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6 \ of THE

STRUCTURE /AND / DISTRIBUTION /

OF

CORAL REEFS

BY

CHARLES DARWIN, M.A., F.R.S., F.G-.S.

WITH THREE PLATES

SECOND EDITION, REVISED —

| LONDON
SMITH, ELDER, & CO., 15 WATERLOO PLACE
1874

All rights reserved s

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 volcano

vi ss PREFACE TO

can affect the whole cireumference 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. 320) that I look at fringing-reefs as a proof of
the recent elevation of the land; but I hawve 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 eases 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

THE SECOND EDITION. Vil

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, Pro-

fessor 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
of the Pelew Islands, which were carefully examined

by him, some account of his objections, and I will here

Vill | PREFACE TO

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 volcano, which
was formerly supposed to exist in Torres Straits. An
account of a remarkable bar of sandstone off Pernam-
buco 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 charts published in

various countries, my map might have been greatly
improved. But I hope that before long some one may

need state of geographical knowledge 3

on a large scale, on nearly ©

i

sas I have done, and in accordance ©

a
47

'
4
v

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 plea-

sure of acknowledging how much I owe to Captain

xii 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 yol-
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 ? : x : ; é é . PAGE!

CHAPTER I.

ATOLLS OR LAGOON ISLANDS.

Section I.—DerscrIPTION OF KEELING ATOLL.

Corals on the outer margin—Zone of Nulliporsee—Exterior reef —Islets—
Coral-conglomerate—Lagoon—Calcareous sediment—Scari and Ho-
luthuriz subsisting on corals—Changes in the condition of the reefs
and islets—Probable subsidence of the atoll—Future state of the
lagoons . : é ‘ : : - : ’ . 7 to 26

Section I].—GENERAL DESCRIPTION OF ATOLLS.

General form and size of atolls, their reefs and islets—External slope—
Zone of Nulliporee—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
jamd, . , : ‘ ; 5 ; x . 27 to 48

Section JIIJ.—AToLis 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—Des-
truction of islets—Connection in the position and submarine foun-
dation of distinct atolls—The apparent disseverment of large atolls—
The Great Chagos Bank—Its submerged condition and extraordinary
structure . : : 5 : : ‘ ‘ . 438 to 5d

X1V . CONTENTS,

CHAPTER Ii.

BARRIER-REEEFS.

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-
reets . : : : 9 : 5 : . PAGE 66 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 sedi-
ment and on worn-down islands—Fringing-reefs affected by
currents of the sea—Coral ceating the bottom of the sea, but not
forming reefs 5 5 : : : : : . 69 to 79

CHAPTER IV.
ON THE GROWTH OF CORAL-REEFS.

Section J.—ON THE DISTRIBUTION OF CORAL-REEFS, AND ON THE
CONDITIONS FAVOURABLE TO THEIR INCREASE Q . 80 to 95

Section II.—On THE RATE OF GROWTH OF CORAL-REEFS 95 to 107

Section IIJ.—On THE DEPTHS AT WHICH REEF-BUILDING CORALS
LIVE . E at ; : : : i . LOT to 16

CHAPTER V.

THEORY OF THE FORMATION OF THE DIFFERENT CLASSES OF
CORAL-REEES.

The atolls of the larger archipelagoes are not formed on submerged
craters, or on banks of sediment—Immense areas interspersed with
atolls—Their subsidence—The effects of storms and earthquakes on
atolls—Recent changes in their state—The origin of barrier-reefs
and of atolls—Their relative forms—The step-formed ledges and

CONTENTS. xV

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, consi-
dered—The probable composition of the lower parts of atolls

PAGE 117 to 155

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—Froofs of
recent elevation where fringing-reefs occur—Oscillations of level—
Absence of active voleanos 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 . . 166 to 193

APPENDIX.

Containing a detailed Description of the Reefs and Islands in the

coloured Map, Plate III. : : ; ; : 197 to 268

GENERAL INDEX bce : f , ; 4 : . 269

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 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 en-
circling ‘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 within each reef; they are in fathoms, of six English feet.

