets of foaming water. The characteristic appearance of this "flat" is
shown in the foregoing woodcut of Whitsunday atoll.
The islets on the reef are first formed between two hundred and three
hundred yards from its outer edge, through the accumulation of a pile of
fragments, thrown together by some unusually strong gale. Their ordinary
width is under a quarter of a mile, and their length varies from a few
yards to several miles. Those on the south-east and windward side of the
atoll, increase solely by the addition of fragments on their outer side;
hence the loose blocks of coral, of which their surface is composed, as
well as the shells mingled with them, almost exclusively consist of those
kinds which live on the outer coast. The highest part of the islets
(excepting hillocks of blown sand, some of which are thirty feet high), is
close to the outer beach (E of the woodcut), and averages from six to ten
feet above ordinary high-water mark. From the outer beach the surface
slopes gently to the shores of the lagoon, which no doubt has been caused
by the breakers the further they have rolled over the reef, having had less
power to throw up fragments. The little waves of the lagoon heap up sand
and fragments of thinly-branched corals on the inner side of the islets on
the leeward side of the atoll; and these islets are broader than those to
windward, some being even eight hundred yards in width; but the land thus
added is very low. The fragments beneath the surface are cemented into a
solid mass, which is exposed as a ledge (D of the woodcut), projecting some
yards in front of the outer shore and from two to four feet high. This
ledge is just reached by the waves at ordinary high-water: it extends in
front of all the islets, and everywhere has a water-worn and scooped
appearance. The fragments of coral which are occasionally cast on the
"flat" are during gales of unusual violence swept together on the beach,
where the waves each day at high-water tend to remove and gradually wear
them down; but the lower fragments having become firmly cemented together
by the percolation of calcareous matter, resist the daily tides longer, and
hence project as a ledge. The cemented mass is generally of a white
colour, but in some few parts reddish from ferruginous matter; it is very
hard, and is sonorous under the hammer; it is obscurely divided by seams,
dipping at a small angle seaward; it consists of fragments of the corals
which grow on the outer margin, some quite and others partially rounded,
some small and others between two and three feet across; and of masses of
previously formed conglomerate, torn up, rounded, and re-cemented; or it
consists of a calcareous sandstone, entirely composed of rounded particles,
generally almost blended together, of shells, corals, the spines of echini,
and other such organic bodies; rocks, of this latter kind, occur on many
shores, where there are no coral reefs. The structure of the coral in the
conglomerate has generally been much obscured by the infiltration of
spathose calcareous matter; and I collected a very interesting series,
beginning with fragments of unaltered coral, and ending with others, where
it was impossible to discover with the naked eye any trace of organic
structure. In some specimens I was unable, even with the aid of a lens,
and by wetting them, to distinguish the boundaries of the altered coral and
spathose limestone. Many even of the blocks of coral lying loose on the
beach, had their central parts altered and infiltrated.
The lagoon alone remains to be described; it is much shallower than that of
most atolls of considerable size. The southern part is almost filled up
with banks of mud and fields of coral, both dead and alive, but there are
considerable spaces, between three and four fathoms, and smaller basins,
from eight to ten fathoms deep. Probably about half its area consists of
sediment, and half of coral-reefs. The corals composing these reefs have a
very different aspect from those on the outside; they are very numerous in
kind, and most of them are thinly branched. Meandrina, however, lives in
the lagoon, and great rounded masses of this coral are numerous, lying
quite or almost loose on the bottom. The other commonest kinds consist of
three closely allied species of true Madrepora in thin branches; of
Seriatapora subulata; two species of Porites (This Porites has somewhat the
habit of P. clavaria, but the branches are not knobbed at their ends. When
alive it is of a yellow colour, but after having been washed in fresh water
and placed to dry, a jet-black slimy substance exuded from the entire
surface, so that the specimen now appears as if it had been dipped in ink.)
with cylindrical branches, one of which forms circular clumps, with the
exterior branches only alive; and lastly, a coral something like an
Explanaria, but with stars on both surfaces, growing in thin, brittle,
stony, foliaceous expansions, especially in the deeper basins of the
lagoon. The reefs on which these corals grow are very irregular in form,
are full of cavities, and have not a solid flat surface of dead rock, like
that surrounding the lagoon; nor can they be nearly so hard, for the
inhabitants made with crowbars a channel of considerable length through
these reefs, in which a schooner, built on the S.E. islet, was floated out.
It is a very interesting circumstance, pointed out to us by Mr. Liesk, that
this channel, although made less than ten years before our visit, was then,
as we saw, almost choked up with living coral, so that fresh excavations
would be absolutely necessary to allow another vessel to pass through it.
The sediment from the deepest parts in the lagoon, when wet, appeared
chalky, but when dry, like very fine sand. Large soft banks of similar,
but even finer grained mud, occur on the S.E. shore of the lagoon,
affording a thick growth of a Fucus, on which turtle feed: this mud,
although discoloured by vegetable matter, appears from its entire solution
in acids to be purely calcareous. I have seen in the Museum of the
Geological Society, a similar but more remarkable substance, brought by
Lieutenant Nelson from the reefs of Bermuda, which, when shown to several
experienced geologists, was mistaken by them for true chalk. On the
outside of the reef much sediment must be formed by the action of the surf
on the rolled fragments of coral; but in the calm waters of the lagoon,
this can take place only in a small degree. There are, however, other and
unexpected agents at work here: large shoals of two species of Scarus, one
inhabiting the surf outside the reef and the other the lagoon, subsist
entirely, as I was assured by Mr. Liesk, the intelligent resident before
referred to, by browsing on the living polypifers. I opened several of
these fish, which are very numerous and of considerable size, and I found
their intestines distended by small pieces of coral, and finely ground
calcareous matter. This must daily pass from them as the finest sediment;
much also must be produced by the infinitely numerous vermiform and
molluscous animals, which make cavities in almost every block of coral.
Dr. J. Allan, of Forres, who has enjoyed the best means of observation,
informs me in a letter that the Holothuriae (a family of Radiata) subsist
on living coral; and the singular structure of bone within the anterior
extremity of their bodies, certainly appears well adapted for this purpose.
The number of the species of Holothuria, and of the individuals which swarm
on every part of these coral-reefs, is extraordinarily great; and many
shiploads are annually freighted, as is well-known, for China with the
trepang, which is a species of this genus. The amount of coral yearly
consumed, and ground down into the finest mud, by these several creatures,
and probably by many other kinds, must be immense. These facts are,
however, of more importance in another point of view, as showing us that
there are living checks to the growth of coral-reefs, and that the almost
universal law of "consumed and be consumed," holds good even with the
polypifers forming those massive bulwarks, which are able to withstand the
force of the open ocean.
Considering that Keeling atoll, like other coral formations, has been
entirely formed by the growth of organic beings, and the accumulation of
their detritus, one is naturally led to inquire how long it has continued,
and how long it is likely to continue, in its present state. Mr. Liesk
informed me that he had seen an old chart in which the present long island
on the S.E. side was divided by several channels into as many islets; and
he assures me that the channels can still be distinguished by the smaller
size of the trees on them. On several islets, also, I observed that only
young cocoa-nut trees were growing on the extremities; and that older and
taller trees rose in regular succession behind them; which shows that these
islets have very lately increased in length. In the upper and south-eastern
part of the lagoon, I was much surprised by finding an irregular
field of at least a mile square of branching corals, still upright, but
entirely dead. They consisted of the species already mentioned; they were
of a brown colour, and so rotten, that in trying to stand on them I sank
halfway up the leg, as if through decayed brushwood. The tops of the
branches were barely covered by water at the time of lowest tide. Several
facts having led me to disbelieve in any elevation of the whole atoll, I
was at first unable to imagine what cause could have killed so large a
field of coral. Upon reflection, however, it appeared to me that the
closing up of the above-mentioned channels would be a sufficient cause; for
before this, a strong breeze by forcing water through them into the head of
the lagoon, would tend to raise its level. But now this cannot happen, and
the inhabitants observe that the tide rises to a less height, during a high
S.E. wind, at the head than at the mouth of the lagoon. The corals, which,
under the former condition of things, had attained the utmost possible
limit of upward growth, would thus occasionally be exposed for a short time
to the sun, and be killed.
Besides the increase of dry land, indicated by the foregoing facts, the
exterior solid reef appears to have grown outwards. On the western side of
the atoll, the "flat" lying between the margin of the reef and the beach,
is very wide; and in front of the regular beach with its conglomerate
basis, there is, in most parts, a bed of sand and loose fragments with
trees growing out of it, which apparently is not reached even by the spray
at high water. It is evident some change has taken place since the waves
formed the inner beach; that they formerly beat against it with violence
was evident, from a remarkably thick and water-worn point of conglomerate
at one spot, now protected by vegetation and a bank of sand; that they beat
against it in the same peculiar manner in which the swell from windward now
obliquely curls round the margin of the reef, was evident from the
conglomerate having been worn into a point projecting from the beach in a
similarly oblique manner. This retreat in the line of action of the
breakers might result, either from the surface of the reef in front of the
islets having been submerged at one time, and afterward having grown
upwards, or from the mounds of coral on the margin having continued to grow
outwards. That an outward growth of this part is in process, can hardly be
doubted from the fact already mentioned of the mounds of Porites with their
summits apparently lately killed, and their sides only three or four inches
lower down thickened by a fresh layer of living coral. But there is a
difficulty on this supposition which I must not pass over. If the whole,
or a large part of the "flat," had been formed by the outward growth of the
margin, each successive margin would naturally have been coated by the
Nulliporae, and so much of the surface would have been of equal height with
the existing zone of living Nulliporae: this is not the case, as may be
seen in the woodcut. It is, however, evident from the abraded state of the
"flat," with its original inequalities filled up, that its surface has been
much modified; and it is possible that the hinder portions of the zone of
Nulliporae, perishing as the reef grows outwards, might be worn down by the
surf. If this has not taken place, the reef can in no part have increased
outwards in breadth since its formation, or at least since the Nulliporae
formed the convex mound on its margin; for the zone thus formed, and which
stands between two and three feet above the other parts of the reef, is
nowhere much above twenty yards in width.
Thus far we have considered facts, which indicate, with more or less
probability, the increase of the atoll in its different parts: there are
others having an opposite tendency. On the south-east side, Lieutenant
Sulivan, to whose kindness I am indebted for many interesting observations,
found the conglomerate projecting on the reef nearly fifty yards in front
of the beach: we may infer from what we see in all other parts of the
atoll, that the conglomerate was not originally so much exposed, but formed
the base of an islet, the front and upper part of which has since been
swept away. The degree to which the conglomerate, round nearly the whole
atoll, has been scooped, broken up, and the fragments cast on the beach, is
certainly very surprising, even on the view that it is the office of
occasional gales to pile up fragments, and of the daily tides to wear them
away. On the western side, also, of the atoll, where I have described a
bed of sand and fragments with trees growing out of it, in front of an old
beach, it struck both Lieutenant Sulivan and myself, from the manner in
which the trees were being washed down, that the surf had lately
recommenced an attack on this line of coast. Appearances indicating a
slight encroachment of the water on the land, are plainer within the
lagoon: I noticed in several places, both on its windward and leeward
shores, old cocoa-nut trees falling with their roots undermined, and the
rotten stumps of others on the beach, where the inhabitants assured us the
cocoa-nut could not now grow. Captain Fitzroy pointed out to me, near the
settlement, the foundation posts of a shed, now washed by every tide, but
which the inhabitants stated, had seven years before stood above high
watermark. In the calm waters of the lagoon, directly connected with a
great, and therefore stable ocean, it seems very improbable that a change
in the currents, sufficiently great to cause the water to eat into the land
on all sides, should have taken place within a limited period. From these
considerations I inferred, that probably the atoll had lately subsided to a
small amount; and this inference was strengthened by the circumstance, that
in 1834, two years before our visit, the island had been shaken by a severe
earthquake, and by two slighter ones during the ten previous years. If,
during these subterranean disturbances, the atoll did subside, the downward
movement must have been very small, as we must conclude from the fields of
dead coral still lipping the surface of the lagoon, and from the breakers
on the western shore not having yet regained the line of their former
action. The subsidence must, also, have been preceded by a long period of
rest, during which the islets extended to their present size, and the
living margin of the reef grew either upwards, or as I believe outwards, to
its present distance from the beach.
Whether this view be correct or not, the above facts are worthy of
attention, as showing how severe a struggle is in progress on these low
coral formations between the two nicely balanced powers of land and water.
With respect to the future state of Keeling atoll, if left undisturbed, we
can see that the islets may still extend in length; but as they cannot
resist the surf until broken by rolling over a wide space, their increase
in breadth must depend on the increasing breadth of the reef; and this must
be limited by the steepness of the submarine flanks, which can be added to
only by sediment derived from the wear and tear of the coral. From the
rapid growth of the coral in the channel cut for the schooner, and from the
several agents at work in producing fine sediment, it might be thought that
the lagoon would necessarily become quickly filled up. Some of this
sediment, however, is transported into the open sea, as appears from the
soundings off the mouth of the lagoon, instead of being deposited within
it. The deposition, moreover, of sediment, checks the growth of coral-reefs,
so that these two agencies cannot act together with full effect in
filling it up. We know so little of the habits of the many different
species of corals, which form the lagoon-reefs, that we have no more
reasons for supposing that their whole surface would grow up as quickly as
the coral did in the schooner-channel, than for supposing that the whole
surface of a peat-moss would increase as quickly as parts are known to do
in holes, where the peat has been cut away. These agencies, nevertheless,
tend to fill up the lagoon; but in proportion as it becomes shallower, so
must the polypifers be subject to many injurious agencies, such as impure
water and loss of food. For instance, Mr. Liesk informed me, that some
years before our visit unusually heavy rain killed nearly all the fish in
the lagoon, and probably the same cause would likewise injure the corals.
The reefs also, it must be remembered, cannot possibly rise above the level
of the lowest spring-tide, so that the final conversion of the lagoon into
land must be due to the accumulation of sediment; and in the midst of the
clear water of the ocean, and with no surrounding high land, this process
must be exceedingly slow.
SECTION 1.II.--GENERAL DESCRIPTION OF ATOLLS.
General form and size of atolls, their reefs and islets.--External slope.--
Zone of Nulliporae.--Conglomerate.--Depth of lagoons.--Sediment.--Reefs
submerged wholly or in part.--Breaches in the reef.--Ledge-formed shores
round certain lagoons.--Conversion of lagoons into land.
I will here give a sketch of the general form and structure of the many
atolls and atoll-formed reefs which occur in the Pacific and Indian Oceans,
comparing them with Keeling atoll. The Maldiva atolls and the Great Chagos
Bank differ in so many respects, that I shall devote to them, besides
occasional references, a third section of this chapter. Keeling atoll may
be considered as of moderate dimensions and of regular form. Of the
thirty-two islands surveyed by Captain Beechey in the Low Archipelago, the
longest was found to be thirty miles, and the shortest less than a mile;
but Vliegen atoll, situated in another part of the same group, appears to
be sixty miles long and twenty broad. Most of the atolls in this group are
of an elongated form; thus Bow Island is thirty miles in length, and on an
average only six in width (See Figure 4, Plate I.), and Clermont Tonnere
has nearly the same proportions. In the Marshall Archipelago (the Ralick
and Radack group of Kotzebue) several of the atolls are more than thirty
miles in length, and Rimsky Korsacoff is fifty-four long, and twenty wide,
at the broadest part of its irregular outline. Most of the atolls in the
Maldiva Archipelago are of great size, one of them (which, however, bears a
double name) measured in a medial and slightly curved line, is no less than
eighty-eight geographical miles long, its greatest width being under
twenty, and its least only nine and a half miles. Some atolls have spurs
projecting from them; and in the Marshall group there are atolls united
together by linear reefs, for instance Menchikoff Island (See Figure 3,
Plate II.), which is sixty miles in length, and consists of three loops
tied together. In far the greater number of cases an atoll consists of a
simple elongated ring, with its outline moderately regular.
The average width of the annular wreath may be taken as about a quarter of
a mile. Captain Beechey (Beechey's "Voyage to the Pacific and Beering's
Straits," chapter viii.) says that in the atolls of the Low Archipelago it
exceeded in no instance half a mile. The description given of the
structure and proportional dimensions of the reef and islets of Keeling
atoll, appears to apply perfectly to nearly all the atolls in the Pacific
and Indian Oceans. The islets are first formed some way back either on the
projecting points of the reef, especially if its form be angular, or on the
sides of the main entrances into the lagoon--that is in both cases, on
points where the breakers can act during gales of wind in somewhat
different directions, so that the matter thrown up from one side may
accumulate against that before thrown up from another. In Lutke's chart of
the Caroline atolls, we see many instances of the former case; and the
occurrence of islets, as if placed for beacons, on the points where there
is a gateway or breach through the reef, has been noticed by several
authors. There are some atoll-formed reefs, rising to the surface of the
sea and partly dry at low water, on which from some cause islets have never
been formed; and there are others on which they have been formed, but have
subsequently been worn away. In atolls of small dimensions the islets
frequently become united into a single horse-shoe or ring-formed strip; but
Diego Garcia, although an atoll of considerable size, being thirteen miles
and a half in length, has its lagoon entirely surrounded, except at the
northern end, by a belt of land, on an average a third of a mile in width.
To show how small the total area of the annular reef and the land is in
islands of this class, I may quote a remark from the voyage of Lutke,
namely, that if the forty-three rings, or atolls, in the Caroline
Archipelago, were put one within another, and over a steeple in the centre
of St. Petersburg, the whole world would not cover that city and its
suburbs.
The form of the bottom off Keeling atoll, which gradually slopes to about
twenty fathoms at the distance of between one and two hundred yards from
the edge of the reef, and then plunges at an angle of 45 deg into
unfathomable depths, is exactly the same (The form of the bottom round the
Marshall atolls in the Northern Pacific is probably similar: Kotzebue
("First Voyage," volume ii., page 16) says: "We had at a small distance
from the reef, forty fathoms depth, which increased a little further so
much that we could find no bottom.") with that of the sections of the
atolls in the Low Archipelago given by Captain Beechey. The nature,
however, of the bottom seems to differ, for this officer (I must be
permitted to express my obligation to Captain Beechey, for the very kind
manner in which he has given me information on several points, and to own
the great assistance I have derived from his excellent published work.)
informs me that all the soundings, even the deepest, were on coral, but he
does not know whether dead or alive. The slope round Christmas atoll (Lat.
1 deg 4' N., 157 deg 45' W.), described by Cook (Cook's "Third Voyage,"
volume ii., chapter 10.), is considerably less, at about half a mile from
the edge of the reef, the average depth was about fourteen fathoms on a
fine sandy bottom, and at a mile, only between twenty and forty fathoms.
It has no doubt been owing to this gentle slope, that the strip of land
surrounding its lagoon, has increased in one part to the extraordinary
width of three miles; it is formed of successive ridges of broken shells
and corals, like those on the beach. I know of no other instance of such
width in the reef of an atoll; but Mr. F.D. Bennett informs me that the
inclination of the bottom round Caroline atoll in the Pacific, is like that
off Christmas Island, very gentle. Off the Maldiva and Chagos atolls, the
inclination is much more abrupt; thus at Heawandoo Pholo, Lieutenant Powell
(This fact is taken from a MS. account of these groups lent me by Captain
Moresby. See also Captain Moresby's paper on the Maldiva atolls in the
"Geographical Journal", volume v., page 401.) found fifty and sixty fathoms
close to the edge of the reef, and at 300 yards distance there was no
bottom with a 300-yard line. Captain Moresby informs me, that at 100
fathoms from the mouth of the lagoon of Diego Garcia, he found no bottom
with 150 fathoms; this is the more remarkable, as the slope is generally
less abrupt in front of channels through a reef, owing to the accumulation
of sediment. At Egmont Island, also, at 150 fathoms from the reef,
soundings were struck with 150 fathoms. Lastly, at Cardoo atoll, only
sixty yards from the reef, no bottom was obtained, as I am informed by
Captain Moresby, with a line of 200 fathoms! The currents run with great
force round these atolls, and where they are strongest, the inclination
appears to be most abrupt. I am informed by the same authority, that
wherever soundings were obtained off these islands, the bottom was
invariably sandy: nor was there any reason to suspect the existence of
submarine cliffs, as there was at Keeling Island. (Off some of the islands
in the Low Archipelago the bottom appears to descend by ledges. Off
Elizabeth Island, which, however, consists of raised coral, Captain Beechey
(page 45, 4to edition) describes three ledges: the first had an easy slope
from the beach to a distance of about fifty yards: the second extended two
hundred yards with twenty-five fathoms on it, and then ended abruptly, like
the first; and immediately beyond this there was no bottom with two hundred
fathoms.) Here then occurs a difficulty; can sand accumulate on a slope,
which, in some cases, appears to exceed fifty-five degrees? It must be
observed, that I speak of slopes where soundings were obtained, and not of
such cases, as that of Cardoo, where the nature of the bottom is unknown,
and where its inclination must be nearly vertical. M. Elie de Beaumont
("Memoires pour servir a une description Geolog. de France," tome iv., page
216.) has argued, and there is no higher authority on this subject, from
the inclination at which snow slides down in avalanches, that a bed of sand
or mud cannot be formed at a greater angle than thirty degrees.
Considering the number of soundings on sand, obtained round the Maldiva and
Chagos atolls, which appears to indicate a greater angle, and the extreme
abruptness of the sand-banks in the West Indies, as will be mentioned in
the Appendix, I must conclude that the adhesive property of wet sand
counteracts its gravity, in a much greater ratio than has been allowed for
by M. Elie de Beaumont. From the facility with which calcareous sand
becomes agglutinated, it is not necessary to suppose that the bed of loose
sand is thick.
Captain Beechey has observed, that the submarine slope is much less at the
extremities of the more elongated atolls in the Low Archipelago, than at
their sides; in speaking of Ducie's Island he says (Beechey's "Voyage," 4to
edition, page 44.) the buttress, as it may be called, which "has the most
powerful enemy (the S.W. swell) to oppose, is carried out much further, and
with less abruptness than the other." In some cases, the less inclination
of a certain part of the external slope, for instance of the northern
extremities of the two Keeling atolls, is caused by a prevailing current
which there accumulates a bed of sand. Where the water is perfectly
tranquil, as within a lagoon, the reefs generally grow up perpendicularly,
and sometimes even overhang their bases; on the other hand, on the leeward
side of Mauritius, where the water is generally tranquil, although not
invariably so, the reef is very gently inclined. Hence it appears that the
exterior angle varies much; nevertheless in the close similarity in form
between the sections of Keeling atoll and of the atolls in the Low
Archipelago, in the general steepness of the reefs of the Maldiva and
Chagos atolls, and in the perpendicularity of those rising out of water
always tranquil, we may discern the effects of uniform laws; but from the
complex action of the surf and currents, on the growing powers of the coral
and on the deposition of sediment, we can by no means follow out all the
results.
Where islets have been formed on the reef, that part which I have sometimes
called the "flat" and which is partly dry at low water, appears similar in
every atoll. In the Marshall group in the North Pacific, it may be
inferred from Chamisso's description, that the reef, where islets have not
been formed on it, slopes gently from the external margin to the shores of
the lagoon; Flinders states that the Australian barrier has a similar
inclination inwards, and I have no doubt it is of general occurrence,
although, according to Ehrenberg, the reefs of the Red Sea offer an
exception. Chamisso observes that "the red colour of the reef (at the
Marshall atolls) under the breakers is caused by a Nullipora, which covers
the stone WHEREVER THE WAVES BEAT; and, under favourable circumstances,
assumes a stalactical form,"--a description perfectly applicable to the
margin of Keeling atoll. (Kotzebue's "First Voyage," volume iii., page
142. Near Porto Praya, in the Cape de Verde Islands, some basaltic rocks,
lashed by no inconsiderable surf, were completely enveloped with a layer of
Nulliporae. The entire surface over many square inches, was coloured of a
peach-blossomed red; the layer, however, was of no greater thickness than
paper. Another kind, in the form of projecting knobs, grew in the same
situation. These Nulliporae are closely related to those described on the
coral-reefs, but I believe are of different species.) Although Chamisso
does not state that the masses of Nulliporae form points or a mound, higher
than the flat, yet I believe that this is the case; for Kotzebue (Kotzebue,
"First Voyage," volume ii., page 16. Lieutenant Nelson, in his excellent
memoir in the Geological Transactions (volume ii., page 105), alludes to
the rocky points mentioned by Kotzebue, and infers that they consist of
Serpulae, which compose incrusting masses on the reefs of Bermudas, as they
likewise do on a sandstone bar off the coast of Brazil (which I have
described in "London Phil. Journal," October 1841). These masses of
Serpulae hold the same position, relatively to the action of the sea, with
the Nulliporae on the coral-reefs in the Indian and Pacific Oceans.), in
another part, speaks of the rocks on the edge of the reef "as visible for
about two feet at low water," and these rocks we may feel quite certain are
not formed of true coral (Captain Moresby, in his valuable paper "on the
Northern atolls of Maldivas" ("Geographical Journal", volume v.), says that
the edges of the reefs there stand above water at low spring-tides.)
Whether a smooth convex mound of Nulliporae, like that which appears as if
artificially constructed to protect the margin of Keeling Island, is of
frequent occurrence round atolls, I know not; but we shall presently meet
with it, under precisely the same form, on the outer edge of the
"barrier-reefs" which encircle the Society Islands.
There appears to be scarcely a feature in the structure of Keeling reef,
which is not of common, if not of universal occurrence, in other atolls.
Thus Chamisso describes (Kotzebue's "First Voyage," volume iii., page 144.)
a layer of coarse conglomerate, outside the islets round the Marshall
atolls which "appears on its upper surface uneven and eaten away." From
drawings, with appended remarks, of Diego Garcia in the Chagos group and of
several of the Maldiva atolls, shown me by Captain Moresby (see also
Moresby on the Northern atolls of the Maldivas, "Geographical Journal",
volume v., page 400.), it is evident that their outer coasts are subject to
the same round of decay and renovation as those of Keeling atoll. From the
description of the atolls in the Low Archipelago, given in Captain
Beechey's "Voyage," it is not apparent that any conglomerate coral-rock was
there observed.
The lagoon in Keeling atoll is shallow; in the atolls of the Low
Archipelago the depth varies from 20 to 38 fathoms, and in the Marshall
Group, according to Chamisso, from 30 to 35; in the Caroline atolls it is
only a little less. Within the Maldiva atolls there are large spaces with
45 fathoms, and some soundings are laid down of 49 fathoms. The greater
part of the bottom in most lagoons, is formed of sediment; large spaces
have exactly the same depth, or the depth varies so insensibly, that it is
evident that no other means, excepting aqueous deposition, could have
leveled the surface so equally. In the Maldiva atolls this is very
conspicuous, and likewise in some of the Caroline and Marshall Islands. In
the former large spaces consist of sand and SOFT CLAY; and Kotzebue speaks
of clay having been found within one of the Marshall atolls. No doubt this
clay is calcareous mud, similar to that at Keeling Island, and to that at
Bermuda already referred to, as undistinguishable from disintegrated chalk,
and which Lieutenant Nelson says is called there pipe-clay. (I may here
observe that on the coast of Brazil, where there is much coral, the
soundings near the land are described by Admiral Roussin, in the "Pilote du
Bresil", as siliceous sand, mingled with much finely comminuted particles
of shells and coral. Further in the offing, for a space of 1,300 miles
along the coast, from the Abrolhos Islands to Maranham, the bottom in many
places is composed of "tuf blanc, mele ou forme de madrepores broyes."
This white substance, probably, is analogous to that which occurs within
the above-mentioned lagoons; it is sometimes, according to Roussin, firm,
and he compares it to mortar.)
Where the waves act with unequal force on the two sides of an atoll, the
islets appear to be first formed, and are generally of greater continuity
on the more exposed shore. The islets, also, which are placed to leeward,
are in most parts of the Pacific liable to be occasionally swept entirely
away by gales, equalling hurricanes in violence, which blow in an opposite
direction to the ordinary trade-wind. The absence of the islets on the
leeward side of atolls, or when present their lesser dimensions compared
with those to windward, is a comparatively unimportant fact; but in several
instances the reef itself on the leeward side, retaining its usual defined
outline, does not rise to the surface by several fathoms. This is the case
with the southern side of Peros Banhos (Plate I., Figure 9) in the Chagos
group, with Mourileu atoll (Frederick Lutke's "Voyage autour du Monde,"
volume ii., page 291. See also his account of Namonouito, below, and the
chart of Oulleay in the Atlas.) in the Caroline Archipelago, and with the
barrier-reef (Plate I., Figure 8) of the Gambier Islands. I allude to the
latter reef, although belonging to another class, because Captain Beechey
was first led by it to observe the peculiarity in the question. At Peros
Banhos the submerged part is nine miles in length, and lies at an average
depth of about five fathoms; its surface is nearly level, and consists of
hard stone, with a thin covering of loose sand. There is scarcely any
living coral on it, even on the outer margin, as I have been particularly
assured by Captain Moresby; it is, in fact, a wall of dead coral-rock,
having the same width and transverse section with the reef in its ordinary
state, of which it is a continuous portion. The living and perfect parts
terminate abruptly, and abut on the submerged portions, in the same manner
as on the sides of an ordinary passage through the reef. The reef to
leeward in other cases is nearly or quite obliterated, and one side of the
lagoon is left open; for instance, at Oulleay (Caroline Archipelago), where
a crescent-formed reef is fronted by an irregular bank, on which the other
half of the annular reef probably once stood. At Namonouito, in the same
Archipelago, both these modifications of the reef concur; it consists of a
great flat bank, with from twenty to twenty-five fathoms water on it; for a
length of more than forty miles on its southern side it is open and without
any reef, whilst on the other sides it is bounded by a reef, in parts
rising to the surface and perfectly characterised, in parts lying some
fathoms submerged. In the Chagos group there are annular reefs, entirely
submerged, which have the same structure as the submerged and defined
portions just described. The Speaker's Bank offers an excellent example of
this structure; its central expanse, which is about twenty-two fathoms
deep, is twenty-four miles across; the external rim is of the usual width
of annular reefs, and is well-defined; it lies between six and eight
fathoms beneath the surface, and at the same depth there are scattered
knolls in the lagoon. Captain Moresby believes the rim consists of dead
rock, thinly covered with sand, and he is certain this is the case with the
external rim of the Great Chagos Bank, which is also essentially a
submerged atoll. In both these cases, as in the submerged portion of the
reef at Peros Banhos, Captain Moresby feels sure that the quantity of
living coral, even on the outer edge overhanging the deep-sea water, is
quite insignificant. Lastly, in several parts of the Pacific and Indian
Oceans there are banks, lying at greater depths than in the cases just
mentioned, of the same form and size with the neighbouring atolls, but with
their atoll-like structure wholly obliterated. It appears from the survey
of Freycinet, that there are banks of this kind in the Caroline
Archipelago, and, as is reported, in the Low Archipelago. When we discuss
the origin of the different classes of coral formations, we shall see that
the submerged state of the whole of some atoll-formed reefs, and of
portions of others, generally but not invariably on the leeward side, and
the existence of more deeply submerged banks now possessing little or no
signs of their original atoll-like structure, are probably the effects of a
uniform cause,--namely, the death of the coral, during the subsidence of
the area, in which the atolls or banks are situated.
There is seldom, with the exception of the Maldiva atolls, more than two or
three channels, and generally only one leading into the lagoon, of
sufficient depth for a ship to enter. in small atolls, there is usually
not even one. Where there is deep water, for instance above twenty
fathoms, in the middle of the lagoon, the channels through the reef are
seldom as deep as the centre,--it may be said that the rim only of the
saucer-shaped hollow forming the lagoon is notched. Mr. Lyell ("Principles
of Geology," volume iii., page 289.) has observed that the growth of the
coral would tend to obstruct all the channels through a reef, except those
kept open by discharging the water, which during high tide and the greater
part of each ebb is thrown over its circumference. Several facts indicate
that a considerable quantity of sediment is likewise discharged through
these channels; and Captain Moresby informs me that he has observed, during
the change of the monsoon, the sea discoloured to a distance off the
entrances into the Maldiva and Chagos atolls. This, probably, would check
the growth of the coral in them, far more effectually than a mere current
of water. In the many small atolls without any channel, these causes have
not prevented the entire ring attaining the surface. The channels, like
the submerged and effaced parts of the reef, very generally though not
invariably occur on the leeward side of the atoll, or on that side,
according to Beechey (Beechey's "Voyage," 4to edition, volume i., page
189.), which, from running in the same direction with the prevalent wind,
is not fully exposed to it. Passages between the islets on the reef,
through which boats can pass at high water, must not be confounded with
ship-channels, by which the annular reef itself is breached. The passages
between the islets occur, of course, on the windward as well as on the
leeward side; but they are more frequent and broader to leeward, owing to
the lesser dimensions of the islets on that side.
At Keeling atoll the shores of the lagoon shelve gradually, where the
bottom is of sediment, and irregularly or abruptly where there are
coral-reefs; but this is by no means the universal structure in other atolls.
Chamisso (Kotzebue's "First Voyage," volume iii., page 142.), speaking in
general terms of the lagoons in the Marshall atolls, says the lead
generally sinks "from a depth of two or three fathoms to twenty or
twenty-four, and you may pursue a line in which on one side of the boat you
may see the bottom, and on the other the azure-blue deep water." The shores
of the lagoon-like channel within the barrier-reef at Vanikoro have a similar
structure. Captain Beechey has described a modification of this structure
(and he believes it is not uncommon) in two atolls in the Low Archipelago,
in which the shores of the lagoon descend by a few, broad, slightly
inclined ledges or steps: thus at Matilda atoll (Beechey's "Voyage," 4to
edition, volume i, page 160. At Whitsunday Island the bottom of the lagoon
slopes gradually towards the centre, and then deepens suddenly, the edge of
the bank being nearly perpendicular. This bank is formed of coral and dead
shells.), the great exterior reef, the surface of which is gently inclined
towards and beneath the surface of the lagoon, ends abruptly in a little
cliff three fathoms deep; at its foot, a ledge forty yards wide extends,
shelving gently inwards like the surface-reef, and terminated by a second
little cliff five fathoms deep; beyond this, the bottom of the lagoon
slopes to twenty fathoms, which is the average depth of its centre. These
ledges seem to be formed of coral-rock; and Captain Beechey says that the
lead often descended several fathoms through holes in them. In some
atolls, all the coral reefs or knolls in the lagoon come to the surface at
low water; in other cases of rarer occurrence, all lie at nearly the same
depth beneath it, but most frequently they are quite irregular,--some with
perpendicular, some with sloping sides,--some rising to the surface, and
others lying at all intermediate depths from the bottom upwards. I cannot,
therefore, suppose that the union of such reefs could produce even one
uniformly sloping ledge, and much less two or three, one beneath the other,
and each terminated by an abrupt wall. At Matilda Island, which offers the
best example of the step-like structure, Captain Beechey observes that the
coral-knolls within the lagoon are quite irregular in their height. We
shall hereafter see that the theory which accounts for the ordinary form of
atolls, apparently includes this occasional peculiarity in their structure.
In the midst of a group of atolls, there sometimes occur small, flat, very
low islands of coral formation, which probably once included a lagoon,
since filled up with sediment and coral-reefs. Captain Beechey entertains
no doubt that this has been the case with the two small islands, which
alone of thirty-one surveyed by him in the Low Archipelago, did not contain
lagoons. Romanzoff Island (in lat. 15 deg S.) is described by Chamisso
(Kotzebue's "First Voyage," volume iii., page 221.) as formed by a dam of
madreporitic rock inclosing a flat space, thinly covered with trees, into
which the sea on the leeward side occasionally breaks. North Keeling atoll
appears to be in a rather less forward stage of conversion into land; it
consists of a horse-shoe shaped strip of land surrounding a muddy flat, one
mile in its longest axis, which is covered by the sea only at high water.
When describing South Keeling atoll, I endeavoured to show how slow the
final process of filling up a lagoon must be; nevertheless, as all causes
do tend to produce this effect, it is very remarkable that not one
instance, as I believe, is known of a moderately sized lagoon being filled
up even to the low water-line at spring-tides, much less of such a one
being converted into land. It is, likewise, in some degree remarkable, how
few atolls, except small ones, are surrounded by a single linear strip of
land, formed by the union of separate islets. We cannot suppose that the
many atolls in the Pacific and Indian Oceans all have had a late origin,
and yet should they remain at their present level, subjected only to the
action of the sea and to the growing powers of the coral, during as many
centuries as must have elapsed since any of the earlier tertiary epochs, it
cannot, I think, be doubted that their lagoons and the islets on their
reef, would present a totally different appearance from what they now do.
This consideration leads to the suspicion that some renovating agency
(namely subsidence) comes into play at intervals, and perpetuates their
original structure.
(DESCRIPTION OF THE PLATES.
PLATE II.--GREAT CHAGOS BANK, NEW CALEDONIA,MENCHIKOFF ATOLL, ETC.
FIGURE 1.--GREAT CHAGOS BANK, in the Indian Ocean; taken from the survey by
Captain Moresby and Lieutenant Powell; the parts which are shaded, with the
exception of two or three islets on the western and northern sides, do not
rise to the surface, but are submerged from four to ten fathoms; the banks
bounded by the dotted lines lie from fifteen to twenty fathoms beneath the
surface, and are formed of sand; the central space is of mud, and from
thirty to fifty fathoms deep.
FIGURE 2.--A vertical section, on the same scale, in an eastern and western
line across the Great Chagos Bank, given for the sake of exhibiting more
clearly its structure.
FIGURE 3.--MENCHIKOFF ATOLL (or lagoon-island), in the Marshall
Archipelago, Northern Pacific Ocean; from Krusenstern's "Atlas of the
Pacific;" originally surveyed by Captain Hagemeister; the depth within the
lagoons is unknown.
FIGURE 4.--MAHLOS MAHDOO ATOLL, together with Horsburgh atoll, in the
Maldiva Archipelago; from the survey by Captain Moresby and Lieutenant
Powell; the white spaces in the middle of the separate small reefs, both on
the margin and in the middle part, are meant to represent little lagoons;
but it was found not possible to distinguish them clearly from the small
islets, which have been formed on these same small reefs; many of the
smaller reefs could not be introduced; the nautical mark (dot over a dash)
over the figures 250 and 200, between Mahlos Mahdoo and Horsburgh atoll and
Powell's island, signifies that soundings were not obtained at these
depths.
FIGURE 5.--NEW CALEDONIA, in the western part of the Pacific; from
Krusenstern's "Atlas," compiled from several surveys; I have slightly
altered the northern point of the reef, in accordance with the "Atlas of
the Voyage of the 'Astrolabe'." In Krusenstern's "Atlas," the reef is
represented by a single line with crosses; I have for the sake of
uniformity added an interior line.
FIGURE 6.--MALDIVA ARCHIPELAGO, in the Indian Ocean; from the survey by
Captain Moresby and Lieutenant Powell.)
SECTION 1.III.--ATOLLS OF THE MALDIVA ARCHIPELAGO--GREAT CHAGOS BANK.
Maldiva Archipelago.--Ring-formed reefs, marginal and central.--Great
depths in the lagoons of the southern atolls.--Reefs in the lagoons all
rising to the surface.--Position of islets and breaches in the reefs, with
respect to the prevalent winds and action of the waves.--Destruction of
islets.--Connection in the position and submarine foundation of distinct
atolls.--The apparent disseverment of large atolls.--The Great Chagos
Bank.--Its submerged condition and extraordinary structure.
Although occasional references have been made to the Maldiva atolls, and to
the banks in the Chagos group, some points of their structure deserve
further consideration. My description is derived from an examination of
the admirable charts lately published from the survey of Captain Moresby
and Lieutenant Powell, and more especially from information which Captain
Moresby has communicated to me in the kindest manner.
The Maldiva Archipelago is 470 miles in length, with an average breadth of
about 50 miles. The form and dimensions of the atolls, and their singular
position in a double line, may be seen, but not well, in the greatly
reduced chart (Figure 6) in Plate II. The dimensions of the longest atoll
in the group (called by the double name of Milla-dou-Madou and
Tilla-dou-Matte) have already been given; it is 88 miles in a medial and
slightly curved line, and is less than 20 miles in its broadest part.
Suadiva, also, is a noble atoll, being 44 miles across in one direction, and
34 in another, and the great included expanse of water has a depth of between
250 and 300 feet. The smaller atolls in this group differ in no respect from
ordinary ones; but the larger ones are remarkable from being breached by
numerous deep-water channels leading into the lagoon; for instance, there
are 42 channels, through which a ship could enter the lagoon of Suadiva.
In the three southern large atolls, the separate portions of reef between
these channels have the ordinary structure, and are linear; but in the
other atolls, especially the more northern ones, these portions are ring-
formed, like miniature atolls. Other ring-formed reefs rise out of the
lagoons, in the place of those irregular ones which ordinarily occur there.
In the reduction of the chart of Mahlos Mahdoo (Plate II., Figure 4), it
was not found easy to define the islets and the little lagoons within each
reef, so that the ring-formed structure is very imperfectly shown; in the
large published charts of Tilla-dou-Matte, the appearance of these rings,
from standing further apart from each other, is very remarkable. The rings
on the margin are generally elongated; many of them are three, and some
even five miles, in diameter; those within the lagoon are usually smaller,
few being more than two miles across, and the greater number rather less
than one. The depth of the little lagoon within these small annular reefs
is generally from five to seven fathoms, but occasionally more; and in Ari
atoll many of the central ones are twelve, and some even more than twelve
fathoms deep. These rings rise abruptly from the platform or bank, on
which they are placed; their outer margin is invariably bordered by living
coral (Captain Moresby informs me that Millepora complanata is one of the
commonest kinds on the outer margin, as it is at Keeling atoll.) within
which there is a flat surface of coral rock; of this flat, sand and
fragments have in many cases accumulated and been converted into islets,
clothed with vegetation. I can, in fact, point out no essential difference
between these little ring-formed reefs (which, however, are larger, and
contain deeper lagoons than many true atolls that stand in the open sea),
and the most perfectly characterised atolls, excepting that the ring-formed
reefs are based on a shallow foundation, instead of on the floor of the
open sea, and that instead of being scattered irregularly, they are grouped
closely together on one large platform, with the marginal rings arranged in
a rudely formed circle.
The perfect series which can be traced from portions of simple linear reef,
to others including long linear lagoons, and from these again to oval or
almost circular rings, renders it probable that the latter are merely
modifications of the linear or normal state. It is conformable with this
view, that the ring-formed reefs on the margin, even where most perfect and
standing furthest apart, generally have their longest axes directed in the
line which the reef would have held, if the atoll had been bounded by an
ordinary wall. We may also infer that the central ring-formed reefs are
modifications of those irregular ones, which are found in the lagoons of
all common atolls. It appears from the charts on a large scale, that the
ring-like structure is contingent on the marginal channels or breaches
being wide; and, consequently, on the whole interior of the atoll being
freely exposed to the waters of the open sea. When the channels are narrow
or few in number, although the lagoon be of great size and depth (as in
Suadiva), there are no ring-formed reefs; where the channels are somewhat
broader, the marginal portions of reef, and especially those close to the
larger channels, are ring-formed, but the central ones are not so; where
they are broadest, almost every reef throughout the atoll is more or less
perfectly ring-formed. Although their presence is thus contingent on the
openness of the marginal channels, the theory of their formation, as we
shall hereafter see, is included in that of the parent atolls, of which
they form the separate portions.
The lagoons of all the atolls in the southern part of the Archipelago are
from ten to twenty fathoms deeper than those in the northern part. This is
well exemplified in the case of Addoo, the southernmost atoll in the group,
for although only nine miles in its longest diameter, it has a depth of
thirty-nine fathoms, whereas all the other small atolls have comparatively
shallow lagoons; I can assign no adequate cause for this difference in
depth. In the central and deepest part of the lagoons, the bottom
consists, as I am informed by Captain Moresby, of stiff clay (probably a
calcareous mud); nearer the border it consists of sand, and in the channels
through the reef, of hard sand-banks, sandstone, conglomerate rubble, and a
little live coral. Close outside the reef and the line joining its
detached portions (where intersected by many channels), the bottom is
sandy, and it slopes abruptly into unfathomable depths. In most lagoons
the depth is considerably greater in the centre than in the channels; but
in Tilla-dou-Matte, where the marginal ring-formed reefs stand far apart,
the same depth is carried across the entire atoll, from the deep-water line
on one side to that on the other. I cannot refrain from once again
remarking on the singularity of these atolls,--a great sandy and generally
concave disc rises abruptly from the unfathomable ocean, with its central
expanse studded and its border symmetrically fringed with oval basins of
coral-rock, just lipping the surface of the sea, sometimes clothed with
vegetation, and each containing a little lake of clear water!
In the southern Maldiva atolls, of which there are nine large ones, all the
small reefs within the lagoons come to the surface, and are dry at low
water spring-tides; hence in navigating them, there is no danger from
submarine banks. This circumstance is very remarkable, as within some
atolls, for instance those of the neighbouring Chagos group, not a single
reef comes to the surface, and in most other cases a few only do, and the
rest lie at all intermediate depths from the bottom upwards. When treating
of the growth of coral I shall again refer to this subject.
Although in the neighbourhood of the Maldiva Archipelago the winds, during
the monsoons, blow during nearly an equal time from opposite quarters, and
although, as I am informed by Captain Moresby, the westerly winds are the
strongest, yet the islets are almost all placed on the eastern side of the
northern atolls, and on the south-eastern side of the southern atolls.
That the formation of the islets is due to detritus thrown up from the
outside, as in the ordinary manner, and not from the interior of the
lagoons, may, I think be safely inferred from several considerations, which
it is hardly worth while to detail. As the easterly winds are not the
strongest, their action probably is aided by some prevailing swell or
current.
In groups of atolls, exposed to a trade-wind, the ship-channels into the
lagoons are almost invariably situated on the leeward or less exposed side
of the reef, and the reef itself is sometimes either wanting there, or is
submerged. A strictly analogous, but different fact, may be observed at
the Maldiva atolls--namely, that where two atolls stand in front of each
other, the breaches in the reef are the most numerous on their near, and
therefore less exposed, sides. Thus on the near sides of Ari and the two
Nillandoo atolls, which face S. Male, Phaleedoo, and Moloque atolls, there
are seventy-three deep-water channels, and only twenty-five on their outer
sides; on the near side of the three latter named atolls there are fifty-
six openings, and only thirty-seven on their outsides. It is scarcely
possible to attribute this difference to any other cause than the somewhat
different action of the sea on the two sides, which would ensue from the
protection afforded by the two rows of atolls to each other. I may here
remark that in most cases, the conditions favourable to the greater
accumulation of fragments on the reef and to its more perfect continuity on
one side of the atoll than on the other, have concurred, but this has not
been the case with the Maldivas; for we have seen that the islets are
placed on the eastern or south-eastern sides, whilst the breaches in the
reef occur indifferently on any side, where protected by an opposite atoll.
The reef being more continuous on the outer and more exposed sides of those
atolls which stand near each other, accords with the fact, that the reef of
the southern atolls is more continuous than that of the northern ones; for
the former, as I am informed by Captain Moresby, are more constantly
exposed than the northern atolls to a heavy surf.
The date of the first formation of some of the islets in this Archipelago
is known to the inhabitants; on the other hand, several islets, and even
some of those which are believed to be very old, are now fast wearing away.
The work of destruction has, in some instances, been completed in ten
years. Captain Moresby found on one water-washed reef the marks of wells
and graves, which were excavated when it supported an islet. In South
Nillandoo atoll, the natives say that three of the islets were formerly
larger: in North Nillandoo there is one now being washed away; and in this
latter atoll Lieutenant Prentice found a reef, about six hundred yards in
diameter, which the natives positively affirmed was lately an island
covered with cocoa-nut trees. It is now only partially dry at low water
spring-tides, and is (in Lieutenant Prentice's words) "entirely covered
with live coral and madrepore." In the northern part, also, of the Maldiva
Archipelago and in the Chagos group, it is known that some of the islets
are disappearing. The natives attribute these effects to variations in the
currents of the sea. For my own part I cannot avoid suspecting that there
must be some further cause, which gives rise to such a cycle of change in
the action of the currents of the great and open ocean.
