JUUUUUL JUUUUUUUUUUUU UU UUUUUUU UU UUUUEE | AN -XAKN | PIP LG a: A ro oe ran Setar Si ‘| UUUUUU DING A BIE OI Dae PAO GAIT OPPO EE ALPIL didi 144 ghd 5 e A 43 2 pee 5 Zz DEN OOD. Ps Vip ids. LIA i OI KOottier 40006, 192.7 THE LIBRARY OF THE UNIVERSITY OF NORTH CAROLINA THE COLLIER COBB COLLECTION QE565 Darwin, Charles D2 189596 The structure and dis- : tribution of coral ... eTORAGE JL Storage’ IT ~ Shelve in sAaner oft. THE LIBRARY OF THE UNIVERSITY OF NORTH CAROLINA WL 1 5 BI! ENDOWED BY THE DIALECTIC AND PHILANTHROPIC SOCIETIES GEOLOGY LIBRARY DATE DUE F =all ose ai fh & Tall 44 AVE Le F GAYLORD 7 be a AJ a “ A a : Se - | -* . . _— 5 7 P : “ ' aad z + é . ae * = . - be . cf =< pa : 7 os i ‘ of 4 : a ” i 4 1 vl rT ’, ‘ . “ ' 2 } ° a x : ; x : j r i, - < = - - - o . ) - . “J P a - > of ~~ , i t le 7 , 4 , i t . ‘ a < ‘ a - | / - : = , ’ t Y f * “2 ane - ¢ ¥, , ry : ‘ i i " i 4 t ie i 7 ‘i , ‘ > : y re . = ? 7 3 i A > : eg - a ® ‘ P ‘ j F ~~ = - * > , % * " “. - ‘ = ;, . : . al ' : y . BY THE SAME AUTHOR. Oriain oF Species By MEANS oF Naturat SELECTION, OR THE PRESERVATION OF FAVORED RACES IN THE STRUG- GLE For Lirk. From sixth and last London edition. 2 vols.,12mo. Cloth, $4.00, Descent oF Man, anp SEtvEcTION IN RELATION TO SEX. With many Illustrations. A new edition. 12mo. Cloth, $3.00. JOURNAL OF RESEARCHES INTO THE Naturat History AND GEOLOGY OF COUNTRIES VISITED DURING THE VOYAGE oF H. M.S. BEAGLE RouND THE WorRLD. New edition. 12mo. Cloth, $2.00. 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THE STRUCTURE AND DISTRIBUTION OF CORAL REEFS BY CHARLES DARWIN, M.A., F.R.S., F.G.S. THIRD EDITION WITH AN APPENDIX BY PROF. T. G. BONNEY, D.Sc., F.R.S., F.G.S. WITH ILLUSTRATIONS NEW YORK D. APPLETON AND COMPANY 1889 Authorized Edition. i PREFACE TO Pat LEIRD:- EDITION. For all that distinguishes the present from the second edition the reader has to thank Professor Bonney. He has added occasional footnotes (distinguished by square brackets), and he has given, in the form of an appendix, a careful summary of the more important memoirs published since 1874. My own contribution is merely the fulfilment of a pleasant duty—the expression of my sincere gratitude to Professor Bonney for the ready kindness with which he undertook a difficult task, and for the care and skill with which he has completed it. I must also be allowed the satisfaction of expressing my obligations to Captain Wharton, R.N., Hydro- grapher to the Admiralty, for an interesting series of notes, which are embodied by Professor Bonney in the present edition. Francis DARWIN. CAMBRIDGE; February 28, 1889. PREFACE TO THE SECOND EDITION. THE FIRST EDITION of this book appeared in 1842, and since then only one important work on the same subject has appeared, namely, in 1872, by Professor Dana, on Corals and Coral-Reefs. In this work he justly says that I have not laid sufficient weight on the mean temperature of the sea, in determining the distribution of coral-reefs; but neither a low tempera- ture nor the presence of mud-banks accounts, as it appears to me, for the absence of coral-reefs through- out certain areas; and we must look to some more recondite cause. Professor Dana, also, insists that voleanic action prevents the growth of coral-reefs much more effectually than I had supposed; but how the heat or poisonous exhalations from a voleano PREFACE TO THE SECOND EDITION. Vil can affect the whole circumference of a large island is not clear. Nor does this fact, if fully established, falsify my generalisation that volcanos in a state of action are not found within the areas of subsidence, whilst they are often present within those of elevation ; for I have not been influenced in my judgment by the absence or presence of coral-reefs round active volcanos ; I have judged only by finding upraised marine remains within the areas of elevation, and by the vicinity of atolls and barrier-reefs with reference to the areas of subsidence. Professor Dana apparently supposes / (p. 820) that I look at fringing-reefs as a proof of the recent elevation of the land; but I have ex- pressly stated that such reefs, as a general rule, indicate that the land has either long remained at the same level or has been recently elevated. Neverthe- less, from upraised recent remains having been found in a large number of cases on coasts which are fringed by coral-reefs, it appears that of these two alternatives recent elevation has been much more frequent than a stationary condition. Professor Dana further believes that many of the lagoon-islands in the Paumotu or Low Archipelago and elsewhere have recently been elevated to a height of a few feet, although originally formed during a period of subsidence; but I shall endeavour to show in the sixth chapter of the present edition that lagoon-islands which have long Vill PREFACE TO remained at a stationary level often present the false appearance of having been slightly elevated. Although I thus demur to some of the remarks and criticisms made by this eminent naturalist, who has examined more coral formations than almost any other man, yet I do not the less admire his work.! It has also afforded me the highest satisfaction to find that he accepts the fundamental proposition that lagoon- islands or atolls, and barrier-reefs, have been formed during periods of subsidence. The late Professor Jukes, in his account of the voyage of H.M.S. Fly, published in 1847, devoted a chapter to the Barrier-Reefs of Australia, and thus concludes: ‘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.’ On the other hand, a distinguished naturalist, Professor Semper, differs much from me, although he seems willing to admit that some atolls and barrier- reefs have been formed in the manner in which I suppose. I will give in the Appendix, under the head ‘ A friendly reply from Professor Dana, contesting some of the points mentioned above, will be found in Nature, Sept. 1874, p. 408. THE SECOND EDITION. 1X of the Pelew Islands, which were carefully examined by him, some account of his objections, and I will here only state that his view does not differ essentially from that of Chamisso, which will hereafter be discussed. It will be seen that the evidence in favour of atolls and barrier-reefs having been formed during sub- sidence is of a cumulative nature; and that it is very difficult to judge with safety respecting any single lagoon-island or barrier-reef, or small group of them, even if the depth outside the reef and the slope of the encircled land are both known. In the present edition I have added some new facts and have revised the whole book ; the latter chapters having been almost re-written. The appended map of - the Pacific and Indian Oceans remains in nearly the same state as before, for I have added only two red and two blue circles. Ihave removed an active vol- cano, which was formerly supposed to exist in Torres Straits. An account of a remarkable bar of sandstone off Pernambuco on the Brazilian coast has been added to the Appendix, as this bar is protected from the wear and tear of the waves by a coating of organic bodies, in the same manner as are most coral-reefs. It also resembles a coral-reef in shape or outline to a curiously deceptive degree. If I had been better situated during the last thirty years, for hearing of recent discoveries in the Pacific, and for consulting xX PREFACE TO THE SECOND EDITION. charts published in various countries, my map might have been greatly improved. But I hope that before long some one may be induced to colour a map on a large scale, on nearly the same principles as I have done, and in accordance with our advanced state of geographical knowledge; for I believe that he would thus arrive at some new and striking generalisations. Down, Brcxenuam, Kent: Pebruary 1874. PREFACE TO THE FIRST EDITION. I SHALL HAVE OCCASION, in many parts of the follow- ing volume, to acknowledge the valuable information I have received from several persons; but I must more particularly express my obligations to Captain R. Moresby, I.N., who conducted the survey of the Red Sea, and of the archipelagoes of low coral-islands in the Indian Ocean. I beg, also, to be permitted to return my best thanks to Captain Beaufort, R.N., for having given me free access to the charts in the Ad- miralty, as well as to Captain Beecher, R.N., for most kindly aiding me in consulting them. My thanks are likewise especially due to Captain Washington, R.N., for his invariable desire to assist me in every possible manner. Having in former publications had the pleasure of acknowledging how much I owe to Captain Xl PREFACE TO THE FIRST EDITION. FitzRoy, for having permitted me to volunteer my services on board H.M.S. Beagle, and for his uniform kindness in giving me assistance in my researches, I can here only repeat my obligations to him. The materials for this volume were nearly ready two years ago; but owing to ill health, its publication has been delayed. The two succeeding Parts—one on the vol- canic islands visited during the voyage of the Beagle, and the other on South America—will appear as soon as they can be prepared. May 2, 1842. CONTENTS. INTRODUCTION : ° ; - ° P ° A e PAGE I CHAPTER I. ATOLLS OR LAGOON ISLANDS. SECTION I.—DESCRIPTION OF KEELING ATOLL, Corals on the outer margin—Zone of Nullipore—Exterior reef—Islets —Coral-conglomerate—Lagoon—Calcareous sediment— Scari and Holuthurie subsisting on corals—Changes in the condition of the reefs and islets—Probable subsidence of the atoll—Future state of the lagoons. : : ‘ * . : ° > 7% to 27 SECTION II.—GENERAL DESCRIPTION OF ATOLLS. General form and size of atolls, their reefs and islets—External slope —fZone of Nullipore—Conglomerate—Depth of lagoons—Sedi- ment—Reefs submerged wholly or in part—Breaches in the reef —Ledge-formed shores round certain lagoons—Conversion of lagoons into land . : : - : : ‘ . 27 to 43 SECTION III.—ATOLLS OF THE MALDIVA ARCHIPELAGO— GREAT CHAGOS BANK. Maldiva Archipelago—Ring-formed reefs, marginal and central— Great depth 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 dissever- ment of large atolls—The Great Chagos Bank—Its submerged condition and extraordinary structure. . . » 43 to 55 2 X1V CONTENTS. 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_. ; : A « PAGE 56 to 68 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 . : : ° ° « 69.t0 79 CHAPTER IV. ON THE GROWTH OF CORAL-REEFS. SECTION I.—ON THE DISTRIBUTION OF CORAL-REEFS, AND ON THE CON- DITIONS FAVOURABLE TO THEIR INCREASE : s - 80 to 95 SECTION II1.—ON THE RATE OF GROWTH OF CORAL-REEFS . 95 to 108 SECTION III.—ON THE DEPTHS AT WHICH REEF-BUILDING CORALS LIVE . ; : s ; “4 ‘ ; - 108 to lls 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 earth- quakes on atolls—Recent changes in their state—The origin of barrier-reefs and of atolls—Their relative forms-—The step-formed CONTENTS. XV 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 subsi- dence required by the theory, considered —The probable composi- tion of the lower parts of atolls : . ~ PAGE 119 to 157 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—Oscilla- tions of level—Absence of active volcanos in the areas of subsi- dence—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 alter- nation with those of elevation—Amount, and slow rate of sub- sidence—Recapitulation . : ; : ° - 158 to 196 APPENDIX [1] Containing a detailed description of the Reefs and Islands in the coloured Map, Plate III. . A . : ° - 199 to 280 [APPENDIX IL] [Summary of the principal contributions to the History of Coral- Reefs since the year 1874] - . : . - 281 to 33: GENERAL INDEX. : ‘ ° ° : Prep es s - 3833 cane vpagehs rae aah e a) eye DESCRIPTION OF THE PLATES. PLATE I. at end of Volume. In the several original surveys, from which the small plans on this plate have been reduced, the coral-reefs are engraved in yery dif- ferent styles. For the sake of uniformity, I have adopted the style used in the charts of the Chagos Archipelago, published by the East India Company, from the survey by Capt. Moresby and Lieut. Powell. The surface of the reef, which dries at low water, is represented by a stippled surface with small crosses: the coral- islets on the reef are marked by small linear unstippled 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, and has been tinted, in order to catch the eye, of a pale blue colour. 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 at- tained 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 withia each reef; they are in fathoms, of six English feet. Fig. 1—Vanrxoro, situated in the western part of the S. Pacific; taken from the survey by Capt. D’Urville in the Astrolabe; the scale is + of an inch to a geographical mile; the soundings on the southern side of the island, namely from 30 to 40 fathoms, XVIll DESCRIPTION OF PLATES. are given from the Voyage of the Chev. Dillon; the other sound- ings 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 repre- sented. 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 imagina- tion the barrier-reef) as a good specimen of an abruptly-sided island, surrounded by a reef of the fringing class. Fig. 2.—Hogoxzv, or Rove, in the Caroline Archipelago; taken from the atlas of the Voyage of the Astrolabe, compiled from the surveys of Captains Duperrey and D’Urville; scale 3, of an inch to a mile; the depth of the immense lagoon-like space within the reef is not known. Fig. 3.—RataTEa, in the Society Archipelago; from the map given in the quarto edition of Cook’s First Voyage; it is probably not accurate; scale +, of an inch to a mile. Fig. 4.—Bow, or Hryov atouy (or lagoon-island), in the Low Arch pelago; from the survey by Capt. Beechey, R.N.; scale ;4 of an inch to a mile; the lagoon is choked up with reefs, but the average greatest depth of about 20 fathoms, is given from the published account of the voyage. Fig. 5.—Botrazona, in the Society Archipelago; from the survey of Capt. Duperrey, in the Coquille; scale + of an inch to a mile; the soundings in this and the following figure have been altered from French feet to English fathoms; height of highest point of the island 4,026 feet. Fig. 6.—Mavrva, in the Society Archipelago; from the survey by Capt. Duperrey inethe Coqwille; scale + of an inch to a mile; height of land about 800 feet. Fig. 7.—Povynipete, or SeNrIAVINE, in the Caroline Archipelago; from the survey by Admiral Lutké; scale } of an inch to a mile. Fig. 8.—Gamprer Istanps, in the southern part of the Low Archi- pelago; from the survey by Capt. Beechey; scale ¢ of an inch to a mile; height of highest island, 1,246 feet; the islands are sur- rounded by extensive and irregular reefs; the reef on the southern side is submerged. Fig. 9.—Pxros Bannos stout (or lagoon-island), in the Chagos group in the Indian Ocean; from the survey by Capt. Moresby and DESCRIPTION OF PLATES. X1X Lieut. Powell; scale } of an inch to a mile; not nearly all the small submerged reefs in the lagoon are represented ; the annular reef on the southern side is submerged. Fig. 10.