Fig. 1.—Vanixkoro, situated in the western part of the S. Pacific; taken
from the survey by Capt. D’Urville in the Astrolabe ; the scale is 3
of an inch to a geographical mile; the soundings on the southern
side of the island, namely from 30 to 40 fathoms, are given from

a

XVill - DESCRIPTION OF PLATES.

the Voyage of the Chev. Dillon; the other soundings are laid down ©
from the survey by D’Urville; height of the summit of the is-
land 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 bar-
rier-reef) as a good specimen of an abruptly-sided island, sur-
rounaed by a reef of the fringing class.

Fig. 2.—Hogorxv, 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 4 of an inch to a mile;

the depth of the immense lagoon-like space within the reef is not
known.

Fig. 3.—Ratarna, in the Society Archipelago; from the map given in
the quarto edition of Cook’s First Voyage; it is probably not accurate; .
scale 35 of an inch to a mile.

Fig. 4.—Bow, or Heycv atott (or lagoon-island), in the Low Archi-
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.—Botazpota, 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 in the Coquille; scale + of an inch to a mile; height
of land about 800 feet.

Fig. 7.—PovynipeETe, or SENIAVINE, in the Caroline Archipelago ; from
the survey by Admiral Lutké ; scale + of an inch to a mile.

Fig. 8.—Gampirer 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 surrounded

by extensive and irregular reefs; the reef on the southern side is
submerged.

by

Fig. 9.—Prros Banuos atTorx (or lagoon-island), in the Chagos group
in the Indian Ocean; from the survey by Capt. Moresby and Lieut.

DESCRIPTION OF PLATES. xX1xX

Powell; scale + of an inch to a mile; not nearly all the small sub-
merged reefs in the lagoon are represented ; the annular reef on the
southern side is submerged.

Fig. 10.—Kzestine, or Cocos aroun (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 Bank, in the Indian Ocean; taken from the
survey by Capt. Moresby and Lieut. Powell ; scale 4 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 380 to 60 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 exhibiting more
clearly its structure.

Fig. 38.—Mencuticorr atoxu (or lagoon-island), in the Marshall Archi-
pelago, northern Pacific Ocean; from Krusenstern’s atlas of the
Pacific ; originally surveyed by Capt. Hagemeister; scale 4+, of an
inch to a mile; the depth within the lagoons is unknown.

Fig. 4.—Mauxos Manpoo Atott, 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.—New Cazeponta, 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 Voyageof the Astrolabe. In Krusenstern’s atlas, the reef is
represented by a single line with crosses; I have for the sake of
nniformity added an interior line; scale 4, of an inch to a mile.

Fig. 6.—Manpiva ArcHirPeLaco, 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.

THE

STRUCTURE AND DISTRIBUTION

OF

CORAL-REEFS.

INTRODUCTION.

THE oBsECT 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 shail 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 every one must be
struck with astonishment, when he first beholds one of

B

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 imsignificant
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

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-
B2

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 Pacifie 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 is 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] have taken the liberty of simplifying the foreground, and leaving
out a mountainous island in the far distance.

INTRODUCTION. fae

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-
graphical order, as far as I possessed information; and
any particular spot may be found by consulting the
Index. -

Several theories have been advanced to explain the
origin of atolls or lagoon-islands, but scarcely one to
account for barrier-reefs. From the limited depths at
which reef-building polypifers can flourish, taken into
consideration with certain other circumstances, we are
compelled to conclude, as it will be seen, that both in
atolls and barrier-reefs, the foundation 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

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.’ ,

1A 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—Scari and Holu-
thurie subsisting on corals—Changes in the condition of the reefs and
islets—Probable. subsidence of the atoll—Future state of the lagoon.

KEELING or Cocos atoll is situated in the Indian Ocean,
in 12° 5’ S. and long. 90° 55’ E.: a reduced chart of it
from the survey of Capt. FitzRoy and the officers of
H.M.S. Beagle, is given in Plate I. fig. 10. The
ereatest width of this atoll is nine miles and a half.
Its structure is in most respects characteristic of the
elass to which it belongs, with the exception of the
shallowness of the lagoon. The accompanying wood~
cut (No. 3) 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 ina
vertical one, as the average greatest height of the land
is only between six and twelve feet above high-water
mark. I will describe the section, commencing with

8 ATOLLS. Cu. I,

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-

heite), that an exposure to the rays of the sun for a very

159 Yards
\ Tee JE8 1

A—Level 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.