Several of the atolls in this Archipelago are so related to each other in
form and position, that at the first glance one is led to suspect that they
have originated in the disseverment of a single one. Male consists of
three perfectly characterised atolls, of which the shape and relative
position are such, that a line drawn closely round all three, gives a
symmetrical figure; to see this clearly, a larger chart is required than
that of the Archipelago in Plate II.; the channel separating the two
northern Male atolls is only little more than a mile wide, and no bottom
was found in it with 100 fathoms. Powell's Island is situated at the
distance of two miles and a half off the northern end of Mahlos Mahdoo (see
Figure 4, Plate II.), at the exact point where the two sides of the latter,
if prolonged, would meet; no bottom, however, was found in the channel with
200 fathoms; in the wider channel between Horsburgh atoll and the southern
end of Mahlos Mahdoo, no bottom was found with 250 fathoms. In these and
similar cases, the relation consists only in the form and position of the
atolls. But in the channel between the two Nillandoo atolls, although
three miles and a quarter wide, soundings were struck at the depth of 200
fathoms; the channel between Ross and Ari atolls is four miles wide, and
only 150 fathoms deep. Here then we have, besides the relation of form, a
submarine connection. The fact of soundings having been obtained between
two separate and perfectly characterised atolls is in itself interesting,
as it has never, I believe, been effected in any of the many other groups
of atolls in the Pacific and Indian seas. In continuing to trace the
connection of adjoining atolls, if a hasty glance be taken at the chart
(Figure 4., Plate II.) of Mahlos Mahdoo, and the line of unfathomable water
be followed, no one will hesitate to consider it as one atoll. But a
second look will show that it is divided by a bifurcating channel, of which
the northern arm is about one mile and three-quarters in width, with an
average depth of 125 fathoms, and the southern one three-quarters of a mile
wide, and rather less deep. These channels resemble in the slope of their
sides and general form, those which separate atolls in every respect
distinct; and the northern arm is wider than that dividing two of the Male
atolls. The ring-formed reefs on the sides of this bifurcating channel are
elongated, so that the northern and southern portions of Mahlos Mahdoo may
claim, as far as their external outline is concerned, to be considered as
distinct and perfect atolls. But the intermediate portion, lying in the
fork of the channel, is bordered by reefs less perfect than those which
surround any other atoll in the group of equally small dimensions. Mahlos
Mahdoo, therefore, is in every respect in so intermediate a condition, that
it may be considered either as a single atoll nearly dissevered into three
portions, or as three atolls almost perfect and intimately connected. This
is an instance of a very early stage of the apparent disseverment of an
atoll, but a still earlier one in many respects is exhibited at Tilla-dou-
Matte. In one part of this atoll, the ring-formed reefs stand so far apart
from each other, that the inhabitants have given different names to the
northern and southern halves; nearly all the rings, moreover, are so
perfect and stand so separate, and the space from which they rise is so
level and unlike a true lagoon, that we can easily imagine the conversion
of this one great atoll, not into two or three portions, but into a whole
group of miniature atolls. A perfect series such as we have here traced,
impresses the mind with an idea of actual change; and it will hereafter be
seen, that the theory of subsidence, with the upward growth of the coral,
modified by accidents of probable occurrence, will account for the
occasional disseverment of large atolls.
The Great Chagos bank alone remains to be described. In the Chagos group
there are some ordinary atolls, some annular reefs rising to the surface
but without any islets on them, and some atoll-formed banks, either quite
submerged, or nearly so. Of the latter, the Great Chagos Bank is much the
largest, and differs in its structure from the others: a plan of it is
given in Plate II., Figure 1, in which, for the sake of clearness, I have
had the parts under ten fathoms deep finely shaded: an east and west
vertical section is given in Figure 2, in which the vertical scale has been
necessarily exaggerated. Its longest axis is ninety nautical miles, and
another line drawn at right angles to the first, across the broadest part,
is seventy. The central part consists of a level muddy flat, between forty
and fifty fathoms deep, which is surrounded on all sides, with the
exception of some breaches, by the steep edges of a set of banks, rudely
arranged in a circle. These banks consist of sand, with a very little live
coral; they vary in breadth from five to twelve miles, and on an average
lie about sixteen fathoms beneath the surface; they are bordered by the
steep edges of a third narrow and upper bank, which forms the rim to the
whole. This rim is about a mile in width, and with the exception of two or
three spots where islets have been formed, is submerged between five and
ten fathoms. It consists of smooth hard rock, covered with a thin layer of
sand, but with scarcely any live coral; it is steep on both sides, and
outwards slopes abruptly into unfathomable depths. At the distance of less
than half a mile from one part, no bottom was found with 190 fathoms; and
off another point, at a somewhat greater distance, there was none with 210
fathoms. Small steep-sided banks or knolls, covered with luxuriantly
growing coral, rise from the interior expanse to the same level with the
external rim, which, as we have seen, is formed only of dead rock. It is
impossible to look at the plan (Figure 1, Plate II.), although reduced to
so small a scale, without at once perceiving that the Great Chagos Bank is,
in the words of Captain Moresby (This officer has had the kindness to lend
me an excellent MS. account of the Chagos Islands; from this paper, from
the published charts, and from verbal information communicated to me by
Captain Moresby, the above account of the Great Chagos Bank is taken.),
"nothing more than a half-drowned atoll." But of what great dimensions,
and of how extraordinary an internal structure? We shall hereafter have to
consider both the cause of its submerged condition, a state common to other
banks in the group, and the origin of the singular submarine terraces,
which bound the central expanse: these, I think, it can be shown, have
resulted from a cause analogous to that which has produced the bifurcating
channel across Mahlos Mahdoo.
CHAPTER II.--BARRIER REEFS.
Closely resemble in general form and structure atoll-reefs.--Width and
depth of the lagoon-channels.--Breaches through the reef in front of
valleys, and generally on the leeward side.--Checks to the filling up of
the lagoon-channels.--Size and constitution of the encircled islands.--
Number of islands within the same reef.--Barrier-reefs of New Caledonia and
Australia.--Position of the reef relative to the slope of the adjoining
land.--Probable great thickness of barrier-reefs.
The term "barrier" has been generally applied to that vast reef which
fronts the N.E. shore of Australia, and by most voyagers likewise to that
on the western coast of New Caledonia. At one time I thought it convenient
thus to restrict the term, but as these reefs are similar in structure, and
in position relatively to the land, to those, which, like a wall with a
deep moat within, encircle many smaller islands, I have classed them
together. The reef, also, on the west coast of New Caledonia, circling
round the extremities of the island, is an intermediate form between a
small encircling reef and the Australian barrier, which stretches for a
thousand miles in nearly a straight line.
The geographer Balbi has in effect described those barrier-reefs, which
encircle moderately sized islands, by calling them atolls with high land
rising from within their central expanse. The general resemblance between
the reefs of the barrier and atoll classes may be seen in the small, but
accurately reduced charts on Plate I. (The authorities from which these
charts have been reduced, together with some remarks on them and
descriptive of the Plates, are given separately.), and this resemblance can
be further shown to extend to every part of the structure. Beginning with
the outside of the reef; many scattered soundings off Gambier, Oualan, and
some other encircled islands, show that close to the breakers there exists
a narrow shelving margin, beyond which the ocean becomes suddenly
unfathomable; but off the west coast of New Caledonia, Captain Kent
(Dalrymple, "Hydrog. Mem." volume iii.) found no bottom with 150 fathoms,
at two ships' length from the reef; so that the slope here must be nearly
as precipitous as off the Maldiva atolls.
I can give little information regarding the kinds of corals which live on
the outer margin. When I visited the reef at Tahiti, although it was low
water, the surf was too violent for me to see the living masses; but,
according to what I heard from some intelligent native chiefs, they
resemble in their rounded and branchless forms, those on the margin of
Keeling atoll. The extreme verge of the reef, which was visible between
the breaking waves at low water, consisted of a rounded, convex,
artificial-like breakwater, entirely coated with Nulliporae, and absolutely
similar to that which I have described at Keeling atoll. From what I heard
when at Tahiti, and from the writings of the Revs. W. Ellis and J.
Williams, I conclude that this peculiar structure is common to most of the
encircled islands of the Society Archipelago. The reef within this mound
or breakwater, has an extremely irregular surface, even more so than
between the islets on the reef of Keeling atoll, with which alone (as there
are no islets on the reef of Tahiti) it can properly be compared. At
Tahiti, the reef is very irregular in width; but round many other encircled
islands, for instance, Vanikoro or Gambier Islands (Figures 1 and 8, Plate
I.), it is quite as regular, and of the same average width, as in true
atolls. Most barrier-reefs on the inner side slope irregularly into the
lagoon-channel (as the space of deep water separating the reef from the
included land may be called), but at Vanikoro the reef slopes only for a
short distance, and then terminates abruptly in a submarine wall, forty
feet high,--a structure absolutely similar to that described by Chamisso in
the Marshall atolls.
In the Society Archipelago, Ellis (Consult, on this and other points, the
"Polynesian Researches," by the Rev. W. Ellis, an admirable work, full of
curious information.) states, that the reefs generally lie at the distance
of from one to one and a half miles, and, occasionally, even at more than
three miles, from the shore. The central mountains are generally bordered
by a fringe of flat, and often marshy, alluvial land, from one to four
miles in width. This fringe consists of coral-sand and detritus thrown up
from the lagoon-channel, and of soil washed down from the hills; it is an
encroachment on the channel, analogous to that low and inner part of the
islets in many atolls which is formed by the accumulation of matter from
the lagoon. At Hogoleu (Figure 2, Plate I.), in the Caroline Archipelago
(See "Hydrographical Mem." and the "Atlas of the Voyage of the
'Astrolabe'," by Captain Dumont D'Urville, page 428.), the reef on the
south side is no less than twenty miles; on the east side, five; and on the
north side, fourteen miles from the encircled high islands.
The lagoon channels may be compared in every respect with true lagoons. In
some cases they are open, with a level bottom of fine sand; in others they
are choked up with reefs of delicately branched corals, which have the same
general character as those within the Keeling atoll. These internal reefs
either stand separately, or more commonly skirt the shores of the included
high islands. The depth of the lagoon-channel round the Society Islands
varies from two or three to thirty fathoms; in Cook's (See the chart in
volume i. of Hawkesworth's 4to edition of "Cook's First Voyage.") chart of
Ulieta, however, there is one sounding laid down of forty-eight fathoms; at
Vanikoro there are several of fifty-four and one of fifty-six and a half
fathoms (English), a depth which even exceeds by a little that of the
interior of the great Maldiva atolls. Some barrier-reefs have very few
islets on them; whilst others are surmounted by numerous ones; and those
round part of Bolabola (Plate I., Figure 5) form a single linear strip.
The islets first appear either on the angles of the reef, or on the sides
of the breaches through it, and are generally most numerous on the windward
side. The reef to leeward retaining its usual width, sometimes lies
submerged several fathoms beneath the surface; I have already mentioned
Gambier Island as an instance of this structure. Submerged reefs, having a
less defined outline, dead, and covered with sand, have been observed (see
Appendix) off some parts of Huaheine and Tahiti. The reef is more
frequently breached to leeward than to windward; thus I find in
Krusenstern's "Memoir on the Pacific," that there are passages through the
encircling reef on the leeward side of each of the seven Society Islands,
which possess ship-harbours; but that there are openings to windward
through the reef of only three of them. The breaches in the reef are
seldom as deep as the interior lagoon-like channel; they generally occur in
front of the main valleys, a circumstance which can be accounted for, as
will be seen in the fourth chapter, without much difficulty. The breaches
being situated in front of the valleys, which descend indifferently on all
sides, explains their more frequent occurrence through the windward side of
barrier-reefs than through the windward side of atolls,--for in atolls
there is no included land to influence the position of the breaches.
It is remarkable, that the lagoon-channels round mountainous islands have
not in every instance been long ago filled up with coral and sediment; but
it is more easily accounted for than appears at first sight. In cases like
that of Hogoleu and the Gambier Islands, where a few small peaks rise out
of a great lagoon, the conditions scarcely differ from those of an atoll,
and I have already shown, at some length, that the filling up of a true
lagoon must be an extremely slow process. Where the channel is narrow, the
agency, which on unprotected coasts is most productive of sediment, namely
the force of the breakers, is here entirely excluded, and the reef being
breached in the front of the main valleys, much of the finer mud from the
rivers must be transported into the open sea. As a current is formed by
the water thrown over the edge of atoll-formed reefs, which carries
sediment with it through the deep-water breaches, the same thing probably
takes place in barrier-reefs, and this would greatly aid in preventing the
lagoon-channel from being filled up. The low alluvial border, however, at
the foot of the encircled mountains, shows that the work of filling up is
in progress; and at Maura (Plate I., Figure 6), in the Society group, it
has been almost effected, so that there remains only one harbour for small
craft.
If we look at a set of charts of barrier-reefs, and leave out in
imagination the encircled land, we shall find that, besides the many points
already noticed of resemblance, or rather of identity in structure with
atolls, there is a close general agreement in form, average dimensions, and
grouping. Encircling barrier-reefs, like atolls, are generally elongated,
with an irregularly rounded, though sometimes angular outline. There are
atolls of all sizes, from less than two miles in diameter to sixty miles
(excluding Tilla-dou-Matte, as it consists of a number of almost
independent atoll-formed reefs); and there are encircling barrier-reefs
from three miles and a half to forty-six miles in diameter,--Turtle Island
being an instance of the former, and Hogoleu of the latter. At Tahiti the
encircled island is thirty-six miles in its longest axis, whilst at Maurua
it is only a little more than two miles. It will be shown, in the last
chapter in this volume, that there is the strictest resemblance in the
grouping of atolls and of common islands, and consequently there must be
the same resemblance in the grouping of atolls and of encircling
barrier-reefs.
The islands lying within reefs of this class, are of very various heights.
Tahiti is 7,000 feet (The height of Tahiti is given from Captain Beechey;
Maurua from Mr. F.D. Bennett ("Geograph. Journ." volume viii., page 220);
Aitutaki from measurements made on board the "Beagle"; and Manouai or
Harvey Island, from an estimate by the Rev. J. Williams. The two latter
islands, however, are not in some respects well characterised examples of
the encircled class.); Maurua about 800; Aitutaki 360, and Manouai only 50.
The geological nature of the included land varies: in most cases it is of
ancient volcanic origin, owing apparently to the fact that islands of this
nature are most frequent within all great seas; some, however, are of
madreporitic limestone, and others of primary formation, of which latter
kind New Caledonia offers the best example. The central land consists
either of one island, or of several: thus, in the Society group, Eimeo
stands by itself; while Taha and Raiatea (Figure 3, Plate I.), both
moderately large islands of nearly equal size, are included in one reef.
Within the reef of the Gambier group there are four large and some smaller
islands (Figure 8, Plate I.); within that of Hogoleu (Figure 2, Plate I.)
nearly a dozen small islands are scattered over the expanse of one vast
lagoon.
After the details now given, it may be asserted that there is not one point
of essential difference between encircling barrier-reefs and atolls: the
latter enclose a simple sheet of water, the former encircle an expanse with
one or more islands rising from it. I was much struck with this fact, when
viewing, from the heights of Tahiti, the distant island of Eimeo standing
within smooth water, and encircled by a ring of snow-white breakers.
Remove the central land, and an annular reef like that of an atoll in an
early stage of its formation is left; remove it from Bolabola, and there
remains a circle of linear coral-islets, crowned with tall cocoa-nut trees,
like one of the many atolls scattered over the Pacific and Indian Oceans.
The barrier-reefs of Australia and of New Caledonia deserve a separate
notice from their great dimensions. The reef on the west coast of New
Caledonia (Figure 5, Plate II.) is 400 miles in length; and for a length of
many leagues it seldom approaches within eight miles of the shore; and near
the southern end of the island, the space between the reef and the land is
sixteen miles in width. The Australian barrier extends, with a few
interruptions, for nearly a thousand miles; its average distance from the
land is between twenty and thirty miles; and in some parts from fifty to
seventy. The great arm of the sea thus included, is from ten to twenty-five
fathoms deep, with a sandy bottom; but towards the southern end, where
the reef is further from the shore, the depth gradually increases to forty,
and in some parts to more than sixty fathoms. Flinders (Flinders' "Voyage
to Terra Australis," volume ii., page 88.) has described the surface of
this reef as consisting of a hard white agglomerate of different kinds of
coral, with rough projecting points. The outer edge is the highest part;
it is traversed by narrow gullies, and at rare intervals is breached by
ship-channels. The sea close outside is profoundly deep; but, in front of
the main breaches, soundings can sometimes be obtained. Some low islets
have been formed on the reef.
(PLATE: UNNAMED, THREE VERTICAL SECTIONS (WOODCUT DIAGRAMS):
1. VANIKORO, from the "Atlas of the Voyage of the 'Astrolabe'," by D.
D'Urville.
2. GAMBIER ISLAND, from Beechey.
3. MAURUA, from the "Atlas of the Voyage of the 'Coquille'," by Duperrey.
The horizontal line is the level of the sea, from which on the right hand a
plummet descends, representing a depth of 200 fathoms, or 1,200 feet. The
vertical shading shows the section of the land, and the horizontal shading
that of the encircling barrier-reef: from the smallness of the scale, the
lagoon-channel could not be represented.
AA.--Outer edge of the coral-reefs, where the sea breaks.
BB.--The shore of the encircled islands.)
There is one important point in the structure of barrier-reefs which must
here be considered. The accompanying diagrams represent north and south
vertical sections, taken through the highest points of Vanikoro, Gambier,
and Maurua Islands, and through their encircling reefs. The scale both in
the horizontal and vertical direction is the same, namely, a quarter of an
inch to a nautical mile. The height and width of these islands is known;
and I have attempted to represent the form of the land from the shading of
the hills in the large published charts. It has long been remarked, even
from the time of Dampier, that considerable degree of relation subsists
between the inclination of that part of the land which is beneath water and
that above it; hence the dotted line in the three sections, probably, does
not widely differ in inclination from the actual submarine prolongation of
the land. If we now look at the outer edge of the reef (AA), and bear in
mind that the plummet on the right hand represents a depth of 1,200 feet,
we must conclude that the vertical thickness of these barrier coral-reefs
is very great.
I must observe that if the sections had been taken in any other direction
across these islands, or across other encircled islands (In the fifth
chapter an east and west section across the Island of Bolabola and its
barrier-reefs is given, for the sake of illustrating another point. The
unbroken line in it (woodcut No. 5) is the section referred to. The scale
is .57 of an inch to a mile; it is taken from the "Atlas of the Voyage of
the 'Coquille'," by Duperrey. The depth of the lagoon-channel is
exaggerated.), the result would have been the same. In the succeeding
chapter it will be shown that reef-building polypifers cannot flourish at
great depths,--for instance, it is highly improbable that they could exist
at a quarter of the depth represented by the plummet on the right hand of
the woodcut. Here there is a great APPARENT difficulty--how were the basal
parts of these barrier-reef formed? It will, perhaps, occur to some, that
the actual reefs formed of coral are not of great thickness, but that
before their first growth, the coasts of these encircled islands were
deeply eaten into, and a broad but shallow submarine ledge thus left, on
the edge of which the coral grew; but if this had been the case, the shore
would have been invariably bounded by lofty cliffs, and not have sloped
down to the lagoon-channel, as it does in many instances. On this view
(The Rev. D. Tyerman and Mr. Bennett ("Journal of Voyage and Travels,"
volume i., page 215) have briefly suggested this explanation of the origin
of the encircling reefs of the Society Islands.), moreover, the cause of
the reef springing up at such a great distance from the land, leaving a
deep and broad moat within, remains altogether unexplained. A supposition
of the same nature, and appearing at first more probable is, that the reefs
sprung up from banks of sediment, which had accumulated round the shore
previously to the growth of the coral; but the extension of a bank to the
same distance round an unbroken coast, and in front of those deep arms of
the sea (as in Raiatea, see Plate II., Figure 3) which penetrate nearly to
the heart of some encircled islands, is exceedingly improbable. And why,
again, should the reef spring up, in some cases steep on both sides like a
wall, at a distance of two, three or more miles from the shore, leaving a
channel often between two hundred and three hundred feet deep, and rising
from a depth which we have reason to believe is destructive to the growth
of coral? An admission of this nature cannot possibly be made. The
existence, also, of the deep channel, utterly precludes the idea of the
reef having grown outwards, on a foundation slowly formed on its outside,
by the accumulation of sediment and coral detritus. Nor, again, can it be
asserted, that the reef-building corals will not grow, excepting at a great
distance from the land; for, as we shall soon see, there is a whole class
of reefs, which take their name from growing closely attached (especially
where the sea is deep) to the beach. At New Caledonia (see Plate II.,
Figure 5) the reefs which run in front of the west coast are prolonged in
the same line 150 miles beyond the northern extremity of the island, and
this shows that some explanation, quite different from any of those just
suggested, is required. The continuation of the reefs on each side of the
submarine prolongation of New Caledonia, is an exceedingly interesting
fact, if this part formerly existed as the northern extremity of the
island, and before the attachment of the coral had been worn down by the
action of the sea, or if it originally existed at its present height, with
or without beds of sediment on each flank, how can we possibly account for
the reefs, not growing on the crest of this submarine portion, but fronting
its sides, in the same line with the reefs which front the shores of the
lofty island? We shall hereafter see, that there is one, and I believe
only one, solution of this difficulty.
One other supposition to account for the position of encircling barrier-reefs
remains, but it is almost too preposterous to be mentioned; namely,
that they rest on enormous submarine craters, surrounding the included
islands. When the size, height, and form of the islands in the Society
group are considered, together with the fact that all are thus encircled,
such a notion will be rejected by almost every one. New Caledonia,
moreover, besides its size, is composed of primitive formations, as are
some of the Comoro Islands (I have been informed that this is the case by
Dr. Allan of Forres, who has visited this group.); and Aitutaki consists of
calcareous rock. We must, therefore, reject these several explanations,
and conclude that the vertical thickness of barrier-reefs, from their outer
edges to the foundation on which they rest (from AA in the section to the
dotted lines) is really great; but in this, there is no difficulty, for it
is not necessary to suppose that the coral has sprung up from an immense
depth, as will be evident when the theory of the upward growth of
coral-reefs, during the slow subsidence of their foundation, is discussed.
CHAPTER III.--FRINGING OR SHORE-REEFS.
Reefs of Mauritius.--Shallow channel within the reef.--Its slow filling
up.--Currents of water formed within it.--Upraised reefs.--Narrow
fringing-reefs in deep seas.--Reefs on the coast of East Africa and of
Brazil.--Fringing-reefs in very shallow seas, round banks of sediment and
on worn-down islands.--Fringing-reefs affected by currents of the sea.--
Coral coating the bottom of the sea, but not forming reefs.
Fringing-reefs, or, as they have been called by some voyagers, shore-reefs,
whether skirting an island or part of a continent, might at first be
thought to differ little, except in generally being of less breadth, from
barrier-reefs. As far as the superficies of the actual reef is concerned
this is the case; but the absence of an interior deep-water channel, and
the close relation in their horizontal extension with the probable slope
beneath the sea of the adjoining land, present essential points of
difference.
The reefs which fringe the island of Mauritius offer a good example of this
class. They extend round its whole circumference, with the exception of
two or three parts (This fact is stated on the authority of the Officier du
Roi, in his extremely interesting "Voyage a l'Isle de France," undertaken
in 1768. According to Captain Carmichael (Hooker's "Bot. Misc." volume
ii., page 316) on one part of the coast there is a space for sixteen miles
without a reef.), where the coast is almost precipitous, and where, if as
is probable the bottom of the sea has a similar inclination, the coral
would have no foundation on which to become attached. A similar fact may
sometimes be observed even in reefs of the barrier class, which follow much
less closely the outline of the adjoining land; as, for instance, on the
south-east and precipitous side of Tahiti, where the encircling reef is
interrupted. On the western side of the Mauritius, which was the only part
I visited, the reef generally lies at the distance of about half a mile
from the shore; but in some parts it is distant from one to two, and even
three miles. But even in this last case, as the coast-land is gently
inclined from the foot of the mountains to the sea-beach, and as the
soundings outside the reef indicate an equally gentle slope beneath the
water, there is no reason for supposing that the basis of the reef, formed
by the prolongation of the strata of the island, lies at a greater depth
than that at which the polypifers could begin constructing the reef. Some
allowance, however, must be made for the outward extension of the corals on
a foundation of sand and detritus, formed from their own wear, which would
give to the reef a somewhat greater vertical thickness, than would
otherwise be possible.
The outer edge of the reef on the western or leeward side of the island is
tolerably well defined, and is a little higher than any other part. It
chiefly consists of large strongly branched corals, of the genus Madrepora,
which also form a sloping bed some way out to sea: the kinds of coral
growing in this part will be described in the ensuing chapter. Between the
outer margin and the beach, there is a flat space with a sandy bottom and a
few tufts of living coral; in some parts it is so shallow, that people, by
avoiding the deeper holes and gullies, can wade across it at low water; in
other parts it is deeper, seldom however exceeding ten or twelve feet, so
that it offers a safe coasting channel for boats. On the eastern and
windward side of the island, which is exposed to a heavy surf, the reef was
described to me as having a hard smooth surface, very slightly inclined
inwards, just covered at low-water, and traversed by gullies; it appears to
be quite similar in structure to the reefs of the barrier and atoll
classes.
The reef of Mauritius, in front of every river and streamlet, is breached
by a straight passage: at Grand Port, however, there is a channel like
that within a barrier-reef; it extends parallel to the shore for four
miles, and has an average depth of ten or twelve fathoms; its presence may
probably be accounted for by two rivers which enter at each end of the
channel, and bend towards each other. The fact of reefs of the fringing
class being always breached in front of streams, even of those which are
dry during the greater part of the year, will be explained, when the
conditions unfavourable to the growth of coral are considered. Low
coral-islets, like those on barrier-reefs and atolls, are seldom formed on
reefs of this class, owing apparently in some cases to their narrowness, and
in others to the gentle slope of the reef outside not yielding many fragments
to the breakers. On the windward side, however, of the Mauritius, two or
three small islets have been formed.
It appears, as will be shown in the ensuing chapter, that the action of the
surf is favourable to the vigorous growth of the stronger corals, and that
sand or sediment, if agitated by the waves, is injurious to them. Hence it
is probable that a reef on a shelving shore, like that of Mauritius, would
at first grow up, not attached to the actual beach, but at some little
distance from it; and the corals on the outer margin would be the most
vigorous. A shallow channel would thus be formed within the reef, and as
the breakers are prevented acting on the shores of the island, and as they
do not ordinarily tear up many fragments from the outside, and as every
streamlet has its bed prolonged in a straight line through the reef, this
channel could be filled up only very slowly with sediment. But a beach of
sand and of fragments of the smaller kinds of coral seems, in the case of
Mauritius, to be slowly encroaching on the shallow channel. On many
shelving and sandy coasts, the breakers tend to form a bar of sand a little
way from the beach, with a slight increase of depth within it; for
instance, Captain Grey (Captain Grey's "Journal of Two Expeditions," volume
i. page 369.) states that the west coast of Australia, in latitude 24 deg.,
is fronted by a sand bar about two hundred yards in width, on which there
is only two feet of water; but within it the depth increases to two
fathoms. Similar bars, more or less perfect, occur on other coasts. In
these cases I suspect that the shallow channel (which no doubt during
storms is occasionally obliterated) is scooped out by the flowing away of
the water thrown beyond the line, on which the waves break with the
greatest force. At Pernambuco a bar of hard sandstone (I have described
this singular structure in the "London and Edinburgh Phil. Mag." October
1841.), which has the same external form and height as a coral-reef,
extends nearly parallel to the coast; within this bar currents, apparently
caused by the water thrown over it during the greater part of each tide,
run strongly, and are wearing away its inner wall. From these facts it can
hardly be doubted, that within most fringing-reefs, especially within those
lying some distance from the land, a return stream must carry away the
water thrown over the outer edge; and the current thus produced, would tend
to prevent the channel being filled up with sediment, and might even deepen
it under certain circumstances. To this latter belief I am led, by finding
that channels are almost universally present within the fringing-reefs of
those islands which have undergone recent elevatory movements; and this
could hardly have been the case, if the conversion of the very shallow
channel into land had not been counteracted to a certain extent.
A fringing-reef, if elevated in a perfect condition above the level of the
sea, ought to present the singular appearance of a broad dry moat within a
low mound. The author ("Voyage a l'Isle de France, par un Officier du
Roi," part i., pages 192, 200.) of an interesting pedestrian tour round the
Mauritius, seems to have met with a structure of this kind: he says
"J'observai que la, ou la mer etale, independamment des rescifs du large,
il y a terre UNE ESPECE D'EFFONCEMENT ou chemin couvert naturel. On y
pourrait mettre du canon," etc. In another place he adds, "Avant de passer
le Cap, on remarque un gros banc de corail eleve de plus de quinze pieds:
c'est une espece de rescif, que la mer abandonne, il regne au pied une
longue flaque d'eau, dont on pourrait faire un bassin pour de petits
vaisseaux." But the margin of the reef, although the highest and most
perfect part, from being most exposed to the surf, would generally during a
slow rise of the land be either partially or entirely worn down to that
level, at which corals could renew their growth on its upper edge. On some
parts of the coast-land of Mauritius there are little hillocks of coral-rock,
which are either the last remnants of a continuous reef, or of low
islets formed on it. I observed that two such hillocks between Tamarin Bay
and the Great Black River; they were nearly twenty feet high, about two
hundred yards from the present beach, and about thirty feet above its
level. They rose abruptly from a smooth surface, strewed with worn
fragments of coral. They consisted in their lower part of hard calcareous
sandstone, and in their upper of great blocks of several species of Astraea
and Madrepora, loosely aggregated; they were divided into irregular beds,
dipping seaward, in one hillock at an angle of 8 deg., and in the other at
18 deg. I suspect that the superficial parts of the reefs, which have been
upraised together with the islands they fringe, have generally been much
more modified by the wearing action of the sea, than those of Mauritius.
Many islands are fringed by reefs quite similar to those of Mauritius (I
may give Cuba, as another instance; Mr. Taylor ("Loudon's Mag. of Nat.
Hist." volume ix., page 449) has described a reef several miles in length
between Gibara and Vjaro, which extends parallel to the shore at the
distance of between half and the third part of a mile, and encloses a space
of shallow water, with a sandy bottom and tufts of coral. Outside the edge
of the reef, which is formed of great branching corals, the depth is six
and seven fathoms. This coast has been upheaved at no very distant
geological period."); but on coasts where the sea deepens very suddenly the
reefs are much narrower, and their limited extension seems evidently to
depend on the high inclination of the submarine slope; a relation, which,
as we have seen, does not exist in reefs of the barrier class. The
fringing-reefs on steep coasts are frequently not more than from fifty to
one hundred yards in width; they have a nearly smooth, hard surface,
scarcely uncovered at low water, and without any interior shoal channel,
like that within those fringing-reefs, which lie at a greater distance from
the land. The fragments torn up during gales from the outer margin are
thrown over the reef on the shores of the island. I may give as instances,
Wateeo, where the reef is described by Cook as being a hundred yards wide;
and Mauti and Elizabeth Islands (Mauti is described by Lord Byron in the
voyage of H.M.S. "Blonde", and Elizabeth Island by Captain Beechey.), where
it is only fifty yards in width: the sea round these islands is very deep.
Fringing-reefs, like barrier-reefs, both surround islands, and front the
shores of continents. In the charts of the eastern coast of Africa, by
Captain Owen, many extensive fringing-reefs are laid down; thus, for a
space of nearly forty miles, from latitude 1 deg 15' to 1 deg 45' S., a
reef fringes the shore at an average distance of rather more than one mile,
and therefore at a greater distance than is usual in reefs of this class;
but as the coast-land is not lofty, and as the bottom shoals very gradually
(the depth being only from eight to fourteen fathoms at a mile and a half
outside the reef), its extension thus far from the land offers no
difficulty. The external margin of this reef is described, as formed of
projecting points, within which there is a space, from six to twelve feet
deep, with patches of living coral on it. At Mukdeesha (latitude 2 deg 1'
N.) "the port is formed," it is said (Owen's "Africa," volume i., page 357,
from which work the foregoing facts are likewise taken.) "by a long reef
extending eastward, four or five miles, within which there is a narrow
channel, with ten to twelve feet of water at low spring-tides;" it lies at
the distance of a quarter of a mile from the shore. Again, in the plan of
Mombas (latitude 4 deg S.), a reef extends for thirty-six miles, at the
distance of from half a mile to one mile and a quarter from the shore;
within it, there is a channel navigable "for canoes and small craft,"
between six and fifteen feet deep: outside the reef the depth is about
thirty fathoms at the distance of nearly half a mile. Part of this reef is
very symmetrical, and has a uniform breadth of two hundred yards.
The coast of Brazil is in many parts fringed by reefs. Of these, some are
not of coral formation; for instance, those near Bahia and in front of
Pernambuco; but a few miles south of this latter city, the reef follows
(See Baron Roussin's "Pilote du Bresil," and accompanying hydrographical
memoir.) so closely every turn of the shore, that I can hardly doubt it is
of coral; it runs at the distance of three-quarters of a mile from the
land, and within it the depth is from ten to fifteen feet. I was assured
by an intelligent pilot that at Ports Frances and Maceio, the outer part of
the reef consists of living coral, and the inner of a white stone, full of
large irregular cavities, communicating with the sea. The bottom of the
sea off the coast of Brazil shoals gradually to between thirty and forty
fathoms, at the distance of between nine and ten leagues from the land.
From the description now given, we must conclude that the dimensions and
structure of fringing-reefs depend entirely on the greater or less
inclination of the submarine slope, conjoined with the fact that
reef-building polypifers can exist only at limited depths. It follows from
this, that where the sea is very shallow, as in the Persian Gulf and in
parts of the East Indian Archipelago, the reefs lose their fringing
character, and appear as separate and irregularly scattered patches, often
of considerable area. From the more vigorous growth of the coral on the
outside, and from the conditions being less favourable in several respects
within, such reefs are generally higher and more perfect in their marginal
than in their central parts; hence these reefs sometimes assume (and this
circumstance ought not to be overlooked) the appearance of atolls; but they
differ from atolls in their central expanse being much less deep, in their
form being less defined, and in being based on a shallow foundation. But
when in a deep sea reefs fringe banks of sediment, which have accumulated
beneath the surface, round either islands or submerged rocks, they are
distinguished with difficulty on the one hand from encircling barrier-reefs,
and on the other from atolls. In the West Indies there are reefs,
which I should probably have arranged under both these classes, had not the
existence of large and level banks, lying a little beneath the surface,
ready to serve as the basis for the attachment of coral, been occasionally
brought into view by the entire or partial absence of reefs on them, and
had not the formation of such banks, through the accumulation of sediment
now in progress, been sufficiently evident. Fringing-reefs sometimes coat,
and thus protect the foundations of islands, which have been worn down by
the surf to the level of the sea. According to Ehrenberg, this has been
extensively the case with the islands in the Red Sea, which formerly ranged
parallel to the shores of the mainland, with deep water within them: hence
the reefs now coating their bases are situated relatively to the land like
barrier-reefs, although not belonging to that class; but there are, as I
believe, in the Red Sea some true barrier-reefs. The reefs of this sea and
of the West Indies will be described in the Appendix. In some cases,
fringing-reefs appear to be considerably modified in outline by the course
of the prevailing currents. Dr. J. Allan informs me that on the east coast
of Madagascar almost every headland and low point of sand has a coral-reef
extending from it in a S.W. and N.E. line, parallel to the currents on that
shore. I should think the influence of the currents chiefly consisted in
causing an extension, in a certain direction, of a proper foundation for
the attachment of the coral. Round many intertropical islands, for
instance the Abrolhos on the coast of Brazil surveyed by Captain Fitzroy,
and, as I am informed by Mr. Cuming, round the Philippines, the bottom of
the sea is entirely coated by irregular masses of coral, which although
often of large size, do not reach the surface and form proper reefs. This
must be owing, either to insufficient growth, or to the absence of those
kinds of corals which can withstand the breaking of the waves.
The three classes, atoll-formed, barrier, and fringing-reefs, together with
the modifications just described of the latter, include all the most
remarkable coral formations anywhere existing. At the commencement of the
last chapter in the volume, where I detail the principles on which the map
(Plate III.) is coloured, the exceptional cases will be enumerated.
CHAPTER IV.--ON THE DISTRIBUTION AND GROWTH OF CORAL-REEFS.
In this chapter I will give all the facts which I have collected, relating
to the distribution of coral-reefs,--to the conditions favourable to their
increase,--to the rate of their growth,--and to the depth at which they are
formed.
These subjects have an important bearing on the theory of the origin of the
different classes of coral-reefs.
SECTION 4.I.--ON THE DISTRIBUTION OF CORAL-REEFS, AND ON THE CONDITIONS
FAVOURABLE TO THEIR INCREASE.
With regard to the limits of latitude, over which coral-reefs extend, I
have nothing new to add. The Bermuda Islands, in 32 deg 15' N., is the
point furthest removed from the equator, in which they appear to exist; and
it has been suggested that their extension so far northward in this
instance is owing to the warmth of the Gulf Stream. In the Pacific, the
Loo Choo Islands, in latitude 27 deg N., have reefs on their shores, and
there is an atoll in 28 deg 30', situated N.W. of the Sandwich Archipelago.
In the Red Sea there are coral-reefs in latitude 30 deg. In the southern
hemisphere coral-reefs do not extend so far from the equatorial sea. In
the Southern Pacific there are only a few reefs beyond the line of the
tropics, but Houtmans Abrolhos, on the western shores of Australia in
latitude 29 deg S., are of coral formation.
The proximity of volcanic land, owing to the lime generally evolved from
it, has been thought to be favourable to the increase of coral-reefs.
There is, however, not much foundation for this view; for nowhere are
coral-reefs more extensive than on the shores of New Caledonia, and of
north-eastern Australia, which consist of primary formations; and in the
largest groups of atolls, namely the Maldiva, Chagos, Marshall, Gilbert,
and Low Archipelagoes, there is no volcanic or other kind of rock,
excepting that formed of coral.
The entire absence of coral-reefs in certain large areas within the
tropical seas, is a remarkable fact. Thus no coral-reefs were observed,
during the surveying voyages of the "Beagle" and her tender on the west
coast of South America south of the equator, or round the Galapagos
Islands. It appears, also, that there are none (I have been informed that
this is the case, by Lieutenant Ryder, R.N., and others who have had ample
opportunities for observation.) north of the equator; Mr. Lloyd, who
surveyed the Isthmus of Panama, remarked to me, that although he had seen
corals living in the Bay of Panama, yet he had never observed any reefs
formed by them. I at first attributed this absence of reefs on the coasts
of Peru and of the Galapagos Islands (The mean temperature of the surface
sea from observations made by the direction of Captain Fitzroy on the
shores of the Galapagos Islands, between the 16th of September and the 20th
of October, 1835, was 68 deg Fahr. The lowest temperature observed was
58.5 deg at the south-west end of Albemarle Island; and on the west coast
of this island, it was several times 62 deg and 63 deg. The mean
temperature of the sea in the Low Archipelago of atolls, and near Tahiti,
from similar observations made on board the "Beagle", was (although further
from the equator) 77.5 deg, the lowest any day being 76.5 deg. Therefore
we have here a difference of 9.5 deg in mean temperature, and 18 deg in
extremes; a difference doubtless quite sufficient to affect the
distribution of organic beings in the two areas.), to the coldness of the
currents from the south, but the Gulf of Panama is one of the hottest
pelagic districts in the world. (Humboldt's "Personal Narrative," volume
vii., page 434.) In the central parts of the Pacific there are islands
entirely free from reefs; in some few of these cases I have thought that
this was owing to recent volcanic action; but the existence of reefs round
the greater part of Hawaii, one of the Sandwich Islands, shows that recent
volcanic action does not necessarily prevent their growth.
In the last chapter I stated that the bottom of the sea round some islands
is thickly coated with living corals, which nevertheless do not form reefs,
either from insufficient growth, or from the species not being adapted to
contend with the breaking waves.
I have been assured by several people, that there are no coral-reefs on the
west coast of Africa (It might be concluded, from a paper by Captain Owen
("Geographical Journal", volume ii., page 89), that the reefs off Cape St.
Anne and the Sherboro' Islands were of coral, although the author states
that they are not purely coralline. But I have been assured by Lieutenant
Holland, R.N., that these reefs are not of coral, or at least that they do
not at all resemble those in the West Indies.), or round the islands in the
Gulf of Guinea. This perhaps may be attributed, in part, to the sediment
brought down by the many rivers debouching on that coast, and to the
extensive mud-banks, which line great part of it. But the islands of St.
Helena, Ascension, the Cape Verdes, St. Paul's, and Fernando Noronha, are,
also, entirely without reefs, although they lie far out at sea, are
composed of the same ancient volcanic rocks, and have the same general
form, with those islands in the Pacific, the shores of which are surrounded
by gigantic walls of coral-rock. With the exception of Bermuda, there is
not a single coral-reef in the central expanse of the Atlantic Ocean. It
will, perhaps, be suggested that the quantity of carbonate of lime in
different parts of the sea, may regulate the presence of reefs. But this
cannot be the case, for at Ascension, the waves charged to excess
precipitate a thick layer of calcareous matter on the tidal rocks; and at
St. Jago, in the Cape Verdes, carbonate of lime not only is abundant on the
shores, but it forms the chief part of some upraised post-tertiary strata.
The apparently capricious distribution, therefore, of coral-reefs, cannot
be explained by any of these obvious causes; but as the study of the
terrestrial and better known half of the world must convince every one that
no station capable of supporting life is lost,--nay more, that there is a
struggle for each station, between the different orders of nature,--we may
conclude that in those parts of the intertropical sea, in which there are
no coral-reefs, there are other organic bodies supplying the place of the
reef-building polypifers. It has been shown in the chapter on Keeling
atoll that there are some species of large fish, and the whole tribe of
Holothuriae which prey on the tenderer parts of the corals. On the other
hand, the polypifers in their turn must prey on some other organic beings;
the decrease of which from any cause would cause a proportionate
destruction of the living coral. The relations, therefore, which determine
the formation of reefs on any shore, by the vigorous growth of the
efficient kinds of coral, must be very complex, and with our imperfect
knowledge quite inexplicable. From these considerations, we may infer that
changes in the condition of the sea, not obvious to our senses, might
destroy all the coral-reefs in one area, and cause them to appear in
another: thus, the Pacific or Indian Ocean might become as barren of
coral-reefs as the Atlantic now is, without our being able to assign any
adequate cause for such a change.
It has been a question with some naturalists, which part of a reef is most
favourable to the growth of coral. The great mounds of living Porites and
of Millepora round Keeling atoll occur exclusively on the extreme verge of
the reef, which is washed by a constant succession of breakers; and living
coral nowhere else forms solid masses. At the Marshall islands the larger
kinds of coral (chiefly species of Astraea, a genus closely allied to
Porites) "which form rocks measuring several fathoms in thickness," prefer,
according to Chamisso (Kotzebue's "First Voyage" (English Translation),
volume iii., pages 142, 143, 331.), the most violent surf. I have stated
that the outer margin of the Maldiva atolls consists of living corals (some
of which, if not all, are of the same species with those at Keeling atoll),
and here the surf is so tremendous, that even large ships have been thrown,
by a single heave of the sea, high and dry on the reef, all on board thus
escaping with their lives.
Ehrenberg (Ehrenberg, "Uber die Natur und Bildung der Corallen Banke im
rothen Meere," page 49.) remarks, that in the Red Sea the strongest corals
live on the outer reefs, and appear to love the surf; he adds, that the
more branched kinds abound a little way within, but that even these in
still more protected places, become smaller. Many other facts having a
similar tendency might be adduced. (In the West Indies, as I am informed
by Captain Bird Allen, R.N., it is the common belief of those, who are best
acquainted with the reefs, that the coral flourishes most, where freely
exposed to the swell of the open sea.) It has, however, been doubted by
MM. Quoy and Gaimard, whether any kind of coral can even withstand, much
less flourish in, the breakers of an open sea ("Annales des Sciences
Naturelles," tome vi., pages 276, 278.--"La ou les ondes sont agitees, les
Lytophytes ne peuvent travailler, parce qu'elles detruiraient leurs
fragiles edifices," etc.): they affirm that the saxigenous lithophytes
flourish only where the water is tranquil, and the heat intense. This
statement has passed from one geological work to another; nevertheless, the
protection of the whole reef undoubtedly is due to those kinds of coral,
which cannot exist in the situations thought by these naturalists to be
most favourable to them. For should the outer and living margin perish, of
any one of the many low coral-islands, round which a line of great breakers
is incessantly foaming, the whole, it is scarcely possible to doubt, would
be washed away and destroyed, in less than half a century. But the vital
energies of the corals conquer the mechanical power of the waves; and the
large fragments of reef torn up by every storm, are replaced by the slow
but steady growth of the innumerable polypifers, which form the living zone
on its outer edge.
From these facts, it is certain, that the strongest and most massive corals
flourish, where most exposed. The less perfect state of the reef of most
atolls on the leeward and less exposed side, compared with its state to
windward; and the analogous case of the greater number of breaches on the
near sides of those atolls in the Maldiva Archipelago, which afford some
protection to each other, are obviously explained by this circumstance. If
the question had been, under what conditions the greater number of species
of coral, not regarding their bulk and strength, were developed, I should
answer,--probably in the situations described by MM. Quoy and Gaimard,
where the water is tranquil and the heat intense. The total number of
species of coral in the circumtropical seas must be very great: in the Red
Sea alone, 120 kinds, according to Ehrenberg (Ehrenberg, "Uber die Natur,"
etc., etc., page 46.), have been observed.
The same author has observed that the recoil of the sea from a steep shore
is injurious to the growth of coral, although waves breaking over a bank
are not so. Ehrenberg also states, that where there is much sediment,
placed so as to be liable to be moved by the waves there is little or no
coral; and a collection of living specimens placed by him on a sandy shore
died in the course of a few days. (Ibid., page 49.) An experiment,
however, will presently be related in which some large masses of living
coral increased rapidly in size, after having been secured by stakes on a
sandbank. That loose sediment should be injurious to the living
polypifers, appears, at first sight, probable; and accordingly, in sounding
off Keeling atoll, and (as will hereafter be shown) off Mauritius, the
arming of the lead invariably came up clean, where the coral was growing
vigorously. This same circumstance has probably given rise to a strange
belief, which, according to Captain Owen (Captain Owen on the Geography of
the Maldiva Islands, "Geographical Journal", volume ii., page 88.), is
general amongst the inhabitants of the Maldiva atolls, namely that corals
have roots, and therefore that if merely broken down to the surface, they
grow up again; but, if rooted out, they are permanently destroyed. By this
means the inhabitants keep their harbours clear; and thus the French
Governor of St. Mary's in Madagascar, "cleared out and made a beautiful
little port at that place." For it is probable that sand would accumulate
in the hollows formed by tearing out the corals, but not on the broken and
projecting stumps, and therefore, in the former case, the fresh growth of
the coral might be thus prevented.
In the last chapter I remarked that fringing-reefs are almost universally
breached, where streams enter the sea. (Lieutenant Wellstead and others
have remarked that this is the case in the Red Sea; Dr. Ruppell ("Reise in
Abyss." Band. i., page 142) says that there are pear-shaped harbours in the
upraised coral-coast, into which periodical streams enter. From this
circumstance, I presume, we must infer that before the upheaval of the
strata now forming the coast-land, fresh water and sediment entered the sea
at these points; and the coral being thus prevented growing, the pear-shaped
harbours were produced.) Most authors have attributed this fact to
the injurious effects of the fresh water, even where it enters the sea only
in small quantity, and during a part of the year. No doubt brackish water
would prevent or retard the growth of coral; but I believe that the mud and
sand which is deposited, even by rivulets when flooded, is a much more
efficient check. The reef on each side of the channel leading into Port
Louis at Mauritius, ends abruptly in a wall, at the foot of which I sounded
and found a bed of thick mud. This steepness of the sides appears to be a
general character in such breaches. Cook (Cook's "First Voyage," volume
ii., page 271 (Hawkesworth's edition).), speaking of one at Raiatea, says,
"like all the rest, it is very steep on both sides." Now, if it were the
fresh water mingling with the salt which prevented the growth of coral, the
reef certainly would not terminate abruptly, but as the polypifers nearest
the impure stream would grow less vigorously than those farther off, so
would the reef gradually thin away. On the other hand, the sediment
brought down from the land would only prevent the growth of the coral in
the line of its deposition, but would not check it on the side, so that the
reefs might increase till they overhung the bed of the channel. The
breaches are much fewer in number, and front only the larger valleys in
reefs of the encircling barrier class. They probably are kept open in the
same manner as those into the lagoon of an atoll, namely, by the force of
the currents and the drifting outwards of fine sediment. Their position in
front of valleys, although often separated from the land by deep water
lagoon-channels, which it might be thought would entirely remove the
injurious effects both of the fresh water and the sediment, will receive a
simple explanation when we discuss the origin of barrier-reefs.
In the vegetable kingdom every different station has its peculiar group of
plants, and similar relations appear to prevail with corals. We have
already described the great difference between the corals within the lagoon
of an atoll and those on its outer margin. The corals, also, on the margin
of Keeling Island occurred in zones; thus the Porites and Millepora
complanata grow to a large size only where they are washed by a heavy sea,
and are killed by a short exposure to the air; whereas, three species of
Nullipora also live amidst the breakers, but are able to survive uncovered
for a part of each tide; at greater depths, a strong Madrepora and
Millepora alcicornis are the commonest kinds, the former appearing to be
confined to this part, beneath the zone of massive corals, minute
encrusting corallines and other organic bodies live. If we compare the
external margin of the reef at Keeling atoll with that on the leeward side
of Mauritius, which are very differently circumstanced, we shall find a
corresponding difference in the appearance of the corals. At the latter
place, the genus Madrepora is preponderant over every other kind, and
beneath the zone of massive corals there are large beds of Seriatopora.
There is also a marked difference, according to Captain Moresby (Captain
Moresby on the Northern Maldiva atolls, "Geographical Journal", volume v.,
page 401.), between the great branching corals of the Red Sea, and those on
the reefs of the Maldiva atolls.
