The structure and distribution of coral reefs

Front matter

AN -XAKN | PIP LG a: A ro oe ran

Setar

Si ‘|

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., Hydrographer 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 temperature nor the presence of mud-banks accounts, as it appears to me, for the absence of coral-reefs throughout 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 expressly 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. Nevertheless, 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 subsidence 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 volcano, 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.

PREFACE

TO

THE FIRST EDITION.

I SHALL HAVE OCCASION, in many parts of the following 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 Admiralty, 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 volcanic islands visited during the voyage of the Beagle, and the other on South America—will appear as soon as they can be prepared.

Contents

INTRODUCTION : ° ; - ° P ° A e PAGE I

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—Sediment—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 disseverment of large atolls—The Great Chagos Bank—Its submerged condition and extraordinary structure. . . » 43 to 55

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 earthquakes 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 subsidence required by the theory, considered —The probable composition 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—Oscillations of level—Absence of active volcanos in the areas of subsidence—Immensity of the areas which have been elevated and have subsided—Their relation to the present distribution of the land—Areas of subsidence elongated, their intersection and alternation with those of elevation—Amount, and slow rate of subsidence—Recapitulation . : ; : ° - 158 to 196

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 different 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 attained by coral-reefs. I have not thought it worth while to introduce all those small and very numerous reefs, which occur within the lagoons of most atolls and within the lagoon-channels of most barrier-reefs, and which stand either isolated, or are attached to the shores of the reef or land. At Peros Banhos none of the lagoon-reefs rise to the surface of the water; a few of them have been introduced, and are marked by plain dotted circles. A few of the deepest soundings are laid down 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 soundings are laid down from the survey by D’Urville; height of the summit of the island is 3,032 feet. The principal small detached reefs within the lagoon-channel have in this instance been represented. The southern shore of the island is narrowly fringed by a reef; if the engraver had carried this reef entirely round both islands, this figure would have served (by leaving out in imagination the barrier-reef) as a good specimen of an abruptly-sided island, surrounded by a reef of the fringing class.

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 Archipelago; from the survey by Capt. Beechey; scale ¢ of an inch to a mile; height of highest island, 1,246 feet; the islands are surrounded 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 Archipelago, 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

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 polypifers, 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 attention; and it is not surprising, for everyone must be struck with astonishment, when he first beholds one of these vast rings of coral-rock, often many leagues in diameter, here and there surmounted by a low verdant island with dazzling white shores, bathed on the outside by the foaming breakers of the ocean, and on the inside surrounding a calm expanse of water, which, from reflection, is 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 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 separate small islands, the word ‘island,’ applied to the whole, is often the cause of confusion; hence I have invariably used in this volume the term ‘ atoll,’ which is the name given to these circular coral formations by their inhabitants in the Indian Ocean, and is synonymous 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 Bolabola,' one of the Society Islands. Here, as in Whitsunday Island, the whole of that part of the reef which is visible is converted into land. This is a circumstance of rare occurrence ; more usually a snow-white line of great breakers, with here and there an islet crowned by cocoa-nut trees, separates the smooth waters of the lagoon-like channel from the waves of the open sea. The barrier reefs of Australia and of New Caledonia, owing to their enormous dimensions, - have excited much attention: in structure and form they resemble those encircling many of the smaller islands in the Pacific Ocean.

With respect to fringing, or shore reefs, there is little in their structure which needs explanation; and their name expresses their comparatively small extension. They differ from barrier reefs in not lying 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.

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 beginning of the last chapter. In the Appendix, every existing coral-reef, except some on the coast of Brazil not included in the map, is briefly described in geosraphical 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 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 Geological Society, and an abstract has appeared in the Proceedings.

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 condition 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 woodcut (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