EA 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.

short time invariably causes their destruction. Hence
it is possible only under the most favourable cireum-
stances, afforded by an unusually low tide and smooth
water, to reach the outer margin, where the coral is
alive. I succeeced only twice in gaining this part, and
found it almost entirely composed of a living Porites,

~ Secr. I. KEELING ATOLL. 9

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 shoreward limit of
existence of those corals which form the outer margin
of the reef. We shall presently see that there are other
organic productions, fitted to bear a somewhat longer
exposure to the air and sun.

Next, but much inferior in importance to the
Porites, is the Millepora complanata.’ It grows in
thick vertical plates, intersecting each other at various

1 This Millepora, (Palmipora of Blainville,) as well as the M. alci-
cornis, possesses the singular property of stinging the skin where it is
delicate, as on the face and arm.

10 ATOLIUS.* ‘Caer.

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 in-
habited by quite a distinct set of corals, generally brittle
and thinly branched; but a Porites, apparently of the
same species with that on the outside, is found there,
although it does not seem to thrive, and certainly does
not attain the thousandth part in bulk of the masses
opposed to the breakers.

The wood-cut (No. 3) shows the form of the bottom
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'The soundings from which this section is laid down were taken
with great care by Captain FitzRoy himself: he used a bell-shaped
lead, having a diameter of four inches, and the armings each time were
eut off and brought on board for me to examine. The arming is a
preparaticn 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 tne bottom be of rock, it brings up an exact impression of its surface.

4
3

Srcr. I. KEELING ATOLL. | rT

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 Mullepora alcicornis
were brought up; and on the arming from an eight-
fathom cast, there was a perfect impression of an
Astrea, apparently alive. J examined the rolled
fragments cast on the beach during gales, in order fur-
ther to ascertain what corals grew outside the reef.

The fragments consisted of many kinds, of which the

Porites already mentioned and a Madrepora, apparently
the M. corymbosa, were the most abundant. As I
searched in vain in the hollows on the reef and in the
lagoon, for a living specimen of this Madrepore, I con-
elude 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 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 in-
dented with coral: an anchor and lead were lost. at the
respective depths of 13 and 16 fathoms. Out of

1 Rea ATOLLS. Cu. I.

twenty-five soundings taken. at a greater depth than
20 fathoms, every one shewed that the bottom was
covered with sand; whereas at a less depth than
12 fathoms, every sounding shewed that it was
exceedingly rugged, and free from all extraneous
particles. Two soundings were obtained at the
depth of 360 fathoms, and several between 200 and
300 fathoms. The sand brought up from these depths
consisted of finely triturated fragments of stony zoo-
phytes, but not, as far as I could distinguish, of a par-
ticle of any lamelliform genus: fragments of shells
were rare.

At a distance of 2,200 yards from the breakers, Cap-
tain FitzRoy found no bottom with a line 7,200 feet in
length; hence the submarine slope of this coral forma-
tion 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 con-
cluded 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, 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

Sect. I. KEELING ATOLL. 13

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 cases is hard, but where formed
of the lichen-like kind, readily yields an impression to
the hammer: the surface is of a reddish colour. These
Nullipore, although able to exist above the limit of
true corals, seem to require to be bathed during the

greater part of each tide by breaking water, for they are

not found in any abundance in the protected hollows on
the back part of the reef, where they might be immersed
either during the whole or an equal proportional time
of each tide. It is remarkable that organic productions
of such extreme simplicity, for the Nulliporz undoubt-
edly belong to one of the lowest classes of the vegetable
kingdom, should be limited to a zone so peculiarly cir-
cumstanced. Hence the layer 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 into a
solid margin, as a continuous smooth convex mound

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
eylindrical, and do not grow thicker at their extremities.

14 ATOLLS. Cu. I

(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 growth of Nulliporee 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 —

lagoon 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
Madrepora, closely allied or identical with M. pociullifera,

is also common. As soon as an islet is formed, and the

waves are prevented from breaking entirely over the

’
~ F Pee
n Pe
< V9 ha reine a in in ee ear a ‘

Seer. I. KEELING ATOLL. | 15

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: it is uncovered
only at low water. I could with difficulty, and only
by the aid of a chisel procure chips of rock from
its surface, and therefore could not ascertain how
much of it is formed by the aggregation of detritus,
and how much by the outward growth of mounds of
corals, similar to those now living on the margin. 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
Nulliporee, 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-
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
afew yards to several miles. Those on the S8.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-

16 shone ATOLLS. Cx.'T.