These facts, which in themselves are deserving of notice, bear, perhaps,
not very remotely, on a remarkable circumstance which has been pointed out
to me by Captain Moresby, namely, that with very few exceptions, none of
the coral-knolls within the lagoons of Peros Banhos, Diego Garcia, and the
Great Chagos Bank (all situated in the Chagos group), rise to the surface
of the water; whereas all those, with equally few exceptions, within
Solomon and Egmont atolls in the same group, and likewise within the large
southern Maldiva atolls, reach the surface. I make these statements, after
having examined the charts of each atoll. In the lagoon of Peros Banhos,
which is nearly twenty miles across, there is only one single reef which
rises to the surface; in Diego Garcia there are seven, but several of these
lie close to the margin of the lagoon, and need scarcely have been
reckoned; in the Great Chagos Bank there is not one. On the other hand, in
the lagoons of some of the great southern Maldiva atolls, although thickly
studded with reefs, every one without exception rises to the surface; and
on an average there are less than two submerged reefs in each atoll; in the
northern atolls, however, the submerged lagoon-reefs are not quite so rare.
The submerged reefs in the Chagos atolls generally have from one to seven
fathoms water on them, but some have from seven to ten. Most of them are
small with very steep sides (Some of these statements were not communicated
to me verbally by Captain Moresby, but are taken from the MS. account
before alluded to, of the Chagos Group.); at Peros Banhos they rise from a
depth of about thirty fathoms, and some of them in the Great Chagos Bank
from above forty fathoms; they are covered, Captain Moresby informs me,
with living and healthy coral, two and three feet high, consisting of
several species. Why then have not these lagoon-reefs reached the surface,
like the innumerable ones in the atolls above named? If we attempt to
assign any difference in their external conditions, as the cause of this
diversity, we are at once baffled. The lagoon of Diego Garcia is not deep,
and is almost wholly surrounded by its reef; Peros Banhos is very deep,
much larger, with many wide passages communicating with the open sea. On
the other hand, of those atolls, in which all or nearly all the lagoon-reefs
have reached the surface, some are small, others large, some shallow,
others deep, some well-enclosed, and others open.
Captain Moresby informs me that he has seen a French chart of Diego Garcia
made eighty years before his survey, and apparently very accurate; and from
it he infers, that during this interval there has not been the smallest
change in the depth on any of the knolls within the lagoon. It is also
known that during the last fifty-one years, the eastern channel into the
lagoon has neither become narrower, nor decreased in depth; and as there
are numerous small knolls of living coral within it, some change might have
been anticipated. Moreover, as the whole reef round the lagoon of this
atoll has been converted into land--an unparalleled case, I believe, in an
atoll of such large size,--and as the strip of land is for considerable
spaces more than half a mile wide--also a very unusual circumstance,--we
have the best possible evidence, that Diego Garcia has remained at its
present level for a very long period. With this fact, and with the
knowledge that no sensible change has taken place during eighty years in
the coral-knolls, and considering that every single reef has reached the
surface in other atolls, which do not present the smallest appearance of
being older than Diego Garcia and Peros Banhos, and which are placed under
the same external conditions with them, one is led to conclude that these
submerged reefs, although covered with luxuriant coral, have no tendency to
grow upwards, and that they would remain at their present levels for an
almost indefinite period.
From the number of these knolls, from their position, size, and form, many
of them being only one or two hundred yards across, with a rounded outline,
and precipitous sides,--it is indisputable that they have been formed by
the growth of coral; and this makes the case much more remarkable. In
Peros Banhos and in the Great Chagos Bank, some of these almost columnar
masses are 200 feet high, and their summits lie only from two to eight
fathoms beneath the surface; therefore, a small proportional amount more of
growth would cause them to attain the surface, like those numerous knolls,
which rise from an equally great depth within the Maldiva atolls. We can
hardly suppose that time has been wanting for the upward growth of the
coral, whilst in Diego Garcia, the broad annular strip of land, formed by
the continued accumulation of detritus, shows how long this atoll has
remained at its present level. We must look to some other cause than the
rate of growth; and I suspect it will be found in the reefs being formed of
different species of corals, adapted to live at different depths.
The Great Chagos Bank is situated in the centre of the Chagos Group, and
the Pitt and Speaker Banks at its two extreme points. These banks resemble
atolls, except in their external rim being about eight fathoms submerged,
and in being formed of dead rock, with very little living coral on it: a
portion nine miles long of the annular reef of Peros Banhos atoll is in the
same condition. These facts, as will hereafter be shown, render it very
probable that the whole group at some former period subsided seven or eight
fathoms; and that the coral perished on the outer margin of those atolls
which are now submerged, but that it continued alive, and grew up to the
surface on those which are now perfect. If these atolls did subside, and
if from the suddenness of the movement or from any other cause, those
corals which are better adapted to live at a certain depth than at the
surface, once got possession of the knolls, supplanting the former
occupants, they would exert little or no tendency to grow upwards. To
illustrate this, I may observe, that if the corals of the upper zone on the
outer edge of Keeling atoll were to perish, it is improbable that those of
the lower zone would grow to the surface, and thus become exposed to
conditions for which they do not appear to be adapted. The conjecture,
that the corals on the submerged knolls within the Chagos atolls have
analogous habits with those of the lower zone outside Keeling atoll,
receives some support from a remark by Captain Moresby, namely, that they
have a different appearance from those on the reefs in the Maldiva atolls,
which, as we have seen, all rise to the surface: he compares the kind of
difference to that of the vegetation under different climates. I have
entered at considerable length into this case, although unable to throw
much light on it, in order to show that an equal tendency to upward growth
ought not to be attributed to all coral-reefs,--to those situated at
different depths,--to those forming the ring of an atoll or those on the
knolls within a lagoon,--to those in one area and those in another. The
inference, therefore, that one reef could not grow up to the surface within
a given time, because another, not known to be covered with the same
species of corals, and not known to be placed under conditions exactly the
same, has not within the same time reached the surface, is unsound.
SECTION 4.II.--ON THE RATE OF GROWTH OF CORAL-REEFS.
The remark made at the close of the last section, naturally leads to this
division of our subject, which has not, I think, hitherto been considered
under a right point of view. Ehrenberg (Ehrenberg, as before cited, pages
39, 46, and 50.) has stated, that in the Red Sea, the corals only coat
other rocks in a layer from one to two feet in thickness, or at most to a
fathom and a half; and he disbelieves that, in any case, they form, by
their own proper growth, great masses, stratum over stratum. A nearly
similar observation has been made by MM. Quoy and Gaimard ("Annales des
Sciences Nat." tom. vi., page 28.), with respect to the thickness of some
upraised beds of coral, which they examined at Timor and some other places.
Ehrenberg (Ehrenberg, ut sup., page 42.) saw certain large massive corals
in the Red Sea, which he imagines to be of such vast antiquity, that they
might have been beheld by Pharaoh; and according to Mr. Lyell (Lyell's
"Principles of Geology," book iii., chapter xviii.) there are certain
corals at Bermuda, which are known by tradition, to have been living for
centuries. To show how slowly coral-reefs grow upwards, Captain Beechey
(Beechey's "Voyage to the Pacific," chapter viii.) has adduced the case of
the Dolphin Reef off Tahiti, which has remained at the same depth beneath
the surface, namely about two fathoms and a half, for a period of
sixty-seven years. There are reefs in the Red Sea, which certainly do not
appear (Ehrenberg, ut sup., page 43.) to have increased in dimensions during
the last half-century, and from the comparison of old charts with recent
surveys, probably not during the last two hundred years. These, and other
similar facts, have so strongly impressed many with the belief of the
extreme slowness of the growth of corals, that they have even doubted the
possibility of islands in the great oceans having been formed by their
agency. Others, again, who have not been overwhelmed by this difficulty,
have admitted that it would require thousands, and tens of thousands of
years, to form a mass, even of inconsiderable thickness; but the subject
has not, I believe, been viewed in the proper light.
That masses of considerable thickness have been formed by the growth of
coral, may be inferred with certainty from the following facts. In the
deep lagoons of Peros Banhos and of the Great Chagos Bank, there are, as
already described, small steep-sided knolls covered with living coral.
There are similar knolls in the southern Maldiva atolls, some of which, as
Captain Moresby assures me, are less than a hundred yards in diameter, and
rise to the surface from a depth of between two hundred and fifty and three
hundred feet. Considering their number, form, and position, it would be
preposterous to suppose that they are based on pinnacles of any rock, not
of coral formation; or that sediment could have been heaped up into such
small and steep isolated cones. As no kind of living coral grows above the
height of a few feet, we are compelled to suppose that these knolls have
been formed by the successive growth and death of many individuals,--first
one being broken off or killed by some accident, and then another, and one
set of species being replaced by another set with different habits, as the
reef rose nearer the surface, or as other changes supervened. The spaces
between the corals would become filled up with fragments and sand, and such
matter would probably soon be consolidated, for we learn from Lieutenant
Nelson ("Geological Transactions," volume v., page 113.), that at Bermuda a
process of this kind takes place beneath water, without the aid of
evaporation. In reefs, also, of the barrier class, we may feel sure, as I
have shown, that masses of great thickness have been formed by the growth
of the coral; in the case of Vanikoro, judging only from the depth of the
moat between the land and the reef, the wall of coral-rock must be at least
three hundred feet in vertical thickness.
It is unfortunate that the upraised coral-islands in the Pacific have not
been examined by a geologist. The cliffs of Elizabeth Island, in the Low
Archipelago, are eighty feet high, and appear, from Captain Beechey's
description, to consist of a homogeneous coral-rock. From the isolated
position of this island, we may safely infer that it is an upraised atoll,
and therefore that it has been formed by masses of coral, grown together.
Savage Island seems, from the description of the younger Forster (Forster's
"Voyage round the World with Cook," volume ii., pages 163, 167.), to have a
similar structure, and its shores are about forty feet high: some of the
Cook Islands also appear (Williams's "Narrative of Missionary Enterprise,"
page 30.) to be similarly composed. Captain Belcher, R.N., in a letter
which Captain Beaufort showed me at the admiralty, speaking of Bow atoll,
says, "I have succeeded in boring forty-five feet through coral-sand, when
the auger became jammed by the falling in of the surrounding CREAMY
matter." On one of the Maldiva atolls, Captain Moresby bored to a depth of
twenty-six feet, when his auger also broke: he has had the kindness to
give me the matter brought up; it is perfectly white, and like finely
triturated coral-rock.
In my description of Keeling atoll, I have given some facts, which show
that the reef probably has grown outwards; and I have found, just within
the outer margin, the great mounds of Porites and of Millepora, with their
summits lately killed, and their sides subsequently thickened by the growth
of the coral: a layer, also, of Nullipora had already coated the dead
surface. As the external slope of the reef is the same round the whole of
this atoll, and round many other atolls, the angle of inclination must
result from an adaption between the growing powers of the coral, and the
force of the breakers, and their action on the loose sediment. The reef,
therefore, could not increase outwards, without a nearly equal addition to
every part of the slope, so that the original inclination might be
preserved, and this would require a large amount of sediment, all derived
from the wear of corals and shells, to be added to the lower part.
Moreover, at Keeling atoll, and probably in many other cases, the different
kinds of corals would have to encroach on each other; thus the Nulliporae
cannot increase outwards without encroaching on the Porites and Millepora
complanata, as is now taking place; nor these latter without encroaching on
the strongly branched Madreporet, the Millepora alcicornis, and some
Astraeas; nor these again without a foundation being formed for them within
the requisite depth, by the accumulation of sediment. How slow, then, must
be the ordinary lateral or outward growth of such reefs. But off Christmas
atoll, where the sea is much more shallow than is usual, we have good
reason to believe that, within a period not very remote, the reef has
increased considerably in width. The land has the extraordinary breadth of
three miles; it consists of parallel ridges of shells and broken corals,
which furnish "an incontestable proof," as observed by Cook (Cook's "Third
Voyage," book III., chapter x.), "that the island has been produced by
accessions from the sea, and is in a state of increase." The land is
fronted by a coral-reef, and from the manner in which islets are known to
be formed, we may feel confident that the reef was not three miles wide,
when the first, or most backward ridge, was thrown up; and, therefore, we
must conclude that the reef has grown outwards during the accumulation of
the successive ridges. Here then, a wall of coral-rock of very
considerable breadth has been formed by the outward growth of the living
margin, within a period during which ridges of shells and corals, lying on
the bare surface, have not decayed. There can be little doubt, from the
account given by Captain Beechey, that Matilda atoll, in the Low
Archipelago, has been converted in the space of thirty-four years, from
being, as described by the crew of a wrecked whaling vessel, a "reef of
rocks" into a lagoon-island, fourteen miles in length, with "one of its
sides covered nearly the whole way with high trees." (Beechey's "Voyage to
the Pacific," chapter vii. and viii.) The islets, also, on Keeling atoll,
it has been shown, have increased in length, and since the construction of
an old chart, several of them have become united into one long islet; but
in this case, and in that of Matilda atoll, we have no proof, and can only
infer as probable, that the reef, that is the foundation of the islets, has
increased as well as the islets themselves.
After these considerations, I attach little importance, as indicating the
ordinary and still less the possible rate of OUTWARD growth of coral-reefs,
to the fact that certain reefs in the Red Sea have not increased during a
long interval of time; or to other such cases, as that of Ouluthy atoll in
the Caroline group, where every islet, described a thousand years before by
Cantova was found in the same state by Lutke (F. Lutke's "Voyage autour du
Monde." In the group Elato, however, it appears that what is now the islet
Falipi, is called in Cantova's Chart, the Banc de Falipi. It is not stated
whether this has been caused by the growth of coral, or by the accumulation
of sand.),--without it could be shown that, in these cases, the conditions
were favourable to the vigorous and unopposed growth of the corals living
in the different zones of depth, and that a proper basis for the extent of
the reef was present. The former conditions must depend on many
contingencies, and in the deep oceans where coral formations most abound, a
basis within the requisite depth can rarely be present.
Nor do I attach any importance to the fact of certain submerged reefs, as
those off Tahiti, or those within Diego Garcia not now being nearer the
surface than they were many years ago, as an indication of the rate under
favourable circumstances of the UPWARD growth of reefs; after it has been
shown, that all the reefs have grown to the surface in some of the Chagos
atolls, but that in neighbouring atolls which appear to be of equal
antiquity and to be exposed to the same external conditions, every reef
remains submerged; for we are almost driven to attribute this to a
difference, not in the rate of growth, but in the habits of the corals in
the two cases.
In an old-standing reef, the corals, which are so different in kind on
different parts of it, are probably all adapted to the stations they
occupy, and hold their places, like other organic beings, by a struggle one
with another, and with external nature; hence we may infer that their
growth would generally be slow, except under peculiarly favourable
circumstances. Almost the only natural condition, allowing a quick upward
growth of the whole surface of a reef, would be a slow subsidence of the
area in which it stood; if, for instance, Keeling atoll were to subside two
or three feet, can we doubt that the projecting margin of live coral, about
half an inch in thickness, which surrounds the dead upper surfaces of the
mounds of Porites, would in this case form a concentric layer over them,
and the reef thus increase upwards, instead of, as at present, outwards?
The Nulliporae are now encroaching on the Porites and Millepora, but in
this case might we not confidently expect that the latter would, in their
turn, encroach on the Nulliporae? After a subsidence of this kind, the sea
would gain on the islets, and the great fields of dead but upright corals
in the lagoon, would be covered by a sheet of clear water; and might we not
then expect that these reefs would rise to the surface, as they anciently
did when the lagoon was less confined by islets, and as they did within a
period of ten years in the schooner-channel, cut by the inhabitants? In
one of the Maldiva atolls, a reef, which within a very few years existed as
an islet bearing cocoa-nut trees, was found by Lieutenant Prentice
"ENTIRELY COVERED WITH LIVE CORAL AND MADREPORE." The natives believe that
the islet was washed away by a change in the currents, but if, instead of
this, it had quietly subsided, surely every part of the island which
offered a solid foundation, would in a like manner have become coated with
living coral.
Through steps such as these, any thickness of rock, composed of a singular
intermixture of various kinds of corals, shells, and calcareous sediment,
might be formed; but without subsidence, the thickness would necessarily be
determined by the depth at which the reef-building polypifers can exist.
If it be asked, at what rate in years I suppose a reef of coral favourably
circumstanced could grow up from a given depth; I should answer, that we
have no precise evidence on this point, and comparatively little concern
with it. We see, in innumerable points over wide areas, that the rate has
been sufficient, either to bring up the reefs from various depths to the
surface, or, as is more probable, to keep them at the surface, during
progressive subsidences; and this is a much more important standard of
comparison than any cycle of years.
It may, however, be inferred from the following facts, that the rate in
years under favourable circumstances would be very far from slow. Dr.
Allan, of Forres, has, in his MS. Thesis deposited in the library of the
Edinburgh University (extracts from which I owe to the kindness of Dr.
Malcolmson), the following account of some experiments, which he tried
during his travels in the years 1830 to 1832 on the east coast of
Madagascar. "To ascertain the rise and progress of the coral-family, and
fix the number of species met with at Foul Point (latitude 17 deg 40')
twenty species of coral were taken off the reef and planted apart on a
sand-bank THREE FEET DEEP AT LOW WATER. Each portion weighed ten pounds,
and was kept in its place by stakes. Similar quantities were placed in a
clump and secured as the rest. This was done in December 1830. In July
following, each detached mass was nearly level with the sea at low water,
quite immovable, and several feet long, stretching as the parent reef, with
the coast current from north to south. The masses accumulated in a clump
were found equally increased, but some of the species in such unequal
ratios, as to be growing over each other." The loss of Dr. Allan's
magnificent collection by shipwreck, unfortunately prevents its being known
to what genera these corals belonged; but from the numbers experimented on,
it is certain that all the more conspicuous kinds must have been included.
Dr. Allan informs me, in a letter, that he believes it was a Madrepora,
which grew most vigorously. One may be permitted to suspect that the level
of the sea might possibly have been somewhat different at the two stated
periods; nevertheless, it is quite evident that the growth of the ten-pound
masses, during the six or seven months, at the end of which they were found
immovably fixed (It is stated by De la Beche ("Geological Manual," page
143), on the authority of Mr. Lloyd, who surveyed the Isthmus of Panama,
that some specimens of Polypifers, placed by him in a sheltered pool of
water, were found in the course of a few days firmly fixed by the secretion
of a stony matter, to the bottom) and several feet in length, must have
been very great. The fact of the different kinds of coral, when placed in
one clump, having increased in extremely unequal ratios, is very
interesting, as it shows the manner in which a reef, supporting many
species of coral, would probably be affected by a change in the external
conditions favouring one kind more than another. The growth of the masses
of coral in N. and S. lines parallel to the prevailing currents, whether
due to the drifting of sediment or to the simple movement of the water, is,
also, a very interesting circumstance.
A fact, communicated to me by Lieutenant Wellstead, I.N., in some degree
corroborates the result of Dr. Allan's experiments: it is, that in the
Persian Gulf a ship had her copper bottom encrusted in the course of twenty
months with a layer of coral, TWO FEET in thickness, which it required
great force to remove, when the vessel was docked: it was not ascertained
to what order this coral belonged. The case of the schooner-channel choked
up with coral in an interval of less than ten years, in the lagoon of
Keeling atoll, should be here borne in mind. We may also infer, from the
trouble which the inhabitants of the Maldiva atolls take to root out, as
they express it, the coral-knolls from their harbours, that their growth
can hardly be very slow. (Mr. Stutchbury ("West of England Journal", No.
I., page 50.) has described a specimen of Agaricia, "weighing 2 lbs. 9 oz.,
which surrounds a species of oyster, whose age could not be more than two
years, and yet is completely enveloped by this dense coral." I presume
that the oyster was living when the specimen was procured; otherwise the
fact tells nothing. Mr. Stutchbury also mentions an anchor, which had
become entirely encrusted with coral in fifty years; other cases, however,
are recorded of anchors which have long remained amidst coral-reefs without
having become coated. The anchor of the "Beagle", in 1832, after having
been down exactly one month at Rio de Janeiro, was so thickly coated by two
species of Tubularia, that large spaces of the iron were entirely
concealed; the tufts of this horny zoophyte were between two and three
inches in length. It has been attempted to compute, but I believe
erroneously, the rate of growth of a reef, from the fact mentioned by
Captain Beechey, of the Chama gigas being embedded in coral-rock. But it
should be remembered, that some species of this genus invariably live, both
whilst young and old, in cavities, which the animal has the power of
enlarging with its growth. I saw many of these shells thus embedded in the
outer "flat" of Keeling atoll, which is composed of dead rock; and
therefore the cavities in this case had no relation whatever with the
growth of coral. M. Lesson, also, speaking of this shell (Partie Zoolog.
"Voyage de la 'Coquille'"), has remarked, "que constamment ses valves
etaient engages completement dans la masse des Madrepores.")
From the facts given in this section, it may be concluded, first, that
considerable thicknesses of rock have certainly been formed within the
present geological area by the growth of coral and the accumulation of its
detritus; and, secondly, that the increase of individual corals and of
reefs, both outwards or horizontally and upwards or vertically, under the
peculiar conditions favourable to such increase, is not slow, when referred
either to the standard of the average oscillations of level in the earth's
crust, or to the more precise but less important one of a cycle of years.
SECTION 4.III.--ON THE DEPTHS AT WHICH REEF-BUILDING POLYPIFERS CAN LIVE.
I have already described in detail, which might have appeared trivial, the
nature of the bottom of the sea immediately surrounding Keeling atoll; and
I will now describe with almost equal care the soundings off the
fringing-reefs of Mauritius. I have preferred this arrangement, for the sake
of grouping together facts of a similar nature. I sounded with the wide
bell-shaped lead which Captain Fitzroy used at Keeling Island, but my
examination of the bottom was confined to a few miles of coast (between
Port Louis and Tomb Bay) on the leeward side of the island. The edge of
the reef is formed of great shapeless masses of branching Madrepores, which
chiefly consist of two species,--apparently M. corymbosa and pocillifera,--
mingled with a few other kinds of coral. These masses are separated from
each other by the most irregular gullies and cavities, into which the lead
sinks many feet. Outside this irregular border of Madrepores, the water
deepens gradually to twenty fathoms, which depth generally is found at the
distance of from half to three-quarters of a mile from the reef. A little
further out the depth is thirty fathoms, and thence the bank slopes rapidly
into the depths of the ocean. This inclination is very gentle compared
with that outside Keeling and other atolls, but compared with most coasts
it is steep. The water was so clear outside the reef, that I could
distinguish every object forming the rugged bottom. In this part, and to a
depth of eight fathoms, I sounded repeatedly, and at each cast pounded the
bottom with the broad lead, nevertheless the arming invariably came up
perfectly clean, but deeply indented. From eight to fifteen fathoms a
little calcareous sand was occasionally brought up, but more frequently the
arming was simply indented. In all this space the two Madrepores above
mentioned, and two species of Astraea, with rather large stars, seemed the
commonest kinds (Since the preceding pages were printed off, I have
received from Mr. Lyell a very interesting pamphlet, entitled "Remarks upon
Coral Formations," etc., by J. Couthouy, Boston, United States, 1842.
There is a statement (page 6), on the authority of the Rev. J. Williams,
corroborating the remarks made by Ehrenberg and Lyell (page 71 of this
volume), on the antiquity of certain individual corals in the Red Sea and
at Bermuda; namely, that at Upolu, one of the Navigator Islands,
"particular clumps of coral are known to the fishermen by name, derived
from either some particular configuration or tradition attached to them,
and handed down from time immemorial." With respect to the thickness of
masses of coral-rock, it clearly appears, from the descriptions given by
Mr. Couthouy (pages 34, 58) that Mangaia and Aurora Islands are upraised
atolls, composed of coral rock: the level summit of the former is about
three hundred feet, and that of Aurora Island is two hundred feet above the
sea-level.); and it must be noticed that twice at the depth of fifteen
fathoms, the arming was marked with a clean impression of an Astraea.
Besides these lithophytes, some fragments of the Millepora alcicornis,
which occurs in the same relative position at Keeling Island, were brought
up; and in the deeper parts there were large beds of a Seriatopora,
different from S. subulata, but closely allied to it. On the beach within
the reef, the rolled fragments consisted chiefly of the corals just
mentioned, and of a massive Porites, like that at Keeling atoll, of a
Meandrina, Pocillopora verrucosa, and of numerous fragments of Nullipora.
From fifteen to twenty fathoms the bottom was, with few exceptions, either
formed of sand, or thickly covered with Seriatopora: this delicate coral
seems to form at these depths extensive beds unmingled with any other kind.
At twenty fathoms, one sounding brought up a fragment of Madrepora
apparently M. pocillifera, and I believe it is the same species (for I
neglected to bring specimens from both stations) which mainly forms the
upper margin of the reef; if so, it grows in depths varying from 0 to 20
fathoms. Between 20 and 23 fathoms I obtained several soundings, and they
all showed a sandy bottom, with one exception at 30 fathoms, when the
arming came up scooped out, as if by the margin of a large Caryophyllia.
Beyond 33 fathoms I sounded only once; and from 86 fathoms, at the distance
of one mile and a third from the edge of the reef, the arming brought up
calcareous sand with a pebble of volcanic rock. The circumstance of the
arming having invariably come up quite clean, when sounding within a
certain number of fathoms off the reefs of Mauritius and Keeling atoll
(eight fathoms in the former case, and twelve in the latter) and of its
having always come up (with one exception) smoothed and covered with sand,
when the depth exceeded twenty fathoms, probably indicates a criterion, by
which the limits of the vigorous growth of coral might in all cases be
readily ascertained. I do not, however, suppose that if a vast number of
soundings were obtained round these islands, the limit above assigned would
be found never to vary, but I conceive the facts are sufficient to show,
that the exceptions would be few. The circumstance of a GRADUAL change, in
the two cases, from a field of clean coral to a smooth sandy bottom, is far
more important in indicating the depth at which the larger kinds of coral
flourish than almost any number of separate observations on the depth, at
which certain species have been dredged up. For we can understand the
gradation, only as a prolonged struggle against unfavourable conditions.
If a person were to find the soil clothed with turf on the banks of a
stream of water, but on going to some distance on one side of it, he
observed the blades of grass growing thinner and thinner, with intervening
patches of sand, until he entered a desert of sand, he would safely
conclude, especially if changes of the same kind were noticed in other
places, that the presence of the water was absolutely necessary to the
formation of a thick bed of turf: so may we conclude, with the same
feeling of certainty, that thick beds of coral are formed only at small
depths beneath the surface of the sea.
I have endeavoured to collect every fact, which might either invalidate or
corroborate this conclusion. Captain Moresby, whose opportunities for
observation during his survey of the Maldiva and Chagos Archipelagoes have
been unrivalled, informs me, that the upper part or zone of the steep-sided
reefs, on the inner and outer coasts of the atolls in both groups,
invariably consists of coral, and the lower parts of sand. At seven or
eight fathoms depth, the bottom is formed, as could be seen through the
clear water, of great living masses of coral, which at about ten fathoms
generally stand some way apart from each other, with patches of white sand
between them, and at a little greater depth these patches become united
into a smooth steep slope, without any coral. Captain Moresby, also,
informs me in support of his statement, that he found only decayed coral on
the Padua Bank (northern part of the Laccadive group) which has an average
depth between twenty-five and thirty-five fathoms, but that on some other
banks in the same group with only ten or twelve fathoms water on them (for
instance, the Tillacapeni bank), the coral was living.
With regard to the coral-reefs in the Red Sea, Ehrenberg has the following
passage:--"The living corals do not descend there into great depths. On
the edges of islets and near reefs, where the depth was small, very many
lived; but we found no more even at six fathoms. The pearl-fishers at
Yemen and Massaua asserted that there was no coral near the pearl-banks at
nine fathoms depth, but only sand. We were not able to institute any more
special researches." (Ehrenberg, "Uber die Natur," etc., page 50.) I am,
however, assured both by Captain Moresby and Lieutenant Wellstead, that in
the more northern parts of the Red Sea, there are extensive beds of living
coral at a depth of twenty-five fathoms, in which the anchors of their
vessels were frequently entangled. Captain Moresby attributes the less
depth, at which the corals are able to live in the places mentioned by
Ehrenberg, to the greater quantity of sediment there; and the situations,
where they were flourishing at the depth of twenty-five fathoms, were
protected, and the water was extraordinarily limpid. On the leeward side
of Mauritius where I found the coral growing at a somewhat greater depth
than at Keeling atoll, the sea, owing apparently to its tranquil state, was
likewise very clear. Within the lagoons of some of the Marshall atolls,
where the water can be but little agitated, there are, according to
Kotzebue, living beds of coral in twenty-five fathoms. From these facts,
and considering the manner in which the beds of clean coral off Mauritius,
Keeling Island, the Maldiva and Chagos atolls, graduated into a sandy
slope, it appears very probable that the depth, at which reef-building
polypifers can exist, is partly determined by the extent of inclined
surface, which the currents of the sea and the recoiling waves have the
power to keep free from sediment.
MM. Quoy and Gaimard ("Annales des Sci. Nat." tom. vi.) believe that the
growth of coral is confined within very limited depths; and they state that
they never found any fragment of an Astraea (the genus they consider most
efficient in forming reefs) at a depth above twenty-five or thirty feet.
But we have seen that in several places the bottom of the sea is paved with
massive corals at more than twice this depth; and at fifteen fathoms (or
twice this depth) off the reefs of Mauritius, the arming was marked with
the distinct impression of a living Astraea. Millepora alcicornis lives in
from 0 to 12 fathoms, and the genera Madrepora and Seriatopora from 0 to 20
fathoms. Captain Moresby has given me a specimen of Sideropora scabra
(Porites of Lamarck) brought up alive from 17 fathoms. Mr. Couthouy
("Remarks on Coral Formations," page 12.) states that he has dredged up on
the Bahama banks considerable masses of Meandrina from 16 fathoms, and he
has seen this coral growing in 20 fathoms. A Caryophyllia, half an inch in
diameter, was dredged up alive from 80 fathoms off Juan Fernandez (latitude
33 deg S.) by Captain P.P. King (I am indebted to Mr. Stokes for having
kindly communicated this fact to me, together with much other valuable
information.): this is the most remarkable fact with which I am
acquainted, showing the depth at which a genus of corals often found on
reefs, can exist.
We ought, however, to feel less surprise at this fact, as Caryophyllia
alone of the lamelliform genera, ranges far beyond the tropics; it is found
in Zetland (Fleming's "British Animals," genus Caryophyllia.) in Latitude
60 deg N. in deep water, and I procured a small species from Tierra del
Fuego in Latitude 53 deg S. Captain Beechey informs me, that branches of
pink and yellow coral were frequently brought up from between twenty and
twenty-five fathoms off the Low atolls; and Lieutenant Stokes, writing to
me from the N.W. coast of Australia, says that a strongly branched coral
was procured there from thirty fathoms; unfortunately it is not known to
what genera these corals belong.
(I will record in the form of a note all the facts that I have been able to
collect on the depths, both within and without the tropics, at which those
corals and corallines can live, which there is no reason to suppose ever
materially aid in the construction of a reef.
(In the following list the name of the Zoophyte is followed by the depth in
fathoms, the country and degrees S. latitude, and the authority. Where no
authority is given, the observation is Darwin's own.)
SERTULARIA, 40, Cape Horn 66.
CELLARIA, 40, Cape Horn 66.
CELLARIA, A minute scarlet encrusting species, found living, 190, Keeling
Atoll, 12.
CELLARIA, An allied, small stony sub-generic form, 48, St Cruz Riv. 50.
A coral allied to VINCULARIA, with eight rows of cells, 40, Cape Horn.
TUBULIPORA, near to T. patima, 40, Cape Horn.
TUBULIPORA, near to T. patima, 94, East Chiloe 43.
CELLEPORA, several species, and allied sub-generic forms, 40, Cape Horn.
CELLEPORA, several species, and allied sub-generic forms, 40 and 57, Chonos
Archipelago 45.
CELLEPORA, several species, and allied sub-generic forms, 48, St Cruz 50.
ESCHARA, 30, Tierra del Fuego 53.
ESCHARA, 48, St Cruz R. 50.
RETEPORA, 40, Cape Horn.
RETEPORA, 100, Cape of Good Hope 34, Quoy and Gaimard, "Ann. Scien. Nat."
tome vi., page 284.
MILLEPORA, a strong coral with cylindrical branches, of a pink colour,
about two inches high, resembling in the form of its orifices M. aspera of
Lamarck, 94 and 30, E. Chiloe 43, Tierra del Fuego 53.
CORALIUM, 120, Barbary 33 N., Peyssonel in paper read to Royal Society May
1752.
ANTIPATHES, 16, Chonos 45.
GORGONIA (or an allied form), 160, Abrolhos on the coast of Brazil 18,
Captain Beechey informed me of this fact in a letter.
Ellis ("Nat. Hist. of Coralline," page 96) states that Ombellularia was
procured in latitude 79 deg N. STICKING to a LINE from the depth of 236
fathoms; hence this coral either must have been floating loose, or was
entangled in stray line at the bottom. Off Keeling atoll a compound
Ascidia (Sigillina) was brought up from 39 fathoms, and a piece of sponge,
apparently living, from 70, and a fragment of Nullipora also apparently
living from 92 fathoms. At a greater depth than 90 fathoms off this coral
island, the bottom was thickly strewed with joints of Halimeda and small
fragments of other Nulliporae, but all dead. Captain B. Allen, R.N.,
informs me that in the survey of the West Indies it was noticed that
between the depth of 10 and 200 fathoms, the sounding lead very generally
came up coated with the dead joints of a Halimeda, of which he showed me
specimens. Off Pernambuco, in Brazil, in about twelve fathoms, the bottom
was covered with fragments dead and alive of a dull red Nullipora, and I
infer from Roussin's chart, that a bottom of this kind extends over a wide
area. On the beach, within the coral-reefs of Mauritius, vast quantities
of fragments of Nulliporae were piled up. From these facts it appears,
that these simply organized bodies are amongst the most abundant
productions of the sea.)
Although the limit of depth, at which each particular kind of coral ceases
to exist, is far from being accurately known; yet when we bear in mind the
manner in which the clumps of coral gradually became infrequent at about
the same depth, and wholly disappeared at a greater depth than twenty
fathoms, on the slope round Keeling atoll, on the leeward side of the
Mauritius, and at rather less depth, both without and within the atolls of
the Maldiva and Chagos Archipelagoes; and when we know that the reefs round
these islands do not differ from other coral formations in their form and
structure, we may, I think, conclude that in ordinary cases, reef-building
polypifers do not flourish at greater depths than between twenty and thirty
fathoms.
It has been argued ("Journal of the Royal Geographical Society," 1831, page
218.) that reefs may possibly rise from very great depths through the means
of small corals, first making a platform for the growth of the stronger
kinds. This, however, is an arbitrary supposition: it is not always
remembered, that in such cases there is an antagonist power in action,
namely, the decay of organic bodies, when not protected by a covering of
sediment, or by their own rapid growth. We have, moreover, no right to
calculate on unlimited time for the accumulation of small organic bodies
into great masses. Every fact in geology proclaims that neither the land,
nor the bed of the sea retain for indefinite periods the same level. As
well might it be imagined that the British Seas would in time become choked
up with beds of oysters, or that the numerous small corallines off the
inhospitable shores of Tierra del Fuego would in time form a solid and
extensive coral-reef.
CHAPTER V.--THEORY OF THE FORMATION OF THE DIFFERENT CLASSES OF
CORAL-REEFS.
The atolls of the larger archipelagoes are not formed on submerged craters,
or on banks of sediment.--Immense areas interspersed with atolls.--Their
subsidence.--The effects of storms and earthquakes on atolls.--Recent
changes in their state.--The origin of barrier-reefs and of atolls.--Their
relative forms.--The step-formed ledges and walls round the shores of some
lagoons.--The ring-formed reefs of the Maldiva atolls.--The submerged
condition of parts or of the whole of some annular reefs.--The disseverment
of large atolls.--The union of atolls by linear reefs.--The Great Chagos
Bank.--Objections from the area and amount of subsidence required by the
theory, considered.--The probable composition of the lower parts of atolls.
The naturalists who have visited the Pacific, seem to have had their
attention riveted by the lagoon-islands, or atolls,--those singular rings
of coral-land which rise abruptly out of the unfathomable ocean--and have
passed over, almost unnoticed, the scarcely less wonderful encircling
barrier-reefs. The theory most generally received on the formation of
atolls, is that they are based on submarine craters; but where can we find
a crater of the shape of Bow atoll, which is five times as long as it is
broad (Plate I., Figure 4); or like that of Menchikoff Island (Plate II.,
Figure 3.), with its three loops, together sixty miles in length; or like
Rimsky Korsacoff, narrow, crooked, and fifty-four miles long; or like the
northern Maldiva atolls, made up of numerous ring-formed reefs, placed on
the margin of a disc,--one of which discs is eighty-eight miles in length,
and only from ten to twenty in breadth? It is, also, not a little
improbable, that there should have existed as many craters of immense size
crowded together beneath the sea, as there are now in some parts atolls.
But this theory lies under a greater difficulty, as will be evident, when
we consider on what foundations the atolls of the larger archipelagoes
rest: nevertheless, if the rim of a crater afforded a basis at the proper
depth, I am far from denying that a reef like a perfectly characterised
atoll might not be formed; some such, perhaps, now exist; but I cannot
believe in the possibility of the greater number having thus originated.
An earlier and better theory was proposed by Chamisso (Kotzebue's "First
Voyage," volume iii., page 331.); he supposes that as the more massive
kinds of corals prefer the surf, the outer portions, in a reef rising from
a submarine basis, would first reach the surface and consequently form a
ring. But on this view it must be assumed, that in every case the basis
consists of a flat bank; for if it were conically formed, like a
mountainous mass, we can see no reason why the coral should spring up from
the flanks, instead of from the central and highest parts: considering the
number of the atolls in the Pacific and Indian Oceans, this assumption is
very improbable. As the lagoons of atolls are sometimes even more than
forty fathoms deep, it must, also, be assumed on this view, that at a depth
at which the waves do not break, the coral grows more vigorously on the
edges of a bank than on its central part; and this is an assumption without
any evidence in support of it. I remarked, in the third chapter, that a
reef, growing on a detached bank, would tend to assume an atoll-like
structure; if, therefore, corals were to grow up from a bank, with a level
surface some fathoms submerged, having steep sides and being situated in a
deep sea, a reef not to be distinguished from an atoll, might be formed: I
believe some such exist in the West Indies. But a difficulty of the same
kind with that affecting the crater theory, runners, as we shall presently
see, this view inapplicable to the greater number of atolls.
No theory worthy of notice has been advanced to account for those
barrier-reefs, which encircle islands of moderate dimensions. The great
reef which fronts the coast of Australia has been supposed, but without any
special facts, to rest on the edge of a submarine precipice, extending
parallel to the shore. The origin of the third class or of fringing-reefs
presents, I believe, scarcely any difficulty, and is simply consequent on
the polypifers not growing up from great depths, and their not flourishing
close to gently shelving beaches where the water is often turbid.
What cause, then, has given to atolls and barrier-reefs their
characteristic forms? Let us see whether an important deduction will not
follow from the consideration of these two circumstances, first, the
reef-building corals flourishing only at limited depths; and secondly, the
vastness of the areas interspersed with coral-reefs and coral-islets, none
of which rise to a greater height above the level of the sea, than that
attained by matter thrown up by the waves and winds. I do not make this
latter statement vaguely; I have carefully sought for descriptions of every
island in the intertropical seas; and my task has been in some degree
abridged by a map of the Pacific, corrected in 1834 by MM. D'Urville and
Lottin, in which the low islands are distinguished from the high ones (even
from those much less than a hundred feet in height) by being written
without a capital letter; I have detected a few errors in this map,
respecting the height of some of the islands, which will be noticed in the
Appendix, where I treat of coral formations in geographical order. To the
Appendix, also, I must refer for a more particular account of the data on
which the statements on the next page are grounded. I have ascertained,
and chiefly from the writings of Cook, Kotzebue, Bellinghausen, Duperrey,
Beechey, and Lutke, regarding the Pacific; and from Moresby (See also
Captain Owen's and Lieutenant Wood's papers in the "Geographical Journal",
on the Maldiva and Laccadive Archipelagoes. These officers particularly
refer to the lowness of the islets; but I chiefly ground my assertion
respecting these two groups, and the Chagos group, from information
communicated to me by Captain Moresby.) with respect to the Indian Ocean,
that in the following cases the term "low island" strictly means land of
the height commonly attained by matter thrown up by the winds and the waves
of an open sea. If we draw a line (the plan I have always adopted) joining
the external atolls of that part of the Low Archipelago in which the
islands are numerous, the figure will be a pointed ellipse (reaching from
Hood to Lazaref Island), of which the longer axis is 840 geographical
miles, and the shorter 420 miles; in this space (I find from Mr. Couthouy's
pamphlet (page 58) that Aurora Island is about two hundred feet in height;
it consists of coral-rock, and seems to have been formed by the elevation
of an atoll. It lies north-east of Tahiti, close without the line bounding
the space coloured dark blue in the map appended to this volume. Honden
Island, which is situated in the extreme north-west part of the Low
Archipelago, according to measurements made on board the "Beagle", whilst
sailing by, is 114 feet from the SUMMIT OF THE TREES to the water's edge.
This island appeared to resemble the other atolls of the group.) none of
the innumerable islets united into great rings rise above the stated level.
The Gilbert group is very narrow, and 300 miles in length. In a prolonged
line from this group, at the distance of 240 miles, is the Marshall
Archipelago, the figure of which is an irregular square, one end being
broader than the other; its length is 520 miles, with an average width of
240; these two groups together are 1,040 miles in length, and all their
islets are low. Between the southern end of the Gilbert and the northern
end of Low Archipelago, the ocean is thinly strewed with islands, all of
which, as far as I have been able to ascertain, are low; so that from
nearly the southern end of the Low Archipelago, to the northern end of the
Marshall Archipelago, there is a narrow band of ocean, more than 4,000
miles in length, containing a great number of islands, all of which are
low. In the western part of the Caroline Archipelago, there is a space of
480 miles in length, and about 100 broad, thinly interspersed with low
islands. Lastly, in the Indian Ocean, the archipelago of the Maldivas is
470 miles in length, and 60 in breadth; that of the Laccadives is 150 by
100 miles; as there is a low island between these two groups, they may be
considered as one group of 1,000 miles in length. To this may be added the
Chagos group of low islands, situated 280 miles distant, in a line
prolonged from the southern extremity of the Maldivas. This group,
including the submerged banks, is 170 miles in length and 80 in breadth.
So striking is the uniformity in direction of these three archipelagoes,
all the islands of which are low, that Captain Moresby, in one of his
papers, speaks of them as parts of one great chain, nearly 1,500 miles
long. I am, then, fully justified in repeating, that enormous spaces, both
in the Pacific and Indian Oceans, are interspersed with islands, of which
not one rises above that height, to which the waves and winds in an open
sea can heap up matter.
On what foundations, then, have these reefs and islets of coral been
constructed? A foundation must originally have been present beneath each
atoll at that limited depth, which is indispensable for the first growth of
the reef-building polypifers. A conjecture will perhaps be hazarded, that
the requisite bases might have been afforded by the accumulation of great
banks of sediment, which owing to the action of superficial currents (aided
possibly by the undulatory movement of the sea) did not quite reach the
surface,--as actually appears to have been the case in some parts of the
West Indian Sea. But in the form and disposition of the groups of atolls,
there is nothing to countenance this notion; and the assumption without any
proof, that a number of immense piles of sediment have been heaped on the
floor of the great Pacific and Indian Oceans, in their central parts far
remote from land, and where the dark blue colour of the limpid water
bespeaks its purity, cannot for one moment be admitted.
The many widely-scattered atolls must, therefore, rest on rocky bases. But
we cannot believe that the broad summit of a mountain lies buried at the
depth of a few fathoms beneath every atoll, and nevertheless throughout the
immense areas above-named, with not one point of rock projecting above the
level of the sea; for we may judge with some accuracy of mountains beneath
the sea, by those on the land; and where can we find a single chain several
hundred miles in length and of considerable breadth, much less several such
chains, with their many broad summits attaining the same height, within
from 120 to 180 feet? If the data be thought insufficient, on which I have
grounded my belief, respecting the depth at which the reef-building
polypifers can exist, and it be assumed that they can flourish at a depth
of even one hundred fathoms, yet the weight of the above argument is but
little diminished, for it is almost equally improbable, that as many
submarine mountains, as there are low islands in the several great and
widely separated areas above specified, should all rise within six hundred
feet of the surface of the sea and not one above it, as that they should be
of the same height within the smaller limit of one or two hundred feet. So
highly improbable is this supposition, that we are compelled to believe,
that the bases of the many atolls did never at any one period all lie
submerged within the depth of a few fathoms beneath the surface, but that
they were brought into the requisite position or level, some at one period
and some at another, through movements in the earth's crust. But this
could not have been effected by elevation, for the belief that points so
numerous and so widely separated were successively uplifted to a certain
level, but that not one point was raised above that level, is quite as
improbable as the former supposition, and indeed differs little from it.
It will probably occur to those who have read Ehrenberg's account of the
Reefs of the Red Sea, that many points in these great areas may have been
elevated, but that as soon as raised, the protuberant parts were cut off by
the destroying action of the waves: a moment's reflection, however, on the
basin-like form of the atolls, will show that this is impossible; for the
upheaval and subsequent abrasion of an island would leave a flat disc,
which might become coated with coral, but not a deeply concave surface;
moreover, we should expect to see, in some parts at least, the rock of the
foundation brought to the surface. If, then, the foundations of the many
atolls were not uplifted into the requisite position, they must of
necessity have subsided into it; and this at once solves every difficulty
(The additional difficulty on the crater hypothesis before alluded to, will
now be evident; for on this view the volcanic action must be supposed to
have formed within the areas specified a vast number of craters, all rising
within a few fathoms of the surface, and not one above it. The supposition
that the craters were at different times upraised above the surface, and
were there abraded by the surf and subsequently coated by corals, is
subject to nearly the same objections with those given above in this
paragraph; but I consider it superfluous to detail all the arguments
opposed to such a notion. Chamisso's theory, from assuming the existence
of so many banks, all lying at the proper depth beneath the water, is also
vitally defective. The same observation applies to an hypothesis of
Lieutenant Nelson's ("Geolog. Trans." volume v., page 122), who supposes
that the ring-formed structure is caused by a greater number of germs of
corals becoming attached to the declivity, than to the central plateau of a
submarine bank: it likewise applies to the notion formerly entertained
(Forster's "Observ." page 151), that lagoon-islands owe their peculiar form
to the instinctive tendencies of the polypifers. According to this latter
view, the corals on the outer margin of the reef instinctively expose
themselves to the surf in order to afford protection to corals living in
the lagoon, which belong to other genera, and to other families!), for we
may safely infer, from the facts given in the last chapter, that during a
gradual subsidence the corals would be favourably circumstanced for
building up their solid frame works and reaching the surface, as island
after island slowly disappeared. Thus areas of immense extent in the
central and most profound parts of the great oceans, might become
interspersed with coral-islets, none of which would rise to a greater
height than that attained by detritus heaped up by the sea, and
nevertheless they might all have been formed by corals, which absolutely
required for their growth a solid foundation within a few fathoms of the
surface.
It would be out of place here to do more than allude to the many facts,
showing that the supposition of a gradual subsidence over large areas is by
no means improbable. We have the clearest proof that a movement of this
kind is possible, in the upright trees buried under the strata many
thousand feet in thickness; we have also every reason for believing that
there are now large areas gradually sinking, in the same manner as others
are rising. And when we consider how many parts of the surface of the
globe have been elevated within recent geological periods, we must admit
that there have been subsidences on a corresponding scale, for otherwise
the whole globe would have swollen. It is very remarkable that Mr. Lyell
("Principles of Geology," sixth edition, volume iii., page 386.), even in
the first edition of his "Principles of Geology," inferred that the amount
of subsidence in the Pacific must have exceeded that of elevation, from the
area of land being very small relatively to the agents there tending to
form it, namely, the growth of coral and volcanic action. But it will be
asked, are there any direct proofs of a subsiding movement in those areas,
in which subsidence will explain a phenomenon otherwise inexplicable?
This, however, can hardly be expected, for it must ever be most difficult,
excepting in countries long civilised, to detect a movement, the tendency
of which is to conceal the part affected. In barbarous and semi-civilised
nations how long might not a slow movement, even of elevation such as that
now affecting Scandinavia, have escaped attention!
Mr. Williams (Williams's "Narrative of Missionary Enterprise," page 31.)
insists strongly that the traditions of the natives, which he has taken
much pains in collecting, do not indicate the appearance of any new
islands: but on the theory of a gradual subsidence, all that would be
apparent would be, the water sometimes encroaching slowly on the land, and
the land again recovering by the accumulation of detritus its former
extent, and perhaps sometimes the conversion of an atoll with coral islets
on it, into a bare or into a sunken annular reef. Such changes would
naturally take place at the periods when the sea rose above its usual
limits, during a gale of more than ordinary strength; and the effects of
the two causes would be hardly distinguishable. In Kotzebue's "Voyage"
there are accounts of islands, both in the Caroline and Marshall
Archipelagoes, which have been partly washed away during hurricanes; and
Kadu, the native who was on board one of the Russian vessels, said "he saw
the sea at Radack rise to the feet of the cocoa-nut trees; but it was
conjured in time." (Kotzebue's "First Voyage," volume iii., page 168.) A
storm lately entirely swept away two of the Caroline islands, and converted
them into shoals; it partly, also, destroyed two other islands. (M.
