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diff --git a/old/2690.txt b/old/2690.txt new file mode 100644 index 0000000..bdeae10 --- /dev/null +++ b/old/2690.txt @@ -0,0 +1,8984 @@ +*The Project Gutenburg Etext of Coral Reefs, by Charles Darwin* + +Copyright laws are changing all over the world, be sure to check +the copyright laws for your country before posting these files!! + +Please take a look at the important information in this header. +We encourage you to keep this file on your own disk, keeping an +electronic path open for the next readers. Do not remove this. + + +**Welcome To The World of Free Plain Vanilla Electronic Texts** + +**Etexts Readable By Both Humans and By Computers, Since 1971** + +*These Etexts Prepared By Hundreds of Volunteers and Donations* + +Information on contacting Project Gutenberg to get Etexts, and +further information is included below. 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FOR PUBLIC DOMAIN ETEXTS*Ver.04.29.93*END* + + + + + +This etext was prepared by Sue Asscher <asschers@dingoblue.net.au> + + + + + +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 + |