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 30 feet high), is
close to the outer beach ( of the wood-cut) and aver-
ages from six to ten feet above ordinary high-water |
water. 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 very low. The fragments beneath the surface
are cemented into a solid mass, which is exposed as a
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
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

Sect. I. KEELING ATOLL. irs

parts reddish from ferruginous matter: it is very hard
and sonorous under the hammer: itis 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
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

C

18 ; ATOLLS. Ca. a

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
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 8.E. islet, was floated out. It is
avery 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

1This Porites has somewhat the habit of P. clavaria, but the
branches are not knobbed at their ends. When alive it is of a yellow
colour, but after having been washed in fresh water and placed to dry,
a jet-black slimy substance exuded from the entire surface, so that the
specimen now appears as if it had been dipped in ink,

Sect. I. KEELING ATOLL. 19

*

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 8.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
caleareous.. I have seen in the Museum of the Geolo-
gical Society, a similar but more remarkable substance,
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 ina
small degree. There are, however, other and unex-
pected 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 con-
siderable size, and I found their intestines distended
by small pieces of coral, and finely ground calcareous
matter. This must daily pass from them as the finest
sediment ; much also must be produced by the infinitely
numerous vermiform and molluscous animals which
make cavities in almost every block of coral. Dr. J.

c 2

20 ATOLLS. CHIME,

Allan of Forres, who has enjoyed the best means of
observation, informs me in a letter, that the Holuthuriz
(a family of Radiata), subsist on living coral; and the
singular structure of hone 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
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 S.E. side was divided by several channels into as
many islets; and he assures me that the channels can
still be distinguished by the smaller size of the trees

Sucr. I. KEELING AIOLL. : 21

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-
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 S.E. wind, at
the head than at the mouth of the lagoon. 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

22 ATOLLS. BOs stad he

erown 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 reyular 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
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,

9
ia
v" sisssmanianinesaaasainaallel

ee

Saar Te KEELING ATOLL. = ee

each successive margin would naturally have been
coated by the Nulliporee, 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 Nullipors formed the convex mound on its
margin: for the zone thus formed, and which stands
between two and three feet above the other parts of
the reef, is nowhere much above twenty yards in
width.

Thus far we have considered facts, which indicate,
with more or less probability, an increase in the
diameter of the atoll; but there are others having an
opposite tendency. On the S.E. side, Lieut. Sulivan,
to whose kindness I am indebted for many interesting
observations, found the conglomerate (D, in wood cut)
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-

24 ATOLLS. Cul,

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
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

Secr. I. KEELING ATOLL. ; 95

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-

elude from the fields of dead coral still lipping the

surface of the lagoon, and from the breakers on the
western shore not having yet regained the line of their

former action. ‘The subsidence must, also, have been

preceded by a long period of rest, during which the
islets extended to their present size, and the living
margin of the reef grew either upwards, or as I believe
outwards, to its present distance from the beach.
Whether this view be correct or not, the above
facts are worthy of attention, as showing how severe a
struggle is in progress on these low coral-formations
between the two nicely balanced. powers of land and
water. With respect to the future state of Keeling
atoll, if left undisturbed, we can see that the islets
may still extend in length; but as they cannot resist
the surf until 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-

26 | ATOLLS. Ca.

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
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.

Sect. IT. 27

SECTION SECOND.

General form and size of atolls, their reefs and islets —External 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 land.

I witt 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
eroup, 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 I.),
and Clermont Tonnere has nearly the same proportions.
In the Marshall Archipelago (the Ralick and Radack
eroup 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-

28 ATOLLS. Cm. 1

ever, bears a double name), measured in a medial and
slightly curved line, is no less than eighty-eight geo-
graphical miles long, its greatest width being under
twenty, and its least only nine and a half miles. Some
atolls have spurs projecting from them; and in the
Marshall group there are atolls united together by
linear reefs, for instance Menchioff Island (See Fig. 3,
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 aboit 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
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
ease; and the occurrence of islets, as if placed for
beacons, on the points where there is a gateway or

1 Beechey’s Voyage to the Pacific and Beering’s Straits, chap. vill.

Sect. II. ATOLLS. 29

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.