Desmoulins in "Comptes Rendus," 1840, page 837.) According to a tradition
which was communicated to Captain Fitzroy, it is believed in the Low
Archipelago, that the arrival of the first ship caused a great inundation,
which destroyed many lives. Mr. Stutchbury relates, that in 1825, the
western side of Chain Atoll, in the same group, was completely devastated
by a hurricane, and not less than 300 lives lost: "in this instance it was
evident, even to the natives, that the hurricane alone was not sufficient
to account for the violent agitation of the ocean." ("West of England
Journal", No. I., page 35.) That considerable changes have taken place
recently in some of the atolls in the Low Archipelago, appears certain from
the case already given of Matilda Island: with respect to Whitsunday and
Gloucester Islands in this same group, we must either attribute great
inaccuracy to their discoverer, the famous circumnavigator Wallis, or
believe that they have undergone a considerable change in the period of
fifty-nine years, between his voyage and that of Captain Beechey's.
Whitsunday Island is described by Wallis as "about four miles long, and
three wide," now it is only one mile and a half long. The appearance of
Gloucester Island, in Captain Beechey's words (Beechey's "Voyage to the
Pacific," chapter vii., and Wallis's "Voyage in the 'Dolphin'," chapter
iv.), has been accurately described by its discoverer, but its present form
and extent differ materially." Blenheim reef, in the Chagos group,
consists of a water-washed annular reef, thirteen miles in circumference,
surrounding a lagoon ten fathoms deep: on its surface there were a few
worn patches of conglomerate coral-rock, of about the size of hovels; and
these Captain Moresby considered as being, without doubt, the last remnants
of islets; so that here an atoll has been converted into an atoll-formed
reef. The inhabitants of the Maldiva Archipelago, as long ago as 1605,
declared, "that the high tides and violent currents were diminishing the
number of the islands" (See an extract from Pyrard's Voyage in Captain
Owen's paper on the Maldiva Archipelago, in the "Geographical Journal",
volume ii., page 84.): and I have already shown, on the authority of
Captain Moresby, that the work of destruction is still in progress; but
that on the other hand the first formation of some islets is known to the
present inhabitants. In such cases, it would be exceedingly difficult to
detect a gradual subsidence of the foundation, on which these mutable
structures rest.
Some of the archipelagoes of low coral-islands are subject to earthquakes:
Captain Moresby informs me that they are frequent, though not very strong,
in the Chagos group, which occupies a very central position in the Indian
Ocean, and is far from any land not of coral formation. One of the islands
in this group was formerly covered by a bed of mould, which, after an
earthquake, disappeared, and was believed by the residents to have been
washed by the rain through the broken masses of underlying rock; the island
was thus rendered unproductive. Chamisso (See Chamisso, in Kotzebue's
"First Voyage," volume iii., pages 182 and 136.) states, that earthquakes
are felt in the Marshall atolls, which are far from any high land, and
likewise in the islands of the Caroline Archipelago. On one of the latter,
namely Oulleay atoll, Admiral Lutke, as he had the kindness to inform me,
observed several straight fissures about a foot in width, running for some
hundred yards obliquely across the whole width of the reef. Fissures
indicate a stretching of the earth's crust, and, therefore, probably
changes in its level; but these coral-islands, which have been shaken and
fissured, certainly have not been elevated, and, therefore, probably they
have subsided. In the chapter on Keeling atoll, I attempted to show by
direct evidence, that the island underwent a movement of subsidence, during
the earthquakes lately felt there.
The facts stand thus;--there are many large tracts of ocean, without any
high land, interspersed with reefs and islets, formed by the growth of
those kinds of corals, which cannot live at great depths; and the existence
of these reefs and low islets, in such numbers and at such distant points,
is quite inexplicable, excepting on the theory, that the bases on which the
reefs first became attached, slowly and successively sank beneath the level
of the sea, whilst the corals continued to grow upwards. No positive facts
are opposed to this view, and some general considerations render it
probable. There is evidence of change in form, whether or not from
subsidence, on some of these coral-islands; and there is evidence of
subterranean disturbances beneath them. Will then the theory, to which we
have thus been led, solve the curious problem,--what has given to each
class of reef its peculiar form?
(PLATE: WOODCUT NO. 4.
AA--Outer edge of the reef at the level of the sea.
BB--Shores of the island.
A'A'--Outer edge of the reef, after its upward growth during a period of
subsidence.
CC--The lagoon-channel between the reef and the shores of the now encircled
land.
B'B'--The shores of the encircled island.
N.B.--In this, and the following woodcut, the subsidence of the land could
only be represented by an apparent rise in the level of the sea.
PLATE: WOODCUT NO. 5.
A'A'--Outer edges of the barrier-reef at the level of the sea. The
cocoa-nut trees represent coral-islets formed on the reef.
CC--The lagoon-channel.
B'B'--The shores of the island, generally formed of low alluvial land and
of coral detritus from the lagoon-channel.
A"A"--The outer edges of the reef now forming an atoll.
C'--The lagoon of the newly formed atoll. According to the scale, the
depth of the lagoon and of the lagoon-channel is exaggerated.)
Let us in imagination place within one of the subsiding areas, an island
surrounded by a "fringing-reef,"--that kind, which alone offers no
difficulty in the explanation of its origin. Let the unbroken lines and
the oblique shading in the woodcut (No. 4) represent a vertical section
through such an island; and the horizontal shading will represent the
section of the reef. Now, as the island sinks down, either a few feet at a
time or quite insensibly, we may safely infer from what we know of the
conditions favourable to the growth of coral, that the living masses bathed
by the surf on the margin of the reef, will soon regain the surface. The
water, however, will encroach, little by little, on the shore, the island
becoming lower and smaller, and the space between the edge of the reef and
the beach proportionately broader. A section of the reef and island in
this state, after a subsidence of several hundred feet, is given by the
dotted lines: coral-islets are supposed to have been formed on the new
reef, and a ship is anchored in the lagoon-channel. This section is in
every respect that of an encircling barrier-reef; it is, in fact, a section
taken (The section has been made from the chart given in the "Atlas of the
Voyage of the 'Coquille'." The scale is .57 of an inch to a mile. The
height of the island, according to M. Lesson, is 4,026 feet. The deepest
part of the lagoon-channel is 162 feet; its depth is exaggerated in the
woodcut for the sake of clearness.) east and west through the highest point
of the encircled island of Bolabola; of which a plan is given in Plate I.,
Figure 5. The same section is more clearly shown in the following woodcut
(No. 5) by the unbroken lines. The width of the reef, and its slope, both
on the outer and inner side, will have been determined by the growing
powers of the coral, under the conditions (for instance the force of the
breakers and of the currents) to which it has been exposed; and the
lagoon-channel will be deeper or shallower, in proportion to the growth of
the delicately branched corals within the reef, and to the accumulation of
sediment, relatively, also, to the rate of subsidence and the length of the
intervening stationary periods.
It is evident in this section, that a line drawn perpendicularly down from
the outer edge of the new reef to the foundation of solid rock, exceeds by
as many feet as there have been feet of subsidence, that small limit of
depth at which the effective polypifers can live--the corals having grown
up, as the whole sank down, from a basis formed of other corals and their
consolidated fragments. Thus the difficulty on this head, which before
seemed so great, disappears.
As the space between the reef and the subsiding shore continued to increase
in breadth and depth, and as the injurious effects of the sediment and
fresh water borne down from the land were consequently lessened, the
greater number of the channels, with which the reef in its fringing state
must have been breached, especially those which fronted the smaller
streams, will have become choked up with the growth of coral: on the
windward side of the reef, where the coral grows most vigorously, the
breaches will probably have first been closed. In barrier-reefs,
therefore, the breaches kept open by draining the tidal waters of the
lagoon-channel, will generally be placed on the leeward side, and they will
still face the mouths of the larger streams, although removed beyond the
influence of their sediment and fresh water;--and this, it has been shown,
is commonly the case.
Referring to the diagram shown above, in which the newly formed barrier-reef
is represented by unbroken lines, instead of by dots as in the former
woodcut, let the work of subsidence go on, and the doubly pointed hill will
form two small islands (or more, according to the number of the hills)
included within one annular reef. Let the island continue subsiding, and
the coral-reef will continue growing up on its own foundation, whilst the
water gains inch by inch on the land, until the last and highest pinnacle
is covered, and there remains a perfect atoll. A vertical section of this
atoll is shown in the woodcut by the dotted lines;--a ship is anchored in
its lagoon, but islets are not supposed yet to have been formed on the
reef. The depth of the lagoon and the width and slope of the reef, will
depend on the circumstances just referred to under barrier-reefs. Any
further subsidence will produce no change in the atoll, except perhaps a
diminution in its size, from the reef not growing vertically upwards; but
should the currents of the sea act violently upon it, and should the corals
perish on part or on the whole of its margin, changes would result during
subsidence which will be presently noticed. I may here observe, that a
bank either of rock or of hardened sediment, level with the surface of the
sea, and fringed with living coral, would (if not so small as to allow the
central space to be quickly filled up with detritus) by subsidence be
converted immediately into an atoll, without passing, as in the case of a
reef fringing the shore of an island, through the intermediate form of a
barrier-reef. If such a bank lay a few fathoms submerged, the simple
growth of the coral (as remarked in the third chapter) without the aid of
subsidence, would produce a structure scarcely to be distinguished from a
true atoll; for in all cases the corals on the outer margin of a reef, from
having space and being freely exposed to the open sea, will grow vigorously
and tend to form a continuous ring whilst the growth of the less massive
kinds on the central expanse, will be checked by the sediment formed there,
and by that washed inwards by the breakers; and as the space becomes
shallower, their growth will, also, be checked by the impurities of the
water, and probably by the small amount of food brought by the enfeebled
currents, in proportion to the surface of living reefs studded with
innumerable craving mouths: the subsidence of a reef based on a bank of
this kind, would give depth to its central expanse or lagoon, steepness to
its flanks, and through the free growth of the coral, symmetry to its
outline:--I may here repeat that the larger groups of atolls in the Pacific
and Indian Oceans cannot be supposed to be founded on banks of this nature.
If, instead of the island in the diagram, the shore of a continent fringed
by a reef had subsided, a great barrier-reef, like that on the north-east
coast of Australia, would have necessarily resulted; and it would have been
separated from the main land by a deep-water channel, broad in proportion
to the amount of subsidence, and to the less or greater inclination of the
neighbouring coast-line. The effect of the continued subsidence of a great
barrier-reef of this kind, and its probable conversion into a chain of
separate atolls, will be noticed, when we discuss the apparent progressive
disseverment of the larger Maldiva atolls.
We now are able to perceive that the close similarity in form, dimensions,
structure, and relative position (which latter point will hereafter be more
fully noticed) between fringing and encircling barrier-reefs, and between
these latter and atolls, is the necessary result of the transformation,
during subsidence of the one class into the other. On this view, the three
classes of reefs ought to graduate into each other. Reefs having
intermediate character between those of the fringing and barrier classes do
exist; for instance, on the south-west coast of Madagascar, a reef extends
for several miles, within which there is a broad channel from seven to
eight fathoms deep, but the sea does not deepen abruptly outside the reef.
Such cases, however, are open to some doubts, for an old fringing-reef,
which had extended itself a little on a basis of its own formation, would
hardly be distinguishable from a barrier-reef, produced by a small amount
of subsidence, and with its lagoon-channel nearly filled up with sediment
during a long stationary period. Between barrier-reefs, encircling either
one lofty island or several small low ones, and atolls including a mere
expanse of water, a striking series can be shown: in proof of this, I need
only refer to the first plate in this volume, which speaks more plainly to
the eye, than any description could to the ear. The authorities from which
the charts have been engraved, together with some remarks on them and
descriptive of the plates, are given above. At New Caledonia (Plate II.,
Figure 5.) the barrier-reefs extend for 150 miles on each side of the
submarine prolongation of the island; and at their northern extremity they
appear broken up and converted into a vast atoll-formed reef, supporting a
few low coral-islets: we may imagine that we here see the effects of
subsidence actually in progress, the water always encroaching on the
northern end of the island, towards which the mountains slope down, and the
reefs steadily building up their massive fabrics in the lines of their
ancient growth.
We have as yet only considered the origin of barrier-reefs and atolls in
their simplest form; but there remain some peculiarities in structure and
some special cases, described in the two first chapters, to be accounted
for by our theory. These consist--in the inclined ledge terminated by a
wall, and sometimes succeeded by a second ledge with a wall, round the
shores of certain lagoons and lagoon-channels; a structure which cannot, as
I endeavoured to show, be explained by the simple growing powers of the
corals,--in the ring or basin-like forms of the central reefs, as well as
of the separate marginal portions of the northern Maldiva atolls,--in the
submerged condition of the whole, or of parts of certain barrier and
atoll-formed reefs; where only a part is submerged, this being generally to
leeward,--in the apparent progressive disseverment of some of the Maldiva
atolls,--in the existence of irregularly formed atolls, some being tied
together by linear reefs, and others with spurs projecting from them,--and,
lastly, in the structure and origin of the Great Chagos Bank.
STEP-FORMED LEDGES ROUND CERTAIN LAGOONS.
If we suppose an atoll to subside at an extremely slow rate, it is
difficult to follow out the complex results. The living corals would grow
up on the outer margin; and likewise probably in the gullies and deeper
parts of the bare surface of the annular reef; the water would encroach on
the islets, but the accumulation of fresh detritus might possibly prevent
their entire submergence. After a subsidence of this very slow nature, the
surface of the annular reef sloping gently into the lagoon, would probably
become united with the irregular reefs and banks of sand, which line the
shores of most lagoons. Should, however, the atoll be carried down by a
more rapid movement, the whole surface of the annular reef, where there was
a foundation of solid matter, would be favourably circumstanced for the
fresh growth of coral; but as the corals grew upwards on its exterior
margin, and the waves broke heavily on this part, the increase of the
massive polypifers on the inner side would be checked from the want of
water. Consequently, the exterior parts would first reach the surface, and
the new annular reef thus formed on the old one, would have its summit
inclined inwards, and be terminated by a subaqueous wall, formed by the
upward growth of the coral (before being much checked), from the inner edge
of the solid parts of the old reef. The inner portion of the new reef,
from not having grown to the surface, would be covered by the waters of the
lagoon. Should a subsidence of the same kind be repeated, the corals would
again grow up in a wall, from all the solid parts of the resunken reef,
and, therefore, not from within the sandy shores of the lagoon; and the
inner part of the new annular reef would, from being as before checked in
its upward growth, be of less height than the exterior parts, and therefore
would not reach the surface of the lagoon. In this case the shores of the
lagoon would be surrounded by two inclined ledges, one beneath the other,
and both abruptly terminated by subaqueous cliffs. (According to Mr.
Couthouy (page 26) the external reef round many atolls descends by a
succession of ledges or terraces. He attempts, I doubt whether
successfully, to explain this structure somewhat in the same manner as I
have attempted, with respect to the internal ledges round the lagoons of
some atolls. More facts are wanted regarding the nature both of the
interior and exterior step-like ledges: are all the ledges, or only the
upper ones, covered with living coral? If they are all covered, are the
kinds different on the ledges according to the depth? Do the interior and
exterior ledges occur together in the same atolls; if so, what is their
total width, and is the intervening surface-reef narrow, etc.?)
THE RING OR BASIN-FORMED REEFS OF THE NORTHERN MALDIVA ATOLLS.
I may first observe, that the reefs within the lagoons of atolls and within
lagoon-channels, would, if favourably circumstanced, grow upwards during
subsidence in the same manner as the annular rim; and, therefore, we might
expect that such lagoon-reefs, when not surrounded and buried by an
accumulation of sediment more rapid than the rate of subsidence, would rise
abruptly from a greater depth than that at which the efficient polypifers
can flourish: we see this well exemplified in the small abruptly-sided
reefs, with which the deep lagoons of the Chagos and Southern Maldiva
atolls are studded. With respect to the ring or basin-formed reefs of the
Northern Maldiva atolls, it is evident, from the perfectly continuous
series which exists that the marginal rings, although wider than the
exterior or bounding reef of ordinary atolls, are only modified portions of
such a reef; it is also evident that the central rings, although wider than
the knolls or reefs which commonly occur in lagoons, occupy their place.
The ring-like structure has been shown to be contingent on the breaches
into the lagoon being broad and numerous, so that all the reefs which are
bathed by the waters of the lagoon are placed under nearly the same
conditions with the outer coast of an atoll standing in the open sea.
Hence the exterior and living margins of these reefs must have been
favourably circumstanced for growing outwards, and increasing beyond the
usual breadth; and they must likewise have been favourably circumstanced
for growing vigorously upwards, during the subsiding movements, to which by
our theory the whole archipelago has been subjected; and subsidence with
this upward growth of the margins would convert the central space of each
little reef into a small lagoon. This, however, could only take place with
those reefs, which had increased to a breadth sufficient to prevent their
central spaces from being almost immediately filled up with the sand and
detritus driven inwards from all sides: hence it is that few reefs, which
are less than half a mile in diameter, even in the atolls where the
basin-like structure is most strikingly exhibited, include lagoons. This
remark, I may add, applies to all coral-reefs wherever found. The
basin-formed reefs of the Maldiva Archipelago may, in fact, be briefly
described, as small atolls formed during subsidence over the separate
portions of large and broken atolls, in the same manner as these latter were
formed over the barrier-reefs, which encircled the islands of a large
archipelago now wholly submerged.
SUBMERGED AND DEAD REEFS.
In the second section of the first chapter, I have shown that there are in
the neighbourhood of atolls, some deeply submerged banks, with level
surfaces; that there are others, less deeply but yet wholly submerged,
having all the characters of perfect atolls, but consisting merely of dead
coral-rock; that there are barrier-reefs and atolls with merely a portion
of their reef, generally on the leeward side, submerged; and that such
portions either retain their perfect outline, or they appear to be quite
effaced, their former place being marked only by a bank, conforming in
outline with that part of the reef which remains perfect. These several
cases are, I believe, intimately related together, and can be explained by
the same means. There, perhaps, exist some submerged reefs, covered with
living coral and growing upwards, but to these I do not here refer.
As we see that in those parts of the ocean, where coral-reefs are most
abundant, one island is fringed and another neighbouring one is not
fringed; as we see in the same archipelago, that all the reefs are more
perfect in one part of it than in another, for instance, in the southern
half compared with the northern half of the Maldiva Archipelago, and
likewise on the outer coasts compared with the inner coasts of the atolls
in this same group, which are placed in a double row; as we know that the
existence of the innumerable polypifers forming a reef, depends on their
sustenance, and that they are preyed on by other organic beings; and,
lastly, as we know that some inorganic causes are highly injurious to the
growth of coral, it cannot be expected that during the round of change to
which earth, air, and water are exposed, the reef-building polypifers
should keep alive for perpetuity in any one place; and still less can this
be expected, during the progressive subsidences, perhaps at some periods
more rapid than at others, to which by our theory these reefs and islands
have been subjected and are liable. It is, then, not improbable that the
corals should sometimes perish either on the whole or on part of a reef; if
on part, the dead portion, after a small amount of subsidence, would still
retain its proper outline and position beneath the water. After a more
prolonged subsidence, it would probably form, owing to the accumulation of
sediment, only the margin of a flat bank, marking the limits of the former
lagoon. Such dead portions of reef would generally lie on the leeward side
(Mr. Lyell, in the first edition of his "Principles of Geology," offered a
somewhat different explanation of this structure. He supposes that there
has been subsidence; but he was not aware that the submerged portions of
reef were in most cases, if not in all, dead; and he attributes the
difference in height in the two sides of most atolls, chiefly to the
greater accumulation of detritus to windward than to leeward. But as
matter is accumulated only on the backward part of the reef, the front part
would remain of the same height on both sides. I may here observe that in
most cases (for instance, at Peros Banhos, the Gambier group and the Great
Chagos Bank), and I suspect in all cases, the dead and submerged portions
do not blend or slope into the living and perfect parts, but are separated
from them by an abrupt line. In some instances small patches of living
reef rise to the surface from the middle of the submerged and dead parts.),
for the impure water and fine sediment would more easily flow out from the
lagoon over this side of the reef, where the force of the breakers is less
than to windward; and therefore the corals would be less vigorous on this
side, and be less able to resist any destroying agent. It is likewise
owing to this same cause, that reefs are more frequently breached to
leeward by narrow channels, serving as by ship-channels, than to windward.
If the corals perished entirely, or on the greater part of the
circumference of an atoll, an atoll-shaped bank of dead rock, more or less
entirely submerged, would be produced; and further subsidence, together
with the accumulation of sediment, would often obliterate its atoll-like
structure, and leave only a bank with a level surface.
In the Chagos group of atolls, within an area of 160 miles by 60, there are
two atoll-formed banks of dead rock (besides another very imperfect one),
entirely submerged; a third, with merely two or three very small pieces of
living reef rising to the surface; and a fourth, namely, Peros Banhos
(Plate I., Figure 9), with a portion nine miles in length dead and
submerged. As by our theory this area has subsided, and as there is
nothing improbable in the death, either from changes in the state of the
surrounding sea or from the subsidence being great or sudden, of the corals
on the whole, or on portions of some of the atolls, the case of the Chagos
group presents no difficulty. So far indeed are any of the above-mentioned
cases of submerged reefs from being inexplicable, that their occurrence
might have been anticipated on our theory, and as fresh atolls are supposed
to be in progressive formation by the subsidence of encircling barrier-reefs,
a weighty objection, namely that the number of atolls must be
increasing infinitely, might even have been raised, if proofs of the
occasional destruction and loss of atolls could not have been adduced.
THE DISSEVERMENT OF THE LARGER MALDIVA ATOLLS.
The apparent progressive disseverment in the Maldiva Archipelago of large
atolls into smaller ones, is, in many respects, an important consideration,
and requires an explanation. The graduated series which marks, as I
believe, this process, can be observed only in the northern half of the
group, where the atolls have exceedingly imperfect margins, consisting of
detached basin-formed reefs. The currents of the sea flow across these
atolls, as I am informed by Captain Moresby, with considerable force, and
drift the sediment from side to side during the monsoons, transporting much
of it seaward; yet the currents sweep with greater force round their
flanks. It is historically known that these atolls have long existed in
their present state; and we can believe, that even during a very slow
subsidence they might thus remain, the central expanse being kept at nearly
its original depth by the accumulation of sediment. But in the action of
such nicely balanced forces during a progressive subsidence (like that, to
which by our theory this archipelago has been subjected), it would be
strange if the currents of the sea should never make a direct passage
across some one of the atolls, through the many wide breaches in their
margins. If this were once effected, a deep-water channel would soon be
formed by the removal of the finer sediment, and the check to its further
accumulation; and the sides of the channel would be worn into a slope like
that on the outer coasts, which are exposed to the same force of the
currents. In fact, a channel precisely like that bifurcating one which
divides Mahlos Mahdoo (Plate II., Figure 4.), would almost necessarily be
formed. The scattered reefs situated near the borders of the new
ocean-channel, from being favourably placed for the growth of coral, would,
by their extension, tend to produce fresh margins to the dissevered portions;
such a tendency is very evident (as may be seen in the large published
chart) in the elongated reefs on the borders of the two channels
intersecting Mahlos Mahdoo. Such channels would become deeper with
continued subsidence, and probably from the reefs not growing up
perpendicularly, somewhat broader. In this case, and more especially if
the channels had been formed originally of considerable breadth, the
dissevered portions would become perfect and distinct atolls, like Ari and
Ross atolls (Plate II., Figure 6), or like the two Nillandoo atolls, which
must be considered as distinct, although related in form and position, and
separated from each other by channels, which though deep have been sounded.
Further subsidence would render such channels unfathomable, and the
dissevered portions would then resemble Phaleedoo and Moluque atolls, or
Mahlos Mahdoo and Horsburgh atolls (Plate II., Figure 4), which are related
to each other in no respect except in proximity and position. Hence, on
the theory of subsidence, the disseverment of large atolls, which have
imperfect margins (for otherwise their disseverment would be scarcely
possible), and which are exposed to strong currents, is far from being an
improbable event; and the several stages, from close relation to entire
isolation in the atolls of the Maldiva Archipelago, are readily explicable.
We might go even further, and assert as not improbable, that the first
formation of the Maldiva Archipelago was due to a barrier-reef, of nearly
the same dimensions with that of New Caledonia (Plate II., Figure 5), for
if, in imagination, we complete the subsidence of that great island, we
might anticipate from the present broken condition of the northern portion
of the reef, and from the almost entire absence of reefs on the eastern
coast, that the barrier-reef after repeated subsidences, would become
during its upward growth separated into distinct portions; and these
portions would tend to assume an atoll-like structure, from the coral
growing with vigour round their entire circumferences, when freely exposed
to an open sea. As we have some large islands partly submerged with
barrier-reefs marking their former limits, such as New Caledonia, so our
theory makes it probable that there should be other large islands wholly
submerged; and these, we may now infer, would be surmounted, not by one
enormous atoll, but by several large elongated ones, like the atolls in the
Maldiva group; and these again, during long periods of subsidence, would
sometimes become dissevered into smaller atolls. I may add, that both in
the Marshall and Caroline Archipelagoes, there are atolls standing close
together, which have an evident relationship in form: we may suppose, in
such cases, either that two or more encircled islands originally stood
close together, and afforded bases for two or more atolls, or that one
atoll has been dissevered. From the position, as well as form, of three
atolls in the Caroline Archipelago (the Namourrek and Elato group), which
are placed in an irregular circle, I am strongly tempted to believe that
they have originated by the process of disseverment. (The same remark is,
perhaps, applicable to the islands of Ollap, Fanadik, and Tamatam in the
Caroline Archipelago, of which charts are given in the atlas of Duperrey's
voyage: a line drawn through the linear reefs and lagoons of these three
islands forms a semicircle. Consult also, the atlas of Lutke's voyage; and
for the Marshall group that of Kotzebue; for the Gilbert group consult the
atlas of Duperrey's voyage. Most of the points here referred to may,
however, be seen in Krusenstern's general Atlas of the Pacific.)
IRREGULARLY FORMED ATOLLS.
In the Marshall group, Musquillo atoll consists of two loops united in one
point; and Menchikoff atoll is formed of three loops, two of which (as may
be seen in Figure 3, Plate II.) are connected by a mere ribbon-shaped reef,
and the three together are sixty miles in length. In the Gilbert group
some of the atolls have narrow strips of reef, like spurs, projecting from
them. There occur also in parts of the open sea, a few linear and straight
reefs, standing by themselves; and likewise some few reefs in the form of
crescents, with their extremities more or less curled inwards. Now, the
upward growth of a barrier-reef which fronted only one side of an island,
or one side of an elongated island with its extremities (of which cases
exist), would produce after the complete subsidence of the land, mere
strips or crescent or hook-formed reefs: if the island thus partially
fronted became divided during subsidence into two or more islands, these
islands would be united together by linear reefs; and from the further
growth of the coral along their shores together with subsidence, reefs of
various forms might ultimately be produced, either atolls united together
by linear reefs, or atolls with spurs projecting from them. Some, however,
of the more simple forms above specified, might, as we have seen, be
equally well produced by the coral perishing during subsidence on part of
the circumference of an atoll, whilst on the other parts it continued to
grow up till it reached the surface.
THE GREAT CHAGOS BANK.
I have already shown that the submerged condition of the Great Chagos Bank
(Plate II., Figure 1, with its section Figure 2), and of some other banks
in the Chagos group, may in all probability be attributed to the coral
having perished before or during the movements of subsidence, to which this
whole area by our theory has been subjected. The external rim or upper
ledge (shaded in the chart), consists of dead coral-rock thinly covered
with sand; it lies at an average depth of between five and eight fathoms,
and perfectly resembles in form the annular reef of an atoll. The banks of
the second level, the boundaries of which are marked by dotted lines in the
chart, lie from about fifteen to twenty fathoms beneath the surface; they
are several miles broad, and terminate in a very steep slope round the
central expanse. This central expanse I have already described, as
consisting of a level muddy flat between thirty and forty fathoms deep.
The banks of the second level, might at first sight be thought analogous to
the internal step-like ledge of coral-rock which borders the lagoons of
some atolls, but their much greater width, and their being formed of sand,
are points of essential difference. On the eastern side of the atoll some
of the banks are linear and parallel, resembling islets in a great river,
and pointed directly towards a great breach on the opposite side of the
atoll; these are best seen in the large published chart. I inferred from
this circumstance, that strong currents sometimes set directly across this
vast bank; and I have since heard from Captain Moresby that this is the
case. I observed, also, that the channels or breaches through the rim,
were all of the same depth as the central lagoon-like space into which they
lead; whereas the channels into the other atolls of the Chagos group, and
as I believe into most other large atolls, are not nearly as deep as their
lagoons: for instance at Peros Banhos, the channels are only of the same
depth, namely between ten and twenty fathoms, as the bottom of the lagoon
for a space about a mile and a half in width round its shores, whilst the
central expanse of the lagoon is from thirty-five to forty fathoms deep.
Now, if an atoll during a gradual subsidence once became entirely
submerged, like the Great Chagos Bank, and therefore no longer exposed to
the surf, very little sediment could be formed from it; and consequently
the channels leading into the lagoon from not being filled up with drifted
sand and coral detritus, would continue increasing in depth, as the whole
sank down. In this case, we might expect that the currents of the open
sea, instead of any longer sweeping round the submarine flanks, would flow
directly through the breaches across the lagoon, removing in their course
the finer sediment, and preventing its further accumulation. We should
then have the submerged reef forming an external and upper rim of rock, and
beneath this portion of the sandy bottom of the old lagoon, intersected by
deep-water channels or breaches, and thus formed into separate marginal
banks; and these would be cut off by steep slopes, overhanging the central
space, worn down by the passage of the oceanic currents.
By these means, I have scarcely any doubt that the Great Chagos Bank has
originated,--a structure which at first appeared to me far more anomalous
than any I had met with. The process of formation is nearly the same with
that, by which Mahlos Mahdoo had been trisected; but in the Chagos Bank the
channels of the oceanic currents entering at several different quarters,
have united in a central space.
This great atoll-formed bank appears to be in an early stage of
disseverment; should the work of subsidence go on, from the submerged and
dead condition of the whole reef, and the imperfection of the south-east
quarter a mere wreck would probably be left. The Pitt's Bank, situated not
far southward, appears to be precisely in this state; it consists of a
moderately level, oblong bank of sand, lying from 10 to 20 fathoms beneath
the surface, with two sides protected by a narrow ledge of rock which is
submerged between 5 and 8 fathoms. A little further south, at about the
same distance as the southern rim of the Great Chagos Bank is from the
northern rim, there are two other small banks with from 10 to 20 fathoms on
them; and not far eastward soundings were struck on a sandy bottom, with
between 110 and 145 fathoms. The northern portion with its ledge-like
margin, closely resembles any one segment of the Great Chagos Bank, between
two of the deep-water channels, and the scattered banks, southward appear
to be the last wrecks of less perfect portions.
I have examined with care the charts of the Indian and Pacific Oceans, and
have now brought before the reader all the examples, which I have met with,
of reefs differing from the type of the class to which they belong; and I
think it has been satisfactorily shown, that they are all included in our
theory, modified by occasional accidents which might have been anticipated
as probable. In this course we have seen, that in the lapse of ages
encircling barrier-reefs are occasionally converted into atolls, the name
of atoll being properly applicable, at the moment when the last pinnacle of
encircled land sinks beneath the surface of the sea. We have, also, seen
that large atolls during the progressive subsidence of the areas in which
they stand, sometimes become dissevered into smaller ones; at other times,
the reef-building polypifers having entirely perished, atolls are converted
into atoll-formed banks of dead rock; and these again through further
subsidence and the accumulation of sediment modified by the force of the
oceanic currents, pass into level banks with scarcely any distinguishing
character. Thus may the history of an atoll be followed from its first
origin, through the occasional accidents of its existence, to its
destruction and final obliteration.
OBJECTIONS TO THE THEORY OF THE FORMATION OF ATOLLS AND BARRIER-REEFS.
The vast amount of subsidence, both horizontally or in area, and vertically
or in depth, necessary to have submerged every mountain, even the highest,
throughout the immense spaces of ocean interspersed with atolls, will
probably strike most people as a formidable objection to my theory. But as
continents, as large as the spaces supposed to have subsided, have been
raised above the level of the sea,--as whole regions are now rising, for
instance, in Scandinavia and South America,--and as no reason can be
assigned, why subsidences should not have occurred in some parts of the
earth's crust on as great a scale both in extent and amount as those of
elevation, objections of this nature strike me as of little force. The
remarkable point is that movements to such an extent should have taken
place within a period, during which the polypifers have continued adding
matter on and above the same reefs. Another and less obvious objection to
the theory will perhaps be advanced from the circumstance, of the lagoons
within atolls and within barrier-reefs never having become in any one
instance during prolonged subsidences of a greater depth than sixty
fathoms, and seldom more than forty fathoms; but we already admit, if the
theory be worth considering, that the rate of subsidence has not exceeded
that of the upward growth of the coral on the exterior margin; we are,
therefore, only further required to admit, that the subsidence has not
exceeded in rate the filling up of the interior spaces by the growth of the
corals living there, and by the accumulation of sediment. As this filling
up must take place very slowly within barrier-reefs lying far from the
land, and within atolls which are of large dimensions and which have open
lagoons with very few reefs, we are led to conclude that the subsidence
thus counter-balanced, must have been slow in an extraordinary degree; a
conclusion which accords with our only means, namely, with what is known of
the rate and manner of recent elevatory movements, of judging by analogy
what is the probable rate of subsidence.
In this chapter it has, I think, been shown, that the theory of subsidence,
which we were compelled to receive from the necessity of giving to the
corals, in certain large areas, foundations at the requisite depth,
explains both the normal structure and the less regular forms of those two
great classes of reefs, which have justly excited the astonishment of all
persons who have sailed through the Pacific and Indian Oceans. But further
to test the truth of the theory, a crowd of questions will occur to the
reader: Do the different kinds of reefs, which have been produced by the
same kind of movement, generally lie within the same areas? What is their
relation of form and position,--for instance, do adjoining groups of
atolls, and the separate atolls in these groups, bear the same relation to
each other which islands do in common archipelagoes? Have we reason to
believe, that where there are fringing-reefs, there has not lately been
subsidence; or, for it is almost our only way of ascertaining this point,
are there frequently proofs of recent elevation? Can we by this means
account for the presence of certain classes of reefs in some large areas,
and their entire absence in others? Do the areas which have subsided, as
indicated by the presence of atolls and barrier-reefs, and the areas which
have remained stationary or have been upraised, as shown by fringing-reefs,
bear any determinate relation to each other; and are the dimensions of
these areas such as harmonise with the greatness of the subterranean
changes, which, it must be supposed, have lately taken place beneath them?
Is there any connection between the movements thus indicated, and recent
volcanic action? All these questions ought to receive answers in
accordance with the theory; and if this can be satisfactorily shown, not
only is the theory confirmed, but as deductions, the answers are in
themselves important. Under this latter point of view, these questions
will be chiefly considered in the following chapter.
(I may take this opportunity of briefly considering the appearances, which
would probably be presented by a vertical and deep section across a coral
formation (referring chiefly to an atoll), formed by the upward growth of
coral during successive subsidences. This is a subject worthy of
attention, as a means of comparison with ancient coral-strata. The
circumferential parts would consist of massive species, in a vertical
position, with their interstices filled up with detritus; but this would be
the part most subject to subsequent denudation and removal. It is useless
to speculate how large a portion of the exterior annular reef would consist
of upright coral, and how much of fragmentary rock, for this would depend
on many contingencies,--such as on the rate of subsidence, occasionally
allowing a fresh growth of coral to cover the whole surface, and on the
breakers having force sufficient to throw fragments over this same space.
The conglomerate which composes the base of the islets, would (if not
removed by denudation together with the exterior reef on which it rests) be
conspicuous from the size of the fragments,--the different degrees in which
they have been rounded,--the presence of fragments of conglomerate torn up,
rounded, and recemented,--and from the oblique stratification. The corals
which lived in the lagoon-reefs at each successive level, would be
preserved upright, and they would consist of many kinds, generally much
branched. In this part, however, a very large proportion of the rock (and
in some cases nearly all of it) would be formed of sedimentary matter,
either in an excessively fine, or in a moderately coarse state, and with
the particles almost blended together. The conglomerate which was formed
of rounded pieces of the branched corals, on the shores of the lagoon,
would differ from that formed on the islets and derived from the outer
coast; yet both might have accumulated very near each other. I have seen a
conglomerate limestone from Devonshire like a conglomerate now forming on
the shores of the Maldiva atolls. The stratification taken as a whole,
would be horizontal; but the conglomerate beds resting on the exterior
reef, and the beds of sandstone on the shores of the lagoon (and no doubt
on the external flanks) would probably be divided (as at Keeling atoll and
at Mauritius) by numerous layers dipping at considerable angles in
different directions. The calcareous sandstone and coral-rock would almost
necessarily contain innumerable shells, echini, and the bones of fish,
turtle, and perhaps of birds; possibly, also, the bones of small saurians,
as these animals find their way to the islands far remote from any
continent. The large shells of some species of Tridacna would be found
vertically imbedded in the solid rock, in the position in which they lived.
We might expect also to find a mixture of the remains of pelagic and
littoral animals in the strata formed in the lagoon, for pumice and the
seeds of plants are floated from distant countries into the lagoons of many
atolls: on the outer coast of Keeling atoll, near the mouth of the lagoon,
the case of a pelagic Pteropodous animal was brought up on the arming of
the sounding lead. All the loose blocks of coral on Keeling atoll were
burrowed by vermiform animals; and as every cavity, no doubt, ultimately
becomes filled with spathose limestone, slabs of the rock taken from a
considerable depth, would, if polished, probably exhibit the excavations of
such burrowing animals. The conglomerate and fine-grained beds of coral-rock
would be hard, sonorous, white and composed of nearly pure calcareous
matter; in some few parts, judging from the specimens at Keeling atoll,
they would probably contain a small quantity of iron. Floating pumice and
scoriae, and occasionally stones transported in the root of trees (see my
"Journal of Researches," page 549) appear the only sources, through which
foreign matter is brought to coral-formations standing in the open ocean.
The area over which sediment is transported from coral-reefs must be
considerable: Captain Moresby informs me that during the change of
monsoons the sea is discoloured to a considerable distance off the Maldiva
and Chagos atolls. The sediment of fringing and barrier coral-reefs must
be mingled with the mud, which is brought down from the land, and is
transported seaward through the breaches, which occur in front of almost
every valley. If the atolls of the larger archipelagoes were upraised, the
bed of the ocean being converted into land, they would form flat-topped
mountains, varying in diameter from a few miles (the smallest atolls being
worn away) to sixty miles; and from being horizontally stratified and of
similar composition, they would, as Mr. Lyell has remarked, falsely appear
as if they had originally been united into one vast continuous mass. Such
great strata of coral-rock would rarely be associated with erupted volcanic
matter, for this could only take place, as may be inferred from what
follows in the next chapter, when the area, in which they were situated,
commenced to rise, or at least ceased to subside. During the enormous
period necessary to effect an elevation of the kind just alluded to, the
surface would necessarily be denuded to a great thickness; hence it is
highly improbable that any fringing-reef, or even any barrier-reef, at
least of those encircling small islands, would be preserved. From this
same cause, the strata which were formed within the lagoons of atolls and
lagoon-channels of barrier-reefs, and which must consist in a large part of
sedimentary matter, would more often be preserved to future ages, than the
exterior solid reef, composed of massive corals in an upright position;
although it is on this exterior part that the present existence and further
growth of atolls and barrier-reefs entirely depend.
CHAPTER VI.--ON THE DISTRIBUTION OF CORAL-REEFS WITH REFERENCE TO THE
THEORY OF THEIR FORMATION.
(DESCRIPTION OF THE PLATES.
PLATE III.--MAP SHOWING THE DISTRIBUTION OF CORAL-REEFS AND ACTIVE
VOLCANOES.
The principles, on which this map was coloured, are explained in the
beginning of Chapter VI.; and the authorities for each particular spot are
detailed in the Appendix to "Coral Reefs." The names not printed in upper
case in the Index refer to the Appendix.)
Description of the coloured map.--Proximity of atolls and barrier-reefs.--
Relation in form and position of atolls with ordinary islands.--Direct
evidence of subsidence difficult to be detected.--Proofs of recent
elevation where fringing-reefs occur.--Oscillations of level.--Absence of
active volcanoes in the areas of subsidence.--Immensity of the areas which
have been elevated and have subsided.--Their relation to the present
distribution of the land.--Areas of subsidence elongated, their
intersection and alternation with those of elevation.--Amount and slow rate
of the subsidence.--Recapitulation.
It will be convenient to give here a short account of the appended map
(Plate III.) [Inasmuch as the coloured map would have proved too costly to
be given in this series, the indications of colour have been replaced by
numbers referring to the dotted groups of reefs, etc. The author's
original wording, however, is retained in full, as it will be easy to refer
to the map by the numbers, and thus the flow of the narrative is
undisturbed.]: a fuller one, with the data for colouring each spot, is
reserved for the Appendix; and every place there referred to may be found
in the Index. A larger chart would have been desirable; but, small as the
adjoined one is, it is the result of many months' labour. I have
consulted, as far as I was able, every original voyage and map; and the
colours were first laid down on charts on a larger scale. The same blue
colour, with merely a difference in the depth of tint, is used for atolls
or lagoon-islands, and barrier-reefs, for we have seen, that as far as the
actual coral-formation is concerned, they have no distinguishing character.
Fringing-reefs have been coloured red, for between them on the one hand,
and barrier-reefs and atolls on the other, there is an important
distinction with respect to the depth beneath the surface, at which we are
compelled to believe their foundations lie. The two distinct colours,
therefore, mark two great types of structure.
The DARK BLUE COLOUR [represented by (3) in our plate] represents atolls
and submerged annular reefs, with deep water in their centres. I have
coloured as atolls, a few low and small coral-islands, without lagoons; but
this has been done only when it clearly appeared that they originally
contained lagoons, since filled up with sediment: when there were not good
grounds for this belief, they have been left uncoloured.
The PALE BLUE COLOUR [represented by (2)] represents barrier-reefs. The
most obvious character of reefs of this class is the broad and deep-water
moat within the reef: but this, like the lagoons of small atolls, is
liable to become filled up with detritus and with reefs of delicately
branched corals: when, therefore, a reef round the entire circumference of
an island extends very far into a profoundly deep sea, so that it can
hardly be confounded with a fringing-reef which must rest on a foundation
of rock within a small depth, it has been coloured pale blue, although it
does not include a deep-water moat: but this has only been done rarely,
and each case is distinctly mentioned in the Appendix.
The RED COLOUR (4) represents reefs fringing the land quite closely where
the sea is deep, and where the bottom is gently inclined extending to a
moderate distance from it, but not having a deep-water moat or lagoon-like
space parallel to the shore. It must be remembered that fringing-reefs are
frequently BREACHED in front of rivers and valleys by deepish channels,
where mud has been deposited. A space of thirty miles in width has been
coloured round or in front of the reefs of each class, in order that the
colours might be conspicuous on the appended map, which is reduced to so
small a scale.
The VERMILLION SPOTS, and streaks (1) represent volcanoes now in action, or
historically known to have been so. They are chiefly laid down from Von
Buch's work on the Canary Islands; and my reasons for making a few
alterations are given in the note below.
(I have also made considerable use of the geological part of Berghaus'
"Physical Atlas." Beginning at the eastern side of the Pacific, I have
added to the number of the volcanoes in the southern part of the
Cordillera, and have coloured Juan Fernandez according to observations
collected during the voyage of the "Beagle" ("Geological Transactions,"
volume v., page 601.) I have added a volcano to Albemarle Island, one of
the Galapagos Archipelago (the author's "Journal of Researches," page 457).
In the Sandwich group there are no active volcanoes, except at Hawaii; but
the Rev. W. Ellis informs me, there are streams of lava apparently modern
on Maui, having a very recent appearance, which can be traced to the
craters whence they flowed. The same gentleman informs me, that there is
no reason to believe that any active volcano exists in the Society
Archipelago; nor are there any known in the Samoa or Navigator group,
although some of the streams of lava and craters there appear recent. In
the Friendly group, the Rev. J. Williams says ("Narrative of Missionary
Enterprise," page 29) that Toofoa and Proby Islands are active volcanoes.
I infer from Hamilton's "Voyage in the 'Pandora'" (Page 95), that Proby
Island is synonymous with Onouafou, but I have not ventured to colour it.
There can be no doubt respecting Toofoa, and Captain Edwards (Von Buch,
page 386) found the lava of recent eruption at Amargura still smoking.
Berghaus marks four active volcanoes actually within the Friendly group;
but I do not know on what authority: I may mention that Maurelle describes
Latte as having a burnt-up appearance: I have marked only Toofoa and
Amargura. South of the New Hebrides lies Matthews Rock, which is drawn and
described as an active crater in the "Voyage of the 'Astrolabe'." Between
it and the volcano on the eastern side of New Zealand, lies Brimstone
Island, which from the high temperature of the water in the crater, may be
ranked as active (Berghaus "Vorbemerk," II Lief. S. 56). Malte Brun,
volume xii., page 231, says that there is a volcano near port St. Vincent
in New Caledonia. I believe this to be an error, arising from a smoke seen
on the OPPOSITE coast by Cook ("Second Voyage," volume ii., page 23) which
smoke went out at night. The Mariana Islands, especially the northern
ones, contain many craters (see Freycinet's "Hydrog. Descript.") which are
not active. Von Buch, however, states (page 462) on the authority of La
Peyrouse, that there are no less than seven volcanoes between these islands
and Japan. Gemelli Creri (Churchill's "Collect." volume iv., page 458),
says there are two active volcanoes in latitude 23 deg 30', and in latitude
24 deg: but I have not coloured them. From the statements in Beechey's
"Voyage" (page 518, 4to edition) I have coloured one in the northern part
of the Bonin group. M. S. Julien has clearly made out from Chinese
manuscripts not very ancient ("Comptes Rendus," 1840, page 832), that there
are two active volcanoes on the eastern side of Formosa. In Torres
Straits, on Cap Island (9 deg 48' S., 142 deg 39' E.) a volcano was seen
burning with great violence in 1793 by Captain Bampton (see Introduction to
Flinders' "Voyage," page 41). Mr. M'Clelland (Report of Committee for
investigating Coal in India, page 39) has shown that the volcanic band
passing through Barren Island must be extended northwards. It appears by
an old chart, that Cheduba was once an active volcano (see also "Silliman's
North American Journal", volume xxxviii., page 385). In Berghaus'
"Physical Atlas," 1840, No. 7 of Geological Part, a volcano on the coast of
Pondicherry is said to have burst forth in 1757. Ordinaire ("Hist. Nat.
des Volcans," page 218) says that there is one at the mouth of the Persian
Gulf, but I have not coloured it, as he gives no particulars. A volcano in
Amsterdam, or St. Paul's, in the southern part of the Indian Ocean, has
been seen ("Naut. Mag." 1838, page 842) in action. Dr. J. Allan, of
Forres, informs me in a letter, that when he was at Joanna, he saw at night
flames apparently volcanic, issuing from the chief Comoro Island, and that
the Arabs assured him that they were volcanic, adding that the volcano
burned more during the wet season. I have marked this as a volcano, though
with some hesitation, on account of the possibility of the flame arising
from gaseous sources.)
The uncoloured coasts consist, first and chiefly, of those, where there are
no coral-reefs, or such small portions as to be quite insignificant.
Secondly, of those coasts where there are reefs, but where the sea is very
shallow, for in this case the reefs generally lie far from the land, and
become very irregular, in their forms: where they have not become
irregular, they have been coloured. thirdly, if I had the means of
ascertaining the fact, I should not colour a reef merely coating the edges
of a submarine crater, or of a level submerged bank; for such superficial
formations differ essentially, even when not in external appearance, from
reefs whose foundations as well as superficies have been wholly formed by
the growth of coral. Fourthly, in the Red Sea, and within some parts of
the East Indian Archipelago (if the imperfect charts of the latter can be
trusted), there are many scattered reefs, of small size, represented in the
chart by mere dots, which rise out of deep water: these cannot be arranged
under either of the three classes: in the Red Sea, however, some of these
little reefs, from their position, seem once to have formed parts of a
continuous barrier. There exist, also, scattered in the open ocean, some
linear and irregularly formed strips of coral-reef, which, as shown in the
last chapter, are probably allied in their origin to atolls; but as they do
not belong to that class, they have not been coloured; they are very few in
number and of insignificant dimensions. Lastly, some reefs are left
uncoloured from the want of information respecting them, and some because
they are of an intermediate structure between the barrier and fringing
classes. The value of the map is lessened, in proportion to the number of
reefs which I have been obliged to leave uncoloured, although, in a
theoretical point of view, few of them present any great difficulty: but
their number is not very great, as will be found by comparing the map with
the statements in the Appendix. I have experienced more difficulty in
colouring fringing-reefs than in colouring barrier-reefs, as the former,
from their much less dimensions, have less attracted the attention of
navigators. As I have had to seek my information from all kinds of
sources, and often from indirect ones, I do not venture to hope that the
map is free from many errors. Nevertheless, I trust it will give an
approximately correct view of the general distribution of the coral-reefs
over the whole world (with the exception of some fringing-reefs on the
coast of Brazil, not included within the limits of the map), and of their
arrangement into the three great classes, which, though necessarily very
imperfect from the nature of the objects classified, have been adopted by
most voyagers. I may further remark, that the dark blue colour represents
land entirely composed of coral-rock; the pale blue, land with a wide and
thick border of coral-rock; and the red, a mere narrow fringe of
coral-rock.