—Kzeruine, or Cocos atoun (or lagoon-island), in the Indian Ocean; from the survey by Capt. FitzRoy; scale + of an inch to a mile; 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 N.W. side is broken, and blends into a shoal sand-bank, on which the sea breaks. PLATE II. at end of Volume. Fig. 1.—Great Cuacos Banr, in the Indian Ocean; taken from the survey by Capt. Moresby and Lieut. Powell; scale = of an inch to a mile (same scale as Hogoleu, in Plate I.); 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 4 to 10 fathoms; the banks bounded by the dotted lines lie from 15 to 20 fathoms beneath the surface, and are formed of sand; the central space is of mud, and from 30 to 50 fathoms deep. Fig. 2.—A vertical section, on the same scale, in an EK. and W. line across the Great Chagos Bank, given for the sake of exhibiting more clearly its structure. Fig. 3.—MeEncutcorr atouu (or lagoon-island), in the Marshall Archi- pelago, northern Pacific Ocean; from Krusenstern’s atlas of the Pacific; originally surveyed by Capt. Hagemeister; scale 34, of an inch to a mile; the depth within the lagoons is unknown. Tig. 4.—Mantos Maupoo atoun, together with Horsburgh atoll, in the Maldiva Archipelago; from the survey by Capt. Moresby and Lieut. Powell; scale 4, of an inch to a mile; 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 (+) 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. XX DESCRIPTION OF PLATES. Fig. 5.—Nuw Carxpont, 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; scale 3, of an inch to a mile. Tig. 6.—Matpriva ArcuIpenaco, in the Indian Ocean; from the survey by Capt. Moresby and Lieut. Powell; scale 4, of an inch to a mile. PLATE III. at beginning of Volume. The principles on which this map is coloured are explained in the beginning of Chapter VI.; and the authorities for colouring each particular spot are detailed in the Appendix. The names printed in italics in the Index refer to the Appendix. Pe ne ° i) ‘ow ¥ ‘ a y f es + . ‘ ‘ f * 4 ‘ 4 . . is ’ | ‘ my] aft vi ; - t ' ! 4 . « 1 en BG We : . PR = 4 ; i i ‘ , 3 ‘ * ’ t 4 ’ « ea! ‘é » 1 Plate 3._ SHEWING THE DISTRIBUTION OF THE DIFFERENT KINDS OF CORAL REEFS, TOGETHER WITH THE POSITION OF THE ACTIVE VOLCANOS IN THE MAP. (SEE NOTE IN LEFT HAND CORNER) 30 A\6 6O ‘ ‘ 9/0 10/5 120 13/5 12;0 Mediter a =a an ei —— Ch ae ai a a F = 0 a a | = Li) = = = = = = = SI =u = = = = Cea = = I = — ranean e. ‘ ci =. * South] + GespoT *Moorl | cas eae Fe ] = | a ze A | | | LPatrocinio + a 7 ‘atrocinio | @ ‘ | fy | Bonin sima. | peer I : | pe a 3 | Lisianskil” | el 2 » SebastianLobos I | tt | Gulf of} Mexico | . Canton "+" | | | 3 pone | ‘ ee : -FolgerI. e fz | : ace, coo, Hoes et eo, P ‘ Are rae Gulf of Bengal Papal. | Se Kevilla Gigedo IF ere ee “4 ° + Smith + [ - ® f | +? Bunker I: 2 — - E = = a we — x a 7 — oe ee oe = =e ee ue —— = b a = sae a <_ a ABYSSYNTA t | a < Mani : | ‘ & Gaspar Rico rie 7 scares ye a ub oir See ee Ee ( a 4 mt Ga | ; Bah ot Manele © Ae Jom! | . Andaman [2 9@:® ‘20 ») - Laceadives @ | : @ y | | A C ] F ] Cc oe aan Marshall . | * cB) 1 | 3 Nitobar i Vs¥ Ceylon 4 | @ geo &* Palmyral@ | . Natunas a Pasa Maldivas | ) ; | @ ey @ | @ Oristnast 7 | | o- = ee | Equator | | == aes ‘2 ate Equa tox | Gilbert If th a Ble — —z ) | Galapagos “he | . © (80) Sydney I - | SAu i Chagos ugustine] @) + , 2 ‘ = | ts . Nicel? @” or, Va | = tac é @ % © de Matha & S!Cruz If @: basi hens ae aitez | Gulf ik ak 8 3) Rotolma | TE; Siovarrow] —~ 4 | ahh . A? _ 1 : z eel = =e ee ”y ee PAE Oe: Tae a (| San —— : = : 4 2) Cargados Garajos | 6 | SL Viti I! MLS’ Maria = ~ . - e . | 7 | @ Lb | = or Ady, \Palmerstond? | I N pai |A N Or Gl BAN | | a °, | ~ Rodriquez WE) &@ | 6...) | & nd) | Bourbon T Wes | “Isles | | @ | | PitoairnT* @ ducer + +| Salas y Gomex | ) E aye AT EasterT * wks . | | Houatguay Norfollel O G Torr = [\ N 30 | Aa ge ee re a aK | Swan R. Bright blue Atolle or lagoon Is.¥ | | ee dl Fale blue Barr Reefs | | | hed Fringing fects |g Amsterdam! | CHowe | | Tamilion spots & streal:s active voleanoes Prull | | | ; Blase Straw / | N.B. for farther partiaidars see beginnings of Chap.lVT. and Appendivc | $ | | B. for further p 2 ” hap VI. and Append | | | = — - - J _ at y cit Pm ti aust r wi I I E ETI ra ua ro yo — pa et es es pe ie ~- Be — a — re — seat — ve — en a ey er ee ee — ey — a es ee pe er — a Pes pes — py — re — er [ae a ep a — et — eo — ° Alt ; 7/6 East. of 1G5 150 gir . 30 bo 6|O 5 LS 0 Pha : | 15)5 West of 1210 Greenwich 10!5 910 : - = = = ae Published by Smith, Elder, & Co. Waterloo Place. TUE STRUCTURE AND DISTRIBUTION OF CORAL-REEFS. INTRODUCTION. Tne opsect of this volume is to describe from my own observation and the works of others, the principal kinds of coral-reefs, and to explain the origin of their peculiar forms. I shall not here treat of the poly- pifers, which construct these vast works, except as to their distribution, and 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 atten- tion; and it is not surprising, for everyone must be struck with astonishment, when he first beholds one of 2, INTRODUCTION. 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 out- side by the foaming breakers of the ocean, and on the inside surrounding a calm expanse of water, which, from reflection, is generally 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 coral-polypifers, 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, enuironné d’un grand bane de pierre tout autour, n’y ayant point d’artifice humain.’ The above sketch of Whitsunday Island, in the S. Pacific, taken from Capt. Beechey’s admirable Voyage, although excellent of its kind, gives but a INTRODUCTION, 3 faint idea of the singular aspect of one of these lagoon-islands. 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 sepa- rate 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 coral formations by their inhabitants in the Indian Ocean, and is syn- onymous with ‘ lagoon-island.’ Barrier-reefs, when encircling small islands, have been comparatively little noticed by voyagers; but Nowe: 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 Bo- 4 INTRODUCTION. labola,' one of the Society Islands. Here, as in Whit- sunday Island, the whole of that part of the reef which is visible is converted into land. This is a circum- stance 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 ex- tension. They differ from barrier reefs in not lying far from the shore, and in not having within them 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 1s very shallow; these are allied in most respects to fringing reefs, but 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 1 I have taken the liberty of simplifying the foreground, and leaving out a mountainous island in the far distance. INTRODUCTION. 5 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 on the map (Plate III.) represent this main division, as explained in the be- ginning 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 geo- sraphical 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 ean 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 to 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 3 6 INTRODUCTION. with relation to the areas of recent elevation, and to the points subject to volcanic eruptions, fully accord with this theory of their origin.! 1 A brief account of my views on coral formations, now published in my Journal of Researches, was read May 31, 1837, before the Geo- logical Society, and an abstract has appeared in the Proceedings. CHAPTER I. ATOLLS OR LAGOON-ISLANDS. SECTION FIRST, KEELING ATOLL. Corals on the outer margin—Zone of Nullipore—Exterior reef— Islets —Coral-conglomerate —Lagoon—Calcareous sediment — Scart and Holuthurie subsisting on corals—Changes in the con- dition of the reefs and islets—Probable subsidence of the atoll— Future state of the lagoon. Krewine or Cocos atoll is situated in the Indian Ocean, in 12° 5’ §. and long. 90° 55’ E.: areduced chart of it, from the survey of Capt. FitzRoy and the officers of H.M.S8. Beagle, is given in Plate I. fig. 10. The oreatest 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 wood- cut (No. 8) 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 8 ATOLLS. Cu. L mark. I will describe the section, commencing with the outer margin. But 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, an intelligent resident on these islands, as well as by some chiefs at Tahiti (Ota- 150 Yards | ne ae Shs rate ee oi A—tLevel of the sea at low water: where the letter A is placed, the depth is 25 fathoms, and the distance rather more than 150 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. neite), that an exposure to the rays of the sun for a very short time invariably causes their destruction.! Hence 1 (This would be true of certain genera or species, but according to the observations made during the voyage of the Challenger (Re- ports xvi. pp. 23, 36), and by Mr. Guppy (Proc. R. 8. Edin. xiii. p. 863), there are some which can bear exposure for a limited time, perhaps two or three hours, especially if occasionally wetted by the spray.] Srcr. I. KEELING ATOLL. 9 it is possible only under the most favourable circum- stances, 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 Astrea, 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 up- ward 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 shore- ward 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 10 ATOLLS. Cu. L Porites, is the Millepora complanata.! It grows in thick vertical plates, intersecting each other at various angles, and forms an exceedingly strong honey-combed mass, which generally assumes 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 in- variably 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 cer- tainly does not attain the thousandth part in bulk of the masses opposed to the breakers. The wood-cut (No. 8) shows the form of the bot- tom outside the reef: the water deepens very gradually for a space of between one and two hundred yards wide, to a depth of 25 fathoms (A in section), beyond which the sides plunge into the unfathomable ocean at an angle of 45°?. To the depth of ten or twelve 1 This Millepora, (Palmipora of Blainville,) as well as the M. alci- cormis, possesses the singular property of stinging the skin where it is delicate, as on the face and arm. 2 The soundings from which this section is laid down were taken with great care by Captain FitzRoy himself: he used a bell-shaped Srer. I. KEELING ATOLL. ll 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 Astrea, apparently alive. JI 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, appa- rently the JZ. corymbosa, were the most abundant. AsI searched in vain in the hollows on the reef and in the lagoon, for a living specimen of this Madrepore, I con- clude that it is confined to a zone outside, and beneath the surface, where it must be very abundant. Frag- ments of the Millepora alcicornis and of an Astrea were also numerous; the former is found, but not in proportionate numbers, in the hollows on the reef; but the Astrea 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 wood-cut 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. TZ ATOLLS. Cu. I. by an uneven line) round and beneath the external margin. Between 12 and 20 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 18 and 16 fathoms. Out of twenty-five soundings taken at a greater depth than 20 fathoms, every one showed that the bottom was covered with sand; whereas at a less depth than 12 fathoms, every sounding showed that it was exceedingly rugged, and free from all extraneous particles. Two soundings were obtained at the depth of 860 fathoms, and several between 200 and 800 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 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 fact of the line having been cut, when between 500 and 600 fathoms were out, Srcr. I. KEELING ATOLL. 13 indicates the probable existence of submarine cliffs of rock. 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.!. 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 casesis hard, but where formed of the lichen-like kind, readily yields an impression to the hammer: the surface is of a reddish colour. These Nullipors, although able to exist above the limit of true corals, seem to require to be bathed during the ereater part of each tide by breaking water, for they are not found inany abundance in the protected hollows on the back part ofthe reef, where they might be immersed during either the whole or an equal proportional time ofeach tide. Itis remarkable that organic productions of such extreme simplicity, for the Nullipore undoubt- edly belong to one of the lowest classes of the vegetable kingdom, should be limited to a zone so peculiarly cir- 1 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. 14 ATOLLS. Cie cumstaneed. Hence thelayer composed by their growth, merely fringes the reef for a space of about 20 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 intoa solid margin, as a continuous smooth convex mound (B in wood-cut) 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 growthof Nullipore on the outer margin, the part most exposed to the breakers, and this must effectually aid in preserving it from being worn down. The wood-cut (at p. 8) 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 the reef, as it occurs where islets have not been formed. It is this reef which essen- tially forms the atoll. In Keeling atoll the ring encloses 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 250 to 500 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 sur- face, into the lagoon: a return stream sets out of the Sor, I, KEELING ATOLU. 15 Jagoon through the main entrance. The most frequent coral in the hollows on the reef is Pocillopora ver- rucosa, which grows in short sinuous plates, or branches, and when alive is of a beautiful pale lake-red: a Mad- repora, closely allied or identical with M. pocillifera, is alsocommon. As soon as an islet is formed, and the waves are prevented from breaking entirely over the reef, the channels and hollows become filled up with fragments cemented together by calcareous matter ; and the surface of the reef is converted into a hard smooth floor (C of wood-cut), like an artificial one of free- ' stone. This flat surface varies in width from 100 to 200, or even 300 yards, and is strewed with a few large fragments of coral torn up during gales : itis 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. No- thing 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 Nullipore, 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 wood-cut of Whit- sunday Atoll. The islets on the reef are first formed between 200 and 300 yards from its outer edge, through the accu- 16 ATOLLS. Cu. I. mulation 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 §.E. and windward side of the atoll, increase solely by the addi- tion 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 exclu- sively consist of those kinds which live on the outer coast. The highest part of the islets (excepting hil- locks of blown sand, some of which are 80 feet high), is close to the outer beach (H of the wood-cut) and aver- ages from six to ten feet above ordinary high-water mark. From the outer beach the surface slopes gently to the shores of the lagoon; and this slope no doubt is due to 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 800 yards in width; but the land thus added is verylow. The fragments beneath the surface are cemented into a solid mass, which is exposed asa ledge (D of the wood-cut), 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 — Sect. I. KEELING ATOLL. {Bed 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 are firmly cemented together by percolated calcareous matter, and they resist the daily tides longer than the loose upper fragments ; and thus a projecting ledge is formed. The cemented mass is generally of a white colour, but in some few parts reddish from ferruginous matter : it is very hard and 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 recemented: or it consists of a calcareous sandstone, entirely composed of rounded particles of shells, corals, the spines of echini, and other organic bodies generally almost blended together ;—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 col- lected an 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 + 18 ATOLLS. Cu. I. 