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

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.’

30 _ATOLLS. Cx. I.

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.
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, Lieut. Powell * found 50 and
60 fathoms close to the edge of the reef, and at 300
yards distance there was no bottom with a 300 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-

1T must be permitted to express my obligation to Captain Beechey,
for the very kind manner in which he has given me information on
several points, and to own the great assistance I have derived from his
excellent published work.

2 Cook’s Third Voyage. vol. i. chap. 10.

$ 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. y. p. 401. ;

Secr. II. ATOLLS. ot

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. I am informed by the same authority,
that wherever soundings were obtained off these
islands, the bottom was invariably sandy: nor was there
any reason to suspect the existence of submarine clifts,
as there was at Keeling Island.!. Here then occurs a
difficulty ;—can sand accumulate on a slope, which, in
some cases, appears to exceed fifty-five degrees? It
must be observed, that I speak of slopes where 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

1 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 twenty-five
fathoms, and then ends abruptly, like the first; and immediately
beyond this there is no bottom with two hundred fathoms.

* Mémoires pour servir 4 une description Géolog. de France, tome
myer p. 216.

oF : ATOLLS. on Cu, I.

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 1t may be called, which ‘has the most
powerful enemy (the 8.W. swell) to oppose, is carried
out much further, and with less abruptness, than the
other.’ In some cages, the less inclination of a certain
part of the external slope, for instance of the northern
extremities of the two Keeling atolls, is caused by a
prevailing current which there accumulates a bed of
sand. Where the water is perfectly tranquil, as within
a lagoon, the reefs generally grow up perpendicularly,
and sometimes even overhang their bases: on the
other hand, on the leeward side of Mauritius, where
the water is generally tranquil although not invariably
so, the reef is very gently inclined. Hence it appears

1 Beechey’s Voyage, 4to. ed. p. 44.

crirtain

Sect. II. _ ATOLLS, 38

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
erowing 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 la-
goon: Flinders states that the Australian barrier has

-a similar inelination inwards, and I have no doubt

it is of general occurrence, although, according to
Ehrenberg, the reefs of the Red Sea offer an exception.

- Chamisso observes that ‘the red colour of the reef (at

the Marshall atolls) under the breakers is caused by a
Nullipora, which covers the stone wherever the waves
beat; and, under favourable circumstances, assumes a
stalactitical form,’—a description perfectly applicable
to the margin of Keeling atoll.! Although Chamisso

1 Kotzebue’s First Voyage, vol. iii. p. 142. Near Porto Praya, in
the Cape de Verd Islands, some basaltic rocks, lashed by no inconsider-

D

34 ATOLLS. Cu. I.

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.?
Whether a smooth convex mound of Nullipore, like
that which appears as if artificially constructed to
protect the margin of Keeling Island, is of frequent
occurrence round atolls, I know not; but we shall
presently meet with it under precisely the same form,
on the outer edge of the ‘ barrier reefs’ which encircle
the Society Islands.

There appears to be scarcely a feature in the
structure of Keeling reef, which is not of common, if
not of universal occurrence, in other atolls. Thus

able 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 situa-
tion, These Nullipore are closely related to those described on the
coral-reefs, but I believe are of different species.

1 Kotzebue’s First Voyage, vol. 11. p. 16. Lieut. Nelson, in his ex-
cellent memoir in the Geological Transactions (vol. ii. p. 105), alludes
to the rocky points mentioned by Kotzebue, and infers that they consist
of Serpulee, which compose incrusting masses on the reefs of Bermudas,
as they likewise do on a sandstone-bar off the coast of Brazil, as de-
scribed 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 position, relatively to the action of

the sea, with the Nullipore on the coral-teefs in the Indian and Pacific

oceans.
2 Capt. Moresby, in his valuable paper ‘On the Northern Atolls of

Maldivas’ (Geographical Journal, vol. v.), says that the edges of the
reefs there stand above water at low spring tides.

Ar [ee

Sect. II. - ATOLLS, on

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 ny, conglomerate
coral-rock was there observed.