Looking now at the map under the theoretical point of view indicated in the
last chapter, the two blue tints signify that the foundations of the reefs
thus coloured have subsided to a considerable amount, at a slower rate than
that of the upward growth of the corals, and that probably in many cases
they are still subsiding. The red signifies that the shores which support
fringing-reefs have not subsided (at least to any considerable amount, for
the effects of a subsidence on a small scale would in no case be
distinguishable); but that they have remained nearly stationary since the
period when they first became fringed by reefs; or that they are now rising
or have been upraised, with new lines of reefs successively formed on them:
these latter alternatives are obviously implied, as newly formed lines of
shore, after elevations of the land, would be in the same state with
respect to the growth of fringing-reefs, as stationary coasts. If during
the prolonged subsidence of a shore, coral-reefs grew for the first time on
it, or if an old barrier-reef were destroyed and submerged, and new reefs
became attached to the land, these would necessarily at first belong to the
fringing class, and, therefore, be coloured red, although the coast was
sinking: but I have no reason to believe, that from this source of error,
any coast has been coloured wrongly with respect to movement indicated.
Well characterised atolls and encircling barrier-reefs, where several occur
in a group, or a single barrier-reef if of large dimensions, leave scarcely
any doubt on the mind respecting the movement by which they have been
produced; and even a small amount of subsequent elevation is soon betrayed.
The evidence from a single atoll or a single encircling barrier-reef, must
be received with some caution, for the former may possibly be based upon a
submerged crater or bank, and the latter on a submerged margin of sediment,
or of worn-down rock. From these remarks we may with greater certainty
infer that the spaces, especially the larger ones, tinted blue in the map,
have subsided, than that the red spaces have remained stationary, or have
been upraised.
ON THE GROUPING OF THE DIFFERENT CLASSES OF REEFS.
Having made these preliminary remarks, I will consider first how far the
grouping of the different kinds of coral-islands and reefs is corroborative
of the truth of the theory. A glance at the map shows that the reefs,
coloured blue and red, produced under widely different conditions, are not
indiscriminately mixed together. Atolls and barrier-reefs, on the other
hand, as may be seen by the two blue tints, generally lie near each other;
and this would be the natural result of both having been produced during
the subsidence of the areas in which they stand. Thus, the largest group
of encircled islands is that of the Society Archipelago; and these islands
are surrounded by atolls, and only separated by a narrow space from the
large group of Low atolls. In the midst of the Caroline atolls, there are
three fine encircled islands. The northern point of the barrier-reef of
New Caledonia seems itself, as before remarked, to form a complete large
atoll. The great Australian barrier is described as including both atolls
and small encircled islands. Captain King (Sailing directions, appended to
volume ii. of his "Surveying Voyage to Australia.") mentions many
atoll-formed and encircling coral-reefs, some of which lie within the
barrier, and others may be said (for instance between latitude 16 deg and
13 deg) to form part of it. Flinders ("Voyage to Terra Australis," volume
ii. page 336.) has described an atoll-formed reef in latitude 10 deg, seven
miles long and from one to three broad, resembling a boot in shape, with
apparently very deep water within. Eight miles westward of this, and
forming part of the barrier, lie the Murray Islands, which are high and are
encircled. In the Corallian Sea, between the two great barriers of
Australia and New Caledonia, there are many low islets and coral-reefs,
some of which are annular, or horse-shoe shaped. Observing the smallness
of the scale of the map, the parallels of latitude being nine hundred miles
apart, we see that none of the large groups of reefs and islands supposed
to have been produced by long-continued subsidence, lie near extensive
lines of coast coloured red, which are supposed to have remained stationary
since the growth of their reefs, or to have been upraised and new lines of
reefs formed on them. Where the red and blue circles do occur near each
other, I am able, in several instances, to show that there have been
oscillations of level, subsidence having preceded the elevation of the red
spots; and elevation having preceded the subsidence of the blue spots: and
in this case the juxtaposition of reefs belonging to the two great types of
structure is little surprising. We may, therefore, conclude that the
proximity in the same areas of the two classes of reefs, which owe their
origin to the subsidence of the earth's crust, and their separation from
those formed during its stationary or uprising condition, holds good to the
full extent, which might have been anticipated by our theory.
As groups of atolls have originated in the upward growth, at each fresh
sinking of the land, of those reefs which primarily fringed the shores of
one great island, or of several smaller ones; so we might expect that these
rings of coral-rock, like so many rude outline charts, will still retain
some traces of the general form, or at least general range, of the land,
round which they were first modelled. That this is the case with the
atolls in the Southern Pacific as far as their range is concerned, seems
highly probable, when we observe that the three principal groups are
directed in north-west and south-east lines, and that nearly all the land
in the S. Pacific ranges in this same direction; namely, N. Western
Australia, New Caledonia, the northern half of New Zealand, the New
Hebrides, Saloman, Navigator, Society, Marquesas, and Austral
archipelagoes: in the Northern Pacific, the Caroline atolls abut against
the north-west line of the Marshall atolls, much in the same manner as the
east and west line of islands from Ceram to New Britain do on New Ireland:
in the Indian Ocean the Laccadive and Maldiva atolls extend nearly parallel
to the western and mountainous coast of India. In most respects, there is
a perfect resemblance with ordinary islands in the grouping of atolls and
in their form: thus the outline of all the larger groups is elongated; and
the greater number of the individual atolls are elongated in the same
direction with the group, in which they stand. The Chagos group is less
elongated than is usual with other groups, and the individual atolls in it
are likewise but little elongated; this is strikingly seen by comparing
them with the neighbouring Maldiva atolls. In the Marshall and Maldiva
archipelagoes, the atolls are ranged in two parallel lines, like the
mountains in a great double mountain-chain. Some of the atolls, in the
larger archipelagoes, stand so near to each other, and have such an evident
relationship in form, that they compose little sub-groups: in the Caroline
Archipelago, one such sub-group consists of Pouynipete, a lofty island
encircled by a barrier-reef, and separated by a channel only four miles and
a half wide from Andeema atoll, with a second atoll a little further off.
In all these respects an examination of a series of charts will show how
perfectly groups of atolls resemble groups of common islands.
ON THE DIRECT EVIDENCE OF THE BLUE SPACES IN THE MAP HAVING SUBSIDED DURING
THE UPWARD GROWTH OF THE REEFS SO COLOURED, AND OF THE RED SPACES HAVING
REMAINED STATIONARY, OR HAVING BEEN UPRAISED.
With respect to subsidence, I have shown in the last chapter, that we
cannot expect to obtain in countries inhabited only by semi-civilised
races, demonstrative proofs of a movement, which invariably tends to
conceal its own evidence. But on the coral-islands supposed to have been
produced by subsidence, we have proofs of changes in their external
appearance--of a round of decay and renovation--of the last vestiges of
land on some--of its first commencement on others: we hear of storms
desolating them to the astonishment of their inhabitants: we know by the
great fissures with which some of them are traversed, and by the
earthquakes felt under others, that subterranean disturbances of some kind
are in progress. These facts, if not directly connected with subsidence,
as I believe they are, at least show how difficult it would be to discover
proofs of such movement by ordinary means. At Keeling atoll, however, I
have described some appearances, which seem directly to show that
subsidence did take place there during the late earthquakes. Vanikoro,
according to Chevalier Dillon (See Captain Dillon's "Voyage in search of La
Peyrouse." M. Cordier in his "Report on the Voyage of the 'Astrolabe'"
(page cxi., volume i.), speaking of Vanikoro, says the shores are
surrounded by reefs of madrepore, "qu'on assure etre de formation
tout-a-fait moderne." I have in vain endeavoured to learn some further
particulars about this remarkable passage. I may here add, that according
to our theory, the island of Pouynipete (Plate I., Figure 7), in the
Caroline Archipelago, being encircled by a barrier-reef, must have
subsided. In the "New S. Wales Lit. Advert." February 1835 (which I have
seen through the favour of Dr. Lloghtsky), there is an account of this
island (subsequently confirmed by Mr. Campbell), in which it is said, "At
the N.E. end, at a place called Tamen, there are ruins of a town, NOW ONLY
accessible by boats, the waves REACHING TO THE STEPS OF The HOUSES."
Judging from this passage, one would be tempted to conclude that the island
must have subsided, since these houses were built. I may, also, here
append a statement in Malte Brun (volume ix., page 775, given without any
authority), that the sea gains in an extraordinary manner on the coast of
Cochin China, which lies in front and near the subsiding coral-reefs in the
China Sea: as the coast is granitic, and not alluvial, it is scarcely
possible that the encroachment of the sea can be owing to the washing away
of the land; and if so, it must be due to subsidence.), is often violently
shaken by earthquakes, and there, the unusual depth of the channel between
the shore and the reef,--the almost entire absence of islets on the reef,--
its wall-like structure on the inner side, and the small quantity of low
alluvial land at the foot of the mountains, all seem to show that this
island has not remained long at its present level, with the lagoon-channel
subjected to the accumulation of sediment, and the reef to the wear and
tear of the breakers. At the Society Archipelago, on the other hand, where
a slight tremor is only rarely felt, the shoaliness of the lagoon-channels
round some of the islands, the number of islets formed on the reefs of
others, and the broad belt of low land at the foot of the mountains,
indicate that, although there must have been great subsidence to have
produced the barrier-reefs, there has since elapsed a long stationary
period.
(Mr. Couthouy states ("Remarks," page 44) that at Tahiti and Eimeo the
space between the reef and the shore has been nearly filled up by the
extension of those coral-reefs, which within most barrier-reefs merely
fringe the land. From this circumstance, he arrives at the same conclusion
as I have done, that the Society Islands since their subsidence, have
remained stationary during a long period; but he further believes that they
have recently commenced rising, as well as the whole area of the Low
Archipelago. He does not give any detailed proofs regarding the elevation
of the Society Islands, but I shall refer to this subject in another part
of this chapter. Before making some further comments, I may observe how
satisfactory it is to me, to find Mr. Couthouy affirming, that "having
personally examined a large number of coral-islands, and also residing
eight months among the volcanic class, having shore and partially
encircling reefs, I may be permitted to state that my own observations have
impressed a conviction of the correctness of the theory of Mr. Darwin."
This gentleman believes, that subsequently to the subsidence by which the
atolls in the Low Archipelago were produced, the whole area has been
elevated to the amount of a few feet; this would indeed be a remarkable
fact; but as far as I am able to judge, the grounds of his conclusion are
not sufficiently strong. He states that he found in almost every atoll
which he visited, the shores of the lagoon raised from eighteen to thirty
inches above the sea-level, and containing imbedded Tridacnae and corals
standing as they grew; some of the corals were dead in their upper parts,
but below a certain line they continued to flourish. In the lagoons, also,
he frequently met with clusters of Madrepore, with their extremities
standing from one inch to a foot above the surface of the water. Now,
these appearances are exactly what I should have expected, without any
subsequent elevation having taken place; and I think Mr. Couthouy has not
borne in mind the indisputable fact, that corals, when constantly bathed by
the surf, can exist at a higher level than in quite tranquil water, as in a
lagoon. As long, therefore, as the waves continued at low water to break
entirely over parts of the annular reef of an atoll, submerged to a small
depth, the corals and shells attached on these parts might continue living
at a level above the smooth surface of the lagoon, into which the waves
rolled; but as soon as the outer edge of the reef grew up to its utmost
possible height, or if the reef were very broad nearly to that height, the
force of the breakers would be checked, and the corals and shells on the
inner parts near the lagoon would occasionally be left dry, and thus be
partially or wholly destroyed. Even in atolls, which have not lately
subsided, if the outer margin of the reef continued to increase in breadth
seaward (each fresh zone of corals rising to the same vertical height as at
Keeling atoll), the line where the waves broke most heavily would advance
outwards, and therefore the corals, which when living near the margin, were
washed by the breaking waves during the whole of each tide, would cease
being so, and would therefore be left on the backward part of the reef
standing exposed and dead. The case of the madrepores in the lagoons with
the tops of their branches exposed, seems to be an analogous fact, to the
great fields of dead but upright corals in the lagoon of Keeling atoll; a
condition of things which I have endeavoured to show, has resulted from the
lagoon having become more and more enclosed and choked up with reefs, so
that during high winds, the rising of the tide (as observed by the
inhabitants) is checked, and the corals, which had formerly grown to the
greatest possible height, are occasionally exposed, and thus are killed:
and this is a condition of things, towards which almost every atoll in the
intervals of its subsidence must be tending. Or if we look to the state of
an atoll directly after a subsidence of some fathoms, the waves would roll
heavily over the entire circumference of the reef, and the surface of the
lagoon would, like the ocean, never be quite at rest, and therefore the
corals in the lagoon, from being constantly laved by the rippling water,
might extend their branches to a little greater height than they could,
when the lagoon became enclosed and protected. Christmas atoll (2 deg N.
latitude) which has a very shallow lagoon, and differs in several respects
from most atolls, possibly may have been elevated recently; but its highest
part appears (Couthouy, page 46) to be only ten feet above the sea-level.
The facts of a second class, adduced by Mr. Couthouy, in support of the
alleged recent elevation of the Low Archipelago, are not all (especially
those referring to a shelf of rock) quite intelligible to me; he believes
that certain enormous fragments of rock on the reef, must have been moved
into their present position, when the reef was at a lower level; but here
again the force of the breakers on any inner point of the reef being
diminished by its outward growth without any change in its level, has not,
I think, been borne in mind. We should, also, not overlook the occasional
agency of waves caused by earthquakes and hurricanes. Mr. Couthouy further
argues, that since these great fragments were deposited and fixed on the
reef, they have been elevated; he infers this from the greatest amount of
erosion not being near their bases, where they are unceasingly washed by
the reflux of the tides, but at some height on their sides, near the line
of high-water mark, as shown in an accompanying diagram. My former remark
again applies here, with this further observation, that as the waves have
to roll over a wide space of reef before they reach the fragments, their
force must be greatly increased with the increasing depth of water as the
tide rises, and therefore I should have expected that the chief line of
present erosion would have coincided with the line of high-water mark; and
if the reef had grown outwards, that there would have been lines of erosion
at greater heights. The conclusion, to which I am finally led by the
interesting observations of Mr. Couthouy is, that the atolls in the Low
Archipelago have, like the Society Islands, remained at a stationary level
for a long period: and this probably is the ordinary course of events,
subsidence supervening after long intervals of rest.)
Turning now to the red colour; as on our map, the areas which have sunk
slowly downwards to great depths are many and large, we might naturally
have been led to conjecture, that with such great changes of level in
progress, the coasts which have been fringed probably for ages (for we have
no reason to believe that coral-reefs are of short duration), would not
have remained all this time stationary, but would frequently have undergone
movements of elevation. This supposition, we shall immediately see, holds
good to a remarkable extent; and although a stationary condition of the
land can hardly ever be open to proof, from the evidence being only
negative, we are, in some degree, enabled to ascertain the correctness of
the parts coloured red on the map, by the direct testimony of upraised
organic remains of a modern date. Before going into the details on this
head (printed in small type), I may mention, that when reading a memoir on
coral formations by MM. Quoy and Gaimard ("Annales des Sciences Nat." tom.
vi., page 279, etc.) I was astonished to find, for I knew that they had
crossed both the Pacific and Indian Oceans, that their descriptions were
applicable only to reefs of the fringing class; but my astonishment ended
satisfactorily, when I discovered that, by a strange chance, all the
islands which these eminent naturalists had visited, though several in
number, namely, the Mauritius, Timor, New Guinea, the Mariana, and Sandwich
Archipelagoes, could be shown by their own statements to have been elevated
within a recent geological era.
In the eastern half of the Pacific, the SANDWICH Islands are all fringed,
and almost every naturalist who has visited them, has remarked on the
abundance of elevated corals and shells, apparently identical with living
species. The Rev. W. Ellis informs me, that he has noticed round several
parts of Hawaii, beds of coral-detritus, about twenty feet above the level
of the sea, and where the coast is low they extend far inland. Upraised
coral-rock forms a considerable part of the borders of Oahu; and at
Elizabeth Island ("Zoology of Captain Beechey's Voyage," page 176. See
also MM. Quoy and Gaimard in "Annales de Scien. Nat." tom. vi.) it composes
three strata, each about ten feet thick. Nihau, which forms the northern,
as Hawaii does the southern end of the group (350 miles in length),
likewise seems to consist of coral and volcanic rocks. Mr. Couthouy
("Remarks on Coral Formations," page 51.) has lately described with
interesting details, several upraised beaches, ancient reefs with their
surfaces perfectly preserved, and beds of recent shells and corals, at the
islands of Maui, Morokai, Oahu, and Tauai (or Kauai) in this group. Mr.
Pierce, an intelligent resident at Oahu, is convinced, from changes which
have taken place within his memory, during the last sixteen years, "that
the elevation is at present going forward at a very perceptible rate." The
natives at Kauai state that the land is there gaining rapidly on the sea,
and Mr. Couthouy has no doubt, from the nature of the strata, that this has
been effected by an elevation of the land.
In the southern part of the Low Archipelago, Elizabeth Island is described
by Captain Beechey (Beechey's "Voyage in the Pacific," page 46, 4to
edition.), as being quite flat, and about eighty feet in height; it is
entirely composed of dead corals, forming a honeycombed, but compact rock.
In cases like this, of an island having exactly the appearance, which the
elevation of any one of the smaller surrounding atolls with a shallow
lagoon would present, one is led to conclude (with little better reason,
however, than the improbability of such small and low fabrics lasting, for
an immense period, exposed to the many destroying agents of nature), that
the elevation has taken place at an epoch not geologically remote. When
merely the surface of an island of ordinary formation is strewed with
marine bodies, and that continuously, or nearly so, from the beach to a
certain height, and not above that height, it is exceedingly improbable
that such organic remains, although they may not have been specially
examined, should belong to any ancient period. It is necessary to bear
these remarks in mind, in considering the evidence of the elevatory
movements in the Pacific and Indian Oceans, as it does not often rest on
specific determinations, and therefore should be received with caution.
Six of the COOK AND AUSTRAL Islands (S.W. of the Society group), are
fringed; of these, five were described to me by the Rev. J. Williams, as
formed of coral-rock, associated with some basalt in Mangaia), and the
sixth as lofty and basaltic. Mangaia is nearly three hundred feet high,
with a level summit; and according to Mr. S. Wilson (Couthouy's "Remarks,"
page 34.) it is an upraised reef; "and there are in the central hollow,
formerly the bed of the lagoon, many scattered patches of coral-rock, some
of them raised to a height of forty feet." These knolls of coral-rock were
evidently once separate reefs in the lagoon of an atoll. Mr. Martens, at
Sydney, informed me that this island is surrounded by a terrace-like plain
at about the height of a hundred feet, which probably marks a pause in its
elevation. From these facts we may infer, perhaps, that the Cook and
Austral Islands have been upheaved at a period probably not very remote.
SAVAGE Island (S.E. of the Friendly group), is about forty feet in height.
Forster ("Observations made during Voyage round the World," page 147.)
describes the plants as already growing out of the dead, but still upright
and spreading trees of coral; and the younger Forster ("Voyage," volume
ii., page 163.) believes that an ancient lagoon is now represented by a
central plain; here we cannot doubt that the elevatory forces have recently
acted. The same conclusion may be extended, though with somewhat less
certainty, to the islands of the FRIENDLY GROUP, which have been well
described in the second and third voyages of Cook. The surface of
Tongatabou is low and level, but with some parts a hundred feet high; the
whole consists of coral-rock, "which yet shows the cavities and
irregularities worn into it by the action of the tides." (Cook's "Third
Voyage" (4to edition), volume i., page 314.) On Eoua the same appearances
were noticed at an elevation of between two hundred and three hundred feet.
Vavao, also, at the opposite or northern end of the group, consists,
according to the Rev. J. Williams, of coral-rock. Tongatabou, with its
northern extensive reefs, resembles either an upraised atoll with one half
originally imperfect, or one unequally elevated; and Anamouka, an atoll
equally elevated. This latter island contains (Ibid., volume i., page
235.) in its centre a salt-water lake, about a mile-and-a-half in diameter,
without any communication with the sea, and around it the land rises
gradually like a bank; the highest part is only between twenty and thirty
feet; but on this part, as well as on the rest of the land (which, as Cook
observes, rises above the height of true lagoon-islands), coral-rock, like
that on the beach, was found. In the NAVIGATOR ARCHIPELAGO, Mr. Couthouy
("Remarks on Coral-Formations," page 50.) found on Manua many and very
large fragments of coral at the height of eighty feet, "on a steep hill-side,
rising half a mile inland from a low sandy plain abounding in marine
remains." The fragments were embedded in a mixture of decomposed lava and
sand. It is not stated whether they were accompanied by shells, or whether
the corals resembled recent species; as these remains were embedded they
possibly may belong to a remote epoch; but I presume this was not the
opinion of Mr. Couthouy. Earthquakes are very frequent in this
archipelago.
Still proceeding westward we come to the NEW HEBRIDES; on these islands,
Mr. G. Bennett (author of "Wanderings in New South Wales"), informs me he
found much coral at a great altitude, which he considered of recent origin.
Respecting SANTA CRUZ, and the SOLOMON ARCHIPELAGO, I have no information;
but at New Ireland, which forms the northern point of the latter chain,
both Labillardiere and Lesson have described large beds of an apparently
very modern madreporitic rock, with the form of the corals little altered.
The latter author ("Voyage de la 'Coquille'," Part. Zoolog.) states that
this formation composes a newer line of coast, modelled round an ancient
one. There only remains to be described in the Pacific, that curved line
of fringed islands, of which the MARIANAS form the main part. Of these
Guam, Rota, Tiniam, Saypan, and some islets farther north, are described by
Quoy and Gaimard (Freycinet's "Voyage autour du Monde." See also the
"Hydrographical Memoir," page 215.), and Chamisso (Kotzebue's "First
Voyage."), as chiefly composed of madreporitic limestone, which attains a
considerable elevation, and is in several cases worn into successively
rising cliffs: the two former naturalists seem to have compared the corals
and shells with the existing ones, and state that they are of recent
species. FAIS, which lies in the prolonged line of the Marianas, is the
only island in this part of the sea which is fringed; it is ninety feet
high, and consists entirely of madreporitic rock. (Lutke's "Voyage,"
volume ii., page 304.)
In the EAST INDIAN ARCHIPELAGO, many authors have recorded proofs of recent
elevation. M. Lesson (Partie Zoolog., "Voyage de la 'Coquille'.") states,
that near Port Dory, on the north coast of New Guinea, the shores are
flanked, to the height of 150 feet, by madreporitic strata of modern date.
He mentions similar formations at Waigiou, Amboina, Bourou, Ceram, Sonda,
and Timor: at this latter place, MM. Quoy and Gaimard ("Ann. des Scien.
Nat." tom. vi., page 281.) have likewise described the primitive rocks, as
coated to a considerable height with coral. Some small islets eastward of
Timor are said in Kolff's "Voyage," (translated by Windsor Earl, chapters
vi., vii.) to resemble small coral islets upraised some feet above the sea.
Dr. Malcolmson informs me that Dr. Hardie found in JAVA an extensive
formation, containing an abundance of shells, of which the greater part
appear to be of existing species. Dr. Jack ("Geolog. Transact." 2nd
series, volume i., page 403. On the Peninsula of Malacca, in front of
Pinang, 5 deg 30' N., Dr. Ward collected some shells, which Dr. Malcolmson
informs me, although not compared with existing species, had a recent
appearance. Dr. Ward describes in this neighbourhood ("Trans. Asiat. Soc."
volume xviii., part ii., page 166) a single water-worn rock, with a
conglomerate of sea-shells at its base, situated six miles inland, which,
according to the traditions of the natives, was once surrounded by the sea.
Captain Low has also described (Ibid., part i., page 131) mounds of shells
lying two miles inland on this line of coast.) has described some upraised
shells and corals, apparently recent, on Pulo Nias off SUMATRA; and Marsden
relates in his history of this great island, that the names of many
promontories, show that they were originally islands. On part of the west
coast of BORNEO and at the SOOLOO Islands, the form of the land, the nature
of the soil, and the water-washed rocks, present appearances ("Notices of
the East Indian Arch." Singapore, 1828, page 6, and Append., page 43.)
(although it is doubtful whether such vague evidence is worthy of mention),
of having recently been covered by the sea; and the inhabitants of the
Sooloo Islands believe that this has been the case. Mr. Cuming, who has
lately investigated, with so much success, the natural history of the
PHILIPPINES, found near Cabagan, in Luzon, about fifty feet above the level
of the R. Cagayan, and seventy miles from its mouth, a large bed of fossil
shells: these, he informs me, are of the same species with those now
existing on the shores of the neighbouring islands. From the accounts
given us by Captain Basil Hall and Captain Beechey (Captain B. Hall,
"Voyage to Loo Choo," Append., pages xxi. and xxv. Captain Beechey's
"Voyage," page 496.) of the lines of inland reefs, and walls of coral-rock
worn into caves, above the present reach of the waves, at the LOO CHOO
Islands, there can be little doubt that they have been upraised at no very
remote period.
Dr. Davy describes the northern province of CEYLON ("Travels in Ceylon,"
page 13. This madreporitic formation is mentioned by M. Cordier in his
report to the Institute (May 4th, 1839), on the voyage of the "Chevrette",
as one of immense extent, and belonging to the latest tertiary period.) as
being very low, and consisting of a limestone with shells and corals of
very recent origin; he adds, that it does not admit of a doubt that the sea
has retired from this district even within the memory of man. There is
also some reason for believing that the western shores of India, north of
Ceylon, have been upraised within the recent period. (Dr. Benza, in his
"Journey through the N. Circars" (the "Madras Lit. and Scient. Journ."
volume v.) has described a formation with recent fresh-water and marine
shells, occurring at the distance of three or four miles from the present
shore. Dr. Benza, in conversation with me, attributed their position to a
rise of the land. Dr. Malcolmson, however (and there cannot be a higher
authority on the geology of India) informs me that he suspects that these
beds may have been formed by the mere action of the waves and currents
accumulating sediment. From analogy I should much incline to Dr. Benza's
opinion.) MAURITIUS has certainly been upraised within the recent period,
as I have stated in the chapter on fringing-reefs. The northern extremity
of MADAGASCAR is described by Captain Owen (Owen's "Africa," volume ii.,
page 37, for Madagascar; and for S. Africa, volume i., pages 412 and 426.
Lieutenant Boteler's narrative contains fuller particulars regarding the
coral-rock, volume i., page 174, and volume ii., pages 41 and 54. See also
Ruschenberger's "Voyage round the World," volume i., page 60.) as formed of
madreporitic rock, as likewise are the shores and outlying islands along an
immense space of EASTERN AFRICA, from a little north of the equator for
nine hundred miles southward. Nothing can be more vague than the
expression "madreporitic rock;" but at the same time it is, I think,
scarcely possible to look at the chart of the linear islets, which rise to
a greater height than can be accounted for by the growth of coral, in front
of the coast, from the equator to 2 deg S., without feeling convinced that
a line of fringing-reefs has been elevated at a period so recent, that no
great changes have since taken place on the surface of this part of the
globe. Some, also, of the higher islands of madreporitic rock on this
coast, for instance Pemba, have very singular forms, which seem to show the
combined effect of the growth of coral round submerged banks, and their
subsequent upheaval. Dr. Allan informs me that he never observed any
elevated organic remains on the SEYCHELLES, which come under our fringed
class.
The nature of the formations round the shores of the RED SEA, as described
by several authors, shows that the whole of this large area has been
elevated within a very recent tertiary epoch. A part of this space in the
appended map, is coloured blue, indicating the presence of barrier-reefs:
on which circumstance I shall presently make some remarks. Ruppell
(Ruppell, "Reise in Abyssinien," Band i., s. 141.) states that the tertiary
formation, of which he has examined the organic remains, forms a fringe
along the shores with a uniform height of from thirty and forty feet from
the mouth of the Gulf of Suez to about latitude 26 deg; but that south of
26 deg, the beds attain only the height of from twelve to fifteen feet.
This, however, can hardly be quite accurate; although possibly there may be
a decrease in the elevation of the shores in the middle parts of the Red
Sea, for Dr. Malcolmson (as he informs me) collected from the cliffs of
Camaran Island (latitude 15 deg 30' S.) shells and corals, apparently
recent, at a height between thirty and forty feet; and Mr. Salt ("Travels
in Abyssinia") describes a similar formation a little southward on the
opposite shore at Amphila. Moreover, near the mouth of the Gulf of Suez,
although on the coast opposite to that on which Dr. Ruppell says that the
modern beds attain a height of only thirty to forty feet, Mr. Burton
(Lyell's "Principles of Geology," 5th edition, volume iv., page 25.) found
a deposit replete with existing species of shells, at the height of 200
feet. In an admirable series of drawings by Captain Moresby, I could see
how continuously the cliff-bounded low plains of this formation extended
with a nearly equable height, both on the eastern and western shores. The
southern coast of Arabia seems to have been subjected to the same elevatory
movement, for Dr. Malcolmson found at Sahar low cliffs containing shells
and corals, apparently of recent species.
The PERSIAN GULF abounds with coral-reefs; but as it is difficult to
distinguish them from sand-banks in this shallow sea, I have coloured only
some near the mouth; towards the head of the gulf Mr. Ainsworth
(Ainsworth's "Assyria and Babylon," page 217.) says that the land is worn
into terraces, and that the beds contain organic remains of existing forms.
The WEST INDIAN ARCHIPELAGO of "fringed" islands, alone remains to be
mentioned; evidence of an elevation within a late tertiary epoch of nearly
the whole of this great area, may be found in the works of almost all the
naturalists who have visited it. I will give some of the principal
references in a note. (On Florida and the north shores of the Gulf of
Mexico, Rogers' "Report to Brit. Assoc." volume iii., page 14.--On the
shores of Mexico, Humboldt, "Polit. Essay on New Spain," volume i., page
62. (I have also some corroborative facts with respect to the shores of
Mexico.)--Honduras and the Antilles, Lyell's "Principles," 5th edition,
volume iv., page 22.--Santa Cruz and Barbadoes, Prof. Hovey, "Silliman's
Journal", volume xxxv., page 74.--St. Domingo, Courrojolles, "Journ de
Phys." tom. liv., page 106.--Bahamas, "United Service Journal", No. lxxi.,
pages 218 and 224. Jamaica, De la Beche, "Geol. Man." page 142.--Cuba,
Taylor in "Lond. and Edin. Mag." volume xi., page 17. Dr. Daubeny also, at
a meeting of the Geolog. Soc., orally described some very modern beds lying
on the N.W. parts of Cuba. I might have added many other less important
references.)
It is very remarkable on reviewing these details, to observe in how many
instances fringing-reefs round the shores, have coincided with the
existence on the land of upraised organic remains, which seem, from
evidence more or less satisfactory, to belong to a late tertiary period.
It may, however, be objected, that similar proofs of elevation, perhaps,
occur on the coasts coloured blue in our map: but this certainly is not
the case with the few following and doubtful exceptions.
The entire area of the Red Sea appears to have been upraised within a
modern period; nevertheless I have been compelled (though on unsatisfactory
evidence, as given in the Appendix) to class the reefs in the middle part,
as barrier-reefs; should, however, the statements prove accurate to the
less height of the tertiary bed in this middle part, compared with the
northern and southern districts, we might well suspect that it had subsided
subsequently to the general elevation by which the whole area has been
upraised. Several authors (Ellis, in his "Polynesian Researches," was the
first to call attention to these remains (volume i., page 38), and the
tradition of the natives concerning them. See also Williams, "Nar. of
Missionary Enterprise," page 21; also Tyerman and G. Bennett, "Journal of
Voyage," volume i., page 213; also Mr. Couthouy's "Remarks," page 51; but
this principal fact, namely, that there is a mass of upraised coral on the
narrow peninsula of Tiarubu, is from hearsay evidence; also Mr. Stutchbury,
"West of England Journal," No. i., page 54. There is a passage in Von
Zach, "Corres. Astronom." volume x., page 266, inferring an uprising at
Tahiti, from a footpath now used, which was formerly impassable; but I
particularly inquired from several native chiefs, whether they knew of any
change of this kind, and they were unanimous in giving me an answer in the
negative.) have stated that they have observed shells and corals high up on
the mountains of the Society Islands,--a group encircled by barrier-reefs,
and, therefore, supposed to have subsided: at Tahiti Mr. Stutchbury found
on the apex of one of the highest mountains, between 5,000 and 7,000 feet
above the level of the sea, "a distinct and regular stratum of semi-fossil
coral." At Tahiti, however, other naturalists, as well as myself, have
searched in vain at a low level near the coast, for upraised shells or
masses of coral-reef, where if present they could hardly have been
overlooked. From this fact, I concluded that probably the organic remains
strewed high up on the surface of the land, had originally been embedded in
the volcanic strata, and had subsequently been washed out by the rain. I
have since heard from the Rev. W. Ellis, that the remains which he met
with, were (as he believes) interstratified with an argillaceous tuff; this
likewise was the case with the shells observed by the Rev. D. Tyerman at
Huaheine. These remains have not been specifically examined; they may,
therefore, and especially the stratum observed by Mr. Stutchbury at an
immense height, be contemporaneous with the first formation of the Society
Islands, and be of any degree of antiquity; or they may have been deposited
at some subsequent, but probably not very recent, period of elevation; for
if the period had been recent, the entire surface of the coast land of
these islands, where the reefs are so extensive, would have been coated
with upraised coral, which certainly is not the case. Two of the Harvey,
or Cook Islands, namely, Aitutaki and Manouai, are encircled by reefs,
which extend so far from the land, that I have coloured them blue, although
with much hesitation, as the space within the reef is shallow, and the
outline of the land is not abrupt. These two islands consist of coral-rock;
but I have no evidence of their recent elevation, besides, the
improbability of Mangaia, a fringed island in the same group (but distant
170 miles), having retained its nearly perfect atoll-like structure, during
any immense lapse of time after its upheaval. The Red Sea, therefore, is
the only area in which we have clear proofs of the recent elevation of a
district, which, by our theory (although the barrier-reefs are there not
well characterised), has lately subsided. But we have no reason to be
surprised at oscillation, of level of this kind having occasionally taken
place. There can be scarcely any doubt that Savage, Aurora (Aurora Island
is described by Mr. Couthouy ("Remarks," page 58); it lies 120 miles
north-east of Tahiti; it is not coloured in the appended map, because it does
not appear to be fringed by living reefs. Mr. Couthouy describes its summit
as "presenting a broad table-land which declines a few feet towards the
centre, where we may suppose the lagoon to have been placed." It is about
two hundred feet in height, and consists of reef-rock and conglomerate,
with existing species of coral embedded in it. The island has been
elevated at two successive periods; the cliffs being marked halfway up with
a horizontal water-worn line of deep excavations. Aurora Island seems
closely to resemble in structure Elizabeth Island, at the southern end of
the Low Archipelago.), and Mangaia Islands, and several of the islands in
the Friendly group, existed originally as atolls, and these have
undoubtedly since been upraised to some height above the level of the sea;
so that by our theory, there has here, also, been an oscillation of level,
--elevation having succeeded subsidence, instead of, as in the middle part
of the Red Sea and at the Harvey Islands, subsidence having probably
succeeded recent elevation.
It is an interesting fact, that Fais, which, from its composition, form,
height, and situation at the western end of the Caroline Archipelago, one
is strongly induced to believe existed before its upheaval as an atoll,
lies exactly in the prolongation of the curved line of the Mariana group,
which we know to be a line of recent elevation. I may add, that Elizabeth
Island, in the southern part of the Low Archipelago, which seems to have
had the same kind of origin as Fais, lies near Pitcairn Island, the only
one in this part of the ocean which is high, and at the same time not
surrounded by an encircling barrier-reef.
ON THE ABSENCE OF ACTIVE VOLCANOES IN THE AREAS OF SUBSIDENCE, AND ON THEIR
FREQUENT PRESENCE IN THE AREAS OF ELEVATION.
Before making some concluding remarks on the relations of the spaces
coloured blue and red, it will be convenient to consider the position on
our map of the volcanoes historically known to have been in action. It is
impossible not to be struck, first with the absence of volcanoes in the
great areas of subsidence tinted pale and dark blue,--namely, in the
central parts of the Indian Ocean, in the China Sea, in the sea between the
barriers of Australia and New Caledonia, in the Caroline, Marshall,
Gilbert, and Low Archipelagoes; and, secondly, with the coincidence of the
principal volcanic chains with the parts coloured red, which indicates the
presence of fringing-reefs; and, as we have just seen, the presence in most
cases of upraised organic remains of a modern date. I may here remark that
the reefs were all coloured before the volcanoes were added to the map, or
indeed before I knew of the existence of several of them.
The volcano in Torres Strait, at the northern point of Australia, is that
which lies nearest to a large subsiding area, although situated 125 miles
within the outer margin of the actual barrier-reef. The Great Comoro
Island, which probably contains a volcano, is only twenty miles distant
from the barrier-reef of Mohila; Ambil volcano, in the Philippines, is
distant only a little more than sixty miles from the atoll-formed Appoo
reef: and there are two other volcanoes in the map within ninety miles of
circles coloured blue. These few cases, which thus offer partial
exceptions to the rule, of volcanoes being placed remote from the areas of
subsidence, lie either near single and isolated atolls, or near small
groups of encircled islands; and these by our theory can have, in few
instances, subsided to the same amount in depth or area, as groups of
atolls. There is not one active volcano within several hundred miles of an
archipelago, or even a small group of atolls. It is, therefore, a striking
fact that in the Friendly Archipelago, which owes its origin to the
elevation of a group of atolls, two volcanoes, and, perhaps, others are
known to be in action: on the other hand, on several of the encircled
islands in the Pacific, supposed by our theory to have subsided, there are
old craters and streams of lava, which show the effects of past and ancient
eruptions. In these cases, it would appear as if the volcanoes had come
into action, and had become extinguished on the same spots, according as
the elevating or subsiding movements prevailed.
There are some other coasts on the map, where volcanoes in a state of
action concur with proofs of recent elevation, besides those coloured red
from being fringed by coral-reefs. Thus I hope to show in a future volume,
that nearly the whole line of the west coast of South America, which forms
the greatest volcanic chain in the world, from near the equator for a space
of between 2,000 and 3,000 miles southward, has undergone an upward
movement during a late geological period. The islands on the north-western
shores of the Pacific, which form the second greatest volcanic chain, are
very imperfectly known; but Luzon, in the Philippines, and the Loo Choo
Islands, have been recently elevated; and at Kamtschatka (At Sedanka, in
latitude 58 deg N. (Von Buch's "Descrip. des Isles Canaries," page 455).
In a forthcoming part, I shall give the evidence referred to with respect
to the elevation of New Zealand.) there are extensive tertiary beds of
modern date. Evidence of the same nature, but not very satisfactory, may
be detected in Northern New Zealand where there are two volcanoes. The
co-existence in other parts of the world of active volcanoes, with upraised
beds of a modern tertiary origin, will occur to every geologist. (During
the subterranean disturbances which took place in Chile, in 1835, I have
shown ("Geolog. Trans." 2nd Ser., vol. v., page 606) that at the same
moment that a large district was upraised, volcanic matter burst forth at
widely separated points, through both new and old vents.) Nevertheless,
until it could be shown that volcanoes were inactive, or did not exist in
subsiding areas, the conclusion that their distribution depended on the
nature of the subterranean movements in progress, would have been
hazardous. But now, viewing the appended map, it may, I think, be
considered as almost established, that volcanoes are often (not necessarily
always) present in those areas where the subterranean motive power has
lately forced, or is now forcing outwards, the crust of the earth, but that
they are invariably absent in those, where the surface has lately subsided
or is still subsiding. (We may infer from this rule, that in any old
deposit, which contains interstratified beds of erupted matter, there was
at the period, and in the area of its formation, a TENDENCY to an upward
movement in the earth's surface, and certainly no movement of subsidence.)
ON THE RELATIONS OF THE AREAS OF SUBSIDENCE AND ELEVATION.
The immense surfaces on the map, which, both by our theory and by the plain
evidence of upraised marine remains, have undergone a change of level
either downwards or upwards during a late period, is a most remarkable
fact. The existence of continents shows that the areas have been immense
which at some period have been upraised; in South America we may feel sure,
and on the north-western shores of the Indian Ocean we may suspect, that
this rising is either now actually in progress, or has taken place quite
recently. By our theory, we may conclude that the areas are likewise
immense which have lately subsided, or, judging from the earthquakes
occasionally felt and from other appearances, are now subsiding. The
smallness of the scale of our map should not be overlooked: each of the
squares on it contains (not allowing for the curvature of the earth)
810,000 square miles. Look at the space of ocean from near the southern
end of the Low Archipelago to the northern end of the Marshall Archipelago,
a length of 4,500 miles, in which, as far as is known, every island, except
Aurora which lies just without the Low Archipelago, is atoll-formed. The
eastern and western boundaries of our map are continents, and they are
rising areas: the central spaces of the great Indian and Pacific Oceans,
are mostly subsiding; between them, north of Australia, lies the most
broken land on the globe, and there the rising parts are surrounded and
penetrated by areas of subsidence (I suspect that the Arru and Timor-laut
Islands present an included small area of subsidence, like that of the
China Sea, but I have not ventured to colour them from my imperfect
information, as given in the Appendix.), so that the prevailing movements
now in progress, seem to accord with the actual states of surface of the
great divisions of the world.
The blue spaces on the map are nearly all elongated; but it does not
necessarily follow from this (a caution, for which I am indebted to Mr.
Lyell), that the areas of subsidence were likewise elongated; for the
subsidence of a long, narrow space of the bed of the ocean, including in it
a transverse chain of mountains, surmounted by atolls, would only be marked
on the map by a transverse blue band. But where a chain of atolls and
barrier-reefs lies in an elongated area, between spaces coloured red, which
therefore have remained stationary or have been upraised, this must have
resulted either from the area of subsidence having originally been
elongated (owing to some tendency in the earth's crust thus to subside), or
from the subsiding area having originally been of an irregular figure, or
as broad as long, and having since been narrowed by the elevation of
neighbouring districts. Thus the areas, which subsided during the
formation of the great north and south lines of atolls in the Indian
Ocean,--of the east and west line of the Caroline atolls,--and of the
north-west and south-east line of the barrier-reefs of New Caledonia and
Louisiade, must have originally been elongated, or if not so, they must
have since been made elongated by elevations, which we know to belong to a
recent period.
I infer from Mr. Hopkins' researches ("Researches in Physical Geology,"
Transact. Cambridge Phil. Soc., volume vi, part i.), that for the formation
of a long chain of mountains, with few lateral spurs, an area elongated in
the same direction with the chain, must have been subjected to an elevatory
movement. Mountain-chains, however, when already formed, although running
in very different directions, it seems (For instance in S. America from
latitude 34 deg, for very many degrees southward there are upraised beds
containing recent species of shells, on both the Atlantic and Pacific side
of the continent, and from the gradual ascent of the land, although with
very unequal slopes, on both sides towards the Cordillera, I think it can
hardly be doubted that the entire width has been upraised in mass within
the recent period. In this case the two W.N.W. and E.S.E. mountain-lines,
namely the Sierra Ventana and the S. Tapalguen, and the great north and
south line of the Cordillera have been together raised. In the West Indies
the N. and S. line of the Eastern Antilles, and the E. and W. line of
Jamaica, appear both to have been upraised within the latest geological
period.) may be raised together by a widely-acting force: so, perhaps,
mountain-chains may subside together. Hence, we cannot tell, whether the
Caroline and Marshall Archipelagoes, two groups of atolls running in
different directions and meeting each other, have been formed by the
subsidence of two areas, or of one large area, including two distinct lines
of mountains. We have, however, in the southern prolongation of the
Mariana Islands, probable evidence of a line of recent elevation having
intersected one of recent subsidence. A view of the map will show that,
generally, there is a tendency to alternation in the parallel areas
undergoing opposite kinds of movement; as if the sinking of one area
balanced the rising of another.
The existence in many parts of the world of high table-land, proves that
large surfaces have been upraised in mass to considerable heights above the
level of the ocean; although the highest points in almost every country
consist of upturned strata, or erupted matter: and from the immense spaces
scattered with atolls, which indicate that land originally existed there,
although not one pinnacle now remains above the level of the sea, we may
conclude that wide areas have subsided to an amount, sufficient to bury not
only any formerly existing table-land, but even the heights formed by
fractured strata, and erupted matter. The effects produced on the land by
the later elevatory movements, namely, successively rising cliffs, lines of
erosion, and beds of literal shells and pebbles, all requiring time for
their production, prove that these movements have been very slow; we can,
however, infer this with safety, only with respect to the few last hundred
feet of rise. But with reference to the whole vast amount of subsidence,
necessary to have produced the many atolls widely scattered over immense
spaces, it has already been shown (and it is, perhaps, the most interesting
conclusion in this volume), that the movements must either have been
uniform and exceedingly slow, or have been effected by small steps,
separated from each other by long intervals of time, during which the
reef-constructing polypifers were able to bring up their solid frameworks
to the surface. We have little means of judging whether many considerable
oscillations of level have generally occurred during the elevation of large
tracts; but we know, from clear geological evidence, that this has
frequently taken place; and we have seen on our map, that some of the same
islands have both subsided and been upraised. I conclude, however, that
most of the large blue spaces, have subsided without many and great
elevatory oscillations, because only a few upraised atolls have been
observed: the supposition that such elevations have taken place, but that
the upraised parts have been worn down by the surf, and thus have escaped
observation, is overruled by the very considerable depth of the lagoons of
all the larger atolls; for this could not have been the case, if they had
suffered repeated elevations and abrasion. From the comparative
observations made in these latter pages, we may finally conclude, that the
subterranean changes which have caused some large areas to rise, and others
to subside, have acted in a very similar manner.
RECAPITULATION.
In the three first chapters, the principal kinds of coral-reefs were
described in detail, and they were found to differ little, as far as
relates to the actual surface of the reef. An atoll differs from an
encircling barrier-reef only in the absence of land within its central
expanse; and a barrier-reef differs from a fringing-reef, in being placed
at a much greater distance from the land with reference to the probable
inclination of its submarine foundation, and in the presence of a deep-water
lagoon-like space or moat within the reef. In the fourth chapter the
growing powers of the reef-constructing polypifers were discussed; and it
was shown, that they cannot flourish beneath a very limited depth. In
accordance with this limit, there is no difficulty respecting the
foundations on which fringing-reefs are based; whereas, with barrier-reefs
and atolls, there is a great apparent difficulty on this head; in
barrier-reefs from the improbability of the rock of the coast or of banks of
sediment extending, in every instance, so far seaward within the required
depth;--and in atolls, from the immensity of the spaces over which they are
interspersed, and the apparent necessity for believing that they are all
supported on mountain-summits, which although rising very near to the
surface-level of the sea, in no one instance emerge above it. To escape
this latter most improbable admission, which implies the existence of
submarine chains of mountains of almost the same height, extending over
areas of many thousand square miles, there is but one alternative; namely,
the prolonged subsidence of the foundations, on which the atolls were
primarily based, together with the upward growth of the reef-constructing
corals. On this view every difficulty vanishes; fringing reefs are thus
converted into barrier-reefs; and barrier-reefs, when encircling islands,
are thus converted into atolls, the instant the last pinnacle of land sinks
beneath the surface of the ocean.
Thus the ordinary forms and certain peculiarities in the structure of
atolls and barrier-reefs can be explained;--namely, the wall-like structure
on their inner sides, the basin or ring-like shape both of the marginal and
central reefs in the Maldiva atolls--the union of some atolls as if by a
ribbon--the apparent disseverment of others--and the occurrence, in atolls
as well as in barrier-reefs, of portions of reef, and of the whole of some
reefs, in a dead and submerged state, but retaining the outline of living
reefs. Thus can be explained the existence of breaches through barrier-reefs
in front of valleys, though separated from them by a wide space of
deep water; thus, also, the ordinary outline of groups of atolls and the
relative forms of the separate atolls one to another; thus can be explained
the proximity of the two kinds of reefs formed during subsidence, and their
separation from the spaces where fringing-reefs abound. On searching for
other evidence of the movements supposed by our theory, we find marks of
change in atolls and in barrier-reefs, and of subterranean disturbances
under them; but from the nature of things, it is scarcely possible to
detect any direct proofs of subsidence, although some appearances are
strongly in favour of it. On the fringed coasts, however, the presence of
upraised marine bodies of a recent epoch, plainly show, that these coasts,
instead of having remained stationary, which is all that can be directly
inferred from our theory, have generally been elevated.
Finally, when the two great types of structure, namely barrier-reefs and
atolls on the one hand, and fringing-reefs on the other, were laid down in
colours on our map, a magnificent and harmonious picture of the movements,
which the crust of the earth has within a late period undergone, is
presented to us. We there see vast areas rising, with volcanic matter
every now and then bursting forth through the vents or fissures with which
they are traversed. We see other wide spaces slowly sinking without any
volcanic outburst, and we may feel sure, that this sinking must have been
immense in amount as well as in area, thus to have buried over the broad
face of the ocean every one of those mountains, above which atolls now
stand like monuments, marking the place of their former existence.
