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COEALi REEFS



THE



STRUCTURE AND DISTRIBUTION



OF



COEAL REEFS



BY

CHARLES DAEWIN, M.A., F. E. S., F.G.S.



THIRD EDITION
WITH AN APPENDIX BY PROF. T. G. BONNET, D.Sc, F.R.S., F.G.I



WITH ILLUSTRATIONS



NEW YORK
D. APPLETON AND COMPANY

1898



Authorized Edition,



PEEFAOE

TO

THE THIED 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
Bkill with which he has completed it.

I must also be allowed the satisfaction of expressing
my obligations to Captain Wharton, E.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.



^^ ^-'^^^oSS



PEE FACE

TO

THE SECOND EDITION.



The fiest 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-Eeefs. 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
volcanic action prevents the growth of coral-reefs
much more effectually than I had supposed; but
how the heat or poisonous exhalations from a volcano



PREFACE TO THE SECOND EDITION. Vll

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 marme 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. 320) 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



VI 11 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
voj^age of H.M.S. Fly, published in 1847, devoted a
chapter to the Barrier-Eeefs 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. IX

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. I have removed an active vol-
cano, which was formerly suj)posed 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



X 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, Eeckenham, Kent :
Febrtiary 1871.



PEEFACE



THE FIEST 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
E. Moresby, I.N., who conducted the survey of the
Eed 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, E.N., for
having given me free access to the charts in the Ad-
miralty, as weU as to Captain Beecher, E.N., for most
kindly aiding me in consulting them. My thanks are
likewise especially due to Captain Washington, E.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



xii PREFACE TO THE FIRST EDITION.

FitzEoy, 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 page 1

CHAPTEE I.
ATOLLS OR LAGOON ISLANDS.

SECTION I. DESCRIPTION OF IvEELING ATOLIi.

Corals on the outer margin — Zone of Nulliporaa — Exterior reef —Islets
— Coral-conglomerate— Lagoon — Calcareous sediment— Scari and
Holutlniria) subsisting on corals — Changes in the condition of tho
reefs and islets — Probable subsidence of the atoU— Future state
of the lagoons 7 to 27

SECTION II. — GENERAIj DESCRIPTION OF ATOLLS.

General form and size of atolls, their reefs and islets — External slope
— Zone of Nulliporie — Conglomerate — Depth of lagoons — Sedi-
ment — Eeefs submerged wholly or in part — Breaches in the reef
— Ledge -formed shores round certain lagoons — Conversion of
lagoons into land . . . . . . . . 27 to 43

SECTION in. ATOLLS OF THE MALDIVA ARCHIPELAGO

GREAT CHAGOS BANK.

Maldiva Archipelago — Eing-formed reefs, marginal and central —
Great depth in the lagoons of the southern atolls — Eeefs 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



XIV CONTENTS.

CHAPTER II.

BAERIER-KEEFS.

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 —
Larrier-reefs of New Caledonia and Australia— Position of the
reef relative to the slope of the adjoining land — Probable great
thickness of barrier-reefs paok 56 to 08

CHAPTER in.

FEINGIXG OE SHORE REEFS.

Eeefs of Mauritius— Shallow channel within the reef — Its slow
tilling up — Currents of water formed within it — Upraised reefs
— Narrow fringlng-reefs in deep seas — Eeefs 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
aO"ected by currents of the sea — Coral coating the bottom of the
sea, but not forming reefs . . . . . . 69 to 79

CHAPTEP. IV.
ON THE GROWTH OF CORAL-REEFS.

BECTIOX 1. — ON THE DISTRIBUTION OF COHAl-EEEFS, AND ON THE CON-
DITIONS FAVOUKAELE TO THEIR INCREASE . . . 80 tO 95

SECTION n. — ON THE RATE OF GROWTH OF COKAL-REEFS . 95 tO 108

SECTION III. — ON THE DEPTHS AT WHICH RKEF - BUILDING CORALS

LIVE 108 to 118

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 eilects of storms and earth-
ijuakes 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 Bauk^-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 OP CORAL-REEFS WITH REFERENCE
TO THE THEORY OP 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 — Eecapitulation 158 to 196

APPENDIX [I.]