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, from three to four fathoms, and smaller basins from eight to ten fathoms deep. Probably about half its area consists of sedi- ment, and half of coral-reefs. The corals composing these reefs have a very different aspect from those on the outside: they are numerous in kind, and most of them are thinly branched. Meandrina, however, lives in the lagoon, and many great rounded masses of this coral lie loose or almost loose on the bottom. The other most common species are three closely allied species of true Madrepora with thin branches ; Seriatapora subulata; two species of Porites? with cylindrical branches, one of which forms circular clumps, with only the exterior branches alive; and lastly, a coral something like an Explanaria, but with stars on both surfaces, growing in thin, brittle, stony, foliaceous ’ [Dead coral still lying on the beach has been found to contain at least 5 per cent. of carbonate of magnesia, though only a very small quantity is present in fresh coral (usually less than 1 per cent.). In old coral-rock as much as 38-07 per cent. has been found (Dana’s Corals and Coral Islands, ch. vi. § 9).] * This Porites has somewhat the habit of P. clavaria, but the branches are not knobbed at their ends. When alive it is of 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. Secr. I. KEELING ATOLL. 19 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 surround- ing the lagoon; nor can they be nearly so hard, for the inhabitants by the aid of crowbars made a channel of considerable length through these reefs, in which a schooner, built on the §.E. islet, was floated out. Itis 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 §.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 sub- stance, brought by Lieut. Nelson from the reefs of Bermuda, which, when shewn 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 asmall degree. There are, however, 20 ATOLLS. Cu. I. 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 cal- careous 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 Holuthurize (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 Holuthuria, and of the individuals which swarm on every part of these coral-reefs, is extraordinarily great; and many ship-loads are, as is well known, annually freighted for China with trepang, which is a species of this genus. The amount of coral yearly consumed, and ground down into the finest mud, by these several crea- tures, and probably by many other kinds, must be immense. These facts are, however, of more importance } (Mr. Guppy, Proc. R. 8. Edin. xiii. p. 894, expresses the opinion that the Holothurians do not subsist on the living coral, but obtain nutriment from swallowing the sand and detrital material, of which broken coral forms a large constituent.] Srot. I, KEELING ATOLL. oF 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 ‘consume 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 for- mations, has been entirely formed by the growth of organic beings, and the accumulation of their detritus, one is naturally led to enquire, how long it has con- tinued, 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 8... 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 half way 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 eleva- hy ATOLLS Cu. I. tion 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 §.E. wind, at the head than at the mouth of thelagoon. The corals, which, under the former condition of things, had at- tained 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 srown 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 vegeta- tion 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 Srcr. I. KEELING ATOLL. oo reef, was evident from the conglomerate having been worn into a point projecting from the beach in a simi- larly oblique manner. This retreat in the line of action of the breakers may have resulted, either from the surface of the reef in front of the islets having formerly been submerged, and afterwards having been raised by accumulated fragments, or from the mounds of coral on the margin having grown outwards. That an outward growth of this part is in process, can hardly be doubted from the existence 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 in 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 Nullipore, and so much of the surface would have been of equal height with the existing zone of living Nullipore: this is not the case, as may be seen in the wood-cut. It is, however, evident from the abraded state of the ‘flat,’ with its original ine- qualities filled up, that its surface has been much modified; and it is possible that the inner portions of the zone of Nullipore, perishing as the reef grows out- wards, 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 Nullipore formed the convex mound on its margin: for the zone thus formed, which stands 24 ATOLLS. Cu. I. 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, an increase in the diameter of the atoll; but there are others having an opposite tendency. On the §.E. side, Lieut. Sulivan, to whose kindness I am indebted for many interesting observations, found the conglomerate (D, in wood-cut p. 8) projecting on the reef nearly fifty yards in front of the islets: we may infer from what we elsewhere see 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 frag- ments 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 Lieut. 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 en- croachment 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 Sect. I. KEELING ATOLL. 25 stumps of others on the beach, where the inhabitants assured us the cocoa-nut could not now grow. Capt. 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 water-mark. 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 con- clude 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 26 ATOLLS. Cu. L 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 it is broken by rolling over a wide space, their increase in breadth must depend on the in- creasing 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, in- stead 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 up the lagoon. We know so little of the habits of the many different species of corals which form the lagoon-reefs, that we have no more reason 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 Scr. IT, ATOLLS. 27 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 remem- bered, 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 SECOND. General form and size of atolls, their reefs and islets—Ezternal slope—Zone of Nullipore—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 lai d. I wit 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, compar- ing 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 Capt. 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 28 ATOLLS. Cu. I. 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 Fig. 4, Plate L.), 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, how- ever, bears a double name), measured in a medial and slightly curved line, is no less than eighty-eight geo- eraphical miles long, its greatest width being under twenty, and its least only nineand 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 Menchioff Island (See Fig. 8, Plate II.), which is sixty miles in length, and consists of three loops tied together. In far the greater num- ber of cases an atoll consists of a simple elongated ring, with its outline moderately regular. The average width of the annular reef may be taken at about a quarter of a mile. Capt. Beechey! 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 islands of Keeling atoll, appears to apply perfectly to nearly all the atolls in the Pacific and Indian ' Beechey’s Voyage to the Pacific and Behring’s Straits, chap. viii. Sret. II, ATOLLS. 29 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 en- trances 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 Lutké’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 annu- lar reef and the land is in islands of this class, I may quote a remark from the voyage of Lutké, namely, that if the forty-three rings, or atolls, in the Caroline Archi- pelago were put one within another, and over a steeple in the centre of St. Petersburg, the whole would not cover that city and its suburbs. 5 30 ATOLLS. Cui The form of the bottom, as given by Captain Beechey in his sections of the atolls in the Low Archipelago, exactly coincides with that already de- scribed in Keeling atoll: it 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° into unfathomable depths.! The nature, however, of the bottom seems to differ, for this officer? 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° 4’ N., 157° 45’ W.), de- scribed by Cook,? 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. 1 The slope of the,bottom round the Marshall atolls in the Northern Pacific is probably similar: Kotzebue (First Voyage, vol. ii. p. 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.’ 2 Tmust 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 tc own the great assistance I have derived from his excellent published work. * Cook’s Third Voyage, vol. ii. chap. 10. Srcr. IL. ATOLLS. 31 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, Licut. Powell ! found 50 and 60 fathoms close to the edge of the reef, and at 300 yards distance there was no bottom with a 800 yard line. Capt. 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 remark- able, 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 two hundred fathoms! The currents run with great force round these atolls, and where they are strongest, the inclination appears to be most abrupt. Iam informed by the same authority, that wherever soundings were obtained off these is- lands, the bottom was invariably sandy: nor was there any reason to suspect the existence of submarine cliffs, as there was at Keeling Island.? Here, then, occurs a 1 This fact is taken from a MS. account of these groups lent me by Capt. Moresby. See also Capt. Moresby’s paper on the Maldiva atolls in the Geographical Journal, vol. v. p. 401. 2 Off some of the atolls in the Low Archipelago the bottom appears to descend by ledges. Off Elizabeth Island, which consists of raised coral-rock, Capt. Beechey (p. 45, quarto ed.) describes three ledges: the first slopes gently from the beach to a distance of about fifty yards; the second extends two hundred yards with a depth of an ATOLLS. Cu. I. 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 sound- ings 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! 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 appear to indicate a greater angle, and the ex- treme 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. Capt. 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? the buttress, as it may be called, which ‘has the most twenty-five fathoms, and then ends abruptly, like the first; and immediately beyond this there is no bottom with two hundred fathoms. 1 Mémoires pour servir & une description Géolog. de France, tome iv. p. 216. * Beechey’s Voyage, 4to. ed. p. 44. Srer. II. ATOLLS. 33 powerful enemy (the §.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 go, the reef is very gently inclined. Hence it appears that the exterior angle is much varied. We can, however, discern the effects of uniform laws in the close similarity in form between the sections of Keel- ing 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; 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 called the ‘ flat,’ and which is partly dry at low water, appears similar in every atoll. In the Marshall group in the N. 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: ot ATOLLS. Cu. I. Flinders states that the Australian barrier has a similar inclination inwards, and I have no doubt it is of general occurrence, although, according to Khren- berg, 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 stalactitical form,’—a description perfectly applicable to the margin of Keeling atoll. Although Chamisso does not state that the masses of Nullipore form points or a mound, higher than the flat, yet I believe that this is the case; for Kotzebue,” 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 certain are not formed of true coral. 1 Kotzebue’s First Voyage, vol. iii. p. 142. Near Porto Praya, in the Cape de Verde Islands, some basaltic rocks, lashed by no incon- siderable surf, were completely enveloped with a layer of Nullipora. The entire surface over many square inches, was coloured of a peach- blossom 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 Nullipore are closely related to those de- scribed on the coral-reefs, but I believe are of different species. 2 Kotzebue’s First Voyage, vol. ii. p. 16. Lieut. Nelson, in his excellent memoir in the Geological Transactions (vol. ii. p. 105), alludes to the rocky points mentioned by Kotzebue, and infers that they consist of Serpule, which compose incrusting masses on the reefs of Bermudas, as they likewise do on a sandstone-bar off the coast of Brazil, as described by me in the London Phil. Journal, Oct. 1841. I have added my description as a short supplement to the present volume. These masses of Serpule hold the same posi- tion, relatively to the action of the sea, with the Nulliporw on the coral-reefs in the Indian and Pacific oceans. 8 Capt. Moresby, in his valuable paper ‘On the Northern Atolls Szcr. II. ATOLLS. GO Whether a smooth convex mound of Nullipora, 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 en- circle 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! 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,? 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 88 fathoms, and in the Marshall Group, according to Chamisso, from 80 to 35: in the Caroline atolls it of Maldivas’ (Geographical Journal, vol. v.), says that the edges of the reefs there stand above water at low spring tides. 