Reflecting how powerful an agent with respect to denudation, and
consequently to the nature and thickness of the deposits in accumulation,
the sea must ever be, when acting for prolonged periods on the land, during
either its slow emergence or subsidence; reflecting, also, on the final
effects of these movements in the interchange of land and ocean-water on
the climate of the earth, and on the distribution of organic beings, I may
be permitted to hope, that the conclusions derived from the study of
coral-formations, originally attempted merely to explain their peculiar
forms, may be thought worthy of the attention of geologists.
APPENDIX.
CONTAINING A DETAILED DESCRIPTION OF THE REEFS AND ISLANDS IN PLATE III.
In the beginning of the last chapter I stated the principles on which the
map is coloured. There only remains to be said, that it is an exact copy
of one by M. C. Gressier, published by the Depot General de la Marine, in
1835. The names have been altered into English, and the longitude has been
reduced to that of Greenwich. The colours were first laid down on accurate
charts, on a large scale. The data, on which the volcanoes historically
known to have been in action, have been marked with vermillion, were given
in a note to the last chapter. I will commence my description on the
eastern side of the map, and will describe each group of islands
consecutively, proceeding westward across the Pacific and Indian Oceans,
but ending with the West Indies.
The WESTERN SHORES OF AMERICA appear to be entirely without coral-reefs;
south of the equator the survey of the "Beagle", and north of it, the
published charts show that this is the case. Even in the Bay of PANAMA,
where corals flourish, there are no true coral-reefs, as I have been
informed by Mr. Lloyd. There are no coral-reefs in the GALAPAGOS
Archipelago, as I know from personal inspection; and I believe there are
none on the COCOS, REVILLA-GIGEDO, and other neighbouring islands.
CLIPPERTON rock, 10 deg N., 109 deg W., has lately been surveyed by Captain
Belcher; in form it is like the crater of a volcano. From a drawing
appended to the MS. plan in the Admiralty, it evidently is not an atoll.
The eastern parts of the Pacific present an enormous area, without any
islands, except EASTER, and SALA, and GOMEZ Islands, which do not appear to
be surrounded by reefs.
THE LOW ARCHIPELAGO.
This group consists of about eighty atolls: it will be quite superfluous
to refer to descriptions of each. In D'Urville and Lottin's chart, one
island (WOLCHONSKY) is written with a capital letter, signifying, as
explained in a former chapter, that it is a high island; but this must be a
mistake, as the original chart by Bellinghausen shows that it is a true
atoll. Captain Beechey says of the thirty-two groups which he examined (of
the greater number of which I have seen beautiful MS. charts in the
Admiralty), that twenty-nine now contain lagoons, and he believes the other
three originally did. Bellinghausen (see an account of his Russian voyage,
in the "Biblioth. des Voyages," 1834, page 443) says, that the seventeen
islands which he discovered resembled each other in structure, and he has
given charts on a large scale of all of them. Kotzebue has given plans of
several; Cook and Bligh mention others; a few were seen during the voyage
of the "Beagle"; and notices of other atolls are scattered through several
publications. The ACTAEON group in this archipelago has lately been
discovered ("Geographical Journal", volume vii., page 454); it consists of
three small and low islets, one of which has a lagoon. Another lagoon-island
has been discovered ("Naut. Mag." 1839, page 770), in 22 deg 4' S.,
and 136 deg 20' W. Towards the S.E. part of the group, there are some
islands of different formation: ELIZABETH Island is described by Beechey
(page 46, 4to edition) as fringed by reefs, at the distance of between two
and three hundred yards; coloured red. PITCAIRN Island, in the immediate
neighbourhood, according to the same authority, has no reefs of any kind,
although numerous pieces of coral are thrown up on the beach; the sea close
to its shore is very deep (see "Zool. of Beechey's Voyage," page 164); it
is left uncoloured. GAMBIER Islands (see Plate I., Figure 8), are
encircled by a barrier-reef; the greatest depth within is thirty-eight
fathoms; coloured pale blue. AURORA Island, which lies N.E. of Tahiti
close to the large space coloured dark blue in the map, has been already
described in a note (page 71), on the authority of Mr. Couthouy; it is an
upraised atoll, but as it does not appear to be fringed by living reefs, it
is left uncoloured.
The SOCIETY Archipelago is separated by a narrow space from the Low
Archipelago; and in their parallel direction they manifest some relation to
each other. I have already described the general character of the reefs of
these fine encircled islands. In the "Atlas of the 'Coquille's' Voyage"
there is a good general chart of the group, and separate plans of some of
the islands. TAHITI, the largest island in the group, is almost
surrounded, as seen in Cook's chart, by a reef from half a mile to a mile
and a half from the shore, with from ten to thirty fathoms within it. Some
considerable submerged reefs lying parallel to the shore, with a broad and
deep space within, have lately been discovered ("Naut. Mag." 1836, page
264) on the N.E. coast of the island, where none are laid down by Cook. At
EIMEO the reef "which like a ring surrounds it, is in some places one or
two miles distant from the shore, in others united to the beach" (Ellis,
"Polynesian Researches," volume i., page 18, 12mo edition). Cook found
deep water (twenty fathoms) in some of the harbours within the reef. Mr.
Couthouy, however, states ("Remarks," page 45) that both at Tahiti and
Eimeo, the space between the barrier-reef and the shore, has been almost
filled up,--"a nearly continuous fringing-reef surrounding the island, and
varying from a few yards to rather more than a mile in width, the lagoons
merely forming canals between this and the sea-reef," that is the
barrier-reef. TAPAMANOA is surrounded by a reef at a considerable distance
from the shore; from the island being small it is breached, as I am informed
by the Rev. W. Ellis, only by a narrow and crooked boat channel. This is the
lowest island in the group, its height probably not exceeding 500 feet. A
little way north of Tahiti, the low coral-islets of TETUROA are situated;
from the description of them given me by the Rev. J. Williams (the author
of the "Narrative of Missionary Enterprise"), I should have thought they
had formed a small atoll, and likewise from the description given by the
Rev. D. Tyerman and G. Bennett ("Journal of Voyage and Travels," volume i.,
page 183), who say that ten low coral-islets "are comprehended within one
general reef, and separated from each other by interjacent lagoons;" but as
Mr. Stutchbury ("West of England Journal," volume i., page 54) describes it
as consisting of a mere narrow ridge, I have left it uncoloured. MAITEA,
eastward of the group, is classed by Forster as a high encircled island;
but from the account given by the Rev. D. Tyerman and G. Bennett (volume
i., page 57) it appears to be an exceedingly abrupt cone, rising from the
sea without any reef; I have left it uncoloured. It would be superfluous
to describe the northern islands in this group, as they may be well seen in
the chart accompanying the 4to edition of Cook's "Voyages," and in the
"Atlas of the 'Coquille's' Voyage." MAURUA is the only one of the northern
islands, in which the water within the reef is not deep, being only four
and a half fathoms; but the great width of the reef, stretching three miles
and a half southward of the land (which is represented in the drawing in
the "Atlas of the 'Coquille's' Voyage" as descending abruptly to the water)
shows, on the principle explained in the beginning of the last chapter,
that it belongs to the barrier class. I may here mention, from information
communicated to me by the Rev. W. Ellis, that on the N.E. side of HUAHEINE
there is a bank of sand, about a quarter of a mile wide, extending parallel
to the shore, and separated from it by an extensive and deep lagoon; this
bank of sand rests on coral-rock, and undoubtedly was originally a living
reef. North of Bolabola lies the atoll of TOUBAI (Motou-iti of the
"'Coquille's' Atlas") which is coloured dark blue; the other islands,
surrounded by barrier-reefs, are pale blue; three of them are represented
in Figures 3, 4, and 5, in Plate I. There are three low coral-groups lying
a little E. of the Society Archipelago, and almost forming part of it,
namely BELLINGHAUSEN, which is said by Kotzebue ("Second Voyage," volume
ii., page 255), to be a lagoon-island; MOPEHA, which, from Cook's
description ("Second Voyage," book iii., chapter i.), no doubt is an atoll;
and the SCILLY Islands, which are said by Wallis ("Voyage," chapter ix.) to
form a GROUP of LOW islets and shoals, and, therefore, probably, they
compose an atoll: the two former have been coloured blue, but not the
latter.
MENDANA OR MARQUESAS GROUP.
These islands are entirely without reefs, as may be seen in Krusenstern's
Atlas, making a remarkable contrast with the adjacent group of the Society
Islands. Mr. F.D. Bennett has given some account of this group, in the
seventh volume of the "Geographical Journal". He informs me that all the
islands have the same general character, and that the water is very deep
close to their shores. He visited three of them, namely, DOMINICANA,
CHRISTIANA, and ROAPOA; their beaches are strewed with rounded masses of
coral, and although no regular reefs exist, yet the shore is in many places
lined by coral-rock, so that a boat grounds on this formation. Hence these
islands ought probably to come within the class of fringed islands and be
coloured red; but as I am determined to err on the cautious side, I have
left them uncoloured.
COOK OR HARVEY AND AUSTRAL ISLAND.
PALMERSTON Island is minutely described as an atoll by Captain Cook during
his voyage in 1774; coloured blue. AITUTAKI was partially surveyed by the
"Beagle" (see map accompanying "Voyages of 'Adventure' and 'Beagle'"); the
land is hilly, sloping gently to the beach; the highest point is 360 feet;
on the southern side the reef projects five miles from the land: off this
point the "Beagle" found no bottom with 270 fathoms: the reef is
surmounted by many low coral-islets. Although within the reef the water is
exceedingly shallow, not being more than a few feet deep, as I am informed
by the Rev. J. Williams, nevertheless, from the great extension of this
reef into a profoundly deep ocean, this island probably belongs, on the
principle lately adverted to, to the barrier class, and I have coloured it
pale blue; although with much hesitation.--MANOUAI or HARVEY Island. The
highest point is about fifty feet: the Rev. J. Williams informs me that
the reef here, although it lies far from the shore, is less distant than at
Aitutaki, but the water within the reef is rather deeper: I have also
coloured this pale blue with many doubts.--Round MITIARO Island, as I am
informed by Mr. Williams, the reef is attached to the shore; coloured red.
--MAUKI or Maouti; the reef round this island (under the name of Parry
Island, in the "Voyage of H.M.S. 'Blonde'," page 209) is described as a
coral-flat, only fifty yards wide, and two feet under water. This
statement has been corroborated by Mr. Williams, who calls the reef
attached; coloured red.--AITU, or Wateeo; a moderately elevated hilly
island, like the others of this group. The reef is described in Cook's
"Voyage," as attached to the shore, and about one hundred yards wide;
coloured red.--FENOUA-ITI; Cook describes this island as very low, not more
than six or seven feet high (volume i., book ii., chapter iii, 1777); in
the chart published in the "'Coquille's' Atlas," a reef is engraved close
to the shore: this island is not mentioned in the list given by Mr.
Williams (page 16) in the "Narrative of Missionary Enterprise;" nature
doubtful. As it is so near Atiu, it has been unavoidably coloured red.--
RAROTONGA; Mr. Williams informs me that it is a lofty basaltic island with
an attached reef; coloured red.--There are three islands, ROUROUTI,
ROXBURGH, and HULL, of which I have not been able to obtain any account,
and have left them uncoloured. Hull Island, in the French chart, is
written with small letters as being low.--MANGAIA; height about three
hundred feet; "the surrounding reef joins the shore" (Williams,
"Narrative," page 18); coloured red.--RIMETARA; Mr. Williams informs me
that the reef is rather close to the shore; but, from information given me
by Mr. Ellis, the reef does not appear to be quite so closely attached to
it as in the foregoing cases: the island is about three hundred feet high
("Naut. Mag." 1839, page 738); coloured red.--RURUTU; Mr. Williams and Mr.
Ellis inform me that this island has an attached reef; coloured red. It is
described by Cook under the name of Oheteroa: he says it is not
surrounded, like the neighbouring islands by a reef; he must have meant a
distant reef.--TOUBOUAI; in Cook's chart ("Second Voyage," volume ii., page
2) the reef is laid down in part one mile, and in part two miles from the
shore. Mr. Ellis ("Polynes. Res." volume iii., page 381) says the low land
round the base of the island is very extensive; and this gentleman informs
me that the water within the reef appears deep; coloured blue.--RAIVAIVAI,
or Vivitao; Mr. Williams informs me that the reef is here distant: Mr.
Ellis, however, says that this is certainly not the case on one side of the
island; and he believes that the water within the reef is not deep; hence I
have left it uncoloured.--LANCASTER Reef, described in "Naut. Mag." 1833
(page 693), as an extensive crescent-formed coral-reef. I have not
coloured it.--RAPA, or Oparree; from the accounts given of it by Ellis and
Vancouver, there does not appear to be any reef.--I. DE BASS is an
adjoining island, of which I cannot find any account.--KEMIN Island;
Krusenstern seems hardly to know its position, and gives no further
particulars.
ISLANDS BETWEEN THE LOW AND GILBERT ARCHIPELAGOES.
CAROLINE Island (10 deg S., 150 deg W.) is described by Mr. F.D. Bennett
("Geographical Journal", volume vii., page 225) as containing a fine
lagoon; coloured blue.--FLINT Island (11 deg S., 151 deg W.); Krusenstern
believes that it is the same with Peregrino, which is described by Quiros
(Burney's "Chron. Hist." volume ii., page 283) as "a cluster of small
islands connected by a reef, and forming a lagoon in the middle;" coloured
blue.--WOSTOCK is an island a little more than half a mile in diameter, and
apparently quite flat and low, and was discovered by Bellinghausen; it is
situated a little west of Caroline Island, but it is not placed on the
French charts; I have not coloured it, although I entertain little doubt
from the chart of Bellinghausen, that it originally contained a small
lagoon.--PENRHYN Island (9 deg S., 158 deg W.); a plan of it in the "Atlas
of the First Voyage" of Kotzebue, shows that it is an atoll; blue.--
SLARBUCK Island (5 deg S., 156 deg W.) is described in Byron's "Voyage in
the 'Blonde'" (page 206) as formed of a flat coral-rock, with no trees; the
height not given; not coloured.--MALDEN Island (4 deg S., 154 deg W.); in
the same voyage (page 205) this island is said to be of coral formation,
and no part above forty feet high; I have not ventured to colour it,
although, from being of coral-formation, it is probably fringed; in which
case it should be red.--JARVIS, or BUNKER Island (0 deg 20' S., 160 deg W.)
is described by Mr. F.D. Bennett ("Geographical Journal", volume vii., page
227) as a narrow, low strip of coral-formation; not coloured.--BROOK, is a
small low island between the two latter; the position, and perhaps even the
existence of it is doubtful; not coloured.--PESCADO and HUMPHREY Islands; I
can find out nothing about these islands, except that the latter appears to
be small and low; not coloured.--REARSON, or Grand Duke Alexander's (10 S.,
161 deg W.); an atoll, of which a plan is given by Bellinghausen; blue.--
SOUVOROFF Islands (13 deg S., 163 deg W.); Admiral Krusenstern, in the most
obliging manner, obtained for me an account of these islands from Admiral
Lazareff, who discovered them. They consist of five very low islands of
coral-formation, two of which are connected by a reef, with deep water
close to it. They do not surround a lagoon, but are so placed that a line
drawn through them includes an oval space, part of which is shallow; these
islets, therefore, probably once (as is the case with some of the islands
in the Caroline Archipelago) formed a single atoll; but I have not coloured
them.--DANGER Island (10 deg S., 166 deg W.); described as low by Commodore
Byron, and more lately surveyed by Bellinghausen; it is a small atoll with
three islets on it; blue.--CLARENCE Island (9 deg S., 172 deg W.);
discovered in the "Pandora" (G. Hamilton's "Voyage," page 75): it is said,
"in running along the land, we saw several canoes crossing the LAGOONS;" as
this island is in the close vicinity of other low islands, and as it is
said, that the natives make reservoirs of water in old cocoa-nut trees
(which shows the nature of the land), I have no doubt it is an atoll, and
have coloured it blue. YORK Island (8 deg S., 172 deg W.) is described by
Commodore Byron (chapter x. of his "Voyage") as an atoll; blue.--SYDNEY
Island (4 deg S., 172 deg W.) is about three miles in diameter, with its
interior occupied by a lagoon (Captain Tromelin, "Annal. Marit." 1829, page
297); blue.--PHOENIX Island (4 deg S., 171 deg W.) is nearly circular, low,
sandy, not more than two miles in diameter, and very steep outside
(Tromelin, "Annal. Marit." 1829, page 297); it may be inferred that this
island originally contained a lagoon, but I have not coloured it.--NEW
NANTUCKET (0 deg 15' N., 174 deg W.). From the French chart it must be a
low island; I can find nothing more about it or about MARY Island; both
uncoloured.--GARDNER Island (5 deg S., 174 deg W.) from its position is
certainly the same as KEMIN Island described (Krusenstern, page 435, Appen.
to Mem., published 1827) as having a lagoon in its centre; blue.
ISLANDS SOUTH OF THE SANDWICH ARCHIPELAGO.
CHRISTMAS Island (2 deg N., 157 deg W.). Captain Cook, in his "Third
Voyage" (Volume ii., chapter x.), has given a detailed account of this
atoll. The breadth of the islets on the reef is unusually great, and the
sea near it does not deepen so suddenly as is generally the case. It has
more lately been visited by Mr. F.D. Bennett ("Geographical Journal,"
volume vii., page 226); and he assures me that it is low and of
coral-formation: I particularly mention this, because it is engraved with a
capital letter, signifying a high island, in D'Urville and Lottin's chart.
Mr. Couthouy, also, has given some account of it ("Remarks," page 46) from
the Hawaiian "Spectator"; he believes it has lately undergone a small
elevation, but his evidence does not appear to me satisfactory; the deepest
part of the lagoon is said to be only ten feet; nevertheless, I have
coloured it blue.--FANNING Island (4 deg N., 158 deg W.) according to
Captain Tromelin ("Ann. Maritim." 1829, page 283), is an atoll: his
account as observed by Krusenstern, differs from that given in Fanning's
"Voyage" (page 224), which, however, is far from clear; coloured blue.--
WASHINGTON Island (4 deg N., 159 deg W.) is engraved as a low island in
D'Urville's chart, but is described by Fanning (page 226) as having a much
greater elevation than Fanning Island, and hence I presume it is not an
atoll; not coloured.--PALMYRA Island (6 deg N., 162 deg W.) is an atoll
divided into two parts (Krusenstern's "Mem. Suppl." page 50, also Fanning's
"Voyage," page 233); blue.--SMYTH'S or Johnston's Islands (17 deg N., 170
deg W.). Captain Smyth, R.N., has had the kindness to inform me that they
consist of two very low, small islands, with a dangerous reef off the east
end of them. Captain Smyth does not recollect whether these islets,
together with the reef, surrounded a lagoon; uncoloured.
SANDWICH ARCHIPELAGO.
HAWAII; in the chart in Freycinet's "Atlas," small portions of the coast
are fringed by reefs; and in the accompanying "Hydrog. Memoir," reefs are
mentioned in several places, and the coral is said to injure the cables.
On one side of the islet of Kohaihai there is a bank of sand and coral with
five feet water on it, running parallel to the shore, and leaving a channel
of about fifteen feet deep within. I have coloured this island red, but it
is very much less perfectly fringed than others of the group.--MAUI; in
Freycinet's chart of the anchorage of Raheina, two or three miles of coast
are seen to be fringed; and in the "Hydrog. Memoir," "banks of coral along
shore" are spoken of. Mr. F.D. Bennett informs me that the reefs, on an
average, extend about a quarter of a mile from the beach; the land is not
very steep, and outside the reefs the sea does not become deep very
suddenly; coloured red.--MOROTOI, I presume, is fringed: Freycinet speaks
of the breakers extending along the shore at a little distance from it.
From the chart, I believe it is fringed; coloured red.--OAHU; Freycinet, in
his "Hydrog. Memoir," mentions some of the reefs. Mr. F.D. Bennett informs
me that the shore is skirted for forty or fifty miles in length. There is
even a harbour for ships formed by the reefs, but it is at the mouth of a
valley; red.--ATOOI, in La Peyrouse's charts, is represented as fringed by
a reef, in the same manner as Oahu and Morotoi; and this, as I have been
informed by Mr. Ellis, on part at least of the shore, is of coral-formation:
the reef does not leave a deep channel within; red.--ONEEHOW;
Mr. Ellis believes that this island is also fringed by a coral-reef:
considering its close proximity to the other islands, I have ventured to
colour it red. I have in vain consulted the works of Cook, Vancouver, La
Peyrouse, and Lisiansky, for any satisfactory account of the small islands
and reefs, which lie scattered in a N.W. line prolonged from the Sandwich
group, and hence have left them uncoloured, with one exception; for I am
indebted to Mr. F.D. Bennett for informing me of an atoll-formed reef, in
latitude 28 deg 22', longitude 178 deg 30' W., on which the "Gledstanes"
was wrecked in 1837. It is apparently of large size, and extends in a N.W.
and S.E. line: very few islets have been formed on it. The lagoon seems
to be shallow; at least, the deepest part which was surveyed was only three
fathoms. Mr. Couthouy ("Remarks," page 38) describes this island under the
name of OCEAN island. Considerable doubts should be entertained regarding
the nature of a reef of this kind, with a very shallow lagoon, and standing
far from any other atoll, on account of the possibility of a crater or flat
bank of rock lying at the proper depth beneath the surface of the water,
thus affording a foundation for a ring-formed coral-reef. I have, however,
thought myself compelled, from its large size and symmetrical outline, to
colour it blue.
SAMOA OR NAVIGATOR GROUP.
Kotzebue, in his "Second Voyage," contrasts the structure of these islands
with many others in the Pacific, in not being furnished with harbours for
ships, formed by distant coral-reefs. The Rev. J. Williams, however,
informs me, that coral-reefs do occur in irregular patches on the shores of
these islands; but that they do not form a continuous band, as round
Mangaia, and other such perfect cases of fringed islands. From the charts
accompanying La Peyrouse's "Voyage," it appears that the north shore of
SAVAII, MAOUNA, OROSENGA, and MANUA, are fringed by reefs. La Peyrouse,
speaking of Maouna (page 126), says that the coral-reef surrounding its
shores, almost touches the beach; and is breached in front of the little
coves and streams, forming passages for canoes, and probably even for
boats. Further on (page 159), he extends the same observation to all the
islands which he visited. Mr. Williams in his "Narrative," speaks of a
reef going round a small island attached to OYOLAVA, and returning again to
it: all these islands have been coloured red.--A chart of ROSE Island, at
the extreme west end of the group, is given by Freycinet, from which I
should have thought that it had been an atoll; but according to Mr.
Couthouy ("Remarks," page 43), it consists of a reef, only a league in
circuit, surmounted by a very few low islets; the lagoon is very shallow,
and is strewed with numerous large boulders of volcanic rock. This island,
therefore, probably consists of a bank of rock, a few feet submerged, with
the outer margin of its upper surface fringed with reefs; hence it cannot
be properly classed with atolls, in which the foundations are always
supposed to lie at a depth, greater than that at which the reef-constructing
polypifers can live; not coloured.
BEVERIDGE Reef, 20 deg S., 167 deg W., is described in the "Naut. Mag."
(May 1833, page 442) as ten miles long in a N. and S. line, and eight wide;
"in the inside of the reef there appears deep water;" there is a passage
near the S.W. corner: this therefore seems to be a submerged atoll, and is
coloured blue.
SAVAGE Island, 19 deg S., 170 deg W., has been described by Cook and
Forster. The younger Forster (volume ii., page 163) says it is about forty
feet high: he suspects that it contains a low plain, which formerly was
the lagoon. The Rev. J. Williams informs me that the reef fringing its
shores, resembles that round Mangaia; coloured red.
FRIENDLY ARCHIPELAGO.
PYLSTAART Island. Judging from the chart in Freycinet's "Atlas," I should
have supposed that it had been regularly fringed; but as nothing is said in
the "Hydrog. Memoir" (or in the "Voyage" of Tasman, the discoverer) about
coral-reefs, I have left it uncoloured.--TONGATABOU: In the "Atlas of the
Voyage of the 'Astrolabe'," the whole south side of the island is
represented as narrowly fringed by the same reef which forms an extensive
platform on the northern side. The origin of this latter reef, which might
have been mistaken for a barrier-reef, has already been attempted to be
explained, when giving the proofs of the recent elevation of this island.--
In Cook's charts the little outlying island also of EOAIGEE, is represented
as fringed; coloured red.--EOUA. I cannot make out from Captain Cook's
charts and descriptions, that this island has any reef, although the bottom
of the neighbouring sea seems to be corally, and the island itself is
formed of coral-rock. Forster, however, distinctly ("Observations," page
14) classes it with high islands having reefs, but it certainly is not
encircled by a barrier-reef and the younger Forster ("Voyage," volume i.,
page 426) says, that "a bed of coral-rocks surrounded the coast towards the
landing-place." I have therefore classed it with the fringed islands and
coloured it red. The several islands lying N.W. of Tongatabou, namely
ANAMOUKA, KOMANGO, KOTOU, LEFOUGA, FOA, etc., are seen in Captain Cook's
chart to be fringed by reefs, in several of them are connected together.
From the various statements in the first volume of Cook's "Third Voyage,"
and especially in the fourth and sixth chapters, it appears that these
reefs are of coral-formation, and certainly do not belong to the barrier
class; coloured red.--TOUFOA AND KAO, forming the western part of the
group, according to Forster have no reefs; the former is an active
volcano.--VAVAO. There is a chart of this singularly formed island, by
Espinoza: according to Mr. Williams it consists of coral-rock: the
Chevalier Dillon informs me that it is not fringed; not coloured. Nor are
the islands of LATTE and AMARGURA, for I have not seen plans on a large
scale of them, and do not know whether they are fringed.
NIOUHA, 16 deg S., 174 deg W., or KEPPEL Island of Wallis, or COCOS Island.
From a view and chart of this island given in Wallis's "Voyage" (4to
edition) it is evidently encircled by a reef; coloured blue: it is however
remarkable that BOSCAWEN Island, immediately adjoining, has no reef of any
kind; uncoloured.
WALLIS Island, 13 deg S., 176 deg W., a chart and view of this island in
Wallis's "Voyage" (4to edition) shows that it is encircled. A view of it
in the "Naut. Mag." July 1833, page 376, shows the same fact; blue.
ALLOUFATOU, or HORN Island, ONOUAFU, or PROBY Island, and HUNTER Islands,
lie between the Navigator and Fidji groups. I can find no distinct
accounts of them.
FIDJI or VITI GROUP.
The best chart of the numerous islands of this group, will be found in the
"Atlas of the 'Astrolabe's' Voyage." From this, and from the description
given in the "Hydrog. Memoir," accompanying it, it appears that many of
these islands are bold and mountainous, rising to the height of between
3,000 and 4,000 feet. Most of the islands are surrounded by reefs, lying
far from the land, and outside of which the ocean appears very deep. The
"Astrolabe" sounded with ninety fathoms in several places about a mile from
the reefs, and found no bottom. Although the depth within the reef is not
laid down, it is evident from several expressions, that Captain D'Urville
believes that ships could anchor within, if passages existed through the
outer barriers. The Chevallier Dillon informs me that this is the case:
hence I have coloured this group blue. In the S.E. part lies BATOA, or
TURTLE Island of Cook ("Second Voyage," volume ii., page 23, and chart, 4to
edition) surrounded by a coral-reef, "which in some places extends two
miles from the shore;" within the reef the water appears to be deep, and
outside it is unfathomable; coloured pale blue. At the distance of a few
miles, Captain Cook (Ibid., page 24) found a circular coral-reef, four or
five leagues in circuit, with deep water within; "in short, the bank wants
only a few little islets to make it exactly like one of the half-drowned
isles so often mentioned,"--namely, atolls. South of Batoa, lies the high
island of ONO, which appears in Bellinghausen's "Atlas" to be encircled; as
do some other small islands to the south; coloured pale blue; near Ono,
there is an annular reef, quite similar to the one just described in the
words of Captain Cook; coloured dark blue.
ROTOUMAH, 13 deg S., 179 deg E.--From the chart in Duperrey's "Atlas," I
thought this island was encircled, and had coloured it blue, but the
Chevallier Dillon assures me that the reef is only a shore or fringing one;
red.
INDEPENDENCE Island, 10 deg S., 179 deg E., is described by Mr. G. Bennett,
("United Service Journal," 1831, part ii., page 197) as a low island of
coral-formation, it is small, and does not appear to contain a lagoon,
although an opening through the reef is referred to. A lagoon probably
once existed, and has since been filled up; left uncoloured.
ELLICE GROUP.
OSCAR, PEYSTER, and ELLICE Islands are figured in Arrowsmith's "Chart of
the Pacific" (corrected to 1832) as atolls, and are said to be very low;
blue.--NEDERLANDISCH Island. I am greatly indebted to the kindness of
Admiral Krusenstern, for sending me the original documents concerning this
island. From the plans given by Captains Eeg and Khremtshenko, and from
the detailed account given by the former, it appears that it is a narrow
coral-island, about two miles long, containing a small lagoon. The sea is
very deep close to the shore, which is fronted by sharp coral-rocks.
Captain Eeg compares the lagoon with that of other coral-islands; and he
distinctly says, the land is "very low." I have therefore coloured it
blue. Admiral Krusenstern ("Memoir on the Pacific," Append., 1835) states
that its shores are eighty feet high; this probably arose from the height
of the cocoa-nut trees, with which it is covered, being mistaken for land.
--GRAN COCAL is said in Krusenstern's "Memoir," to be low, and to be
surrounded by a reef; it is small, and therefore probably once contained a
lagoon; uncoloured.--ST. AUGUSTIN. From a chart and view of it, given in
the "Atlas of the 'Coquille's' Voyage," it appears to be a small atoll,
with its lagoon partly filled up; coloured blue.
GILBERT GROUP.
The chart of this group, given in the "Atlas of the 'Coquille's' Voyage,"
at once shows that it is composed of ten well characterised atolls. In
D'Urville and Lottin's chart, SYDENHAM is written with a capital letter,
signifying that it is high; but this certainly is not the case, for it is a
perfectly characterised atoll, and a sketch, showing how low it is, is
given in the "'Coquille's' Atlas." Some narrow strip-like reefs project
from the southern side of DRUMMOND atoll, and render it irregular. The
southern island of the group is called CHASE (in some charts, ROTCHES); of
this I can find no account, but Mr. F.D. Bennett discovered ("Geographical
Journal", volume vii., page 229), a low extensive island in nearly the same
latitude, about three degrees westward of the longitude assigned to
Rotches, but very probably it is the same island. Mr. Bennett informs me
that the man at the masthead reported an appearance of lagoon-water in the
centre; and, therefore, considering its position, I have coloured it blue.
--PITT Island, at the extreme northern point of the group, is left
uncoloured, as its exact position and nature is not known.--BYRON Island,
which lies a little to the eastward, does not appear to have been visited
since Commodore Byron's voyage, and it was then seen only from a distance
of eighteen miles; it is said to be low; uncoloured.
OCEAN, PLEASANT, and ATLANTIC Islands all lie considerably to the west of
the Gilbert group: I have been unable to find any distinct account of
them. Ocean Island is written with small letters in the French chart, but
in Krusenstern's "Memoir" it is said to be high.
MARSHALL GROUP.
We are well acquainted with this group from the excellent charts of the
separate islands, made during the two voyages of Kotzebue: a reduced one
of the whole group may be easily seen in Krusenstern's "Atlas," and in
Kotzebue's "Second Voyage." The group consists (with the exception of two
LITTLE islands which probably have had their lagoon filled up) of a double
row of twenty-three large and well-characterised atolls, from the
examination of which Chamisso has given us his well-known account of
coral-formations. I include GASPAR RICO, or CORNWALLIS Island in this group,
which is described by Chamisso (Kotzebue's "First Voyage," volume iii.,
page 179) "as a low sickle-formed group, with mould only on the windward
side." Gaspard Island is considered by some geographers as a distinct
island lying N.E. of the group, but it is not entered in the chart by
Krusenstern; left uncoloured. In the S.W. part of this group lies BARING
Island, of which little is known (see Krusenstern's "Appendix," 1835, page
149). I have left it uncoloured; but BOSTON Island I have coloured blue,
as it is described (Ibid.) as consisting of fourteen small islands, which,
no doubt, enclose a lagoon, as represented in a chart in the "'Coquille's'
Atlas."--Two islands, AUR KAWEN and GASPAR RICO, are written in the French
chart with capital letters; but this is an error, for from the account
given by Chamisso in Kotzebue's "First Voyage," they are certainly low.
The nature, position, and even existence, of the shoals and small islands
north of the Marshall group, are doubtful.
NEW HEBRIDES.
Any chart, on even a small scale, of these islands, will show that their
shores are almost without reefs, presenting a remarkable contrast with
those of New Caledonia on the one hand, and the Fidji group on the other.
Nevertheless, I have been assured by Mr. G. Bennett, that coral grows
vigorously on their shores; as indeed, will be further shown in some of the
following notices. As, therefore, these islands are not encircled, and as
coral grows vigorously on their shores, we might almost conclude, without
further evidence, that they were fringed, and hence I have applied the red
colour with rather greater freedom than in other instances.--MATTHEW'S
ROCK, an active volcano, some way south of the group (of which a plan is
given in the "Atlas of the 'Astrolabe's' Voyage") does not appear to have
reefs of any kind about it.--ANNATOM, the southernmost of the Hebrides;
from a rough woodcut given in the "United Service Journal" (1831, part
iii., page 190), accompanying a paper by Mr. Bennett, it appears that the
shore is fringed; coloured red.--TANNA. Forster, in his "Observations"
(page 22), says Tanna has on its shores coral-rock and madrepores; and the
younger Forster, in his account (volume ii., page 269) speaking of the
harbour says, the whole S.E. side consists of coral-reefs, which are
overflowed at high-water; part of the southern shore in Cook's chart is
represented as fringed; coloured red.--IMMER is described ("United Service
Journal," 1831, part iii., page 192) by Mr. Bennett as being of moderate
elevation, with cliffs appearing like sandstone: coral grows in patches on
its shore, but I have not coloured it; and I mention these facts, because
Immer might have been thought from Forster's classification
("Observations," page 14), to have been a low island or even an atoll.--
ERROMANGO Island; Cook ("Second Voyage," volume ii., page 45, 4to edition)
speaks of rocks everywhere LINING the coast, and the natives offered to
haul his boat over the breakers to the sandy beach: Mr. Bennett, in a
letter to the Editor of the "Singapore Chron.," alludes to the REEFS on its
shores. It may, I think, be safely inferred from these passages that the
shore is fringed in parts by coral-reefs; coloured red.--SANDWICH Island.
The east coast is said (Cook's "Second Voyage," volume ii., page 41) to be
low, and to be guarded by a chain of breakers. In the accompanying chart
it is seen to be fringed by a reef; coloured red.--MALLICOLLO. Forster
speaks of the reef-bounded shore: the reef is about thirty yards wide, and
so shallow that a boat cannot pass over it. Forster also ("Observations,"
page 23) says, that the rocks of the sea-shore consist of madrepore. In
the plan of Sandwich harbour, the headlands are represented as fringed;
coloured red.--AURORA and PENTECOST Islands, according to Bougainville,
apparently have no reefs; nor has the large island of S. ESPIRITU, nor
BLIGH Island or BANKS' Islands, which latter lie to the N.E. of the
Hebrides. But in none of these cases, have I met with any detailed account
of their shores, or seen plans on a large scale; and it will be evident,
that a fringing-reef of only thirty or even a few hundred yards in width,
is of so little importance to navigation, that it will seldom be noticed,
excepting by chance; and hence I do not doubt that several of these
islands, now left uncoloured, ought to be red.
SANTA CRUZ GROUP.
VANIKORO (Figure 1, Plate I.) offers a striking example of a barrier-reef:
it was first described by the Chevalier Dillon, in his voyage, and was
surveyed in the "Astrolabe"; coloured pale blue.--TIKOPIA and FATAKA
Islands appear, from the descriptions of Dillon and D'Urville, to have no
reefs; ANOUDA is a low, flat island, surrounded by cliffs ("'Astrolabe'
Hydrog." and Krusenstern, "Mem." volume ii., page 432); these are
uncoloured. TOUPOUA (OTOOBOA of Dillon) is stated by Captain Tromelin
("Annales Marit." 1829, page 289) to be almost entirely included in a reef,
lying at the distance of two miles from the shore. There is a space of
three miles without any reef, which, although indented with bays, offers no
anchorage from the extreme depth of the water close to the shore: Captain
Dillon also speaks of the reefs fronting this island; coloured blue.--
SANTA-CRUZ. I have carefully examined the works of Carteret,
D'Entrecasteaux, Wilson, and Tromelin, and I cannot discover any mention of
reefs on its shores; left uncoloured.--TINAKORO is a constantly active
volcano without reefs.--MENDANA ISLES (mentioned by Dillon under the name
of MAMMEE, etc.); said by Krusenstern to be low, and intertwined with
reefs. I do not believe they include a lagoon; I have left them
uncoloured.--DUFF'S Islands compose a small group directed in a N.W. and
S.E. band; they are described by Wilson (page 296, "Miss. Voy." 4to
edition), as formed by bold-peaked land, with the islands surrounded by
coral-reefs, extending about half a mile from the shore; at a distance of a
mile from the reefs he found only seven fathoms. As I have no reason for
supposing there is deep water within these reefs, I have coloured them red.
KENNEDY Island, N.E. of Duff's. I have been unable to find any account of
it.
NEW CALEDONIA.
The great barrier-reefs on the shores of this island have already been
described (Figure 5, Plate II.). They have been visited by Labillardiere,
Cook, and the northern point by D'Urville; this latter part so closely
resembles an atoll that I have coloured it dark blue. The LOYALTY group is
situated eastward of this island; from the chart and description given in
the "Voyage of the 'Astrolabe'," they do not appear to have any reefs;
north of this group, there are some extensive low reefs (called ASTROLABE
and BEAUPRE,) which do not seem to be atoll-formed; these are left
uncoloured.
AUSTRALIAN BARRIER-REEF.
The limits of this great reef, which has already been described, have been
coloured from the charts of Flinders and King. In the northern parts, an
atoll-formed reef, lying outside the barrier, has been described by Bligh,
and is coloured dark blue. In the space between Australia and New
Caledonia, called by Flinders the Corallian Sea, there are numerous reefs.
Of these, some are represented in Krusenstern's "Atlas" as having an
atoll-like structure; namely, BAMPTON shoal, FREDERIC, VINE or Horse-shoe,
and ALERT reefs; these have been coloured dark blue.
LOUISIADE.
The dangerous reefs which front and surround the western, southern, and
northern coasts of this so-called peninsula and archipelago, seem evidently
to belong to the barrier class. The land is lofty, with a low fringe on
the coast; the reefs are distant, and the sea outside them profoundly deep.
Nearly all that is known of this group is derived from the labours of
D'Entrecasteaux and Bougainville: the latter has represented one
continuous reef ninety miles long, parallel to the shore, and in places as
much as ten miles from it; coloured pale blue. A little distance northward
we have the LAUGHLAN Islands, the reefs round which are engraved in the
"Atlas of the Voyage of the 'Astrolabe'," in the same manner as in the
encircled islands of the Caroline Archipelago, the reef is, in parts, a
mile and a half from the shore, to which it does not appear to be attached;
coloured blue. At some little distance from the extremity of the Louisiade
lies the WELLS reef, described in G. Hamilton's "Voyage in H.M.S.
'Pandora'" (page 100): it is said, "We found we had got embayed in a
double reef, which will soon be an island." As this statement is only
intelligible on the supposition of the reef being crescent or horse-shoe
formed, like so many other submerged annular reefs, I have ventured to
colour it blue.
SOLOMON ARCHIPELAGO.
The chart in Krusenstern's "Atlas" shows that these islands are not
encircled, and as coral appears from the works of Surville, Bougainville,
and Labillardiere, to grow on their shores, this circumstance, as in the
case of the New Hebrides, is a presumption that they are fringed. I cannot
find out anything from D'Entrecasteaux's "Voyage," regarding the southern
islands of the group, so have left them uncoloured.--MALAYTA Island in a
rough MS. chart in the Admiralty has its northern shore fringed.--YSABEL
Island, the N.E. part of this island, in the same chart, is also fringed:
Mendana, speaking (Burney, volume i., page 280) of an islet adjoining the
northern coast, says it is surrounded by reefs; the shores, also of Port
Praslin appear regularly fringed.--CHOISEUL Island. In Bougainville's
"Chart of Choiseul Bay," parts of the shores are fringed by coral-reefs.--
BOUGAINVILLE Island. According to D'Entrecasteaux the western shore
abounds with coral-reefs, and the smaller islands are said to be attached
to the larger ones by reefs; all the before-mentioned islands have been
coloured red.--BOUKA Islands. Captain Duperrey has kindly informed me in a
letter that he passed close round the northern side of this island (of
which a plan is given in his "Atlas of the 'Coquille's' Voyage"), and that
it was "garnie d'une bande de recifs a fleur d'eau adherentes au rivage;"
and he infers, from the abundance of coral on the islands north and south
of Bouka, that the reef probably is of coral; coloured red.
Off the north coast of the Solomon Archipelago there are several small
groups which are little known; they appear to be low, and of
coral-formation; and some of them probably have an atoll-like structure; the
Chevallier Dillon, however, informs me that this is not the case with the
B. de CANDELARIA.--OUTONG JAVA, according to the Spanish navigator,
Maurelle, is thus characterised; but this is the only one which I have
ventured to colour blue.
NEW IRELAND.
The shores of the S.W. point of this island and some adjoining islets, are
fringed by reefs, as may be seen in the "Atlases of the Voyages of the
'Coquille' and 'Astrolabe'." M. Lesson observes that the reefs are open in
front of each streamlet. The DUKE OF YORK'S Island is also fringed; but
with regard to the other parts of NEW IRELAND, NEW HANOVER, and the small
islands lying northward, I have been unable to obtain any information. I
will only add that no part of New Ireland appears to be fronted by distant
reefs. I have coloured red only the above specified portions.
NEW BRITAIN AND THE NORTHERN SHORE OF NEW GUINEA.
From the charts in the "Voyage of the 'Astrolabe'," and from the "Hydrog.
Memoir," it appears that these coasts are entirely without reefs, as are
the SCHOUTEN Islands, lying close to the northern shore of New Guinea. The
western and south-western parts of New Guinea, will be treated of when we
come to the islands of the East Indian Archipelago.
ADMIRALTY GROUP.
From the accounts by Bougainville, Maurelle, D'Entrecasteaux, and the
scattered notices collected by Horsburgh, it appears, that some of the many
islands composing it, are high, with a bold outline; and others are very
low, small and interlaced with reefs. All the high islands appear to be
fronted by distant reefs rising abruptly from the sea, and within some of
which there is reason to believe that the water is deep. I have therefore
little doubt they are of the barrier class.--In the southern part of the
group we have ELIZABETH Island, which is surrounded by a reef at the
distance of a mile; and two miles eastward of it (Krusenstern, "Append."
1835, page 42) there is a little island containing a lagoon.--Near here,
also lies CIRCULAR-REEF (Horsburgh, "Direct." volume i., page 691, 4th
edition), "three or four miles in diameter having deep water inside with an
opening at the N.N.W. part, and on the outside steep to." I have from
these data, coloured the group pale blue, and CIRCULAR-REEF dark blue.--the
ANACHORITES, ECHEQUIER, and HERMITES, consist of innumerable low islands of
coral-formation, which probably have atoll-like forms; but not being able
to ascertain this, I have not coloured them, nor DUROUR Island, which is
described by Carteret as low.
The CAROLINE ARCHIPELAGO is now well-known, chiefly from the hydrographical
labours of Lutke; it contains about forty groups of atolls, and three
encircled islands, two of which are engraved in Figures 2 and 7, Plate I.
Commencing with the eastern part; the encircling reef round UALEN appears
to be only about half a mile from the shore; but as the land is low and
covered with mangroves ("Voyage autour du Monde," par F. Lutke, volume i.,
page 339), the real margin has not probably been ascertained. The extreme
depth in one of the harbours within the reef is thirty-three fathoms (see
charts in "Atlas of 'Coquille's' Voyage"), and outside at half a mile
distant from the reef, no bottom was obtained with two hundred and fifty
fathoms. The reef is surmounted by many islets, and the lagoon-like
channel within is mostly shallow, and appears to have been much encroached
on by the low land surrounding the central mountains; these facts show that
time has allowed much detritus to accumulate; coloured pale blue.--
POUYNIPETE, or Seniavine. In the greater part of the circumference of this
island, the reef is about one mile and three quarters distant; on the north
side it is five miles off the included high islets. The reef is broken in
several places; and just within it, the depth in one place is thirty
fathoms, and in another, twenty-eight, beyond which, to all appearance,
there was "un porte vaste et sur" (Lutke, volume ii., page 4); coloured
pale blue.--HOGOLEU or ROUG. This wonderful group contains at least
sixty-two islands, and its reef is one hundred and thirty-five miles in
circuit. Of the islands, only a few, about six or eight (see "Hydrog.
Descrip." page 428, of the "Voyage of the 'Astrolabe'," and the large
accompanying chart taken chiefly from that given by Duperrey) are high, and
the rest are all small, low, and formed on the reef. The depth of the great
interior lake has not been ascertained; but Captain D'Urville appears to have
entertained no doubt about the possibility of taking in a frigate. The
reef lies no less than fourteen miles distant from the northern coasts of
the interior high islands, seven from their western sides, and twenty from
the southern; the sea is deep outside. This island is a likeness on a
grand scale to the Gambier group in the Low Archipelago. Of the groups of
low (In D'Urville and Lottin's chart, Peserare is written with capital
letters; but this evidently is an error, for it is one of the low islets on
the reef of Namonouyto (see Lutke's charts)--a regular atoll.) islands
forming the chief part of the Caroline Archipelago, all those of larger
size, have the true atoll-structure (as may be seen in the "Atlas" by
Captain Lutke), and some even of the very small ones, as MACASKILL and
DUPERREY, of which plans are given in the "Atlas of the 'Coquille's'
Voyage." There are, however, some low small islands of coral-formation,
namely OLLAP, TAMATAM, BIGALI, SATAHOUAL, which do not contain lagoons; but
it is probable that lagoons originally existed, but have since filled up:
Lutke (volume ii., page 304) seems to have thought that all the low
islands, with only one exception, contained lagoons. From the sketches,
and from the manner in which the margins of these islands are engraved in
the "Atlas of the Voyage of the 'Coquille'," it might have been thought
that they were not low; but by a comparison with the remarks of Lutke
(volume ii., page 107, regarding Bigali) and of Freycinet ("Hydrog. Memoir
'L'Uranie' Voyage," page 188, regarding Tamatam, Ollap, etc.), it will be
seen that the artist must have represented the land incorrectly. The most
southern island in the group, namely PIGUIRAM, is not coloured, because I
have found no account of it. NOUGOUOR, or MONTE VERDISON, which was not
visited by Lutke, is described and figured by Mr. Bennett ("United Service
Journal," January 1832) as an atoll. All the above-mentioned islands have
been coloured blue.
WESTERN PART OF THE CAROLINE ARCHIPELAGO.
FAIS Island is ninety feet high, and is surrounded, as I have been informed
by Admiral Lutke, by a narrow reef of living coral, of which the broadest
part, as represented in the charts, is only 150 yards; coloured red.--
PHILIP Island., I believe, is low; but Hunter, in his "Historical Journal,"
gives no clear account of it; uncoloured.--ELIVI; from the manner in which
the islets on the reefs are engraved, in the "Atlas of the 'Astrolabe's'
Voyage," I should have thought they were above the ordinary height, but
Admiral Lutke assures me this is not the case: they form a regular atoll;
coloured blue.--GOUAP (EAP of Chamisso), is a high island with a reef (see
chart in "Voyage of the 'Astrolabe'"), more than a mile distant in most
parts from the shore, and two miles in one part. Captain D'Urville thinks
that there would be anchorage ("Hydrog. Descript. 'Astrolabe' Voyage," page
436) for ships within the reef, if a passage could be found; coloured pale
blue.--GOULOU, from the chart in the "'Astrolabe's' Atlas," appears to be
an atoll. D'Urville ("Hydrog. Descript." page 437) speaks of the low
islets on the reef; coloured dark blue.
PELEW ISLANDS.
Krusenstern speaks of some of the islands being mountainous; the reefs are
distant from the shore, and there are spaces within them, and not opposite
valleys, with from ten to fifteen fathoms. According to a MS. chart of the
group by Lieutenant Elmer in the Admiralty, there is a large space within
the reef with deepish water; although the high land does not hold a central
position with respect to the reefs, as is generally the case, I have little
doubt that the reefs of the Pelew Islands ought to be ranked with the
barrier class, and I have coloured them pale blue. In Lieutenant Elmer's
chart there is a horseshoe-formed shoal, laid down thirteen miles N.W. of
Pelew, with fifteen fathoms within the reef, and some dry banks on it;
coloured dark blue.--SPANISH, MARTIRES, SANSEROT, PULO ANNA and MARIERE
Islands are not coloured, because I know nothing about them, excepting that
according to Krusenstern, the second, third, and fourth mentioned, are low,
placed on coral-reefs, and therefore, perhaps, contain lagoons; but Pulo
Mariere is a little higher.