The lagoon in Keeling atoll is shallow: in the
atolls of the Low Archipelago the depth varies from 20
to 38 fathoms, and in the Marshall Group, according

to Chamisso, from 30 to 35: in the Caroline atolls it

is only a little less. Within the Maldiva atolls there
are large spaces with 45 fathoms, and some soundings
are laid down 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

1 Kotzebue’s First Voyage, vol, iii, p. 144.
? See also Moresby on the Northern Atolls of the Maldivas, Geogra-

phical Journal, vol. v. p. 400.

Dp 2

36 ATOLLS. Ca. I

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 oe is
ealled 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
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

iT 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 composed of ‘tuf blanc,
mélé ou formé de madrépores broyés.’ This white substance, probably
is analogous to that which occurs within the above-mentioned lagoons ;
if is sometimes, according to Roussin, firm, os he compares it to
mortar,

Pak 8

Szcr. IT. : ATOLLS. 37

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 : it is, in fact, a wall of dead coral-rock,
having the same width and transverse section with the

_reef in its ordinary state, of which it is a continuous

portion. The living and perfect parts terminate
abruptly, and abut on the submerged portions, in
the same manner as 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 of the reef 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

1 Frederic Lutke’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.

_parts rising to the surface and perfectly characterised,
in parts lying some fathoms submerged. In the Chagos
| group there are annular reefs entirely submerged, which
have the same structure as the submerged and defined
portions just described. The Speaker’s Bank offers an
excellent example of this structure ; its central expanse,
which is about 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
the submerged portion of the reef at Peros Banhos, Capt.
Moresby feels sure that the quantity of hving coral,
even on the outer edge overhanging the deep-sea water,
is quite insignificant. Lastly, in several parts of the
Pacific and Indian Oceans there are banks, lying at
oreater 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

Szcr. II. ATOLLS. 39

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
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-

~ 1 Principles of Geology, vol. ili. p. 289.

40 ATOLLS. | Cx. I.

ing the surface. 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 oceur, of course, on the windward
as well as on the leeward side; but they are more
frequent and broader to leeward, owing to the lesser
dimensions of the islets on that side. |

At Keeling atoll the shores of the lagoon shelve
gradually where the bottom is of sediment, and irregu-
larly or abruptly where there are eoral reefs; but this
is by no means the universal structure in other atolls.
Chamisso,” speaking in general terms of the lagoons in
the Marshall atolls, says the lead generally sinks ‘ from

a depth of two or three fathoms to twenty or twenty-—

four, and you may pursue a line in which on one side
of the boat you may see the bottom, and on the other
the azure-blue deep water.’ The shores of the lagoon-
like channel within the barrier-reef at Vanikoro have
a similar structure. Captain Beechey has described a
modification of this structure (and he believes it is not
uncommon) in two atolls in the Low Archipelago, in
which the shores of the lagoon descend by a few broad,

1 Beechey’s Voyage, 4to ed. vol. i. p. 189. —
? Kotzebue’s First Voyage, vol. iii. p. 142.

ie ace

4
:

Sect. II. ATOLLS. Aj

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 eome to the surface at
low-water; in other cases of rarer oecurrence, all
lie at nearly the same depth beneath it, but most
frequently they are quite irregular—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

1 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 perpendicular.
This bank is formed of coral and dead shells.

42 ‘ ATOLLS. . (Ops aea la

shall hereafter see that the theory which accounts for:

the ordinary form of atolls, apparently inciudes 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° DS») 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 oceasion-
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 surrounding
a muddy flat, one mile in its longest axis, which is
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 upa 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

’ Kotzebue’s First Voyage, vol. iii. p 221.

a
3

‘

———

Stan

iz se
r aie

’

Secr. IT. MALDIVA ATOLLS. 43

strip of land formed by the union of separate islets.
We cannot suppose that the many atolls in the Pacific
and Indian oceans all have had a late origin, and yet
should they remain at their present level, subjected
only to the action of the sea and to the growing powers

of the coral, during as many centuries as must have

elapsed since any of the earlier tertiary epochs, it can-
not, I think, be doubted that their lagoons and the
islets on their reef, would present a totally different
appearance from what they now do. This considera-
tion leads to the suspicion that some agency (namely,
subsidence) comes into play at intervals, and renovates
their original structure.

SECTION THIRD.

Maldiva Archipelago—hing-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 in the reefs with
respect to the prevalent winds and action of the waves—Destruction
of islets—Relation in position between distinct atolls—The apparent
disseverment of large atolls—The Great Chagos Bank—Its submerged
condition and extraordinary structure.