1 Kotzebue’s First Voyage, vol. iii. p. 144. 2 See also Moresby on the Northern Atolls of the Maldivas, Geo- graphical Journal, vol. v. p. 400. 36 ATOLLS. Cu. I. is only alittle less. Within the Maldiva atolls there are large spaces with 45 fathoms, and some soundings are laid down at 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 levelled 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 Ber- muda already referred to, as undistinguishable from disintegrated chalk, and which Lieut. Nelson says is called there pipe-clay.! : 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 length on the more exposed shore. The islets, also, which are placed 1 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 Brésil, 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 com- posed of ‘tuf blanc, mélé ou formé de madrépores broyés.’ 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. [Probably the clay is commonly similar to that mentioned by Mr. Guppy (Proc. R. S. Edin. vol. xiii. p- 879 n.) and others. See the abstract of his paper in Appendix II.] Srcr. II. ATOLLS. St to leeward as regards the trade-wind, are in most parts of the Pacific liable to be occasionally swept entirely away by gales, equalling hurricanes in vio- lence, which blow in the opposite direction. The absence of islets on the leeward side of atolls, or, when present, their lesser dimensions compared with those to windward, is a comparatively unimportant fact ; but it is remarkable that in several instances the reef itself, although retaining its usual defined outline, does not rise to the surface by several fathoms on the leeward side. This is the case with the - southern side of Peros Banhos (Plate I. fig. 9) in the Chagos group, with Mourileu atoll! in the Caroline Archipelago, and with the barrier reef (Plate I. fig. 8) of the Gambier Islands, where Captain Beechey was first led to observe the peculiarity in 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: itis, 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 1 Frederic Lutké’s Voyage autour du Monde, vol. ii. p. 291. See also his account of Namonouito, at pp. 97 and 105, and the chart of Oulleay in the Atlas. 38 ATOLLS. Cu. I. the same manner as occurs where there is a 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 Archi- pelago), 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 ofthereef concur ; it consists of a great flat bank, with from 20 to 25 fathoms of water on it; for a length of more than 40 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 thereare 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 22 fathoms deep, is 24 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 that the rim consists of dead rock thinly covered with sand, and he is certain that 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 thesubmerged portion of the reefat Peros Banhos, Capt. Moresby feels sure that the quantity of living coral, even on the outer edge overhanging the deep-sea water, Sxcr. II. ATOLLS. 39 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 are 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 20 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. Sir C. Lyell! 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 1 Principles of Geology, vol. iii. p. 289. [Vol. ii. p. 609, ed. 1872.] 40 ATOLLS. Cu. LE high tide and the greater part of each ebb is thrown over a large portion of its circumference. Several facts indicate that a considerable quantity of sediment is likewise discharged through these channels; and Captain Moresby has observed, during the change of the monsoon, that the sea is discoloured to some dis- tance off the entrances into the Maldiva and Chagos atolls. This would probably check the growth of the coral in the channels, far more effectually than if they merely discharged a current of water. Where there is not any channel, as in the case of many small atolls, — these causes have not prevented the entire ring attain- ing thesurface. The channels, like the submerged and effaced parts of the reef, occur very generally, though not invariably on the leeward side of the atoll, or on that side, according to Beechey,! which, from extending 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 sradually where the bottom is of sediment, and i irregu- larly or abruptly where there are coral reefs ; but this is by no means the universal structure in other atolls. ' Beechey’s Voyage, 4to ed. vol. i. p. 189. Srcr. II. ATOLLS. 41 Chamisso,! speaking in general terms of the lagoonsin 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,? the great exterior reef, the surface of which is gently inclined inwards, ends abruptly in a little submarine cliff three fathoms deep; at its foot, a ledge 40 yards in width also shelves gently in- wards, like the surface-reef, and terminates in a second little cliff five fathoms deep; beyond this, the bottom of the lagoon slopes to 20 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 1 Kotzebue’s First Voyage, vol. iii. p. 142. 2 Beechey’s Voyage, 4to ed. vol. i. p. 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 perpen- dicular. This bank is formed of coral and dead shells. 6 49 ATOLLS. Crake frequently they are quite irrezular—some with per- pendicular, some with sloping sides—some rising to the surface, and others lying at all intermediate depths from the bottom upwards. I cannot, there- fore, suppose that the union of such reefs could pro- duce even one uniformly sloping ledge, and much less two or three one beneath the other, and each ter- minated 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 sur- veyed by him in the Low Archipelago, did not con- tain lagoons. Romanzoff Island (in lat. 15° §.) is described by Chamisso! as formed by a dam of madre- poritic rock inclosing a flat space, thinly covered with trees, into which the sea on the leeward side occasion- ally breaks. North Keeling atoll appears to be ina rather less forward stage of conversion into land: it consists of a horse-shoe shaped strip of land surround- ing a muddy flat, one mile in its longest axis, which is Kotzebue’s First Voyage, vol. iii. p. 221. Srcr, IL ATOLLS. 43 covered by the sea only at high-water. When de- scribing 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 atollsin 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 can- not, I think, be doubted that their lagoons and the islets on their recf, would present a totally different appearance from what they now do. This considera- tion leads to the suspicion that some agency (namely, subsidence) comes into play at intervals, and renovates their original structure. 44 ATOLLS, Cu. I, SECTION THIRD. Maldwa Archipelago—Ring-formed reefs, marginal and central— Great depth in the lagoons of the southern atolls—Reefs in the lagoons all reach the surface—Position of islets, and breaches im the reefs with respect to the prevalent winds and action of the waves— Destruction of islets—Relation in position between dis- tinct 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 consi- deration. My description is derived from an exami- nation of the admirable charts lately published from the survey of Captain Moresby and Lieut. 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 posi- tion in a double line, may be seen, though imperfectly, in the greatly reduced chart (fig. 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 Sror. ILL. MALDIVA ATOLLS. AS 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 northern ones, these portions are ring-formed lke -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. fig. 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 im- perfectly shown : in the large published charts of Tilla- dou-Matte, the appearance of these rings, from stand- ing 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 46 ATOLLS. . Call stand ; their outer margins are invariably bordered by living coral,! within which there is a flat surface of coral rock; on this flat, sand and fragments have in many cases accumulated and been converted into islets clothed with vegetation. They are indeed larger, and contain deeper lagoons than many true atolls standing in the open sea; and I can point out no essential difference between these little ring-formed reefs and the most perfectly characterised atolls, excepting that they are based on a shallow foundation, instead of on the floor of the ocean, and that instead of being scattered irregularly, they are grouped closely together with the marginal rings arranged in a rudely-formed circle. . The perfect series which can be traced from a linear reef like that surrounding an ordinary atoll, to others which are ring-formed and much elongated but con- taining only a very narrow lagoon, and to others which are oval or almost circular, renders it probable that the latter are merely modifications of a linear and normal reef. The fact that the marginal annular reefs generally have their longest axes directed in the line which the exterior linear reef would have held, agrees with this view. We may also infer that the central annular 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 1 Captain Moresby informs me that Millepora complanata is one of the commonest kinds on the outer margin, as it is at Keeling atoll. Sxct. IIL. MALDIVA ATOLLS, 47 ring-like structure in these central reefs is con- | tingent 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 mar- ginal 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 10 to 20 fathoms deeper than those in the northern part. Thisis well exemplified in the case of Addoo, the southernmost atoll in the group, for although only 9 miles in its longest diameter, it has a depth of 39 fathoms, whereas all the other small atolls have comparatively shallow lagoons ; I can assign no adequate cause for this difference in depth, excepting that the southern part of the Archipelago has subsided to a greater degree or at a quicker rate than the northern part ; and this conclusion agrees well with the fact that, in the Chagos group, lying 280 miles still further southwards, most of the atolls are sunken and 48 ATOLLS. Cu. I. half destroyed with the dead corals. In the central and deepest part of the Maldiva lagoons, the bottom consists, as I am informed by Captain Moresby, of stiff clay (probably a calcareous mud) ; nearer the border it con- sists of sand, and in the channels through the reef, of hard sand-banks, sandstone, conglomerate rubble, anda little live coral. Close outside the reef the bottom is sandy, and 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 singular structure of these atolls, —a great sandy and generally concave disk rises abruptly from the unfathomable ocean, with the central expansestudded and the margins 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 salt 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 Srcr. III. MALDIVA ATOLLS. 49 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 the trade wind, the ship-channels into the lagoons are almost always situated on the leeward or less exposed side of the reef, and the reef itselfis 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 near together, the breaches in the reef are most numerous on the sides which face each other, and are therefore less exposed to the waves. Thus on the sides of Ari and the two Nillandoo atolls which face §. Male, Phaleedoo, and Moloque atolls, there are seventy-three deep-water channels, and only twenty-five on the outer sides ;on thethree latter-named atolls there are fifty-six openings on the near side, and only thirty- 50 ATOLLS. Cu. I. seven on the outside. It is scarcely possible to attri- bute this difference to any other cause than the some- what different action of the sea on the two sides, which would ensue from the mutual protection afforded by the two rows of atolls. 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 cther, accords with the fact, that the reefs of the southern atolls are more continuous than those of the northern ones, for the former, as I am informed by Captain Moresby, are more constantly exposed to a heavy surf than are the northern atolls. 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 Sect. IL MALDIVA ATOLLS. ol now being washed away ; and in this latter atoll Lieut. 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, andis (in Lieut. 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 ; but to see this, a larger chartis required than that of the Archipelago in Plate IJ. 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 another atoll, namely Mahlos Mahdoo (fig. 4), at the exact point where the two sides of the latter, if prolonged, would meet: no bottom, however, 52 ATOLLS. Cu. I. was found in the channel with 200 fathoms: in the wider channel between Horsburgh atoll and thesouth- ern end of Mahlos Mahdoo, no bottom was found with 250 fathoms. In these 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 a submarine connection, besides a relation in position and form. 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 con- nection of adjoining atolls, if a hasty glance be taken at the chart (fig. 4, Plate IT.) 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 northern and southern sides of this bifurecating channel Srcr. ITI. GREAT CHAGOS BANK. 53 are elongated, and so continuous that the northern and southern portions of Mahlos Mahdoo may claim to be considered as distinct atolls. But the reefs of the intermediate portion are less perfect, so that this portion hardly yet resembles a distinct atoll. Mahlos Mahdoo, therefore, is in every respect in an inter- mediate condition, so that it may be considered either as a single atoll nearly dissevered into three portions, or as three atolls almost perfect and intimately con- nected. This is an instance of a very early stage of the apparent disseverment of an atoll, and another 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 series such as we have here traced, impresses the mind with the idea of actual change; and it will hereafter be seen, that the theory of subsidence together with the upward growth of the coral-reefs, modified by accidents of probable occur- rence, accounts for the occasional disseverment of large atolls. The great Chagos Bank alone remains to be de- seribed.! In the Chagos group there are some ordi- ' 1 [See Appendix II.] 54 ATOLLS. Cu. I. nary atolls, some annular reefs rising to the surface but without any islets on them, and some atoll-formed banks either quite or nearly submerged. 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. fig. 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 fig. 2, in which the vertical scale has been neces- sarily exaggerated. Its longest axis is ninety nautical miles, and another line drawn across the broadest part, at right angles to the first, 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 he 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. Tt 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 slopes abruptly outwards 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 Scr. IIL GREAT CHAGOS BANK. 55 ‘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 (fig. 