MARIANA ARCHIPELAGO, or LADRONES.
GUAHAN. Almost the whole of this island is fringed by reefs, which extend
in most parts about a third of a mile from the land. Even where the reefs
are most extensive, the water within them is shallow. In several parts
there is a navigable channel for boats and canoes within the reefs. In
Freycinet's "Hydrog. Mem." there is an account of these reefs, and in the
"Atlas," a map on a large scale; coloured red.--ROTA. "L'ile est presque
entierement entouree des recifs" (page 212, Freycinet's "Hydrog. Mem.").
These reefs project about a quarter of a mile from the shore; coloured
red.--TINIAN. THE EASTERN coast is precipitous, and is without reefs; but
the western side is fringed like the last island; coloured red.--SAYPAN.
The N.E. coast, and likewise the western shores appear to be fringed; but
there is a great, irregular, horn-like reef projecting far from this side;
coloured red.--FARALLON DE MEDINILLA, appears so regularly and closely
fringed in Freycinet's charts, that I have ventured to colour it red,
although nothing is said about reefs in the "Hydrographical Memoir." The
several islands which form the northern part of the group are volcanic
(with the exception perhaps of Torres, which resembles in form the
madreporitic island of Medinilla), and appear to be without reefs.--MANGS,
however, is described (by Freycinet, page 219, "Hydrog.") from some Spanish
charts, as formed of small islands placed "au milieu des nombreux recifs;"
and as these reefs in the general chart of the group do not project so much
as a mile; and as there is no appearance from a double line, of the
existence of deep water within, I have ventured, although with much
hesitation, to colour them red. Respecting FOLGER and MARSHALL Islands
which lie some way east of the Marianas, I can find out nothing, excepting
that they are probably low. Krusenstern says this of Marshall Island; and
Folger Island is written with small letters in D'Urville's chart;
uncoloured.
BONIN OR ARZOBISPO GROUP.
PEEL Island has been examined by Captain Beechey, to whose kindness I am
much indebted for giving me information regarding it: "At Port Lloyd there
is a great deal of coral; and the inner harbour is entirely formed by
coral-reefs, which extend outside the port along the coast." Captain
Beechey, in another part of his letter to me, alludes to the reefs fringing
the island in all directions; but at the same time it must be observed that
the surf washes the volcanic rocks of the coast in the greater part of its
circumference. I do not know whether the other islands of the Archipelago
are fringed; I have coloured Peel Island red.--GRAMPUS Island to the
eastward, does not appear (Meare's "Voyage," page 95) to have any reefs,
nor does ROSARIO Island (from Lutke's chart), which lies to the westward.
Respecting the few other islands in this part of the sea, namely the
SULPHUR Islands, with an active volcano, and those lying between Bonin and
Japan (which are situated near the extreme limit in latitude, at which
reefs are formed), I have not been able to find any clear account.
WEST END OF NEW GUINEA.
PORT DORY. From the charts in the "Voyage of the 'Coquille'," it would
appear that the coast in this part is fringed by coral-reefs; M. Lesson,
however, remarks that the coral is sickly; coloured red.--WAIGIOU. A
considerable portion of the northern shores of these islands is seen in the
charts (on a large scale) in Freycinet's "Atlas" to be fringed by
coral-reefs. Forrest (page 21, "Voyage to New Guinea") alludes to the
coral-reefs lining the heads of Piapis Bay; and Horsburgh (volume ii., page
599, 4th edition), speaking of the islands in Dampier Strait, says "sharp
coral-rocks line their shores;" coloured red.--In the sea north of these
islands, we have GUEDES (or FREEWILL, or ST. DAVID'S), which from the chart
given in the 4to edition of Carteret's "Voyage," must be an atoll.
Krusenstern says the islets are very low; coloured blue.--CARTERET'S SHOALS,
in 2 deg 53' N., are described as circular, with stony points showing all
round, with deeper water in the middle; coloured blue.--AIOU; the plan of
this group, given in the "Atlas of the Voyage of the 'Astrolabe'," shows that
it is an atoll; and, from a chart in Forrest's "Voyage," it appears that
there is twelve fathoms within the circular reef; coloured blue.--The S.W.
coast of New Guinea appears to be low, muddy, and devoid of reefs. The ARRU,
TIMOR-LAUT, and TENIMBER groups have lately been examined by Captain Kolff,
the MS. translation of which, by Mr. W. Earl, I have been permitted to read,
through the kindness of Captain Washington, R.N. These islands are mostly
rather low, and are surrounded by distant reefs (the Ki Islands, however,
are lofty, and, from Mr. Stanley's survey, appear without reefs); the sea
in some parts is shallow, in others profoundly deep (as near Larrat). From
the imperfection of the published charts, I have been unable to decide to
which class these reefs belong. From the distance to which they extend
from the land, where the sea is very deep, I am strongly inclined to
believe they ought to come within the barrier class, and be coloured blue;
but I have been forced to leave them uncoloured.--The last-mentioned groups
are connected with the east end of Ceram by a chain of small islands, of
which the small groups of CERAM-LAUT, GORAM and KEFFING are surrounded by
very extensive reefs, projecting into deep water, which, as in the last
case, I strongly suspect belong to the barrier class; but I have not
coloured them. From the south side of Keffing, the reefs project five
miles (Windsor Earl's "Sailing Direct. for the Arafura Sea," page 9).
CERAM.
In various charts which I have examined, several parts of the coast are
represented as fringed by reefs.--MANIPA Island, between Ceram and Bourou,
in an old MS. chart in the Admiralty, is fringed by a very irregular reef,
partly dry at low water, which I do not doubt is of coral-formation; both
islands coloured red.--BOUROU; parts of this island appear fringed by
coral-reefs, namely, the eastern coast, as seen in Freycinet's chart; and
CAJELI BAY, which is said by Horsburgh (volume ii., page 630) to be lined
by coral-reefs, that stretch out a little way, and have only a few feet
water on them. In several charts, portions of the islands forming the
AMBOINA GROUP are fringed by reefs; for instance, NOESSA, HARENCA, and
UCASTER, in Freycinet's charts. The above-mentioned islands have been
coloured red, although the evidence is not very satisfactory.--North of
Bourou the parallel line of the XULLA Isles extends: I have not been able
to find out anything about them, excepting that Horsburgh (volume ii., page
543) says that the northern shore is surrounded by a reef at the distance
of two or three miles; uncoloured.--MYSOL GROUP; the Kanary Islands are
said by Forrest ("Voyage," page 130) to be divided from each other by deep
straits, and are lined with coral-rocks; coloured red.--GUEBE, lying
between Waigiou and Gilolo, is engraved as if fringed; and it is said by
Freycinet, that all the soundings under five fathoms were on coral;
coloured red.--GILOLO. In a chart published by Dalrymple, the numerous
islands on the western, southern (BATCHIAN and the STRAIT OF PATIENTIA),
and eastern sides appear fringed by narrow reefs; these reefs, I suppose,
are of coral, for it is said in "Malte Brun" (volume xii., page 156), "Sur
les cotes (of Batchian) comme DANS LES PLUPART des iles de cet archipel, il
y a de rocs de medrepores d'une beaute et d'une variete infimies."
Forrest, also (page 50), says Seland, near Batchian, is a little island
with reefs of coral; coloured red.--MORTY Island (north of Gilolo).
Horsburgh (volume ii., page 506) says the northern coast is lined by reefs,
projecting one or two miles, and having no soundings close to them; I have
left it uncoloured, although, as in some former cases, it ought probably to
be pale blue.--CELEBES. The western and northern coasts appear in the
charts to be bold and without reefs. Near the extreme northern point,
however, an islet in the STRAITS OF LIMBE, and parts of the adjoining
shore, appear to be fringed: the east side of the bay of MANADO, has deep
water, and is fringed by sand and coral ("'Astrol.' Voyage," Hydrog. Part,
pages 453-4); this extreme point, therefore, I have coloured red.--Of the
islands leading from this point to Magindanao, I have not been able to find
any account, except of SERANGANI, which appears surrounded by narrow reefs;
and Forrest ("Voyage," page 164) speaks of coral on its shores; I have,
therefore, coloured this island red. To the eastward of this chain lie
several islands; of which I cannot find any account, except of KARKALANG,
which is said by Horsburgh (volume ii., page 504) to be lined by a
dangerous reef, projecting several miles from the northern shore; not
coloured.
ISLANDS NEAR TIMOR.
The account of the following islands is taken from Captain D. Kolff's
"Voyage," in 1825, translated by Mr. W. Earl, from the Dutch.--LETTE has
"reefs extending along shore at the distance of half a mile from the
land."--MOA has reefs on the S.W. part.--LAKOR has a reef lining its shore;
these islands are coloured red.--Still more eastward, LUAN has, differently
from the last-mentioned islands, an extensive reef; it is steep outside,
and within there is a depth of twelve feet; from these facts, it is
impossible to decide to which class this island belongs.--KISSA, off the
point of Timor, has its "shore fronted by a reef, steep too on the outer
side, over which small proahs can go at the time of high water;" coloured
red.--TIMOR; most of the points, and some considerable spaces of the
northern shore, are seen in Freycinet's chart to be fringed by coral-reefs;
and mention is made of them in the accompanying "Hydrog. Memoir;" coloured
red.--SAVU, S.E. of Timor, appears in Flinders' chart to be fringed; but I
have not coloured it, as I do not know that the reefs are of coral.--
SANDALWOOD Island has, according to Horsburgh (volume ii., page 607), a
reef on its southern shore, four miles distant from the land; as the
neighbouring sea is deep, and generally bold, this probably is a barrier-
reef, but I have not ventured to colour it.
N.W. COAST OF AUSTRALIA.
It appears, in Captain King's Sailing Directions ("Narrative of Survey,"
volume ii, pages 325-369), that there are many extensive coral-reefs
skirting, often at considerable distances, the N.W. shores, and
encompassing the small adjoining islets. Deep water, in no instance, is
represented in the charts between these reefs and the land; and, therefore,
they probably belong to the fringing class. But as they extend far into
the sea, which is generally shallow, even in places where the land seems to
be somewhat precipitous; I have not coloured them. Houtman's Abrolhos
(latitude 28 deg S. on west coast) have lately been surveyed by Captain
Wickham (as described in "Naut. Mag." 1841, page 511): they lie on the
edge of a steeply shelving bank, which extends about thirty miles seaward,
along the whole line of coast. The two southern reefs, or islands, enclose
a lagoon-like space of water, varying in depth from five to fifteen
fathoms, and in one spot with twenty-three fathoms. The greater part of
the island has been formed on their inland sides, by the accumulation of
fragments of coral; the seaward face consisting of nearly bare ledges of
rock. Some of the specimens, brought home by Captain Wickham, contained
fragments of marine shells, but others did not; and these closely resembled
a formation at King George's Sound, principally due to the action of the
wind on calcareous dust, which I shall describe in a forthcoming part.
From the extreme irregularity of these reefs with their lagoons, and from
their position on a bank, the usual depth of which is only thirty fathoms,
I have not ventured to class them with atolls, and hence have left them
uncoloured.--ROWLEY SHOALS. These lie some way from the N.W. coast of
Australia: according to Captain King ("Narrative of Survey," volume i.,
page 60), they are of coral-formation. They rise abruptly from the sea,
and Captain King had no bottom with 170 fathoms close to them. Three of
them are crescent-shaped; they are mentioned by Mr. Lyell, on the authority
of Captain King, with reference to the direction of their open sides. "A
third oval reef of the same group is entirely submerged" ("Principles of
Geology," book iii. chapter xviii.); coloured blue.--SCOTT'S REEFS, lying
north of Rowley Shoals, are briefly described by Captain Wickham ("Naut.
Mag." 1841, page 440): they appear to be of great size, of a circular
form, and "with smooth water within, forming probably a lagoon of great
extent." There is a break on the western side, where there probably is an
entrance: the water is very deep off these reefs; coloured blue.
Proceeding westward along the great volcanic chain of the East Indian
Archipelago, SOLOR STRAIT is represented in a chart published by Dalrymple
from a Dutch MS., as fringed; as are parts of FLORES, of ADENARA, and of
SOLOR. Horsburgh speaks of coral growing on these shores; and therefore I
have no doubt that the reefs are of coral, and accordingly have coloured
them red. We hear from Horsburgh (volume ii., page 602) that a coral-flat
bounds the shores of SAPY Bay. From the same authority it appears (page
610) that reefs fringe the island of TIMOR-YOUNG, on the N. shore of
Sumbawa; and, likewise (page 600), that BALLY town in LOMBOCK, is fronted
by a reef, stretching along the shore at a distance of a hundred fathoms,
with channels through it for boats; these places, therefore, have been
coloured red.--BALLY Island. In a Dutch MS. chart on a large scale of
Java, which was brought from that island by Dr. Horsfield, who had the
kindness to show it me at the India House, its western, northern, and
southern shores appear very regularly fringed by a reef (see also
Horsburgh, volume ii., page 593); and as coral is found abundantly there, I
have not the least doubt that the reef is of coral, and therefore have
coloured it red.
JAVA.
My information regarding the reefs of this great island is derived from the
chart just mentioned. The greater part of MADUARA is represented in it as
regularly fringed, and likewise portions of the coast of Java immediately
south of it. Dr. Horsfield informs me that coral is very abundant near
SOURABAYA. The islets and parts of the N. coast of Java, west of POINT
BUANG, or JAPARA, are fringed by reefs, said to be of coral. LUBECK, or
BAVIAN Islands, lying at some distance from the shore of Java, are
regularly fringed by coral-reefs. CARIMON JAVA appears equally so, though
it is not directly said that the reefs are of coral; there is a depth
between thirty and forty fathoms round these islands. Parts of the shores
of SUNDA STRAIT, where the water is from forty to eighty fathoms deep, and
the islets near BATAVIA appear in several charts to be fringed. In the
Dutch chart the southern shore, in the narrowest part of the island, is in
two places fringed by reefs of coral. West of SEGORROWODEE Bay, and the
extreme S.E. and E. portions are likewise fringed by coral-reefs; all the
above-mentioned places coloured red.
MACASSAR STRAIT.
The EAST COAST OF Borneo appears, in most parts, free from reefs, and where
they occur, as on the east coast of PAMAROONG, the sea is very shallow;
hence no part is coloured. In MACASSAR Strait itself, in about latitude 2
deg S., there are many small islands with coral-shoals projecting far from
them. There are also (old charts by Dalrymple) numerous little flats of
coral, not rising to the surface of the water, and shelving suddenly from
five fathoms to no bottom with fifty fathoms; they do not appear to have a
lagoon-like structure. There are similar coral-shoals a little farther
south; and in latitude 4 deg 55' there are two, which are engraved from
modern surveys, in a manner which might represent an annular reef with deep
water inside: Captain Moresby, however, who was formerly in this sea,
doubts this fact, so that I have left them uncoloured: at the same time I
may remark, that these two shoals make a nearer approach to the atoll-like
structure than any other within the E. Indian Archipelago. Southward of
these shoals there are other low islands and irregular coral-reefs; and in
the space of sea, north of the great volcanic chain, from Timor to Java, we
have also other islands, such as the POSTILLIONS, KALATOA, TOKAN-BESSEES,
etc., which are chiefly low, and are surrounded by very irregular and
distant reefs. From the imperfect charts I have seen, I have not been able
to decide whether they belong to the atoll or barrier-classes, or whether
they merely fringe submarine banks, and gently sloping land. In the Bay of
BONIN, between the two southern arms of Celebes, there are numerous coral-
reefs; but none of them seem to have an atoll-like structure. I have,
therefore, not coloured any of the islands in this part of the sea; I think
it, however, exceedingly probable that some of them ought to be blue. I
may add that there is a harbour on the S.E. coast of BOUTON which,
according to an old chart, is formed by a reef, parallel to the shore, with
deep water within; and in the "Voyage of the 'Coquille'," some neighbouring
islands are represented with reefs a good way distant, but I do not know
whether with deep water within. I have not thought the evidence sufficient
to permit me to colour them.
SUMATRA.
Commencing with the west coast and outlying islands, ENGANO Island is
represented in the published chart as surrounded by a narrow reef, and
Napier, in his "Sailing Directions," speaks of the reef being of coral
(also Horsburgh, volume ii., page 115); coloured red.--RAT Island (3 deg
51' S.) is surrounded by reefs of coral, partly dry at low water,
(Horsburgh, volume ii., page 96).--TRIESTE Island (4 deg 2' S.). The shore
is represented in a chart which I saw at the India House, as fringed in
such a manner, that I feel sure the fringe consists of coral; but as the
island is so low, that the sea sometimes flows quite over it (Dampier,
"Voyage," volume i., page 474), I have not coloured it.--PULO DOOA
(latitude 3 deg). In an old chart it is said there are chasms in the reefs
round the island, admitting boats to the watering-place, and that the
southern islet consists of a mass of sand and coral.--PULO PISANG;
Horsburgh (volume ii., page 86) says that the rocky coral-bank, which
stretches about forty yards from the shore, is steep to all round: in a
chart, also, which I have seen, the island is represented as regularly
fringed.--PULO MINTAO is lined with reefs on its west side (Horsburgh,
volume ii., page 107).--PULO BANIAK; the same authority (volume ii., page
105), speaking of a part, says it is faced with coral-rocks.--MINGUIN (3
deg 36' N.). A coral-reef fronts this place, and projects into the sea
nearly a quarter of a mile ("Notices of the Indian Arch." published at
Singapore, page 105).--PULO BRASSA (5 deg 46' N.). A reef surrounds it at
a cable's length (Horsburgh, volume ii., page 60). I have coloured all the
above-specified points red. I may here add, that both Horsburgh and Mr.
Moor (in the "Notices" just alluded to) frequently speak of the numerous
reefs and banks of coral on the west coast of Sumatra; but these nowhere
have the structure of a barrier-reef, and Marsden ("History of Sumatra")
states, that where the coast is flat, the fringing-reefs extend furthest
from it. The northern and southern points, and the greater part of the
east coast, are low, and faced with mud banks, and therefore without coral.
NICOBAR ISLANDS.
The chart represents the islands of this group as fringed by reefs. With
regard to GREAT NICOBAR, Captain Moresby informs me, that it is fringed by
reefs of coral, extending between two and three hundred yards from the
shore. The NORTHERN NICOBARS appear so regularly fringed in the published
charts, that I have no doubt the reefs are of coral. This group,
therefore, is coloured red.
ANDAMAN ISLANDS.
From an examination of the MS. chart, on a large scale, of this island, by
Captain Arch. Blair, in the Admiralty, several portions of the coast appear
fringed; and as Horsburgh speaks of coral-reefs being numerous in the
vicinity of these islands, I should have coloured them red, had not some
expressions in a paper in the "Asiatic Researches" (volume iv., page 402)
led me to doubt the existence of reefs; uncoloured.
The coast of MALACCA, TENASSERIM and the coasts northward, appear in the
greater part to be low and muddy: where reefs occur, as in parts of
MALACCA STRAITS, and near SINGAPORE, they are of the fringing kind; but the
water is so shoal, that I have not coloured them. In the sea, however,
between Malacca and the west coast of Borneo, where there is a greater
depth from forty to fifty fathoms, I have coloured red some of the groups,
which are regularly fringed. The northern NATUNAS and the ANAMBAS Islands
are represented in the charts on a large scale, published in the "Atlas of
the Voyage of the 'Favourite'," as fringed by reefs of coral, with very
shoal water within them.--TUMBELAN and BUNOA Islands (1 deg N.) are
represented in the English charts as surrounded by a very regular fringe.--
ST. BARBES (0 deg 15' N.) is said by Horsburgh (volume ii., page 279) to be
fronted by a reef, over which boats can land only at high water.--The shore
of BORNEO at TUNJONG APEE is also fronted by a reef, extending not far from
the land (Horsburgh, volume ii., page 468). These places I have coloured
red; although with some hesitation, as the water is shallow. I might
perhaps have added PULO LEAT, in Gaspar Strait, LUCEPARA, and CARIMATA; but
as the sea is confined and shallow, and the reefs not very regular, I have
left them uncoloured.
The water shoals gradually towards the whole west coast of BORNEO: I
cannot make out that it has any reefs of coral. The islands, however, off
the northern extremity, and near the S.W. end of PALAWAN, are fringed by
very distant coral-reefs; thus the reefs in the case of BALABAC are no less
than five miles from the land; but the sea, in the whole of this district,
is so shallow, that the reefs might be expected to extend very far from the
land. I have not, therefore, thought myself authorised to colour them.
The N.E. point of Borneo, where the water is very shoal, is connected with
Magindanao by a chain of islands called the SOOLOO ARCHIPELAGO, about which
I have been able to obtain very little information; PANGOOTARAN, although
ten miles long, entirely consists of a bed of coral-rock ("Notices of E.
Indian Arch." page 58): I believe from Horsburgh that the island is low;
not coloured.--TAHOW BANK, in some old charts, appears like a submerged
atoll; not coloured. Forrest ("Voyage," page 21) states that one of the
islands near Sooloo is surrounded by coral-rocks; but there is no distant
reef. Near the S. end of BASSELAN, some of the islets in the chart
accompanying Forrest's "Voyage," appear fringed with reefs; hence I have
coloured, though unwillingly, parts of the Sooloo group red. The sea
between Sooloo and Palawan, near the shoal coast of Borneo, is interspersed
with irregular reefs and shoal patches; not coloured: but in the northern
part of this sea, there are two low islets, CAGAYANES and CAVILLI,
surrounded by extensive coral-reefs; the breakers round the latter
(Horsburgh, volume ii., page 513) extend five or six miles from a sandbank,
which forms the only dry part; these breakers are steep to outside; there
appears to be an opening through them on one side, with four or five
fathoms within: from this description, I strongly suspect that Cavilli
ought to be considered an atoll; but, as I have not seen any chart of it,
on even a moderately large scale, I have not coloured it. The islets off
the northern end of PALAWAN, are in the same case as those off the southern
end, namely they are fringed by reefs, some way distant from the shore, but
the water is exceedingly shallow; uncoloured. The western shore of Palawan
will be treated of under the head of China Sea.
PHILIPPINE ARCHIPELAGO.
A chart on a large scale of APPOO SHOAL, which lies near the S.E. coast of
Mindoro, has been executed by Captain D. Ross: it appears atoll-formed,
but with rather an irregular outline; its diameter is about ten miles;
there are two well-defined passages leading into the interior lagoon, which
appears open; close outside the reef all round, there is no bottom with
seventy fathoms; coloured blue.--MINDORO: the N.W. coast is represented in
several charts, as fringed by a reef, and LUBAN Island is said, by
Horsburgh (volume ii., page 436), to be "lined by a reef."--LUZON: Mr.
Cuming, who has lately investigated with so much success the Natural
History of the Philippines, informs me, that about three miles of the shore
north of Point St. Jago, is fringed by a reef; as are (Horsburgh, volume
ii., page 437) the Three Friars off Silanguin Bay. Between Point Capones
and Playa Honda, the coast is "lined by a coral-reef, stretching out nearly
a mile in some places," (Horsburgh); and Mr. Cuming visited some fringing-
reefs on parts of this coast, namely, near Puebla, Iba, and Mansinglor. In
the neighbourhood of Solon-solon Bay, the shore is lined (Horsburgh ii.,
page 439) by coral-reefs, stretching out a great way: there are also reefs
about the islets off Solamague; and as I am informed by Mr. Cuming, near
St. Catalina, and a little north of it. The same gentleman informs me
there are reefs on the S.E. point of this island in front of Samar,
extending from Malalabon to Bulusan. These appear to be the principal
fringing-reefs on the coasts of Luzon; and they have all been coloured red.
Mr. Cuming informs me that none of them have deep water within; although it
appears from Horsburgh that some few extend to a considerable distance from
the shore. Within the Philippine Archipelago, the shores of the islands do
not appear to be commonly fringed, with the exception of the S. shore of
MASBATE, and nearly the whole of BOHOL; which are both coloured red. On
the S. shore of MAGINDANAO, Bunwoot Island is surrounded (according to
Forrest, "Voyage," page 253), by a coral-reef, which in the chart appears
one of the fringing class. With respect to the eastern coasts of the whole
Archipelago, I have not been able to obtain any account.
BABUYAN ISLANDS.
Horsburgh says (volume ii., page 442), coral-reefs line the shores of the
harbour in Fuga; and the charts show there are other reefs about these
islands. Camiguin has its shore in parts lined by coral-rock (Horsburgh,
page 443); about a mile off shore there is between thirty and thirty-five
fathoms. The plan of Port San Pio Quinto shows that its shores are fringed
with coral; coloured red.--BASHEE Islands: Horsburgh, speaking of the
southern part of the group (volume ii., page 445) says the shores of both
islands are fortified by a reef, and through some of the gaps in it, the
natives can pass in their boats in fine weather; the bottom near the land
is coral-rock. From the published charts, it is evident that several of
these islands are most regularly fringed; coloured red. The northern
islands are left uncoloured, as I have been unable to find any account of
them.--FORMOSA. The shores, especially the western one, seem chiefly
composed of mud and sand, and I cannot make out that they are anywhere
lined by reefs; except in a harbour (Horsburgh, volume ii., page 449) at
the extreme northern point: hence, of course, the whole of this island is
left uncoloured. The small adjoining islands are in the same case.--
PATCHOW, or MADJIKO-SIMA GROUPS. PATCHUSON has been described by Captain
Broughton ("Voy. to the N. Pacific," page 191); he says, the boats, with
some difficulty, found a passage through the coral-reefs, which extend
along the coast, nearly half a mile off it. The boats were well sheltered
within the reef; but it does not appear that the water is deep there.
Outside the reef the depth is very irregular, varying from five to fifty
fathoms; the form of the land is not very abrupt; coloured red.--TAYPIN-
SAN; from the description given (page 195) by the same author, it appears
that a very irregular reef extends, to the distance of several miles, from
the southern island; but whether it encircles a space of deep water is not
evident; nor, indeed, whether these outlying reefs are connected with those
more immediately adjoining the land; left uncoloured. I may here just add
that the shore of KUMI (lying west of Patchow), has a narrow reef attached
to it in the plan of it, in La Peyrouse's "Atlas;" but it does not appear
in the account of the voyage that it is of coral; uncoloured.--LOO CHOO.
The greater part of the coast of this moderately hilly island, is skirted
by reefs, which do not extend far from the shore, and which do not leave a
channel of deep water within them, as may be seen in the charts
accompanying Captain B. Hall's voyage to Loo Choo (see also remarks in
Appendix, pages xxi. and xxv.). There are, however, some ports with deep
water, formed by reefs in front of valleys, in the same manner as happens
at Mauritius. Captain Beechey, in a letter to me, compares these reefs
with those encircling the Society Islands; but there appears to me a marked
difference between them, in the less distance at which the Loo Choo reefs
lie from the land with relation to the probable submarine inclination, and
in the absence of an interior deep water-moat or channel, parallel to the
land. Hence, I have classed these reefs with fringing-reefs, and coloured
them red.--PESCADORES (west of Formosa). Dampier (volume i., page 416),
has compared the appearance of the land to the southern parts of England.
The islands are interlaced with coral-reefs; but as the water is very
shoal, and as spits of sand and gravel (Horsburgh, volume ii., page 450)
extend far out from them, it is impossible to draw any inferences regarding
the nature of the reefs.
CHINA SEA.--Proceeding from north to south, we first meet the PRATAS SHOAL
(latitude 20 deg N.) which, according to Horsburgh (volume ii., page 335),
is composed of coral, is of a circular form, and has a low islet on it.
The reef is on a level with the water's edge, and when the sea runs high,
there are breakers mostly all round, "but the water within seems pretty
deep in some places; although steep-to in most parts outside, there appear
to be several parts where a ship might find anchorage outside the
breakers;" coloured blue.--The PARACELLS have been accurately surveyed by
Captain D. Ross, and charts on a large scale published: but few low islets
have been formed on these shoals, and this seems to be a general
circumstance in the China Sea; the sea close outside the reefs is very
deep; several of them have a lagoon-like structure; or separate islets
(PRATTLE, ROBERT, DRUMMOND, etc.) are so arranged round a moderately
shallow space, as to appear as if they had once formed one large atoll.--
BOMBAY SHOAL (one of the Paracells) has the form of an annular reef, and is
"apparently deep within;" it seems to have an entrance (Horsburgh, volume
ii., page 332) on its west side; it is very steep outside.--DISCOVERY
SHOAL, also is of an oval form, with a lagoon-like space within, and three
openings leading into it, in which there is a depth from two to twenty
fathoms. Outside, at the distance (Horsburgh, volume ii., page 333) of
only twenty yards from the reef, soundings could not be obtained. The
Paracells are coloured blue.--MACCLESFIELD BANK: this is a coral-bank of
great size, lying east of the Paracells; some parts of the bank are level,
with a sandy bottom, but, generally, the depth is very irregular. It is
intersected by deep cuts or channels. I am not able to perceive in the
published charts (its limits, however, are not very accurately known)
whether the central part is deeper, which I suspect is the case, as in the
Great Chagos Bank, in the Indian Ocean; not coloured.--SCARBOROUGH SHOAL:
this coral-shoal is engraved with a double row of crosses, forming a
circle, as if there was deep water within the reef: close outside there
was no bottom, with a hundred fathoms; coloured blue.--The sea off the west
coast of Palawan and the northern part of Borneo is strewed with shoals:
SWALLOW SHOAL, according to Horsburgh (volume ii., page 431) "is formed,
LIKE MOST of the shoals hereabouts, of a belt of coral-rocks, "with a basin
of deep water within."--HALF-MOON SHOAL has a similar structure; Captain D.
Ross describes it, as a narrow belt of coral-rock, "with a basin of deep
water in the centre," and deep sea close outside.--BOMBAY SHOAL appears
(Horsburgh, volume ii., page 432) "to be a basin of smooth water surrounded
by breakers." These three shoals I have coloured blue.--The PARAQUAS
SHOALS are of a circular form, with deep gaps running through them; not
coloured.--A bank gradually shoaling to the depth of thirty fathoms,
extends to a distance of about twenty miles from the northern part of
BORNEO, and to thirty miles from the northern part of PALAWAN. Near the
land this bank appears tolerably free from danger, but a little further out
it is thickly studded with coral-shoals, which do not generally rise quite
to the surface; some of them are very steep-to, and others have a fringe of
shoal-water round them. I should have thought that these shoals had level
surfaces, had it not been for the statement made by Horsburgh "that most of
the shoals hereabouts are formed of a belt of coral." But, perhaps that
expression was more particularly applied to the shoals further in the
offing. If these reefs of coral have a lagoon-like structure, they should
have been coloured blue, and they would have formed an imperfect barrier in
front of Palawan and the northern part of Borneo. But, as the water is not
very deep, these reefs may have grown up from inequalities on the bank: I
have not coloured them.--The coast of CHINA, TONQUIN, and COCHIN-CHINA,
forming the western boundary of the China Sea, appear to be without reefs:
with regard to the two last-mentioned coasts, I speak after examining the
charts on a large scale in the "Atlas of the Voyage of the 'Favourite'."
INDIAN OCEAN.
SOUTH KEELING atoll has been specially described. Nine miles north of it
lies North Keeling, a very small atoll, surveyed by the "Beagle," the
lagoon of which is dry at low water.--CHRISTMAS Island, lying to the east,
is a high island, without, as I have been informed by a person who passed
it, any reefs at all.--CEYLON: a space about eighty miles in length of the
south-western and southern shores of these islands has been described by
Mr. Twynam ("Naut. Mag." 1836, pages 365 and 518); parts of this space
appear to be very regularly fringed by coral-reefs, which extend from a
quarter to half a mile from the shore. These reefs are in places breached,
and afford safe anchorage for the small trading craft. Outside, the sea
gradually deepens; there is forty fathoms about six miles off shore: this
part I have coloured red. In the published charts of Ceylon there appear
to be fringing-reefs in several parts of the south-eastern shores, which I
have also coloured red.--At Venloos Bay the shore is likewise fringed.
North of Trincomalee there are also reefs of the same kind. The sea off
the northern part of Ceylon is exceedingly shallow; and therefore I have
not coloured the reefs which fringe portions of its shores, and the
adjoining islets, as well as the Indian promontory of MADURA.
CHAGOS, MALDIVA, AND LACCADIVE ARCHIPELAGOES.
These three great groups which have already been often noticed, are now
well-known from the admirable surveys of Captain Moresby and Lieutenant
Powell. The published charts, which are worthy of the most attentive
examination, at once show that the CHAGOS and MALDIVA groups are entirely
formed of great atolls, or lagoon-formed reefs, surmounted by islets. In
the LACCADIVE group, this structure is less evident; the islets are low,
not exceeding the usual height of coral-formations (see Lieutenant Wood's
account, "Geographical Journal", volume vi., page 29), and most of the
reefs are circular, as may be seen in the published charts; and within
several of them, as I am informed by Captain Moresby, there is deepish
water; these, therefore, have been coloured blue. Directly north, and
almost forming part of this group, there is a long, narrow, slightly curved
bank, rising out of the depths of the ocean, composed of sand, shells, and
decayed coral, with from twenty-three to thirty fathoms on it. I have no
doubt that it has had the same origin with the other Laccadive banks; but
as it does not deepen towards the centre I have not coloured it. I might
have referred to other authorities regarding these three archipelagoes; but
after the publication of the charts by Captain Moresby, to whose personal
kindness in giving me much information I am exceedingly indebted, it would
have been superfluous.
SAHIA DE MALHA bank consists of a series of narrow banks, with from eight
to sixteen fathoms on them; they are arranged in a semicircular manner,
round a space about forty fathoms deep, which slopes on the S.E. quarter to
unfathomable depths; they are steep-to on both sides, but more especially
on the ocean-side. Hence this bank closely resembles in structure, and I
may add from Captain Moresby's information in composition, the Pitt's Bank
in the Chagos group; and the Pitt's Bank, must, after what has been shown
of the Great Chagos Bank, be considered as a sunken, half-destroyed atoll;
hence coloured blue.--CARGADOS CARAJOS BANK. Its southern portion consists
of a large, curved, coral-shoal, with some low islets on its eastern edge,
and likewise some on the western side, between which there is a depth of
about twelve fathoms. Northward, a great bank extends. I cannot (probably
owing to the want of perfect charts) refer this reef and bank to any
class;--therefore not coloured.--ILE DE SABLE is a little island, lying
west of C. Carajos, only some toises in height ("Voyage of the
'Favourite'," volume i., page 130); it is surrounded by reefs; but its
structure is unintelligible to me. There are some small banks north of it,
of which I can find no clear account.--MAURITIUS. The reefs round this
island have been described in the chapter on fringing-reefs; coloured red.
--RODRIGUEZ. The coral-reefs here are exceedingly extensive; in one part
they project even five miles from the shore. As far as I can make out,
there is no deep-water moat within them; and the sea outside does not
deepen very suddenly. The outline, however, of the land appears to be
("Life of Sir J. Makintosh," volume ii., page 165) hilly and rugged. I am
unable to decide whether these reefs belong to the barrier class; as seems
probable from their great extension, or to the fringing class; uncoloured.
--BOURBON. The greater part of the shores of this island are without
reefs; but Captain Carmichael (Hooker's "Bot. Misc.") states that a
portion, fifteen miles in length, on the S.E. side, is imperfectly fringed
with coral reefs: I have not thought this sufficient to colour the island.
SEYCHELLES.
The rocky islands of primary formation, composing this group, rise from a
very extensive and tolerably level bank, having a depth between twenty and
forty fathoms. In Captain Owen's chart, and in that in the "Atlas of the
Voyage of the 'Favourite'," it appears that the east side of MAHE and the
adjoining islands of ST. ANNE and CERF, are regularly fringed by coral-reefs.
A portion of the S.E. part of CURIEUSE Island, the N., and part of
the S.W. shore of PRASLIN Island, and the whole west side of DIGUE Island,
appear fringed. From a MS. account of these islands by Captain F. Moresby,
in the Admiralty, it appears that SILHOUETTE is also fringed; he states
that all these islands are formed of granite and quartz, that they rise
abruptly from the sea, and that "coral-reefs have grown round them, and
project for some distance." Dr. Allan, of Forres, who visited these
islands, informs me that there is no deep water between the reefs and the
shore. The above specified points have been coloured red. AMIRANTES
Islands: The small islands of this neighbouring group, according to the
MS. account of them by Captain F. Moresby, are situated on an extensive
bank; they consist of the debris of corals and shells; are only about
twenty feet in height, and are environed by reefs, some attached to the
shore, and some rather distant from it.--I have taken great pains to
procure plans and information regarding the several islands lying between
S.E. and S.W. of the Amirantes, and the Seychelles; relying chiefly on
Captain F. Moresby and Dr. Allan, it appears that the greater number,
namely--PLATTE, ALPHONSE, COETIVI, GALEGA, PROVIDENCE, ST. PIERRE, ASTOVA,
ASSOMPTION, and GLORIOSO, are low, formed of sand or coral-rock, and
irregularly shaped; they are situated on very extensive banks, and are
connected with great coral-reefs. Galega is said by Dr. Allan, to be
rather higher than the other islands; and St. Pierre is described by
Captain F. Moresby, as being cavernous throughout, and as not consisting of
either limestone or granite. These islands, as well as the Amirantes,
certainly are not atoll-formed, and they differ as a group from every other
group with which I am acquainted; I have not coloured them; but probably
the reefs belong to the fringing class. Their formation is attributed,
both by Dr. Allan and Captain F. Moresby, to the action of the currents,
here exceedingly violent, on banks, which no doubt have had an independent
geological origin. They resemble in many respects some islands and banks
in the West Indies, which owe their origin to a similar agency, in
conjunction with an elevation of the entire area. In close vicinity to the
several islands, there are three others of an apparently different nature:
first, JUAN DE NOVA, which appears from some plans and accounts to be an
atoll; but from others does not appear to be so; not coloured. Secondly
COSMOLEDO; "this group consists of a ring of coral, ten leagues in
circumference, and a quarter of a mile broad in some places, enclosing a
magnificent lagoon, into which there did not appear a single opening"
(Horsburgh, volume i., page 151); coloured blue. Thirdly, ALDABRA; it
consists of three islets, about twenty-five feet in height, with red cliffs
(Horsburgh, volume i., page 176) surrounding a very shallow basin or
lagoon. The sea is profoundly deep close to the shore. Viewing this
island in a chart, it would be thought an atoll; but the foregoing
description shows that there is something different in its nature; Dr.
Allan also states that it is cavernous, and that the coral-rock has a
vitrified appearance. Is it an upheaved atoll, or the crater of a
volcano?--uncoloured.
COMORO GROUP.
MAYOTTA, according to Horsburgh (volume i., page 216, 4th edition), is
completely surrounded by a reef, which runs at the distance of three, four,
and in some places even five miles from the land; in an old chart,
published by Dalrymple, a depth in many places of thirty-six and thirty-eight
fathoms is laid down within the reef. In the same chart, the space
of open water within the reef in some parts is even more than three miles
wide: the land is bold and peaked; this island, therefore, is encircled by
a well-characterised barrier-reef, and is coloured pale blue.--JOHANNA;
Horsburgh says (volume I. page 217) this island from the N.W. to the S.W.
point, is bounded by a reef, at the distance of two miles from the shore;
in some parts, however, the reef must be attached, since Lieutenant Boteler
("Narr." volume i., page 161) describes a passage through it, within which
there is room only for a few boats. Its height, as I am informed by Dr.
Allan, is about 3,500 feet; it is very precipitous, and is composed of
granite, greenstone, and quartz; coloured blue.--MOHILLA; on the S. side of
this island there is anchorage, in from thirty to forty-five fathoms,
between a reef and the shore (Horsburgh, volume i., page 214); in Captain
Owen's chart of Madagascar, this island is represented as encircled;
coloured blue.--GREAT COMORO Island is, as I am informed by Dr. Allan,
about 8,000 feet high, and apparently volcanic; it is not regularly
encircled; but reefs of various shapes and dimensions, jut out from every
headland on the W., S., and S.E. coasts, inside of which reefs there are
channels, often parallel with the shore, with deep water. On the
north-western coasts the reefs appear attached to the shores. The land near
the coast is in some places bold, but generally speaking it is flat;
Horsburgh says (volume i., page 214) the water is profoundly deep close to
the SHORE, from which expression I presume some parts are without reefs.
From this description I apprehend the reef belongs to the barrier class;
but I have not coloured it, as most of the charts which I have seen,
represent the reefs round it as very much less extensive than round the
other islands in the group.
MADAGASCAR.
My information is chiefly derived from the published charts by Captain
Owen, and the accounts given by him and by Lieutenant Boteler. Commencing
at the S.W. extremity of the island; towards the northern part of the STAR
BANK (in latitude 25 deg S.) the coast for ten miles is fringed by a reef;
coloured red. The shore immediately S. of ST. AUGUSTINE'S BAY appears
fringed; but TULLEAR Harbour, directly N. of it, is formed by a narrow reef
ten miles long, extending parallel to the shore, with from four to ten
fathoms within it. If this reef had been more extensive, it must have been
classed as a barrier-reef; but as the line of coast falls inwards here, a
submarine bank perhaps extends parallel to the shore, which has offered a
foundation for the growth of the coral; I have left this part uncoloured.
From latitude 22 deg 16' to 21 deg 37', the shore is fringed by coral-reefs
(see Lieutenant Boteler's "Narrative," volume ii., page 106), less than a
mile in width, and with shallow water within. There are outlying
coral-shoals in several parts of the offing, with about ten fathoms between
them and the shore, and the depth of the sea one mile and a half seaward, is
about thirty fathoms. The part above specified is engraved on a large
scale; and as in the charts on rather a smaller scale the same fringe of
reef extends as far as latitude 33 deg 15'; I have coloured the whole of
this part of the coast red. The islands of JUAN DE NOVA (in latitude 17
deg S.) appear in the charts on a large scale to be fringed, but I have not
been able to ascertain whether the reefs are of coral; uncoloured. The
main part of the west coast appears to be low, with outlying sandbanks,
which, Lieutenant Boteler (volume ii., page 106) says, "are faced on the
edge of deep water by a line of sharp-pointed coral-rocks." Nevertheless I
have not coloured this part, as I cannot make out by the charts that the
coast itself is fringed. The headlands of NARRENDA and PASSANDAVA Bays (14
deg 40') and the islands in front of RADAMA HARBOUR are represented in the
plans as regularly fringed, and have accordingly been coloured red. With
respect to the EAST COAST OF MADAGASCAR, Dr. Allan informs me in a letter,
that the whole line of coast, from TAMATAVE, in 18 deg 12', to C. AMBER, at
the extreme northern point of the island, is bordered by coral-reefs. The
land is low, uneven, and gradually rising from the coast. From Captain
Owen's charts, also, the existence of these reefs, which evidently belong
to the fringing class, on some parts, namely N. of BRITISH SOUND, and near
NGONCY, of the above line of coast might have been inferred. Lieutenant
Boteler (volume i., page 155) speaks of "the reef surrounding the island of
ST. MARY'S at a small distance from the shore." In a previous chapter I
have described, from the information of Dr. Allan, the manner in which the
reefs extend in N.E. lines from the headlands on this coast, thus sometimes
forming rather deep channels within them, this seems caused by the action
of the currents, and the reefs spring up from the submarine prolongations
of the sandy headlands. The above specified portion of the coast is
coloured red. The remaining S.E. portions do not appear in any published
chart to possess reefs of any kind; and the Rev. W. Ellis, whose means of
information regarding this side of Madagascar have been extensive, informs
me he believes there are none.
EAST COAST OF AFRICA.
Proceeding from the northern part, the coast appears, for a considerable
space, without reefs. My information, I may here observe, is derived from
the survey by Captain Owen, together with his narrative; and that by
Lieutenant Boteler. At MUKDEESHA (10 deg 1' N.) there is a coral-reef
extending four or five miles along the shore (Owen's "Narr." volume i, page
357) which in the chart lies at the distance of a quarter of a mile from
the shore, and has within it from six to ten feet water: this then is a
fringing-reef, and is coloured red. From JUBA, a little S. of the equator,
to LAMOO (in 2 deg 20' S.) "the coast and islands are formed of madrepore"
(Owen's "Narrative," volume i., page 363). The chart of this part
(entitled DUNDAS Islands), presents an extraordinary appearance; the coast
of the mainland is quite straight and it is fronted at the average distance
of two miles, by exceedingly narrow, straight islets, fringed with reefs.
Within the chain of islets, there are extensive tidal flats and muddy bays,
into which many rivers enter; the depths of these spaces varies from one to
four fathoms--the latter depth not being common, and about twelve feet the
average. Outside the chain of islets, the sea, at the distance of a mile,
varies in depth from eight to fifteen fathoms. Lieutenant Boteler ("Narr."
volume i., page 369) describes the muddy bay of PATTA, which seems to
resemble other parts of this coast, as fronted by small, narrow, level
islets formed of decomposing coral, the margin of which is seldom of
greater height than twelve feet, overhanging the rocky surface from which
the islets rise. Knowing that the islets are formed of coral, it is, I
think, scarcely possible to view the coast, and not at once conclude that
we here see a fringing-reef, which has been upraised a few feet: the
unusual depth of from two to four fathoms within some of these islets, is
probably due to muddy rivers having prevented the growth of coral near the
shore. There is, however, one difficulty on this view, namely, that before
the elevation took place, which converted the reef into a chain of islets,
the water must apparently have been still deeper; on the other hand it may
be supposed that the formation of a nearly perfect barrier in front, of so
large an extent of coast, would cause the currents (especially in front of
the rivers), to deepen their muddy beds. When describing in the chapter on
fringing-reefs, those of Mauritius, I have given my reasons for believing
that the shoal spaces within reefs of this kind, must, in many instances,
have been deepened. However this may be, as several parts of this line of
coast are undoubtedly fringed by living reefs, I have coloured it red.--
MALEENDA (3 deg 20' S.). In the plan of the harbour, the south headland
appears fringed; and in Owen's chart on a larger scale, the reefs are seen
to extend nearly thirty miles southward; coloured red.--MOMBAS (4 deg 5'
S.). The island which forms the harbour, "is surrounded by cliffs of
madrepore, capable of being rendered almost impregnable" (Owen's "Narr."
volume i., page 412). The shore of the mainland N. and S. of the harbour,
is most regularly fringed by a coral-reef at a distance from half a mile to
one mile and a quarter from the land; within the reef the depth is from
nine to fifteen feet; outside the reef the depth at rather less than half a
mile is thirty fathoms. From the charts it appears that a space about
thirty-six miles in length, is here fringed; coloured red.--PEMBA (5 deg
S.) is an island of coral-formation, level, and about two hundred feet in
height (Owen's "Narr." volume i., page 425); it is thirty-five miles long,
and is separated from the mainland by a deep sea. The outer coast is
represented in the chart as regularly fringed; coloured red. The mainland
in front of Pemba is likewise fringed; but there also appear to be some
outlying reefs with deep water between them and the shore. I do not
understand their structure, either from the charts or the description,
therefore have not coloured them.--ZANZIBAR resembles Pemba in most
respects; its southern half on the western side and the neighbouring islets
are fringed; coloured red. On the mainland, a little S. of Zanzibar, there
are some banks parallel to the coast, which I should have thought had been
formed of coral, had it not been said (Boteler's "Narr." volume ii., page
39) that they were composed of sand; not coloured.--LATHAM'S BANK is a
small island, fringed by coral-reefs; but being only ten feet high, it has
not been coloured.--MONFEEA is an island of the same character as Pemba;
its outer shore is fringed, and its southern extremity is connected with
Keelwa Point on the mainland by a chain of islands fringed by reefs;
coloured red. The four last-mentioned islands resemble in many respects
some of the islands in the Red Sea, which will presently be described.--
KEELWA. In a plan of the shore, a space of twenty miles N. and S. of this
place is fringed by reefs, apparently of coral: these reefs are prolonged
still further southward in Owen's general chart. The coast in the plans of
the rivers LINDY and MONGHOW (9 deg 59' and 10 deg 7' S.) has the same
structure; coloured red.--QUERIMBA Islands (from 10 deg 40' to 13 deg S.).
A chart on a large scale is given of these islands; they are low, and of
coral-formation (Boteler's "Narr." volume ii., page 54); and generally have
extensive reefs projecting from them which are dry at low water, and which
on the outside rise abruptly from a deep sea: on their insides they are
separated from the continent by a channel, or rather a succession of bays,
with an average depth of ten fathoms. The small headlands on the continent
also have coral-banks attached to them; and the Querimba islands and banks
are placed on the lines of prolongation of these headlands, and are
separated from them by very shallow channels. It is evident that whatever
cause, whether the drifting of sediment or subterranean movements, produced
the headlands, likewise produced, as might have been expected, submarine
prolongations to them; and these towards their outer extremities, have
since afforded a favourable basis for the growth of coral-reefs, and
subsequently for the formation of islets. As these reefs clearly belong to
the fringing class, the Querimba islands have been coloured red.--MONABILA
(13 deg 32' S.). In the plan of this harbour, the headlands outside are
fringed by reefs apparently of coral; coloured red.--MOZAMBIQUE (150 deg
S.) The outer part of the island on which the city is built, and the
neighbouring islands, are fringed by coral-reefs; coloured red. From the
description given in Owen's "Narr." (volume i., page 162), the shore from
MOZAMBIQUE to DELAGOA BAY appears to be low and sandy; many of the shoals
and islets off this line of coast are of coral-formation; but from their
small size and lowness, it is not possible, from the charts, to know
whether they are truly fringed. Hence this portion of coast is left
uncoloured, as are likewise those parts more northward, of which no mention
has been made in the foregoing pages from the want of information.