_ ALTHOUGH occasional references have been made to the
Maldiva atolls and to the banks in the Chagos group, —

some points of their structure deserve further 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

44 ATOLLS. , : Cu. I. 1
; : |

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
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 ehannels 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 like
miniature atolls. Other ring-formed reefs rise out of
the lagoons, in the place of those irregular ones which
ordinarily occur there. In the reduction of the chart
of Mahlos Mahdoo (Plate II. 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-

Sscr. ITI. MALDIVA ATOLLS. 4d

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
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

! Captain Moresby informs me that Millepora complanata is one of
the commonest kinds on the outer margin, as it is at Keeling atoll.

AG ATOLLS. Cx. 1, @
» |b

reef like that surrounding an ordinary atoll, to others
which are ring-formed and much elongated but con-
taining only a very narrow lagoon, 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
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.

Geert. MALDIVA ATOLLS. 47

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. This is 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
half destroyed with the dead corals. In the central and
deepest part of the Maldiva lagoons, the bottom consists,
as Iam informed by Captain Moresby, of stiff clay (pro-
bably a calcareous mud); nearer the border it consists
of sand, and in the channels through the reef, of hard |
sand-banks, sandstone, conglomerate rubble, and a little
live coral. Close outside the reef 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

48 ATOLLS. *Oreae

abruptly from the unfathomable ocean, with the central
expanse studded 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 he at all intermediate depths from the
bottom upwards. When treating of the growth of coral
I shall again refer to this subject.

Although in the neighbourhood of the Maldiva
Archipelago the winds, during the monsoons, blow
during nearly an equal time from opposite quarters,
and although, as I am informed by Captain Moresby,
the westerly winds are the strongest, yet the islets are
almost all placed on the eastern side of the northern
atolls, and on the south-eastern side of the southern
atolls. That the formation of the islets is due to
detritus thrown up from the outside, as in the ordinary
manner, and not from the interior of the lagoons, may,
I think, be safely inferred from several considerations
which it is hardly worth while to detail. As the easterly
winds are not the strongest, their action probably is
aided by some prevailing swell or current.

f

-Szcr. ITI. MALDIVA ATOLLS, 49

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 itself issometimes 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 S. Male, Phaleedoo, and Moloque atolls, there are
seventy-three deep-water channels, and only twenty-five
on the outer sides; on the three latter-named atolls there
are fifty-six openings on the near side, and only thirty-
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 other, accords
with the fact, that the reefs of the southern atolls are

E

50 ATOLLS. Cereal.

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
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, and is (in
Lieut. Prentice’s words) ‘entirely covered with live
coral and madrépore.’ 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

4 pow

Secr. IIT. MALDIVA ATOLLS. 5

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 chart is required than
that of the Archipelago in Plate Il. 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,
was found in the channel with 200 fathoms: in the
wider channel between Horsburgh atoll and.the southern
end of Mahlos Mahdoo, no bottom was found with
250 fathoms. In these 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 atollsis 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 characterized atolls is in itself
interesting, as it has never, I believe, been effected in
any of thé many other groups of atolls in the Pacific
and Indian seas. In continuing to trace the con-
E2

Bo ATOLLS. Cu. I.

nection of adjoining atolls, if a hasty glance be taken
at the chart (fig. 4, Plate II.) of Mahlos Mahdoo and \
the line of unfathomable water be followed, no one ~
will hesitate to consider it as one atoll. But a second
look will show that it is divided by a bifureating
ehannel, 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 bifurcating channel
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 Tuilla-doumatte. 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:

{
\
;
|
|
:
|

u

Ssor. TIT. GREAT CHAGOS BANK. ae

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-
scribed. In the Chagos group there are some ordi-
nary atolls, some annular reefs rising to the surtace
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

La , T= t
‘ f
ay

|

54 ‘a ATOLLS. Cu. 1.

edges of a set of banks rudely arranged in a circle.
These banks consist of sand with a very little live
coral; they vary in breadth from five to twelve miles,
and on an average lie about sixteen fathoms beneath
the surface; they are bordered by the steep edges of a
third narrow and upper bank, which forms the rim to
the whole. This rim is about a mile in width, and with
the exception of two or three spots where islets have
been formed, is submerged between five and ten fathoms.
It consists of smooth hard rock, covered with a thin
layer of sand, but with scarcely any live coral; it is

steep on both sides, and 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
distance, there was none with 210 fathoms. Small
steep-sided banks or knolls, covered with luxuriantly-
erowing coral, rise from the interior expanse to the
same level with the external rim, which, as we have seen,
1s 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

'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.