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,! ‘no- thing 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. ' 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. 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. Tue 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 con- venient 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 tcgether. The reef, also, on the west coast of New Caledonia, circling round the extremities of the island, is an intermediate form between a small en- circling 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 resem- Cu. Il. BARRIER-REEEFS. 57 blance between the reefs of the barrier and atoll classes may be seen in the small, but accurately re- duced charts on Plate I.,! and this resemblance can be further shown to extend to every part of their struc- ture. Beginning with the outside of the reef; many scattered soundings off Gambier, Ualan, and some other encircled islands, show that close to the breakers there exists a narrow shelving margin, beyond which in most cases, the ocean suddenly becomes unfathom- able. Off the west coast of New Caledonia, Captain Kent * found no bottom with 150 fathoms, at two ship’s ‘lengths 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 in- telligent 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, con- sisted of a rounded, convex, artificial-like breakwater, entirely coated with Nullipore, and absolutely similar to that which I have described at Keeling atoll. From what I heard when at Tahiti, and from the ‘ The authorities from which these charts have been reduced, together with some remarks on them, are given in a separately ap- pended page, descriptive of the Plates. 2 Dalrymple, Hydrog. Mem. vol. iii. 5§ BARRIER-REEFS. Cu. II. writings of the Revs. W. Ellis and J. Williams, I con- clude 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 ex- tremely 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 (figs. 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 sepa- rating the reef from the included land may be called,) but at Vanikoro the reef slopes only for a short dis- tance, 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, Hillis! 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, 1 Consult, on this and other points, the Polynesian Researches by the Rev. W. Ellis, an admirable work, full of curious information. Qn. IL BARRIER-REEFS. 59 analogous to that low and inner part of the islets in rany atolls, which is formed by the accumulation of matter from the lagoon. At Hogoleu (fig. 2, Plate I.), in the Caroline Archipelago,! 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 islands. The lagoon-channels may be compared in every respect with true lagoons. In some cases they are open, with a level bottom af fine sand; in others they are choked up with reefs of delicately branched corals, - which have the same general character as those within Keeling atoll. These internal reefs either stand sepa- rately, or more commonly skirt the shores of the in- cluded highislands. The depth of the lagoon-channel round the Society Islands varies from two or three, to thirty fathoms; in Cook’s? chart of Ulietea, how- ever, there is one sounding laid down of 48 fathoms: at Vanikoro there are several of 54 and one of 564 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 L., fig. 5), form a single linear strip. The islets first appear either on the angles of the reef, or on the sides of the breaches 1 See Hydrographical Mem. and the Atlas of the Voyage of the Astrolabe, by Capt. Dumont D’Urville, p. 428. 2 See the chart in vol. i. of Hawkesworth’s 4to ed. of Cook’s First Voyage. 60 BARRIER-REEFS. Cu. I. through it, and are generally most numerous on the windward side. The reef to leeward retaining its usual width, sometimes lies submerged several fathoms be- neath the surface; I have already mentioned Gambier Island as an instance of this structure. Submerged reefs, dead, covered with sand, and with a less defined outline, have been observed (see Appendix I.) off some parts of Huaheine and Tahiti. The reef is more fre- quently breached to leeward than to windward, although this is not so frequent as in the case of atolls. Thus lL find in Krusenstern’s Memoir on the Pacific, that there are passages through the encircling reef on the lee- ward side of the seven Society Islands, which possess ship-harbours; but that there are openings to wind- ward through 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 ac- counted for, as will be seen in the fourth chapter, without much difficulty. The breaches being generally situated in front of the valleys which descend 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. - Itis 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 accounted for without much difficulty. In cases like Cu. II. BARRIER-REEFS. 61 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 lagoon-channel is narrow, that agency, which on unprotected coasts is the most productive of sediment, namely the force of the breakers, is here entirely ex- cluded; and owing to the reef being breached in the front of the main valleys, much of the finer mud from the rivers must be transported into the opensea. The -water which is thrown over the edges of atoll-formed reefs causes a current which carries sediment from the lagoon through the breaches into the sea; and the same thing probably takes place in barrier-reefs. This would greatly aid in preventing the lagoon-channels from being filled up. The low alluvial border, how- eyer, at the foot of the encircled mountains, shows that the work of filling up is in progress; and at Maurua (Plate I., fig. 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 see 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, aver- age dimensions, and grouping. Lncircling reefs, like atolls, are generally elongated, and have an irregularly rounded, though sometimes angular outline. There are 62 BARRIER-REEFS. Cu. II, atolls of all sizes, from less than two miles in diameter to sixty miles (excluding Tilla-dou-Matte, which 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 also be shown in the last chapter, that there is the strictest resemblance between the grouping of atolls and of common islands, and there is the same resemblance between atolls and encircling barrier-reefs. The islands lying within reefs of this class, are of very various heights. Tahiti! is 7,000 feet; Maurua about 800; Aitutaki 8360, and Manouai only 50. The geological nature of the included land also varies; in most cases it is of ancient volcanic origin, owing appa- rently to the fact that islands of this nature are the most frequent within all great seas; some, however, are of madreporitic limestone, and others of primary forma- tion, 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, Himeo stands by itself; while Taha and Raiatea (fig. 3, Plate I.), both 1 The height of Tahiti is given from Captain Beechey; Maurua from Mr. F. D. Bennett (Geograph. Journ. vol. viii. p. 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. Cu. II. BARRIER-REEFS. 63 moderately large islands, of nearly equal size, are in- cluded in one reef. Within the reef of the Gambier eroup there are four large and some smaller islands (fig. 8, Plate I.); within that of Hogoleu (fig. 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 forma- tion is left; remove 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 (fig. 5, Plate II.) is 400 miles in length ; and for a length of many leagues seldom approaches within eight miles of the shore. 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 about eleven hundred miles; its average distance from the land is between twenty and 64 BARRIER-REEFS. Cu. II. thirty miles, but in parts from fifty to ninety. The ereat 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 has described the surface of the reef as consisting of a hard white agglomerate of different kinds of coral, with rough projecting points. A few low islets have been formed on it. The outer edge is the highest part ; it is traversed by narrow gullies, and at intervals by ship-channels. The sea close outside is in most parts profoundly deep; but to the north, near New Guinea, and to the south, the depth is much less, and here the bottom slopes gradually from the reef, as it generally does in front of the ship-channels.} There is one important point in the structure of barrier-reefs which must here be considered. The accompanying diagrams represent north and south ver- tical sections, taken through the highest points of Vani-. koro, Gambier, and Maurua islands, as well as 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 are known; and I have attempted to repre- sent the form of the land from the shading of the hills 1 The foregoing details are taken chiefly from Flinders’ Voyage to Terra Australis, vol. i. p. 88; but these have been corrected by the account given by Prof. Jukes, Narrative of the Voyage of the Fly, vol. i. 1847, chap. xiii. Cu. II. BARRIER REEFS. 69 in the large published charts. It has long been remarked, even from the time of Dampier, that a 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 is probably a moderately accurate repre- sentation of the actual submarine prolongation of the land. If we now look at the outer edge of the reef 30327 B B == SSS mam) Uff YM : ‘ , MN Wii, ae South, North, 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 Coquwille, 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 repre- sented. A A—Outer edge of the coral-reefs, where the sea breaks. BB—tThe shore of the encircled islands, 8 66 BARRIER REEFS. Cu. I. (A A), 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,! 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 above one-eighth of the depth represented by the plummet on the right hand of the woodcut. Here then is a great apparent difficulty—how were the basal parts of these barrier-reefs 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 sea had deeply eaten into the coasts of these encircled islands, and had thus left a broad but shallow sub- marine ledge, on the edges of which the corals 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,? moreover, the cause of the reef springing up at such a great distance from 1 An East and West section across the Island of Bolabola and its barrier-reefs is given in the fifth chapter, for the sake of illustrating another point. 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 Rey. D. Tyerman and Mr. Bennett (Journal of Voyage and Travels, vol. i. p. 215) have briefly suggested this explanation of the origin of the encircling reefs of the Society Islands. Gz. 1 - BARRIER REEFS. 67 - 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 have risen 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 deep arms of the sea (as in Raiatea, see Plate II., fig. 3), which penetrate nearly to the heart of some encircled islands, is exceedingly improbable. And why, again, should the reef, in some cases steep on both _ sides like a wall, spring up at a distance of two, three, or more miles from the shore, leaving a channel often be- tween 200 and 3800 feet deep—a depth which, we have good reason to believe, is too great for the growth of coral? The existence, also, of this same channel pre- cludes the idea of the reef having grown outwards, ona foundation slowly formed by the accumulation ofits own detritus and sediment. 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 (especially where the sea is deep) closely attached to the shore. At New Caledonia (see Plate II., fig. 5), the reefs which run in front of the west coast are prolonged in the same line for 150 miles beyond the northern extremity of the island, and this shows that some explanation, quite different from any one of those just suggested is requisite. If the island had been originally prolonged to this distance, and if the northern 68 BARRIER REEFS. Cu. IL end had been worn away until it was a little beneath the level of the sea, why should the coral-reefs have become attached, not on the central crest, but in the same line with the reefs which still front the existing shores? Weshall 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 reefs remains, but it is almost too pre- posterous to be mentioned ;—namely, that they rest on enormous submarine craters surrounding the in- cluded 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 everyone. New Caledonia, moreover, besides its size, is composed of primitive formations, as are some of the Comoro Islands;' and Aitutaki consists of caleareous rock. We must, therefore, reject the several explanations, and conclude that the vertical thickness of barrier- reefs, from their outer edges to the foundation on which they rest (from A A in the sections No. 4 to the dotted lines), is really great: but this presents no real difficulty, as I hope to show hereafter when the upward growth of coral-reefs, during the slow subsidence of their foundation, is discussed. 1 I have been informed that this is the case by Dr. Allan of Forres, who has visited this group. 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 H. 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 bottom of the sea, but not forming recfs. F'RINGING-REEFS, or, aS they have been called by some yoyagers, shore-reefs, whether skirting an island or part of a continent, at first appear to differ little from barrier-reefs, except that they are generally of less breadth. 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 of the adjoining land beneath the sea, 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' where the coast is almost precipitous, and 1 This fact is stated on the authority of the Officier du Roi, in his extremely interesting ‘ Voyage 4 l’Isle de France,’ undertaken in 1768. According to Captain Carmichael (Hooker’s Bot. Misc., vol. ii. p- 316), on one part of the coast there is a space of sixteen miles without a reef. 70 FRINGING-REEFS. Cu. IIL. 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 §.K. 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. ven 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 a foundation formed of sand and detritus, from the wear of the corals; and this 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 Cu. ITIL FRINGING REEFS. val 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, apparently owing in some cases to their narrow- ness, and in others to the gentle slope of the reef 72 | FRINGING REEFS. Cu. IIL. 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 sedi- ment, 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. . S28 ie aaa Brown lava . . ; : ° » 249 -.: 13506 II. King’s Well No. 2, about half a mile west of Diamond Mill, and 850 yards from the sea-shore. Thickness Depth Feet Feet Sand and coral ° ° ° ° o-oo White coral rock . ° ° . 