Containing a detailed description of the Reefs and Islands in the
coloured Map, Plate III 199 to 260

[APPENDIX IL]

[Summary of the principal contributions to the History of Coral-
lieels since the year 1874] 281 to 8^2

Genekal iNiiKs 333



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 very 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 jjlain dotted
circles. A few of the deepest soundings are laid down within
each reef ; they are in fathoms, of six English feet.

Fig. 1. — Vakikoko, situated in the western part of the S. Pacific;

taken from the survey by Cajjt. 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,

2



XVI U DESCRIPTION OF PLATES.

are given from the Voyage of the Chev. Dillon ; the other sounl-
ings are laid down from the survey by D'Urville ; hciglit 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. — HoGOLEU, or Rouo, 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 ^ of an inch
to a mile ; the depth of the immense lagoon-like space within the
reef is not known.

Fig. 3.— Baiatea, 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 Heyou atoll (or lagoon-island), in the Low Arch
pelago ; from the survey by Capt. Beechey, R.N. ; scale ^j-, 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.— BoLABOLA, in the Society Archipelago ; from the survey of
Capt. Duperrey, in the Goquille ; scale ^ of an inch to a mile ;
the soundings in this and the following figure have been altered
from French feet to Enghsh fathoms ; height of highest point of
the island 4,026 feet.

Fig. G.— Maurua, in the Society Archipelago; from the survey by
Capt. Duperrey in the Goquille; scale I- of an inch to a mile;
height of land about 800 feet.

Fig. 7.— PouYNiPKTE, or Seniavine, in the Caroline Archipelago;
from the survey by Admiral Lutk6 ; scale \ of an inch to a mile.

Fig. 8. — Gambier Islands, 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
aide is submerged.

Fi^r. 9.— Peros Banhos atoll (or lagoon-island), in the ChagoH group
in the Indian Ocean; from the survey by Capt. Moropby ami



DESCRIPTION OF PLATES. XIX

Lieut. Powell ; scale \ of an inch to a mile ; not nearly all tlio
Bmall submerged reefs in the lagoon are represented ; the annular
reef on the southern side is submerged.

Fig. 10. — Keeling, or Cocos atoll (or lagoon-island), in the Indian
Ocean ; from the survey by Capt. FitzPioy ; 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
Bea breaks.

PLATE II. at end of Volume.

Fig. 1. — Great Chagos Bank, 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 E. and W. line
- across the Great Chagos Bank, given for the sake of exnibiting
more clearly its structure.

Fig. 3. — Menchicoff atoll (or lagoon-island), in the Marshall Archi-
pelago, northern Pacific Ocean ; from Krusenstern's atlas of the
Pacific ; originally surveyed by Capt. Hagemeister ; scale ^^ of an
inch to a mile ; the depth within the lagoons is unknown.

Fig. 4. — Mahlos Mahdoo atoll, together with Horsburgh atoll, in
the Maldiva Archipelago ; from the survey by Capt. Moresby and
Lieut. Powell ; scale 55 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
vras found not possible to distinguish them clearly from the small
iislets, which have been formed on these same small reefs ; many
of the smaller reefs could not be introduced; the nautical mark
(-1.) 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. — New Calkdonia, in the western part of the Pacific ; from
Kiusenaterrib 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 Jj of an
inch to a mile.

Fig. G. — Maidiva Archipelago, in the Indian Ocean ; from the survey
by Capt. Moresby and Lieut. Powell; scale ^^ 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.



THE



STRUCTUEE AND DISTEIBUTIOIT



COEAL-EEEFS.



INTRODUCTION.

The object 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 msignificant
coral-polypifers, and when he knows that the solid reef
increases only on the outer edge, which day and night



No. 1.




is lashed by the breakers of an ocean never at
rest. Well did Francois Pyrard de Laval, in the
year 1605, exclaim, * C'est une merueille de voir
chacun de ces atollons, enuironne d'un grand banc de
pierre tout autour, n'y ayant point d'artifice humain.'
The 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. 6

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 ' atoU,' 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

No. 2.




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, owmg 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. Eeefs also occur around
submerged banks of sediment and of worn-down rock ;
and others are scattered quite irregularly where the
sea is very shallow ; these are allied in most respects
to fringing reefs, but are of comparatively httle
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

' I have taken the liberty of simplifying the foreground, and
leaving out a mountainous island in the far distance.



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