PERSIAN GULF.
From the charts lately published on a large scale by the East India
Company, it appears that several parts, especially the southern shores of
this gulf, are fringed by coral-reefs; but as the water is very shallow,
and as there are numerous sandbanks, which are difficult to distinguish on
the chart from reefs, I have not coloured the upper part red. Towards the
mouth, however, where the water is rather deeper, the islands of ORMUZ and
LARRACK, appear so regularly fringed, that I have coloured them red. There
are certainly no atolls in the Persian Gulf. The shores of IMMAUM, and of
the promontory forming the southern headland of the Persian Gulf, seem to
be without reefs. The whole S.W. part (except one or two small patches) of
ARABIA FELIX, and the shores of SOCOTRA appear from the charts and memoir
of Captain Haines ("Geographical Journal," 1839, page 125) to be without
any reefs. I believe there are no extensive coral-reefs on any part of the
coasts of INDIA, except on the low promontory of MADURA (as already
mentioned) in front of Ceylon.
RED SEA.
My information is chiefly derived from the admirable charts published by
the East India Company in 1836, from personal communication with Captain
Moresby, one of the surveyors, and from the excellent memoir, "Uber die
Natur der Corallen-Banken des Rothen Meeres," by Ehrenberg. The plains
immediately bordering the Red Sea seem chiefly to consist of a sedimentary
formation of the newer tertiary period. The shore is, with the exception
of a few parts, fringed by coral-reefs. The water is generally profoundly
deep close to the shore; but this fact, which has attracted the attention
of most voyagers, seems to have no necessary connection with the presence
of reefs; for Captain Moresby particularly observed to me, that, in
latitude 24 deg 10' on the eastern side, there is a piece of coast, with
very deep water close to it, without any reefs, but not differing in other
respects from the usual nature of the coast-line. The most remarkable
feature in the Red Sea is the chain of submerged banks, reefs, and islands,
lying some way from the shore, chiefly on the eastern side; the space
within being deep enough to admit a safe navigation in small vessels. The
banks are generally of an oval form, and some miles in width; but some of
them are very long in proportion to their width. Captain Moresby informs
me that any one, who had not made actual plans of them, would be apt to
think that they were much more elongated than they really are. Many of
them rise to the surface, but the greater number lie from five to thirty
fathoms beneath it, with irregular soundings on them. They consist of sand
and living coral; coral on most of them, according to Captain Moresby,
covering the greater part of their surface. They extend parallel to the
shore, and they are not unfrequently connected in their middle parts by
short transverse banks with the mainland. The sea is generally profoundly
deep quite close to them, as it is near most parts of the coast of the
mainland; but this is not universally the case, for between latitude 15 deg
and 17 deg the water deepens quite gradually from the banks, both on the
eastern and western shores, towards the middle of the sea. Islands in many
parts arise from these banks; they are low, flat-topped, and consist of the
same horizontally stratified formation with that forming the plain-like
margin of the mainland. Some of the smaller and lower islands consist of
mere sand. Captain Moresby informs me, that small masses of rock, the
remnants of islands, are left on many banks where there is now no dry land.
Ehrenberg also asserts that most of the islets, even the lowest, have a
flat abraded basis, composed of the same tertiary formation: he believes
that as soon as the surf wears down the protuberant parts of a bank, just
beneath the level of the sea, the surface becomes protected from further
abrasion by the growth of coral, and he thus accounts for the existence of
so many banks standing on a level with the surface of this sea. It appears
that most of the islands are certainly decreasing in size.
The form of the banks and islands is most singular in the part just
referred to, namely, from latitude 15 deg to 17 deg, where the sea deepens
quite gradually: the DHALAC group, on the western coast, is surrounded by
an intricate archipelago of islets and shoals; the main island is very
irregularly shaped, and it includes a bay seven miles long, by four across,
in which no bottom was found with 252 feet: there is only one entrance
into this bay, half a mile wide, and with an island in front of it. The
submerged banks on the eastern coast, within the same latitudes, round
FARSAN Island, are, likewise, penetrated by many narrow creeks of deep
water; one is twelve miles long, in the form of a hatchet, in which, close
to its broad upper end, soundings were not struck with 360 feet, and its
entrance is only half a mile wide: in another creek of the same nature,
but even with a more irregular outline, there was no bottom with 480 feet.
The island of Farsan, itself, has as singular a form as any of its
surrounding banks. The bottom of the sea round the Dhalac and Farsan
Islands consists chiefly of sand and agglutinated fragments, but, in the
deep and narrow creeks, it consists of mud; the islands themselves consist
of thin, horizontally stratified, modern tertiary beds, containing but
little broken coral (Ruppell, "Reise in Abyssinie," Band. i., S. 247.),
their shores are fringed by living coral-reefs.
From the account given by Ruppell (Ibid., S. 245.) of the manner in which
Dhalac has been rent by fissures, the opposite sides of which have been
unequally elevated (in one instance to the amount of fifty feet), it seems
probable that its irregular form, as well as probably that of Farsan, may
have been partly caused by unequal elevations; but, considering the general
form of the banks, and of the deep-water creeks, together with the
composition of the land, I think their configuration is more probably due
in great part to strong currents having drifted sediment over an uneven
bottom: it is almost certain that their form cannot be attributed to the
growth of coral. Whatever may have been the precise origin of the Dhalac
and Farsan Archipelagoes, the greater number of the banks on the eastern
side of the Red Sea seem to have originated through nearly similar means.
I judge of this from their similarity in configuration (in proof of which I
may instance a bank on the east coast in latitude 22 deg; and although it
is true that the northern banks generally have a less complicated outline),
and from their similarity in composition, as may be observed in their
upraised portions. The depth within the banks northward of latitude 17
deg, is usually greater, and their outer sides shelve more abruptly
(circumstances which seem to go together) than in the Dhalac and Farsan
Archipelagoes; but this might easily have been caused by a difference in
the action of the currents during their formation: moreover, the greater
quantity of living coral, which, according to Captain Moresby, exists on
the northern banks, would tend to give them steeper margins.
From this account, brief and imperfect as it is, we can see that the great
chain of banks on the eastern coast, and on the western side in the
southern portion, differ greatly from true barrier-reefs wholly formed by
the growth of coral. It is indeed the direct conclusion of Ehrenberg
("Uber die," etc., pages 45 and 51), that they are connected in their
origin quite secondarily with the growth of coral; and he remarks that the
islands off the coast of Norway, if worn down level with the sea, and
merely coated with living coral, would present a nearly similar appearance.
I cannot, however, avoid suspecting, from information given me by Dr.
Malcolmson and Captain Moresby, that Ehrenberg has rather under-rated the
influence of corals, in some places at least, on the formation of the
tertiary deposits of the Red Sea.
THE WEST COAST OF THE RED SEA BETWEEN LATITUDE 19 DEG AND 22 DEG.
There are, in this space, reefs, which, if I had known nothing of those in
other parts of the Red Sea, I should unhesitatingly have considered as
barrier-reefs; and, after deliberation, I have come to the same conclusion.
One of these reefs, in 20 deg 15', is twenty miles long, less than a mile
in width (but expanding at the northern end into a disc), slightly sinuous,
and extending parallel to the mainland at the distance of five miles from
it, with very deep water within; in one spot soundings were not obtained
with 205 fathoms. Some leagues further south, there is another linear
reef, very narrow, ten miles long, with other small portions of reef, north
and south, almost connected with it; and within this line of reefs (as well
as outside) the water is profoundly deep. There are also some small linear
and sickle-formed reefs, lying a little way out at sea. All these reefs
are covered, as I am informed by Captain Moresby, by living corals. Here,
then, we have all the characters of reefs of the barrier class; and in some
outlying reefs we have an approach to the structure of atolls. The source
of my doubts about the classification of these reefs, arises from having
observed in the Dhalac and Farsan groups the narrowness and straightness of
several spits of sand and rock: one of these spits in the Dhalac group is
nearly fifteen miles long, only two broad, and it is bordered on each side
with deep water; so that, if worn down by the surf, and coated with living
corals, it would form a reef nearly similar to those within the space under
consideration. There is, also, in this space (latitude 21 deg) a
peninsula, bordered by cliffs, with its extremity worn down to the level of
the sea, and its basis fringed with reefs: in the line of prolongation of
this peninsula, there lies the island of MACOWA (formed, according to
Captain Moresby, of the usual tertiary deposit), and some smaller islands,
large parts of which likewise appear to have been worn down, and are now
coated with living corals. If the removal of the strata in these several
cases had been more complete, the reefs thus formed would have nearly
resembled those barrier-like ones now under discussion. Notwithstanding
these facts, I cannot persuade myself that the many very small, isolated,
and sickle-formed reefs and others, long, nearly straight, and very narrow,
with the water unfathomably deep close round them, could possibly have been
formed by corals merely coating banks of sediment, or the abraded surfaces
of irregularly shaped islands. I feel compelled to believe that the
foundations of these reefs have subsided, and that the corals, during their
upward growth, have given to these reefs their present forms: I may remark
that the subsidence of narrow and irregularly-shaped peninsulas and
islands, such as those existing on the coasts of the Red Sea, would afford
the requisite foundations for the reefs in question.
THE WEST COAST FROM LATITUDE 22 DEG TO 24 DEG.
This part of the coast (north of the space coloured blue on the map) is
fronted by an irregularly shelving bank, from about ten to thirty fathoms
deep; numerous little reefs, some of which have the most singular shapes,
rise from this bank. It may be observed, respecting one of them, in
latitude 23 deg 10', that if the promontory in latitude 24 deg were worn
down to the level of the sea, and coated with corals, a very similar and
grotesquely formed reef would be produced. Many of the reefs on this part
of the coast may thus have originated; but there are some sickle, and
almost atoll-formed reefs lying in deep water off the promontory in
latitude 24 deg, which lead me to suppose that all these reefs are more
probably allied to the barrier or atoll classes. I have not, however,
ventured to colour this portion of coast. ON THE WEST COAST FROM LATITUDE
19 DEG TO 17 DEG (south of space coloured blue on the map), there are many
low islets of very small dimensions, not much elongated, and rising out of
great depths at a distance from the coast; these cannot be classed either
with atolls, or barrier- or fringing-reefs. I may here remark that the
outlying reefs on the west coast, between latitude 19 deg and 24 deg, are
the only ones in the Red Sea, which approach in structure to the true
atolls of the Indian and Pacific Oceans, but they present only imperfect
miniature likenesses of them.
EASTERN COAST.
I have felt the greatest doubt about colouring any portion of this coast,
north of the fringing-reefs round the Farsan Islands in 16 deg 10'. There
are many small outlying coral-reefs along the whole line of coast; but as
the greater number rise from banks not very deeply submerged (the formation
of which has been shown to be only secondarily connected with the growth of
coral), their origin may be due simply to the growth of knolls of corals,
from an irregular foundation situated within a limited depth. But between
latitude 18 deg and 20 deg, there are so many linear, elliptic, and
extremely small reefs, rising abruptly out of profound depths, that the
same reasons, which led me to colour blue a portion of the west coast, have
induced me to do the same in this part. There exist some small outlying
reefs rising from deep water, north of latitude 20 deg (the northern limit
coloured blue), on the east coast; but as they are not very numerous and
scarcely any of them linear, I have thought it right to leave them
uncoloured.
In the SOUTHERN PARTS of the Red Sea, considerable spaces of the mainland,
and of some of the Dhalac islands, are skirted by reefs, which, as I am
informed by Captain Moresby, are of living coral, and have all the
characters of the fringing class. As in these latitudes, there are no
outlying linear or sickle-formed reefs, rising out of unfathomable depths,
I have coloured these parts of the coast red. On similar grounds, I have
coloured red the NORTHERN PARTS OF THE WESTERN COAST (north of latitude 24
deg 30'), and likewise the shores of the chief part of the GULF OF SUEZ.
In the GULF OF ACABA, as I am informed by Captain Moresby there are no
coral-reefs, and the water is profoundly deep.
WEST INDIES.
My information regarding the reefs of this area, is derived from various
sources, and from an examination of numerous charts; especially of those
lately executed during the survey under Captain Owen, R.N. I lay under
particular obligation to Captain Bird Allen, R.N., one of the members of
the late survey, for many personal communications on this subject. As in
the case of the Red Sea, it is necessary to make some preliminary remarks
on the submerged banks of the West Indies, which are in some degree
connected with coral-reefs, and cause considerable doubts in their
classification. That large accumulations of sediment are in progress on
the West Indian shores, will be evident to any one who examines the charts
of that sea, especially of the portion north of a line joining Yucutan and
Florida. The area of deposition seems less intimately connected with the
debouchement of the great rivers, than with the course of the sea-currents;
as is evident from the vast extension of the banks from the promontories of
Yucutan and Mosquito.
Besides the coast-banks, there are many of various dimensions which stand
quite isolated; these closely resemble each other, they lie from two or
three to twenty or thirty fathoms under water, and are composed of sand,
sometimes firmly agglutinated, with little or no coral; their surfaces are
smooth and nearly level, shelving only to the amount of a few fathoms, very
gradually all round towards their edges, where they plunge abruptly into
the unfathomable sea. This steep inclination of their sides, which is
likewise characteristic of the coast-banks, is very remarkable: I may give
as an instance, the Misteriosa Bank, on the edges of which the soundings
change in 250 fathoms horizontal distance, from 11 to 210 fathoms; off the
northern point of the bank of Old Providence, in 200 fathoms horizontal
distance, the change is from 19 to 152 fathoms; off the Great Bahama Bank,
in 160 fathoms horizontal distance, the inclination is in many places from
10 fathoms to no bottom with 190 fathoms. On coasts in all parts of the
world, where sediment is accumulating, something of this kind may be
observed; the banks shelve very gently far out to sea, and then terminate
abruptly. The form and composition of the banks standing in the middle
parts of the W. Indian Sea, clearly show that their origin must be chiefly
attributed to the accumulation of sediment; and the only obvious
explanation of their isolated position is the presence of a nucleus, round
which the currents have collected fine drift matter. Any one who will
compare the character of the bank surrounding the hilly island of Old
Providence, with those banks in its neighbourhood which stand isolated,
will scarcely doubt that they surround submerged mountains. We are led to
the same conclusion by examining the bank called Thunder Knoll, which is
separated from the Great Mosquito Bank by a channel only seven miles wide,
and 145 fathoms deep. There cannot be any doubt that the Mosquito Bank has
been formed by the accumulation of sediment round the promontory of the
same name; and Thunder Knoll resembles the Mosquito Bank, in the state of
its surface submerged twenty fathoms, in the inclinations of its sides, in
composition, and in every other respect. I may observe, although the
remark is here irrelevant, that geologists should be cautious in concluding
that all the outlyers of any formation have once been connected together,
for we here see that deposits, doubtless of exactly the same nature, may be
deposited with large valley-like spaces between them.
Linear strips of coral-reefs and small knolls project from many of the
isolated, as well as coast-banks; sometimes they occur quite irregularly
placed, as on the Mosquito Bank, but more generally they form crescents on
the windward side, situated some little distance within the outer edge of
the banks:--thus on the Serranilla Bank they form an interrupted chain
which ranges between two and three miles within the windward margin:
generally they occur, as on Roncador, Courtown, and Anegada Banks, nearer
the line of deep water. Their occurrence on the windward side is
conformable to the general rule, of the efficient kinds of corals
flourishing best where most exposed; but their position some way within the
line of deep water I cannot explain, without it be, that a depth somewhat
less than that close to the outer margin of the banks, is most favourable
to their growth. Where the corals have formed a nearly continuous rim,
close to the windward edge of a bank some fathoms submerged, the reef
closely resembles an atoll; but if the bank surrounds an island (as in the
case of Old Providence), the reef resembles an encircling barrier-reef. I
should undoubtedly have classed some of these fringed banks as imperfect
atolls, or barrier-reefs, if the sedimentary nature of their foundations
had not been evident from the presence of other neighbouring banks, of
similar forms and of similar composition, but without the crescent-like
marginal reef: in the third chapter, I observed that probably some atoll-like
reefs did exist, which had originated in the manner here supposed.
Proofs of elevation within recent tertiary periods abound, as referred to
in the sixth chapter, over nearly the whole area of the West Indies. Hence
it is easy to understand the origin of the low land on the coasts, where
sediment is now accumulating; for instance on the northern part of Yucutan,
and on the N.E. part of Mosquito, where the land is low, and where
extensive banks appear to be in progressive formation. Hence, also, the
origin of the Great Bahama Banks, which are bordered on their western and
southern edges by very narrow, long, singularly shaped islands, formed of
sand, shells, and coral-rock, and some of them about a hundred feet in
height, is easily explained by the elevation of banks fringed on their
windward (western and southern) sides by coral-reefs. On this view,
however, we must suppose either that the chief part of the surfaces of the
great Bahama sandbanks were all originally deeply submerged, and were
brought up to their present level by the same elevatory action, which
formed the linear islands; or that during the elevation of the banks, the
superficial currents and swell of the waves continued wearing them down and
keeping them at a nearly uniform level: the level is not quite uniform;
for, in proceeding from the N.W. end of the Bahama group towards the S.E.
end, the depth of the banks increases, and the area of land decreases, in a
very gradual and remarkable manner. The latter view, namely, that these
banks have been worn down by the currents and swell during their elevation,
seems to me the most probable one. It is, also, I believe, applicable to
many banks, situated in widely distant parts of the West Indian Sea, which
are wholly submerged; for, on any other view, we must suppose, that the
elevatory forces have acted with astonishing uniformity.
The shores of the Gulf of Mexico, for the space of many hundred miles, is
formed by a chain of lagoons, from one to twenty miles in breadth
("Columbian Navigator," page 178, etc.), containing either fresh or salt
water, and separated from the sea by linear strips of sand. Great spaces
of the shores of Southern Brazil (In the "London and Edinburgh
Philosophical Journal," 1841, page 257, I have described a singular bar of
sandstone lying parallel to the coast off Pernambuco in Brazil, which
probably is an analogous formation.), and of the United States from Long
Island (as observed by Professor Rogers) to Florida have the same
character. Professor Rogers, in his "Report to the British Association"
(volume iii., page 13), speculates on the origin of these low, sandy,
linear islets; he states that the layers of which they are composed are too
homogeneous, and contain too large a proportion of shells, to permit the
common supposition of their formation being simply due to matter thrown up,
where it now lies, by the surf: he considers these islands as upheaved
bars or shoals, which were deposited in lines where opposed currents met.
It is evident that these islands and spits of sand parallel to the coast,
and separated from it by shallow lagoons, have no necessary connection with
coral-formations. But in Southern Florida, from the accounts I have
received from persons who have resided there, the upraised islands seem to
be formed of strata, containing a good deal of coral, and they are
extensively fringed by living reefs; the channels within these islands are
in some places between two and three miles wide, and five or six fathoms
deep, though generally (In the ordinary sea-charts, no lagoons appear on
the coast of Florida, north of 26 deg; but Major Whiting ("Silliman's
Journal," volume xxxv., page 54) says that many are formed by sand thrown
up along the whole line of coast from St. Augustine's to Jupiter Inlet.)
they are less in depth than width. After having seen how frequently banks
of sediment in the West Indian Sea are fringed by reefs, we can readily
conceive that bars of sediment might be greatly aided in their formation
along a line of coast, by the growth of corals; and such bars would, in
that case, have a deceptive resemblance with true barrier-reefs.
Having now endeavoured to remove some sources of doubt in classifying the
reefs of the West Indies, I will give my authorities for colouring such
portions of the coast as I have thought myself warranted in doing. Captain
Bird Allen informs me, that most of the islands on the BAHAMA BANKS are
fringed, especially on their windward sides, with living reefs; and hence I
have coloured those, which are thus represented in Captain Owen's late
chart, red. The same officer informs me, that the islands along the
southern part of FLORIDA are similarly fringed; coloured red. CUBA:
Proceeding along the northern coast, at the distance of forty miles from
the extreme S.E. point, the shores are fringed by reefs, which extend
westward for a space of 160 miles, with only a few breaks. Parts of these
reefs are represented in the plans of the harbours on this coast by Captain
Owen; and an excellent description is given of them by Mr. Taylor (Loudon's
"Mag. of Nat. Hist." volume ix., page 449); he states that they enclosed a
space called the "baxo," from half to three-quarters of a mile in width,
with a sandy bottom, and a little coral. In most parts people can wade, at
low water, to the reef; but in some parts the depth is between two and
three fathoms. Close outside the reef, the depth is between six and seven
fathoms; these well-characterised fringing-reefs are coloured red.
Westward of longitude 77 deg 30', on the northern side of Cuba, a great
bank commences, which extends along the coast for nearly four degrees of
longitude. In the place of its commencement, in its structure, and in the
"CAYS," or low islands on its edge, there is a marked correspondence (as
observed by Humboldt, "Pers. Narr." volume vii., page 88) between it and
the Great Bahama and Sal Banks, which lie directly in front. Hence one is
led to attribute the same origin to both these sets of banks; namely, the
accumulation of sediment, conjoined with an elevatory movement, and the
growth of coral on their outward edges; those parts which appear fringed by
living reefs are coloured red. Westward of these banks, there is a portion
of coast apparently without reefs, except in the harbours, the shores of
which seem in the published plans to be fringed. The COLORADO SHOALS (see
Captain Owen's charts), and the low land at the western end of Cuba,
correspond as closely in relative position and structure to the banks at
the extreme point of Florida, as the banks above described on the north
side of Cuba, do to the Bahamas, the depth within the islets and reefs on
the outer edge of the COLORADOS, is generally between two and three
fathoms, increasing to twelve fathoms in the southern part, where the bank
becomes nearly open, without islets or coral-reefs; the portions which are
fringed are coloured red. The southern shore of Cuba is deeply concave,
and the included space is filled up with mud and sandbanks, low islands and
coral-reefs. Between the mountainous ISLE OF PINES and the southern shore
of Cuba, the general depth is only between two and three fathoms; and in
this part small islands, formed of fragmentary rock and broken madrepores
(Humboldt, "Pers. Narr." volume vii. pages 51, 86 to 90, 291, 309, 320),
rise abruptly, and just reach the surface of the sea. From some
expressions used in the "Columbian Navigator" (volume i., part ii., page
94), it appears that considerable spaces along the outer coast of Southern
Cuba are bounded by cliffs of coral-rock, formed probably by the upheaval
of coral-reefs and sandbanks. The charts represent the southern part of
the Isle of Pines as fringed by reefs, which the "Columb. Navig." says
extend some way from the coast, but have only from nine to twelve feet
water on them; these are coloured red.--I have not been able to procure any
detailed description of the large groups of banks and "cays" further
eastward on the southern side of Cuba; within them there is a large
expanse, with a muddy bottom, from eight to twelve fathoms deep; although
some parts of this line of coast are represented in the general charts of
the West Indies, as fringed, I have not thought it prudent to colour them.
The remaining portion of the south coast of Cuba appears to be without
coral-reefs.
YUCUTAN.
The N.E. part of the promontory appears in Captain Owen's charts to be
fringed; coloured red. The eastern coast, from 20 deg to 18 deg is
fringed. South of latitude 18 deg, there commences the most remarkable
reef in the West Indies: it is about one hundred and thirty miles in
length, ranging in a N. and S. line, at an average distance of fifteen
miles from the coast. The islets on it are all low, as I have been
informed by Captain B. Allen; the water deepens suddenly on the outside of
the reef, but not more abruptly than off many of the sedimentary banks:
within its southern extremity (off HONDURAS) the depth is twenty-five
fathoms; but in the more northern parts, the depth soon increases to ten
fathoms, and within the northernmost part, for a space of twenty miles, the
depth is only from one to two fathoms. In most of these respects we have
the characteristics of a barrier-reef; nevertheless, from observing, first,
that the channel within the reef is a continuation of a great irregular
bay, which penetrates the mainland to the depth of fifty miles; and
secondly, that considerable spaces of this barrier-like reef are described
in the charts (for instance, in latitude 16 deg 45' and 16 deg 12') as
formed of pure sand; and thirdly, from knowing that sediment is
accumulating in many parts of the West Indies in banks parallel to the
shore; I have not ventured to colour this reef as a barrier, without
further evidence that it has really been formed by the growth of corals,
and that it is not merely in parts a spit of sand, and in other parts a
worn down promontory, partially coated and fringed by reefs; I lean,
however, to the probability of its being a barrier-reef, produced by
subsidence. To add to my doubts, immediately on the outside of this
barrier-like reef, TURNEFFE, LIGHTHOUSE, and GLOVER reefs are situated, and
these reefs have so completely the form of atolls, that if they had
occurred in the Pacific, I should not have hesitated about colouring them
blue. TURNEFFE REEF seems almost entirely filled up with low mud islets;
and the depth within the other two reefs is only from one to three fathoms.
From this circumstance and from their similarity in form, structure, and
relative position, both to the bank called NORTHERN TRIANGLES, on which
there is an islet between seventy and eighty feet, and to COZUMEL Island,
the level surface of which is likewise between seventy and eighty feet in
height, I consider it more probable that the three foregoing banks are the
worn down bases of upheaved shoals, fringed with corals, than that they are
true atolls, wholly produced by the growth of coral during subsidence; left
uncoloured.
In front of the eastern MOSQUITO coast, there are between latitude 12 deg
and 16 deg some extensive banks (already mentioned, page 148), with high
islands rising from their centres; and there are other banks wholly
submerged, both of which kinds of banks are bordered, near their windward
margins, by crescent-shaped coral-reefs. But it can hardly be doubted, as
was observed in the preliminary remarks, that these banks owe their origin,
like the great bank extending from the Mosquito promontory, almost entirely
to the accumulation of sediment, and not to the growth of corals; hence I
have not coloured them.
CAYMAN ISLAND: this island appears in the charts to be fringed; and
Captain B. Allen informs me that the reefs extend about a mile from the
shore, and have only from five to twelve feet water within them; coloured
red.--JAMAICA: judging from the charts, about fifteen miles of the S.E.
extremity, and about twice that length on the S.W. extremity, and some
portions on the S. side near Kingston and Port Royal, are regularly
fringed, and therefore are coloured red. From the plans of some harbours
on the N. side of Jamaica, parts of the coast appear to be fringed; but as
these are not represented in the charts of the whole island, I have not
coloured them.--ST. DOMINGO: I have not been able to obtain sufficient
information, either from plans of the harbours, or from general charts, to
enable me to colour any part of the coast, except sixty miles from Port de
Plata westward, which seems very regularly fringed; many other parts,
however, of the coast are probably fringed, especially towards the eastern
end of the island.--PUERTO RICO: considerable portions of the southern,
western, and eastern coasts, and some parts of the northern coast, appear
in the charts to be fringed; coloured red.--Some miles in length of the
southern side of the Island of ST. THOMAS is fringed; most of the VIRGIN
GORDA Islands, as I am informed by Mr. Schomburgk, are fringed; the shores
of ANEGADA, as well as the bank on which it stands, are likewise fringed;
these islands have been coloured red. The greater part of the southern
side of SANTA CRUZ appears in the Danish survey to be fringed (see also
Prof. Hovey's account of this island, in "Silliman's Journal," volume
xxxv., page 74); the reefs extend along the shore for a considerable space,
and project rather more than a mile; the depth within the reef is three
fathoms; coloured red.--The ANTILLES, as remarked by Von Buch ("Descrip.
Iles Canaries," page 494), may be divided into two linear groups, the
western row being volcanic, and the eastern of modern calcareous origin; my
information is very defective on the whole group. Of the eastern islands,
BARBUDA and the western coasts of ANTIGUA and MARIAGALANTE appear to be
fringed: this is also the case with BARBADOES, as I have been informed by
a resident; these islands are coloured red. On the shores of the Western
Antilles, of volcanic origin, very few coral-reefs appear to exist. The
island of MARTINIQUE, of which there are beautifully executed French
charts, on a very large scale, alone presents any appearance worthy of
special notice. The south-western, southern, and eastern coasts, together
forming about half the circumference of the island, are skirted by very
irregular banks, projecting generally rather less than a mile from the
shore, and lying from two to five fathoms submerged. In front of almost
every valley, they are breached by narrow, crooked, steep-sided passages.
The French engineers ascertained by boring, that these submerged banks
consisted of madreporitic rocks, which were covered in many parts by thin
layers of mud or sand. From this fact, and especially from the structure
of the narrow breaches, I think there can be little doubt that these banks
once formed living reefs, which fringed the shores of the island, and like
other reefs probably reached the surface. From some of these submerged
banks reefs of living coral rise abruptly, either in small detached
patches, or in lines parallel to, but some way within the outer edges of
the banks on which they are based. Besides the above banks which skirt the
shores of the island, there is on the eastern side a range of linear banks,
similarly constituted, twenty miles in length, extending parallel to the
coast line, and separated from it by a space between two and four miles in
width, and from five to fifteen fathoms in depth. From this range of
detached banks, some linear reefs of living coral likewise rise abruptly;
and if they had been of greater length (for they do not front more than a
sixth part of the circumference of the island), they would necessarily from
their position have been coloured as barrier-reefs; as the case stands they
are left uncoloured. I suspect that after a small amount of subsidence,
the corals were killed by sand and mud being deposited on them, and the
reefs being thus prevented from growing upwards, the banks of madreporitic
rock were left in their present submerged condition.
THE BERMUDA Islands have been carefully described by Lieutenant Nelson, in
an excellent Memoir in the "Geological Transactions" (volume v., part i.,
page 103). In the form of the bank or reef, on one side of which the
islands stand, there is a close general resemblance to an atoll; but in the
following respects there is a considerable difference,--first, in the
margin of the reef not forming (as I have been informed by Mr. Chaffers,
R.N.) a flat, solid surface, laid bare at low water, and regularly bounding
the internal space of shallow water or lagoon; secondly, in the border of
gradually shoaling water, nearly a mile and a half in width, which
surrounds the entire outside of the reef (as is laid down in Captain Hurd's
chart); and thirdly, in the size, height, and extraordinary form of the
islands, which present little resemblance to the long, narrow, simple
islets, seldom exceeding half a mile in breadth, which surmount the annular
reefs of almost all the atolls in the Indian and Pacific Oceans. Moreover,
there are evident proofs (Nelson, Ibid., page 118), that islands similar to
the existing ones, formerly extended over other parts of the reef. It
would, I believe, be difficult to find a true atoll with land exceeding
thirty feet in height; whereas, Mr. Nelson estimates the highest point of
the Bermuda Islands to be 260 feet; if, however, Mr. Nelson's view, that
the whole of the land consists of sand drifted by the winds, and
agglutinated together, were proved correct, this difference would be
immaterial; but, from his own account (page 118), there occur in one place,
five or six layers of red earth, interstratified with the ordinary
calcareous rock, and including stones too heavy for the wind to have moved,
without having at the same time utterly dispersed every grain of the
accompanying drifted matter. Mr. Nelson attributes the origin of these
several layers, with their embedded stones, to as many violent
catastrophes; but further investigation in such cases has generally
succeeded in explaining phenomena of this kind by ordinary and simpler
means. Finally, I may remark, that these islands have a considerable
resemblance in shape to Barbuda in the West Indies, and to Pemba on the
eastern coast of Africa, which latter island is about two hundred feet in
height, and consists of coral-rock. I believe that the Bermuda Islands,
from being fringed by living reefs, ought to have been coloured red; but I
have left them uncoloured, on account of their general resemblance in
external form to a lagoon-island or atoll.
INDEX.
The names not in capitals are all names of places, and refer exclusively to
the Appendix: in well-defined archipelagoes, or groups of islands, the
name of each separate island is not given.
ABROLHOS, Brazil, coated by corals.
Abrolhos (Australia).
ABSENCE of coral-reefs from certain coasts.
Acaba, gulf of.
Admiralty group.
AFRICA, east coast, fringing-reef of.
Madreporitic rock of.
Africa, east coast.
AGE of individual corals.
Aiou.
Aitutaki.
Aldabra.
Alert reef.
Alexander, Grand Duke, island.
ALLAN, Dr., on Holuthuriae feeding on corals.
On quick growth of corals at Madagascar.
On reefs affected by currents.
Alloufatou.
Alphonse.
Amargoura. (Amargura.)
Amboina.
America, west coast.
Amirantes.
Anachorites.
Anambas.
ANAMOUKA, description of.
Anamouka.
Anadaman islands.
Antilles.
Appoo reef.
Arabia Felix.
AREAS, great extent of, interspersed with low islands.
Of subsidence and of elevation.
Of subsidence appear to be elongated.
Of subsidence alternating with areas of elevation.
Arru group.
Arzobispo.
ASCIDIA, depth at which found.
Assomption.
Astova.
Atlantic islands.
ATOLLS, breaches in their reefs.
Dimensions of.
Dimensions of groups of.
Not based on craters or on banks of sediment, or of rock.
Of irregular forms.
Steepness of their flanks.
Width of their reef and islets.
Their lowness.
Lagoons.
General range.
With part of their reef submerged, and theory of.
Augustine, St.
AURORA island, an upraised atoll.
Aurora.
AUSTRAL islands, recently elevated.
Austral islands.
Australia, N.W. coast.
AUSTRALIAN barrier-reef.
Australian barrier.
Babuyan group.
Bahama banks.
Balahac.
Bally.
Baring.
BARRIER-REEF of Australia.
Of New Caledonia.
BARRIER-REEFS, breaches through.
Not based on worn down margin of rock.
On banks of sediment.
On submarine craters.
Steepness of their flanks.
Their probable vertical thickness.
Theory of their formation.
Bampton shoal.
Banks islands.
Banks in the West Indies.
Bashee islands.
Bass island.
Batoa.
Beaupre reef.
BEECHEY, Captain, obligations of the author to.
On submerged reefs.
Account of Matilda island.
BELCHER, Captain, on boring through coral-reef.
Belize reef, off.
Bellinghausen.
Bermuda islands.
Beveridge reef.
Bligh.
BOLABOLA, view of.
Bombay shoal.
Bonin Bay.
Bonin group.
BORINGS through coral-reefs.
BORNEO, W. coast, recently elevated.
Borneo, E. coast.
S.W. and W. coast
N. coast.
Western bank.
Boscawen.
Boston.
Bouka.
Bourbon.
Bourou.
Bouton.
BRAZIL, fringing-reefs on coast of.
BREACHES through barrier-reefs.
Brook.
Bunker.
Bunoa.
BYRON.
Cagayanes.
Candelaria.
Cargados Carajos.
Caroline archipelago.
Caroline island.
Carteret shoal.
CARYOPHYLLIA, depth at which it lives.
Cavilli.
Cayman island.
Celebes.
Ceram.
CEYLON, recently elevated.
Ceylon.
CHAGOS Great Bank, description and theory of.
CHAGOS group.
Chagos group.
CHAMA-SHELLS embedded in coral-rock.
CHAMISSO, on corals preferring the surf.
CHANGES in the state of Keeling atoll.
Of atolls.
CHANNELS leading into the lagoons of atolls.
Into the Maldiva atolls.
Through barrier-reefs.
Chase.
China sea.
CHRISTMAS atoll.
Christmas atoll.
Christmas island (Indian Ocean).
Clarence.
Clipperton rock.
COCOS, or Keeling atoll.
Cocos (or Keeling).
Cocos island (Pacific).
COCHIN China, encroachments of the sea on the coast.
Cochin China.
Coetivi.
Comoro group.
COMPOSITION of coral-formations.
CONGLOMERATE coral-rock on Keeling atoll.
On other atolls.
Coral-rock.
COOK islands, recently elevated.
Cook islands.
CORAL-BLOCKS bored by vermiform animals.
CORAL-REEFS, their distribution and absence from certain areas.
Destroyed by loose sediment.
CORAL-ROCK at Keeling atoll.
Mauritius.
Organic remains of.
CORALS dead but upright in Keeling lagoon.
Depths at which they live.
Off Keeling atoll.
Killed by a short exposure.
Living in the lagoon of Keeling atoll.
Quick growth of, in Keeling lagoon.
Merely coating the bottom of the sea.
Standing exposed in the Low archipelago.
CORALLIAN sea.
Corallian sea.
Cornwallis.
Cosmoledo.
COUTHOUY, Mr., alleged proofs of recent elevation of the Low archipelago.
On coral-rock at Mangaia and Aurora islands.
On external ledges round coral-islands.
Remarks confirmatory of the author's theory.
CRESCENT-FORMED reefs.
Cuba.
CUMING, Mr., on the recent elevation of the Philippines.
Dangerous, or Low archipelago.
Danger islands.
DEPTHS at which reef-building corals live.
At Mauritius, the Red Sea, and in the Maldiva archipelago.
At which other corals and corallines can live.
Dhalac group.
DIEGO GARCIA, slow growth of reef.
DIMENSIONS of the larger groups of atolls.
DISSEVERMENT of the Maldiva atolls, and theory of.
DISTRIBUTION of coral-reefs.
Domingo, St.
DORY, Port, recently elevated.
Dory, Port.
Duff islands.
Durour.
Eap.
EARTHQUAKES at Keeling atoll.
In groups of atolls.
In Navigator archipelago.
EAST INDIAN ARCHIPELAGO, recently elevated.
Easter.
Echequier.
EHRENBERG, on the banks of the Red Sea.
On depths at which corals live in the Red Sea.
On corals preferring the surf.
On the antiquity of certain corals.
Eimeo.
ELEVATED reef of Mauritius.
ELEVATIONS, recent proofs of.
Immense areas of.
Elivi.
ELIZABETH island.
Recently elevated.
Elizabeth island.
Ellice group.
ENCIRCLED ISLANDS, their height.
Geological composition.
EOUA, description of.
Eoua.
ERUPTED MATTER probably not associated with thick masses of coral-rock.
FAIS, recently elevated.
Fais.
Fanning.
Farallon de Medinilla.
Farson group.
Fataka.
FIJI archipelago.
FISH, feeding on corals.
Killed in Keeling lagoon by heavy rain.
FISSURES across coral-islands.
FITZROY, Captain, on a submerged shed at Keeling atoll.
On an inundation in the Low archipelago.
Flint.
Flores.
Florida.
Folger.
Formosa.
FORSTER, theory of coral-formations.
Frederick reef.
Freewill.
FRIENDLY group recently elevated.
Friendly archipelago.
FRINGING-REEFS, absent where coast precipitous.
Breached in front of streams.
Described by MM. Quoy and Gaimard.
Not closely attached to shelving coasts.
Of east coast of Africa.
Of Cuba.
Of Mauritius.
On worn down banks of rock.
On banks of sediment.
Their appearance when elevated.
Their growth influenced by currents.
By shallowness of sea.
Galapagos archipelago.
Galega.
GAMBIER islands, section of.
Gambier islands.
Gardner.
Gaspar rico.
GEOLOGICAL COMPOSITION of coral-formations.
Gilbert archipelago.
Gilolo.
Glorioso.
GLOUCESTER Island.
Glover reef.
Gomez.
Gouap.
Goulou.
Grampus.
Gran Cocal.
GREAT CHAGOS BANK, description and theory of.
GREY, Captain, on sandbars.
GROUPING of the different classes of reefs.
Guedes.
HALL, Captain B., on Loo Choo.
HARVEY islands, recently elevated.
HEIGHT of encircled islands.
Hermites.
Hervey or Cook islands.
Hogoleu.
HOLOTHURIAE (Holuthuriae) feeding on coral.
HOUDEN island, height of.
Honduras, reef off.
Horn.
Houtman Abrolhos.
HUAHEINE; alleged proofs of its recent elevation.
Huaheine.
Humphrey.
Hunter.
HURRICANES, effects of, on coral-islands.
Immaum.
Independence.
INDIA, west coast, recently elevated.
India.
IRREGULAR REEFS in shallow seas.
ISLETS of coral-rock, their formation.
Their destruction in the Maldiva atolls.
Jamaica.
Jarvis.
JAVA, recently elevated.
Java.
Johnston island.
Juan de Nova.
Juan de Nova (Madagascar).
Kalatoa.
KAMTSCHATKA, proofs of its recent elevation.
Karkalang.
KEELING atoll, section of reef.
Keeling, south atoll.
North atoll.
Keffing.
Kemin.
Kennedy.
Keppel.
Kumi.
Laccadive group.
LADRONES, or Marianas, recently elevated.
Ladrones archipelago.
LAGOON of Keeling atoll.
LAGOONS bordered by inclined ledges and walls, and theory of their
formation.
Of small atolls filled up with sediment.
LAGOON-CHANNELS within barrier-reefs.
LAGOON-REEFS, all submerged in some atolls, and rising to the surface in
others.
Lancaster reef.
Latte.
Lauglan islands.
LEDGES round certain lagoons.
Lette.
Lighthouse reef.
LLOYD, Mr., on corals refixing themselves.
LOO CHOO, recently elevated.
Loo Choo.
Louisiade.
LOW ARCHIPELAGO, alleged proofs of its recent elevation.
Low archipelago.
LOWNESS of coral-islands.
Loyalty group.
Lucepara.
LUTKE, Admiral, on fissures across coral-islands.
LUZON, recently elevated.
Luzon.
LYELL, Mr., on channels into the lagoons of atolls.
On the lowness of their leeward sides.
On the antiquity of certain corals.
On the apparent continuity of distinct coral-islands.
On the recently elevated beds of the Red Sea.
On the outline of the areas of subsidence.
Macassar strait.
Macclesfield bank.
MADAGASCAR, quick growth of corals at.
Madreporitic rock of.
Madagascar.
Madjiko-sima.
Madura (Java).
Madura (India).
MAHLOS MAHDOO, theory of formation.
MALACCA, recently elevated.
Malacca.
MALCOLMSON, Dr., on recent elevation of W. coast of India.
On recent elevation of Camaran island.
Malden.
MALDIVA atolls, and theory of their formation.
Steepness of their flanks.
Growth of coral at.
Maldiva archipelago.
MANGAIA island.
Recently elevated.
Mangaia.
Mangs.
MARIANAS, recently elevated.
Mariana archipelago.
Mariere.
Marquesas archipelago.
Marshall archipelago.
Marshall island.
Martinique.
Martires.
MARY'S ST., in Madagascar, harbour made in reefs.
Mary island.
Matia, or Aurora.
MATILDA atoll.
MAURITIUS, fringing-reefs of.
Depths at which corals live there.
Recently elevated.
Mauritius.
MAURUA, section of.
Maurua.
MENCHIKOFF atoll.
Mendana archipelago.
Mendana isles.
Mexico, gulf of.
MILLEPORA COMPLANATA at Keeling atoll.
Mindoro.
Mohilla. (Mohila.)
MOLUCCA islands, recently elevated.
Mopeha.
MORESBY, Captain, on boring through coral-reefs.
Morty.
Mosquito coast.
MUSQUILLO atoll.
Mysol.
NAMOURREK group.
Natunas.
NAVIGATOR archipelago, elevation of.
Navigator archipelago.
Nederlandisch.
NELSON, Lieutenant, on the consolidation of coral-rocks under water.
Theory of coral-formations.
On the Bermuda islands.
New Britain.
NEW CALEDONIA, steepness of its reefs.
Barrier-reef of.
New Caledonia.
New Guinea (E. end).
New Guinea (W. end).
New Hanover.
NEW HEBRIDES, recently elevated.
New Hebrides.
NEW IRELAND, recently elevated.
New Ireland.
New Nantucket.
Nicobar islands.
Niouha.
NULLIPORAE at Keeling atoll.
On the reefs of atolls.
On barrier-reefs.
Their wide distribution and abundance.
OBJECTIONS to the theory of subsidence.
Ocean islands.
Ono.
Onouafu. (Onouafou.)
Ormuz.
Oscar group.
OSCILLATIONS of level.
Ouallan, or Ualan. (Oualan.)
OULUTHY atoll.
Outong Java.
Palawan, S.W. coast.
N.W. coast.
Western bank.
Palmerston.
Palmyra.
Paracells.
Paraquas.
Patchow.
Pelew islands.
PEMBA island, singular form of.
Pemba.
Penrhyn.
Peregrino.
PERNAMBUCO, bar of sandstone at.
PERSIAN gulf, recently elevated.
Persian gulf.
PESCADO.
Pescadores.
Peyster group.
Philip.
PHILIPPINE archipelago, recently elevated.
Philippine archipelago.
Phoenix.
Piguiram.
Pitcairn.
PITT'S bank.
Pitt island.
Platte.
Pleasant.
PORITES, chief coral on margin of Keeling atoll.
Postillions.
POUYNIPETE.
Its probable subsidence.
Pouynipete.
Pratas shoal.
Proby.
Providence.
Puerto Rico.
Pulo Anna.
PUMICE floated to coral-islands.
Pylstaart.
PYRARD DE LAVAL, astonishment at the atolls in the Indian Ocean.
QUOY AND GAIMARD, depths at which corals live.
Description of reefs applicable only to fringing-reefs.
RANGE of atolls.
Rapa.
Rearson.
RED SEA, banks of rock coated by reefs.
Proofs of its recent elevation.
Supposed subsidence of.
Red Sea.
REEFS, irregular in shallow seas.
Rising to the surface in some lagoons and all submerged in others.
Their distribution.
Their absence from some coasts.
Revilla-gigedo.
RING-FORMED REEFS of the Maldiva atolls, and theory of.
Rodriguez.
Rosario.
Rose island.
Rotches.
Rotoumah.
Roug.
Rowley shoals.
RUPPELL, Dr., on the recent deposits of Red Sea.
Sable, ile de.
Sahia de Malha.
St. Pierre.
Sala.
Salomon archipelago. (Solomon.)
SAMOA, or Navigator archipelago, elevation of.
Samoa archipelago.
SAND-BARS parallel to coasts.
Sandal-wood.
SANDWICH archipelago, recently elevated.
Sandwich archipelago.
Sanserot.
Santa-Cruz group.
SAVAGE island, recently elevated.
Savage.
Savu.
Saya, or Sahia de Malha.
Scarborough shoal.
SCARUS feeding on corals.
Schouten.
Scilly.
SCORIAE floated to coral-islands.
Scott's reef.
SECTIONS of islands encircled by barrier-reefs.
Of Bolabola.
SEDIMENT in Keeling lagoon.
In other atolls.
Injurious to corals.
Transported from coral-islands far seaward.
Seniavine.
Serangani.
Seychelles.
SHIP-BOTTOM quickly coated with coral.
SMYTH island.
SOCIETY archipelago, stationary condition of.
Alleged proofs of recent elevation.
Society archipelago.
Socotra.
Solor.
SOOLOO islands, recently elevated.
Sooloo islands.
Souvaroff.
Spanish.
SPONGE, depths at which found.
Starbuck. (Slarbuck.)
STONES transported in roots of trees.
STORMS, effects of, on coral-islands.
STUTCHBURY, Mr., on the growth of an Agaricia.
On upraised corals in Society archipelago.
SUBSIDENCE of Keeling atoll.
Extreme slowness of.
Areas of, apparently elongated.
Areas of immense.
Great amount of.
Suez, gulf of.
Sulphur islands.
SUMATRA, recently elevated.
Sumatra.
Sumbawa.
SURF favourable to the growth of massive corals.
Swallow shoal.
Sydney island.
TAHITI, alleged proofs of its recent elevation.
Tahiti.
TEMPERATURE of the sea at the Galapagos archipelago.
Tenasserim.
Tenimber island.
Teturoa.
THEORIES on coral-formations.
THEORY OF subsidence, and objections to.
THICKNESS, vertical, of barrier-reefs.
Thomas, St.
Tikopia.
TIMOR, recently elevated.
Timor.
Timor-laut.
Tokan-Bessees.
Tongatabou.
Tonquin.
Toubai.
Toufoa. (Toofoa.)
Toupoua.
TRADITIONS OF CHANGE in coral-islands.
TRIDACNAE embedded in coral-rock.
Left exposed in the Low archipelago.
TUBULARIA, quick growth of.
Tumbelan.
Turneffe reef.
Turtle.
Ualan.
VANIKORO, section of.
Its state and changes in its reefs.
Vanikoro.
Vine reef.
Virgin Gorda.
Viti archipelago.
VOLCANIC islands, with living corals on their shores.
Matter, probably not associated with thick masses of coral-rock.
VOLCANOES, authorities for their position on the map.
Their presence determined by the movements in progress.
Absent or extinct in the areas of subsidence.
Waigiou.
Wallis island.
Washington.
Well's reef.
WELLSTEAD, Lieutenant, account of a ship coated with corals.
WEST INDIES, banks of sediment fringed by reefs.
Recently elevated.
West Indies.
WHITSUNDAY island, view of.
Changes in its state.
WILLIAMS, Rev. J., on traditions of the natives regarding coral-islands.
On antiquity of certain corals.
Wolchonsky.
Wostock.
Xulla islands.
York island.
Yucutan, coast of.
ZONES of different kinds of corals outside the same reefs.
End of The Project Gutenburg Etext of Coral Reefs, by Charles Darwin
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