2 (RR eek a Yellow sand . . ° . ° o | 4B eres Hard lava ; ° ° ° ° o 47 elas White coral rock . ° ° ° - 110 . 260 Blue clay : ‘ . . . - 25 .. 285 Tough clay and coral . ° ° - - 65 . 850 Blue clay , , ° ‘ ° 6 30 areoae Hard coral rock . 0 Nite ia, “ete = AO eee Soft coral < ‘ eit ote - 380 ., 450 MASAMARHU ISLAND. 323 King’s Well, etc.—continued. Thickness Depth Feet Feet Tough clay . : A ° ° ° 5 . 455 White coral rock . . ° ° - 40 . 495 Toughclay . * : . ° poe molars Se astaen 7915 White coral rock . : ‘ . - 100 . 625 Toughclay . . . ° . ° 5 . 630 Coral and clay . ° . . Pia i he hey it Tough clay . 7 ° . ee oo oc hee Black sand . ‘ . ° . . 2° 0-730 Lava . ° ° : ° 4 - 120 . 850 III. Well in Thomas Square, Honolulu. Thickness Depth Feet Feet Soil 6 feet, with 6 feet of black sand, and clay 4 feet . . . . . aoe hid Rate White coral rock , . . ° - 200 . 216 Brown clay . . . ° . oop nd ech es ebU Coral rock : ° ° ° ° og hr se atU Brown clay . . ° F : - 60 . 330 White coral rock : ° ° s\... 00s he uos0 Brown clay . . . - 80 . 460 Bed rock or lava, Pensirnced ; ‘ mea + Beer ST The evidence of these borings, which is corroborated by others quoted in the paper, points to a very consider- able subsidence in this region, to the amount of at least 800 feet, and in all probability of considerably more than 1,000 feet. Moreover, the ‘hard coral rock, like marble’ (No. I.) can hardly be anything but a ‘fossil reef’ ; the base of this, it will be observed, after some upheaval, is even now at a depth of 825 feet (1874 fathoms), and the reef has a continuous thickness of 505 feet (full 84 fathoms). The above abstracts may suffice, I hope, to give a fair representation of the arguments for and against Mr. Dar- win’s theory, which have been advanced during the last fourteen years. That theory is regarded by some enthu- siastic opponents, as already on the threshold of the limbo i ¢ , 324 APPENDIX II. appointed for exploded hypotheses. To this opinion I cannot declare myself a convert, for reasons which, in conclusion, I shall endeavour to indicate. First, however, I may remark that certain of Mr. Darwin’s critics occasionally appear to have perused his book with overmuch haste, and to have overlooked the fact that he admits such possibilities as local upheavals, the lateral growth of reefs, and modes of formation similar to those asserted for the Florida reefs ;! that in short, most of the causes on which stress has been laid by his critics have been already noticed by him, so that he differs from them, not in overlooking such causes, but in assigning to them a subordinate value. Moreover, it may not be unfair to call attention to the want of unanimity among his opponents : some advocating solution as a primary cause in the shaping of atolls, while others rely chiefly on the mode of growth of the polyps. Such a divergence obviously does not prove Mr. Darwin right, but it does indicate that as yet no other hypothesis has been able to secure a general acceptance, and that the problem still demands the exercise of cautious induction, which was his method of procedure, and does not justify the over-confident boldness of assertion which has characterised at least one critic of his work. The chief arguments which have been advanced against Mr. Darwin’s theory, as it appears to me, may be thus summarised :—1. That such evidence as can be obtained in regions where extensive coral reefs exist is favourable to upheaval rather than to subsidence. 2. That lateral srowth is a most important factor in the formation of a reef, the polyps, as they advance, being supported on a founda- tion composed partly of the broken fragments of the reef, partly of other marine organisms, and that by means of the latter deeply submerged banks are sometimes augmented vertically until they are brought within the zone of reef-coral 1 See pp. 22, 23, 79, 120, 121, 174, ete. SUMMARY OF ARGUMENTS. 325 life. 8. That lagoons and lagoon channels are materially enlarged by the destruction of dead coral through the solvent effects of sea-water. 4. That in the past history of the earth we find no evidence in favour of the formation of coral reefs in areas of subsidence, or in other words that fossil coral reefs are less than some 25 fathoms thick. 1. Much stress is evidently laid upon the fact that many coral islands afford evidences of a certain amount of upheaval. This amount, in most cases, is but slight, and its significance appears to me to have been exaggerated. Undoubtedly, it proves that the record which is the most obvious indicates an upward and not a downward motion, but in so doing it introduces a difficulty which will presently be noticed. These indications, however, do not of themselves prove a general upheaval, but only oscilla- tion. Every geologist is aware that movements in any given direction are frequently neither uniform nor contin- uous. For instance, no one doubts that the western coast of Scandinavia, and, in a less degree, that of Great Britain, have very considerably subsided since the sculpture of their leading physical features, and yet from the Land’s End to the North Cape we constantly find proofs that the latest movements have been in an upward direction. Even in the case of the more important, but much rarer, upheaval of reefs, as at the island of Cuba, the coral masses are so thick that we must assume the practical arrest of all up- ward movement during the growth of the reef. In this case also, if the coral reef be only a sort of cap concealing a hill of pre-existent rock, we may reasonably be surprised that the ‘ ashlar-work ’ of coral limestone has in no case so far yielded to the action of the atmospheric agencies as to lay bare its inner support. Doubtless there are many reefs to which either explan- ation might be applied, but there are some which, unless coral polyps can build at depths much greater than 25 326 APPENDIX II. fathoms, can only be explained by subsidence. It is sought to elude this difficulty by supposing that the reef builders, under specially favourable circumstances, may commence: operations at depths considerably greater than the usual limit. It is indeed true that reef corals are sometimes dredged alive from depths much exceeding 25 fathoms, but the result of all recent researches has certainly been to confirm the general correctness of this bathymetrical limit, and the proposed evasion of the difficulty is at present a mere hypothesis, which bears a suspicious resem- blance to the epicycles devised to prop up the Ptolemaic system of astronomy. While the existence of ‘ continental rocks,’ as they may be called, in oceanic islands would have almost proved a general subsidence, I do not see that the frequent occur- rence of volcanic rocks is seriously opposed to it. The arrangement of the majority of coral islands, whether wholly composed of organic material, or incrusting a nucleus of volcanic rocks, is indicative of lines of weakness in the earth’s crust, which would give rise to movements in either direction, and in each case the islands would be connected with extruded masses of volcanic rocks, ejected at various points along these lines.! Thus, we have to consider which of two hypotheses is the more probable: (a) that mounds thus formed have, in the majority of cases, failed to reach the surface, but have nevertheless generally arrived within a comparatively short distance of that goal; or (0), that they, after having in many cases overtopped the surface, have again subsided. The latter, 1 It must not be forgotten that though the peaks of mountain ranges are frequently composed of ‘ continental rocks,’ instances are by no means wanting, as in the Andes, Caucasus, &c., where the higher portions are volcanic. In more insulated mountain masses, as those of Etna, Kilimanjaro, Ararat, of some of the islands of the Malay Archipelago and on the western coast of North America, we have instances of volcanoes forming the highest part of the land. SUMMARY. OF ARGUMENTS. 328 I must confess, seems to me the more probable, especially when we remember that subsidence very commonly occurs in a district when it has recently ceased to be the scene of volcanic disturbances on a large scale. 2. In regard to the lateral spreading of reefs, like a ‘ fairy ring,’ as it has been happily expressed, there is no doubt that, as has been admitted by Mr. Darwin,! some augmen- tation may occur in this way; but to regard this as a factor of prime importance in the development of a reef seems tome _ to import new and serious difficulties. Let us assume that the submarine mound or shoal on which the reef is founded remains at rest during the whole period of the growth of the latter, and that this commences on the area (regarded, for simplicity, as a plain) included within the bathymetrical contour line of 25 fathoms. For a considerable period, until the edge of the reef arrives within a few fathoms, pro- bably less than ten (see p. 315), of the surface of the sea— ~ that is, for full three-fifths of its whole vertical growth-— the exterior slopes will only be augmented by the accu- mulation of marine organisms, a process which cannot be rapid. Hence, for a considerable time, until the reef itself has completed the greater part of its growth, and begins to augment the talus with its own ruins, the process of laying the foundation for a new coral growth, and thus the lateral spreading of the reef, will be slow. Consider, then, the case of a reef where this process has begun, and for simplicity regard it as a cylinder cap- ping a flat-topped cone. Obviously, if the reef begin to spread laterally, the volume of the foundation required to support the new growth increases far more rapidly than the area from which material can be supplied. Hence, as the reef advances outwards, the rate of increase will rapidly diminish, unless we suppose either an extraordi- nary annual destruction of growing coral, or an increased 1 See pp, 22, 67, 70 of this work. 328 APPENDIX II. accumulation of other organisms. Moreover, unless we rely on solution for enlarging the lagoon, this will remain of its original size, and thus will be small in comparison with the ultimate area of the atoll. No doubt, for a time, as the reef is approaching the surface of the sea, the more rapid growth of the coral at its outer margin will cause it to be saucer-like in section, and thus somewhat enlarge the lagoon, but as soon as the upward growth ceases this process is arrested and the atoll can only spread laterally and thus must increase in breadth, while the lagoon, if there be no solution, tends rather to diminish in size. It is, however, stated on good authority! that coral growth, as a rule, is by no means entirely arrested in a lagoon, and we cannot suppose that so long as there is free passage for a considerable stratum of water above the reef{—1z.e. so long as there are soundings of 8 or 10 fathoms over it—the polyps on its inner part will suffer materially from want of food or properly aérated water. Hence the lagoon will not be formed at all until the reef has made some progress upwards, so that it should always be com- paratively shallow, not exceeding a few fathoms in maxi- mum depth. From the above considerations it appears to me that the ‘fairy-ring’ hypothesis is inadequate unless it be inseparably linked with that of ‘ solution.’ At this period we may not unfitly notice another con- sideration which has been urged, viz. that many shoals, chiefly of volcanic origin, which lie at too great a depth to be colonised by reef-building polyps, may be raised up to the proper level by the accumulation of marine organisms. That this may sometimes occur cannot be denied, but it must be remembered that, unless the shoal lie at a very moderate depth below the required level, the process of accumulation will be extremely slow. Mr. Murray’s estimate of the quantity of carbonate of lime present in the 1 See pp. 302, 318 of this work. — Re ; A Raa Me THE SOLUTION THEORY. 329 minute organisms which inhabit the upper stratum of the Ocean water seems at first considerable, but when we estimate its thickness in a given area, this proves to be extremely small. Hence, unless we assign a very brief existence to each individual, and thus suppose a heavy rain of shells on the ocean floor, the foundation for the future reef will rise but slowly, and its initiation, in the case of those which now exist, must be carried back to a rather remote epoch. Here, again, we may inquire whether a cause, which must not be wholly overlooked, has not, through an error - in mental perspective, been brought into undue prominence. 3. The solution theory, which indeed by no means meets with universal acceptance among Mr. Darwin’s critics, appears to me beset with considerable difficulties. The solubility of carbonate of lime in ocean-water cannot of course be denied; but is there satisfactory evidence that this is a factor of primary importance to the case of a coral reef? The apparently rapid solution of calcareous organisms at great depths has but little bearing on what occurs at small depths, and the good preservation of the ‘olobigerina ooze’ down to depths of some 2,000 fathoms, in itself indicates that solution to any important amount takes place under very exceptional conditions. The rottenness ‘frequently noted in dead coral is mainly due to the decom- position of the animal tissues with which the mineral constituent is incorporated: thus the process is one of disintegration more than of solution. The dead coral is no doubt to some extent dissolved, but it mainly forms a sand or mud. This of course, in some cases, will be swept out by currents into the open ocean, and thus the coral will be removed from its place of growth, but it may well be doubted whether this substitute for a true solution will be for long a factor of prime importance in the genesis of a lagoon. There is moreover some evidence directly opposed to the theory of solution at a moderate depth, as, for 30 330 APPENDIX II. example, the blocks of recent limestone which were dredged by Professor A. Agassiz off the Florida reef.! Under what circumstances, then, will the sea-water act as a solvent on the dead coral? I think we must reply, When the fluid is rather rapidly altering its position in regard to the sub- © stance attacked. Thus rain and streams are important solvents, and so might be breaking waves or tidal ebb and flow, but when the water is at rest or is only spreading with a slow, diffusive movement, its solvent action is ex- tremely slight. For instance, chalk often is, and must often have been, saturated with water, yet numbers of the minute organisms which enter into its composition are still perfectly distinguishable. The same is true of many other limestones; indeed the effect of water often seems conservative rather than destructive. It sinks down into the body of the rock, carrying with it the carbonate of lime which has been obtained from the exposed superficial part of the mass, but on reaching the level of saturation, when it only percolates by diffusion, it commonly deposits its burden, filling up with mineral calcite the interstices of the organic materials. Hence the comparatively quiet waters of a lagoon would be favourable to the consolidation rather than to the destruction of the dead coral, save only within, a very limited distance from the surface. Moreover, the remains of organisms, when once the interstitial animal tissues have been replaced, appear to be less soluble than the other parts of a rock, as is indicated by the familiar ‘weathering out’ of fossils. Reef rock also appears very apt to assume a solid and semi-crystalline condition (p. 17), and in regard to this we must not overlook a peculiarity of coral which, as it seems to me, has an important bearing on the subject. Dead coral is very readily converted into dolomite, which is a much less soluble salt than calcite. Further, the conditions which 1 See p. 288 of this work. a2 i GEOLOGICAL EVIDENCE. 331 would prevail in a lagoon, when its waters had become unsuitable for coral life, would be those which would be exceptionally favourable to the formation of dolomite. It seems, then, from the above considerations that we cannot regard the corrosive effect of sea-water as an agent of more than very secondary importance in modifying the structure of an atoll. 4, In regard to the negative geological evidence. Here we must not overlook two considerations—one that the structure of a coral reef is very commonly more or less ~ composite; broken coral, shells, &c., forming a part, and sometimes predominating when from one cause or another the growth of the polyps is temporarily checked (p. 155) ; hence in some cases, what is really a continuous reef may be supposed, if only an occasional section be visible, to be a series of thin reefs—the other (the more important and general) that the characteristic structure of dead coral becomes rapidly inconspicuous and may be only discover- able in thin sections under the microscope. Where dolo- mitisation has occurred it may be actually obliterated, for the molecular changes involved in the process are often sufficient to destroy every trace of an organism. We may thus be prevented from recognising many ancient coral reefs. Moreover, the aporosa and madreporaria, which are now the chief reef-builders, have only become common since the conclusion of Paleozoic ages, so that the largest volume of the geological history of the earth is excluded from consideration, because in the times which it covers the habits of the reef-builders may have been different. Reefs also, it must be remembered, are restricted at the present day to almost tropical regions, so that, notwithstand- ing any variation of climate, they must always have been less frequent and less luxuriant in northern latitudes—that is to say, in those regions with which geologists are best acquainted, Still, instances of thick reefs of comparatively 332 APPENDIX It. late date are on record,! and if those geologists are right who consider the Schlern dolomites as being to a great extent due to reef-building corals, we have in the Triassic deposits of the Italian Tyrol reefs thick enough to satisfy the most exacting requirements. It is then, I think, premature to regard the theory which was advanced by Mr. Darwin, and has received the approval of an observer of such an exceptional experience as Professor Dana, as conclusively disproved by the results of the more recent investigations. That this theory may have been expressed in terms a little too comprehensive, that there may be a larger number of exceptional cases than was at first supposed, is quite possible. This, however, is the almost inevitable lot of every great generalisation. Its author concentrates, and rightly concentrates, his atten- tion on the salient features, as one who gazes first at a mountain group fixes his eyes upon the principal peaks and for a time pays little attention to, perhaps even under-estimates the importance of, the subordinate ranges ; nevertheless his conception of the physical structure of the region, though modified, is not overthrown by the work of subsequent travellers. This may prove to be the case in regard to the present controversy. It may very possibly be found that, as remarked by Mr. Bourne, the history cf coral reefs is more varied and complicated than was at first supposed, but it seems to me that, as the evidence at present stands, it is insufficient to justify a decision adverse to Mr. Darwin’s theory as a general explanation. l See pp. 809, 322 of this work. Cit a OR Be oe ee es si ali a \ are TC te Koow oie ae ts Note. Seale ¢ of an inch to a mile, except in Fig.£ 2,3 and 4 in which it is io of an inch to a mile, Soundings in fathoms, a few of the deepest being given. \ : ' BOLABOLA Fig. s. (many reefs) =a > fii Pig.4 ' WY RAIATEA | tt higs Bi. SS “HOGOLEU Fug. 2. MAURUA Fig 6. PEROS BANHOS ATOLL Fig. 9. , POD ENT EE LE GAMBIER I°* Fig. 8. +f AY cadena 9 Satur ee REhtr gs KEELING ATOLL? Figl0 . 4 WS 43) F1L.HKO_OSEEES Eee eee eee ) < " Facey, “Mt eerecneeneny, fe ee al Plate 1. SHEWING THE RESEMBLANCE IN FORM BETWEEN BARRIER CORAL- REEFS SURROUNDING MOUNTAINOUS ISLAND S, AND ATOLLS OR LAGOON-ISLANDS . J.&C.Walker, Sculpt Published by Smith, Elder, ( Waterloo Place. 2 | » ig MALDIVA ARCHIP? Fig. 6. One Anchi to 60 mules Mahlos Mahdoo NEW CALEDONIA fies 1g. 4. one inch to 60 miles GREAT CHAGOS BANK Fig. 1. 2 of antnch to a mile The shaded parts are from 4 to.10.f" under water 4 15 | to: 20 f° “ Q Level of the Sea 3 ; Nillandoo 5 8 = East & West Section across the Gr. Chagos Bank 76 miles in length Lig. 2.20 of an inch to a mile i ies lea & ¢ . mettre Gi B- Bte & Re 38 & eee ‘yw. 28" te MAHLOS & fs oo € MAHDOO ATOLL S76? fig. 4. 2% of antnch to amile ee 132 Nailin. Fo kom MENCHICOFF ATOLL Fig. 3. 2% of an Inch to a mile iS eas J&CWalker Sculpt Published by Smith, Elder, & Co. Waterloo Place. MeN Eaux. The names in italics are a!l names of places, and refer exclusively to the Appendix; in : we l-defined archipelagoes, or groups of islands, the name of each separate island is not given. References in square brackets refer to the new appendix. ABR Abrolhos, Brazil, coated by corals, 79 | Abrothos (Australia), 235 Absence of coral-reefs from cer- tain coasts, 81 Acaba, gulf of, 266 Acteon group, 200 Admiralty group, 224 Admiralty islands, [285] Africa, east coast, fringing-reef of, 76. Madreporitic rocks of, 181 Africa, east coast, 254. — Agassiz, Prof. A. on Tortuga and Florida reefs, [287] effect of Gulf stream, [287] growth of Florida reefs, [288] effect of currents on reefs, [289] depth of lagoons, [290] formation of silt [291] Age of individual corals, 96 Awou, 231 Aitutaki, 204 Aldabra, 251 _ Alert reef, 222 Alexander, Grand Duke, island, 207 Allan, Dr. on Holuthurie feeding on corals, 20 ASO Allan, Dr. : on quick growth of corals at Madagascar, 104 on reefs affected by currents, 79 Alloufatou, 214 Alphonse, 250 Amargura, 214 Amboina, 232 America, west coast, 199 Amirantes, 249 Anachorites, 225 Anambas, 240 Anamouka, description of, 177 Anamouka, 213 Andaman islands, 239 Antilles, 274 Appoo reef, 241 Arabia Felix, 260 Areas, great extent of, interspersed with low islands, 122 of subsidence and of eleva- tion, 191 of subsidence appear to be elongated, 191 of subsidence alternating with areas of elevation, 192 Arru group, 231 Arzobispo, 230 Ascension, no reef at, 83 354 INDEX. ASC Ascidia, depth at which found, 117 Assomption, 250 Astova, 250 Atlantic islands, 83, 217 Atolls, breaches in their reefs, 89, 145 dimensions of, 27 dimensions of groups of, 123 not based on craters, or on banks of sediment, or on rock, 119, 124, 125,126,194 of irregular forms, 28, 146 steepness of their flanks, 31, 164, 229 width of their reef and islets, 28 their lowness, 122 lagoons, 35, [285], [318] general range, 167 with part of their reef sub- merged, and theory of, 37, 146, 147 whole reef submerged, and theory of, 38,146, 147, [285] Augustin, St., 217 Aurora island, an upraised atoll, 123 Aurora, 201 Austral islands, recently elevated, 167, 177, 186 Austral islands, 204 Australia, N.W. coast, 234 Australian barrier-reef, 63, 166 Australian barrier, 222 Babuyan group, 243 Bahama banks, 268, 270 Balabac, 240 Balfour, Prof. Bayley, descr'p- tion of Rodriguez, [307] Bally, 236 Bampton shoal, 222 Banks’ wslands, 220 Banks in the West Indies, 267 Barbes, St., 240 Baring, 218 BON Barrier-reef of Australia, 63, 166 of New Caledonia, 63, 67 Barrier-reefs, breaches through, 135 not based on. worn-down margin of rock, 66 on banks ef sediment, 67 on submarine craters, steepness of their flanks, 57, their probable vertical thick- ness, 64, 134 theory of their formation, 133, 137, [297] Bashee islands, 243 Bass island, 206 Batoa, 215 Beaupreé reef, 221 Beechey, Capt., obligations of the author to, 30 on submerged reefs, 37 account of Matilda island, 101 Belcher, Sir E. on boring through coral- reefs, 99 on changes in Chain atoll, 172 on Clipperton rock, 199, 200 Bellinghausen, 203 Bengal, gulf of, elevation of east- ern shores of, 181 Bermuda islands, 275 Beveridge reef, 212 Bligh, 220 Bolabola, view of, 3 Bombay shoal, 245 Bonin bay, 238 Bonin group, 230 Bonney, Prof. T. G., discussion of arguments against Mr. Darwin’s theory, [325] movements of upheaval and subsidence, [326] INDEX. 335 BON Bonney, Prof. T. G., on lateral spreading of reef, [827] on the solution theory, [329] on the geological evidence, (331) » Borings through coral reefs, 99, [322] Borneo, W. coast, recently ele- vated, 180 Borneo, EH. coast, 237 S.W. and W. coast, 240 N. coast, 240 western bank, 240 Boscawen, 214 Boston, 218 Bouka, 223 Bourbon, 249 Bourne, Mr. G. C., Chagos group, [300] on solution of dead coral, [3801] on lateral spreading of reefs, [301] coral growth in lagoons, [302] importance of currents, [304] conclusions, [305] Bourou, 232 Bouton, 238 Brazil, fringing-reefs on coast of, 77 Breaches through barrier-reefs, 135 Brook, 207 Bunker, 207 Bunoa, 240 Byron, 217 Cagayanes, 241 Candelaria, 224 Carbonate of lime in ocean water, [284], [328] Cargados Carajos, 248 Caroline archipelago, 225 Caroline island, 206 Carteret shoal, 231 Caryophyllia, depths at which it lives, 117 CON Cavilli, 241 Cayman island, 273 Celebes, 233 Ceram, 232 Ceylon, recently elevated, 171 Ceylon, 247 Chagos Great bank, [300] Chagos, Great bank, description and theory of, 538, 149, [300] Chagos group, 149 Chagos group, 247, [300] Chama shells embedded in coral rock, 106 Chamisso on corals preferring the surf, 85 Changes in the state of Keeling atoll, 20 of atolls, 130, 170 Channels leading into the lagoons, 59 of atolls, 39, 148, [812] the Maldiva atolls, 48, 49 through barrier-reefs, 135 Chase, 217 China sea, 244 Christmas atoll, 100 Chrisimas atoll, 208 Christmas island (Indian Ocean), 246 Clarence, 207 Clarke, W. B., on recent elevation of the Loyalty islands, 178 Clipperton rock, 199 Cochin China, 246 Cocos, or Keeling atoll, 7, [305] Cocos (or Keeling), 246 Cocos island (Pacific), 199, 214 Coetivi, 250 Comoro group, 251 Composition of coral formations, 155 Conglomerate coral rock on Keeling atoll, 17 coral rock on other atolls, 35 coral rock, 156 336 INDEX. coo Cook islands, recently elevated, 177 Cook islands, 204 Corallian sea, 166 Corallian sea, 222 Coral-reefs, their distribution and ab- sence from certain areas, destroyed by loose sediment, 87 Coral-rock, at Keeling atoll, 16 Mauritius, 74 Metia, 98 organic remains of, 156 at Oahu, [322] Coral-rocks bored by vermiform animals, 20, 156 Corals, dead but upright in Keeling lagoon, 21 depths at which they live, 108, [293], [298], [326] off Keeling atoll, 11 killed by a short exposure, 8 living in the lagoon of Keel- ing atoll, 18 quick growth of, in Keeling lagoon, 17 merely coating the bottom of the sea, 79 standing exposed in the Low archipelago, 170 Cornwallis, 218 Cosmoledo, 250 Couthouy, Mr. alleged proofs of recent eleva- tion of the Low archipe- lago, 170 on external ledges round coral islands, 140 Crescent-formed reefs, 146 Crosby, Mr. W.O., on the raised reefs of Cuba, [308] proofs of ancient subsi- dence, [309] Cuba, 270, [308] Cuming, Mr., on the recent ele- DEP vation of the Philippines, 180 Dana, Prof., on the reef of Hawaii, 82, 175 distribution of coral-reefs as - affected by the tempera- ture of the sea, 85 upraised coral-rock of Metia, 98 39 boring through coral-rock, 99, [322] depth at which corals live, 112 subsidence of the Mendana island, 165, 204 subsidence in the Caroline archipelago, 169 slight recent subsidence of. the Paumotu archipelago, 170 extension of the Hawaii ar- chipelago, 192, 211 Feejee islands, 215 outline of some islands in- dicative of subsidence, [310] distribution of reef-materials, [311] different kinds of reef in Pacific, [312] submarine slopes, [313] local elevations, [314] on soundings at Tahiti, [314] on lateral spreading of reefs, [315] on Florida reefs, [316] on Mr. Murray’s explanation of the foundation of reefs and of ring-shaped atolls, [317] Danger islands, 207 Dangerous or Low archipelago, 200 Depths, at which reef-building corals live, 108, [293], [298], [326] om, a, Ct a A mee INDEX. DEP Depths at Mauritius, the Red Sea, and in the Maldiva archipelago, 113 at which other corals and corallines can live, 116 Dhalac group, 261 Diego Garcia, slow growth of reefs, 92 Dimensions of the larger groups of atolls, 123 Discoyery shoal, 245 Disseverment of the Maldiva atolls, and theory of, 50, 143 Distribution of coral-reefs, 80 Dolomitisation of coral, 18, [330] Domingo, St., 273 Dory Port, recently elevated, 179 Dory Port, 231 Duchassaing on rapid growth of corals, 107 Duff's islands, 221 Durour, 225 Hap, 227 Earthquakes, at Keeling atoll, 25 in groups of atolls, 131 in Navigator archipelago,178 East Indian archipelago recently elevated, 179 Flaster, 200 Fichequier, 225 Ehrenberg, on the banks of the Red Sea, 78, 260 on depth at which corals live in the Red Sea, 113 on corals preferring the surf, on the antiquity of certain corals, 96 Eimeo, 201 Elevated Reef of Mauritius, 74 of Rodriguez, [307] of Cuba, [308] Elevations, recent, proofs of, 175, [291], [314] immense areas of, 190 doF FRI Elivi, 227 Elizabeth island, 98 recently elevated, 176, 186 Elizabeth island, 200 Ellice group, 216 Encircled islands, their height, 62 their geological composition, 62, 68 Koua, description of, 177 Koua, 213 Erupted matter, probably not associated with thick masses of coral rock, 155-157 Fais recently elevated, 179, 191 | fais, 227 Fanning, 209 Farallon de Medinilla, 229 Farsan Group, 262 Fataka, 220 Fidji archipelago, 214, [810] Fish, feeding on corals, 20 killed in Keeling lagoon by heavy rain, 27 Fissures across coral islands 132, 262 FitzRoy, Capt., on a submerged shed at Keeling atoll, 25 on an inundation in the Low archipelago, 130 Flint, 206 Flores, 236, 259 Florida, 270, 275, [287] Folger, 230 Formosa, 243 Forster, theory of coral forma- tions, 127 Frederic reef, 222 Freewill, 231 Friendly group recently elevated, . 177, 186 Friendly archipelago, 212 Fringing-reefs absent where coast preci- pitous, 69 338 INDEX. Fri Fringing-reefs breached in front of streams, 88 described by MM. Quoy and Gaimard, 175 not closely attached to shel- ving coasts, 72 of east coast of Africa, 76 of Cuba, 75 of Mauritius, 69, 71 on worn-down banks of rock, 78 on banks of sediment, 78 their appearance when ele- vated, 74 their growth influenced by currents, 79, [804] by shallowness of sea, 77 Galapagos archipelago, 199 Galega, 250 Gambier islands, section of, 65 Gambier islands, 201 Gardner, 208 Gaspar Rico, 218 Geological composition of coral formations, 156, [331] Gilbert archipelago, 217 Gilolo, 233 Glorioso, 250 Gloucester island, 130 Glover reef, 272 Gomez, 200 Gouap, 227 Goulou, 227 Grampus, 230 Grand Cocal, 216 Graves, on the recent elevation in the Bonin archipelago, 179 Great Chagos bank, description and theory of, 53, 150, [300] Grey, Capt., on sand-bars, 72 Guedes, 231 Guppy, Mr., on Solomon archi- pelago, [291] proofs of upheaval, [291] summary of opinion, [292] IND Guppy, Mr., growth of corals, [292] development of reefs, [293] different kinds of reefs, [294] description of islands, [294] barrier reefs and their for- mation, [297] depth at which reefs begin, [298] ‘ lateral spreading of reefs, [298] removal of dead coral, [299] nature of sea-bed near reefs, [299] on Keeling atoll, [806] various Hales, Mr., on subsidence in the Caroline archipelago, 169 Hall, Capt. B., on Loo Choo, 181 Halstead, Capt., elevation of eastern shore of Gulf of Bengal, 181 Harvey islands recently elevated, 185, 186 Harvey or Cook islands, 204 Height of encircled islands, 62 Hermites, 225 Hogoleu, 226 Holuthurie feeding on corals, Honduras, reef off, 272 Honolulu, boring at, [323] Horn, 214 Houtman’s Abrolhos, 235 — Huaheine, 203 Hull island, 208 Hunvphrey, 207 Hunter, 214 Hurricanes, effects of on coral islands, 129 Immaum, 260 Independence, 216 India, east coast recently ele- vated, 181 Zidia, 260 INDEX. 339 IRR Irregular reefs in shallow seas, 77, 78 Islets of coral-rock, their forma- tion, 15 their destruction in the Mal- diva atolls, 50 Jamaica, 273 Jarvis, 207 Java recently elevated, 180 Java, 236 Johnston island, 209 Juan de Nova, 250 Juan de Nova (Madagascar), 253 Jukes, P of., the barrier reef of Australia, 64 Kalatoa, 237 Kamtschatka, proofs of its recent elevation, 189 Karkalang, 233 Keeling atoll, section of reef, 7, 8, [305] Keeling, - north atoll, 246 south atoll, 246 Keffing, 232 Kemin, 206, 208 Kennedy, 221 Keppel island, 214 Keppell, Capt., on the reefs and elevation of Celebes, 233 King, Capt., on distribution of the different classes of reefs, 165 Kumi, 244 Laccadive group, 247 Ladrones or Marianas, recently elevated, 172 Ladrones archipelago, 229 Lagoon of. Keeling atoll, 18 Lagoons, bordered by inclined ledges and walls, and theory of their formation, 41, 139, [285], [801} of small atolls filled up with gediment, 42 MAC Lagoon-channels within barrier reefs, 59 Lagoon-reefs, all submerged in some atolls and all rising to the surface in others, 90 Lancaster reef, 206 Larrack, 260 Latte, 213 Laughlan islands, 222 Ledges round certain lagoons, 41, 139 Lette, 234 Lighthouse reef, 272 Lloyd, Mr., on corals refixing themselves, 105 Loo Choo recently elevated, 181 Loo Choo, 244 Lowisiade, 222 Low archipelago, alleged proofs of its recent elevation, 170 Low archipelago, 200 Lowness of coral islands, 122 Loyalty group, 221 Loyalty islands, recently elevated, 186, 221 Lucepara, 240 Lutké, Adm., on fissures across coral islands, 132 Luzon recently elevated, 180 Luzon, 241 Lyell, Sir C., on channels into the lagoons of atolls, 39 on the lownessof their lee- ward sides, 148 on the antiquity of certain corals, 96 on the apparent continuity of distinct coral-islands, 157 on the recently elevated beds of the Red Sea, 184 MacAskill Islands, formed of up- raised coral rock, 191, 227 Macassar strait, 237 Macclesfield bank, 245 Maclear, Capt., on Masamarhu Island, [319] 340 INDEX. MAD Madagascar, quick growth of corals at, 104 madreporitic rock of, 181 Madagascar, 252 Madjiko-sima, 243 Madura (India), 247, 260 Madura (Java), 236 Mahlos Mahdoo, theory of for- mation, 144 Maitea, 202 Malacca recently elevated, 180 Malacca, 239 Malcolmson, Dr., on recent elevation of E. coast of India, 181 on recent elevation of Cama- ran island, 183 Malden, 207 Maldiva atolls, and theory of their forma- tion, 44, 141, 142 steepness of their flanks, 30 growth of coral at, 103 Maldiwa archipelago, 247 Mangaia island, recently elevated, 176, 186 Mangaia, 205 Mangs, 230 Manouai island, 204 Marianas, recently elevated, 179 Mariana archipelago, 229 Mariere, 228 Marquesas, subsidence of, 165 Marquesas, 203 Marshall archipelago, 218 Marshall island, 230 Martinique, 274 Martires, 228 Mary’s, St., in Madagascar, har- bour made in reefs, 88 Mary island, 208 Masdmarhu island, sections of, [319] Matilda atoll, 101 Mauki island, 204 Mauritius, fringing-reefs of, 69, 71 depths at which corals there live, 110 NEW Mauritius, recently elevated, 181 Mauritius, 248 Maurua, section of, 65 Maurua, 202 Menchicoff atoll, 28, 146 Mendana isles, 203, 221 Mendana island, subsidence of, 165 Metia, 98, 123, 176, 186 Mexico, gulf of, 269 Millepora complanata at Keeling atoll, 10 Mindoro, 241 Mohilla, 252 Mopeha, 203 Moresby, Capt.,on boring through coral reefs, 99 Morty, 233 Mosquito coast, 273 Murray, Mr., on the structure* and origin of coral reefs and islands, [283] quantity of carbonate of lime present in ocean water, [284] lateral spreading of coral reefs, [285] solvent action of sea-water, [285] summary of conclusions, [286] Musquillo atoll, 146 Mysol, 232 Namourrek group, 145 Natunas, 240 Navigator archipelago, elevation of, 178 Navigator archipelago, 211 Nederlandisch islands, 216 Nelson, Lieut., on the consolidation of coral- rock, under water, 98 theory of coral formations, 127 on the Bermuda islands, 275 New Britain, 224 INDEX. 341 NEW New Caledonia, steepness of its reef, 57 barrier-reef of, 63, 67, 139, 145, 166 New Caledonia, 221 New Guinea (EH. end), 224 New Guinea (W. end), 231 New Hanover, 224 New Hebrides recently elevated, 178 New Hebrides, 218 New Ireland recently elevated, 178 New Ireland, 224 New Nantucket, 208 Nicobar Islands, 239 Niouha, 214 Nullipore, at Keeling atoll, 18 on the reefs of atolls, 34 on barrier reefs, 57 their wide distribution and abundance, 117 Oahu, borings at, [322] Objections to the theory of subsi- dence, 153 Ocean islands, 211, 217 Ono, 215 Onouafu, 214 Ormuz, 260 Oscar group, 216 Oscillations of level, 166,184,193 Oualan or Ualan, 225 Ouluthy atoll, 101 Outong Java, 224 Palawan, S.W. coast, 240 N.W. coast, 241 western bank, 245 Palmerston, 204 Palmyra, 209 Paracells, 245 Paraquas, 246 Patchow, 243 Paumotu archipelago, 170 Paumotu archipelago, 200 81 QUO Peel island, 230 Pelew islands, 227 Pemba island, singular form of, 182 Pemba, 255 Penrhyn, 206 Peregrino, 206 Pernambuco, bar of sandstone at, 73, 277 Persian gulf, recently elevated, 183 Persian gulf, 259 Pescado, 207 Pescadores, 244 Peyster group, 216 Philip, 227 Philippine archipelago, recently elevated, 180 Philippine archipelago, 241 Phenix, 208 Pierre, St., 250 Piguiram, 227 Pitcairn, 201 Pit island, 217 Pitt’s bank, 152 Platte, 250 Pleasant, 217 Porites, chief coral on margin of Keeling atoll, 9 Postillions, 237 Pouynipéte, 168 its probable subsidence, 169 Pouynipéte, 226 Pratas shoal, 244 Proby, 214 Providence, 250 Puerto Rico, 273 Pulo Anno, 228 Pulo Leat, 240 Pumice floatel to coral islands, 157 Pylstaart, 212 Pyrard de Laval, astonishment at the atolls in the Indian Ocean, 2 Quoy and Gaimard, depths at which corals live, 114 a42 INDEX, QUO Quoy and Gaimard, description of reefs appli- cable only to fringing- reefs, 174 Raivaivai, 206 Range of atolls, 167 Rapa, 206 Rearson, 207 Red Sea, banks of rock coated by reefs, proofs of its recent elevation, 182 supposed subsidence of, 184, [319] Red Sea, 260 Reefs irregular in shallow seas, 77 rising to the surface in some lagoons and all submerged in others, 91 their distribution, 80 their absence from some coasts, 81 lateral spreading of, 22, [285], [298], [801], [315], [327] formation of, '291] Revilla-gigedo, 199 Ring-formed reefs of the Maldiva atolls, and theory of, 45, 139 Rodriguez, 248, [307] Rosario, 230 : Rose island, 212 Rotches,.217 Roug, 226 Routowmah, 216 Rowley shoals, 235 Riippell, Dr., on the recent de- posits of Red Sea, 183 Sable, Ile de, 248 Sahia de Matha, 248 Salomon archipelago, 223 Samoan, or Navigator archipe- lago, elevation of, 178 SOL Samoa archipelago, 211 Sandalwood, 234 Sandbars, parallel to coasts, 73 - Sandwich archipelago recently elevated, 175 extension of, 191, 211 Sandwich archipelago, 209, [822] Sanserot, 228 Santa Cruz, 220, 274 Savage island recently elevated, 177 Savage, 212 Savu, 234 Saya or Sahia de Malha, 248 Scarborough shoal, 245 Scarus feeding on corals, 20 Schoulon, 224 Scilly, 203 Scori# floated to coral islands, 157 Scott’s reef, 235 Sections, of islands encircled by bar- rier reefs, 66, 133 of Bolabola, 134 Sediment, in Keeling lagoon, 19 in other atolls, 36, 48 injurious to corals, 87 transported from _ coral- islands far seaward, 157 formation of, [291] Semper, Prof., on the Pelew islands, 228 on the reef of the Philippine archipelago, 242 Seniavine, 227 Serangani, 233 Seychelles, 249 Ship-bottom quickly coated with coral, 106 Smyth island, 209 Society archipelago, 165 stationary condition of, 169 alleged proofs of recent ele vation, 185 Society archipelago, 201 Socotra, 260 Solomon archipelago, [291] Solor, 236 INDEX, 343 SOL Solution of dead coral, [286], [299], [301], [329] Sooloo islands, recently elevated, 180 Sooloo islands, 240 Souvoroff, 207 Spallanzani, on growth of coral, 106 Spanish, 228 Starbuck, 206 Stones transported in roots of trees, 157 Storms, effects of on coral-islands, 129 ‘Stutchbury, Mr., on the growth of an Agari- cia, 106 on upraised corals in Society archipelago, 185 Subsidence of Keeling atoll, 23-25 extreme slowness of, 193 areas of, apparently elon- gated, 191 areas of, immense, 190 great amount of, 193 indicated by shape of coast, [313] at Masamarhu island, [319] at Oahu, [322] Suez, gulf of, 266 Sulphur islands, 230 Sumatra, recently elevated, 180 Sumatra, 238 Sumbawa, 236 Surf favourable to the growth of massive corals, 85 Swallow shoal, 245 Sydney island, 208 Tahiti, alleged proofs of its re- cent elevation, 185, [314] Tahiti, 201 Tanasserim, 239 Tapamanoa, 202 Temperature of the sea at the Galapagos archipelago, 82 Tenimber island, 231 Teluroa, 202 voL Theories on coral formations, 119, 127, [286], (290), [298], [305] Theory of subsidence, and objec- tions to, 126, 153, [283], [325] Thickness, vertical, of barrier- reefs, 66, 135 Thomas, St., 274 Tikopia, 220 Timor recently elevated, 180 Timor, 234 Timor-laut, 231 Tokan-Bessees, 237 Tongatabou, description of, 177 Tongatabou, 213 Tonquin, 246 Toubat, 203 Toufoa, 213 Toupoua, 220 Traditions of change in coral- islands, 129 Tridacne, embedded in coral-rock, 156 left exposed in the Low archipelago, 170 Tubularia, quick growth of, 106 Tumbelan, 240 Turneffe reef, 272 Turtle, 215 Ualan, 225 Vanikoro, section of, 65 its state and changes in its reefs, 169 Vanikoro, 220 Vavao, 213 Vine reef, 222 Virgin Gorda, 274 Viti archipelago, 214 Volcanic islands, with living corals on their shores, 81 matter, rarely associated with thick masses of coral- rock, 157 OD. 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BODY AND MIND: An Inquiry into their Connection and Mutual Influence, specially in reference to Mental Disorders. 1 vol., 12mo. Cloth, $1.50. PHYSIOLOGY AND PATITOLOGY OF MIND: PHYSIOLOGY OF THE MIND. New edition. 1 vol., 12mo. Cloth, $2.00. Contents: Chapter I. On the Method of the Study of the Mind.—II. The Mind and the Nervous System.—III. The Spinal Cord, or Tertiary Nervous Centres; or, Nervous Centres of Reflex Action.—IV. Secondary Nervous Centres; or, Sensory Ganglia ; Sensorium Commune.—V. Hemispherical Ganglia ; Cortical Cells of the Cerebral Hemispheres; Ideational Nervous Centres; Primary Nervous Centres; Intellectorium Commune.—VI. The Emotions.—VII. Volition.—VIII. Motor Nervous Centres, or Mo- torium Cummune and Actuation or Effection—IX. Memory and Imagination. PATHOLOGY OF THE MIND. Being the Third Edition of the Second Part of the “Physiology and Pathology of Mind,” recast, enlarged, and rewritten. 1 vol.,12mo. Cloth, $2.00. Con- tents: Chapter I. Sleep and Dreaming.—II. Hypnotism, Somnam- bulism, and Allied States.—III. The Causation and Prevention of Insanity: (A) Etiological—IV. The same continued—vV. The Causation and Prevention of Insanity: (B) Pathological—VI. The Insanity of Early Life.—VII. The Symptomatology of Insanity.— VIII. The same continued.—IX. Clinical Groups of Mental Disease. —X. The Morbid Anatomy of Mental Derangement.—XI. The Treatment of Mental Disorders, RESPONSIBILITY IN MENTAL DISEASE. (International Scientific Series.) 1 vol.,12mo. Cloth, $1.50. “The author is at home in his subject, and presents his views in an almost singularly clear and satisfactory manner. ... The volume is a valuable contri- bution to one of the most difficult and at the same time one of the most impor- tant subjects of investigation at the present day."—New York Observer. “Handles the important topic with masterly power, and its suggestions ar@ practical and of great value.”—Providence Press. New York: D. APPLETON & CO., 1, 8, & 5 Bond Street. D. APPLETON & CO,’S PUBLICATIONS, GEORGE J. ROMANES’S WORKS. MENTAL EVOLUTION IN MAN: Origin of Human Faculty. One vol., 8vo. Cloth, $3.00. This work, which follows ‘Mental Evolution in Animals,” by the same au- thor, considers the probable mode of genesis of the human mind from the mind of lower animals, and attempts to show that there is no distinction of kind be- tween man and brute, but, on the contrary, that such distinctions as do exist all admit of being explained, with respect to their evolution, by adequate psycho- logical analysis. ‘““The vast array of facts, and the sober and solid method of argument em- ployed by Mr. Romanegs, will prove, we think, a great gift to knowledge.”— Saturday Review. JELLY-FISH, STAR-FISH, AND SEA-URCHINS. Being a Research on Primitive Nervous Systems. 12mo. 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Cloth, $2.00. ‘*Mr. Romanes has followed up his careful enumeration of the facts of ‘ Ani- mal Intelligence,’ contributed to the ‘International Scientific Series,’ with a work dealing with the successive stages at which the various mental phenomena appear in the scale of life. ue Peete installment displays the same evidence of industry in collecting facts and caution in co-ordinating them by theory as the former.”—The Atheneum. New York: D. APPLETON & CO., 1, 3, & 5 Bond Street. , nar eae aS ; A st 7 ay P = vw) ; 5 el eax = i { ’ 4 t = a . eo r a / > ’ ’ i] Se ok er ee ee ee E>. i, by q aay, ~~: ee ve « - . a SS PUCCIO AR SERN aA: DERE ERS AAS SAS Sy KS ss \S RS RS RS iN ~ S ‘ SS NANO IVs“rV»ny ga \ \ | \ EAN . > . . ; SOO TAS SS SN SASS ENR S AN : NS AN SU SERRA SENS AS ANAS SERN SN SARC . SSN A < < \ \ \ \ . ce UX A \» A CWO QARA KCC SS a > SS - iy ys: , H i AY fst uA aif! 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