_. eS ee ee

e shall hereafter have to consider both
its submerged condition, a state common

nks in the group, and the origin of the
submarine terraces which bound the central

2; these, I think it can be shown, have resulted

~

1 ce hs
he eer ns 2, ,

*

=e ee he? Sa Pete ek See

ee oe ad SIAL LPOIMETE e it Vee = : ——

4
.

56

, | 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 Cale-
donia 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 together. The reef, also, on the west coast of
New Caledonia, circling round the extremities of the
island, is an intermediate form between a small 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-

ee

iE

~Cu, II. BARRIER REEFS. SF

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, Oualan, 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

1The 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.

:

;
58 BARRIER REEFS. Cu. It. @

: '
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, Ellis! states that the
reefs generally le 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

* Consult, on this and other points, the Polynesian Researches by
the Rey. W. Ellis, an admirable work, full of curious information.

Cu. II. BARRIER REEFS. 59

from the hills; it is an encroachment on the channel,
analogous to that low and inner part of the islets in
many atolls, which is formed by the accumulation of
matter from the lagoon. At Hogoleu (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 of 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
separately, or more commonly skirt the shores of the
included high islands. The depth of the lagoon-chan-
nel 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 I., fig. 5), form a single
linear strip. The islets first appear either on the

1See 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. Cx IL gif

angles of the reef, or on the sides of the breaches —
through it, and are generally most numerous on the
windward side. The reef to leeward retaining its usual
width, sometimes lies submerged several fathoms 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) 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 I
find in Krusenstern’s Memoir on the Pacific, that there
are passages through the eneircling 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.

It is remarkable that the lagoon-channels round
mountainous islands have not in every instance been
long ago filled up with coral and sediment; but it is

: 4
Gu. IT. BARRIER REEFS. 61

accounted for without much difficulty. In cases like
that of Hogoleu and the Gambier Islands, where a few
small peaks rise out of a great lagoon, the conditions
scarcely differ from those of an atoll ; and I have already
shown at some length, that the filling up of a true
lagoon must be an extremely slow process. Where
the 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 open sea. The
water which is thrown over the edges of atoll-formed
teefs 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-~
ever, 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, average
dimensions, and grouping. LEncircling reefs, like
atolls, are generally elongated, and have an irregularly

62 BARRIER REEFS. Cu. II.

rounded, though sometimes angular outline. There are
atolls of all sizes, from less than two miles in diameter
to sixty miles (excluding Tilla~dou-Matte, 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 360, and Manouai only 50. The geo-
logical nature of the included land also varies; in most
cases it is of ancient volcanic origin, owing apparently 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 formation, of which
latter kind New Caledonia offers the best example.
The central land consists either of one island, or of
several; thus in the Society group, Eimeo stands by

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 Rey. J. Williams. The two latter
islands, however, are not in some respects well characterised examples
of the encircled class.

Cu. II. BARRIER REEFS, 63

itself; while Taha and Raiatea (fig. 3, Plate I.), both
moderately large islands, of nearly equal size, are in-
eluded in one reef. Within the reef of the Gambier
group 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 EKimeo 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

GAPS, BARRIER REEFS. | Cx Ih i}

average distance from the land is between twenty and
thirty miles, but in parts from fifty to ninety. The
great arm of the sea thus included, is from ten to

twenty-five fathoms deep, with a sandy bottom; but —

towards the southern end where the reef is further
from the shore, the depth gradually increases to forty,
and in some parts to more than sixty fathoms. Flinders
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-channeis.'

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 seale 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-

1 The foregoing details are taken chiefly from Flinders’ Voyage to
Terra Australis, vol. 11. 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. 65

sent the form of the land from the shading of the hills

in the large published charts. It has long been
remarked, even from the time of Dampier, that a
i 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

| 303245
\ rem Me Lt TB a
7 MT,

) Y) ii i ame |
ii

7

y

B

B wt CLIT IIE GTETSET pT rc eae eee ey Ri te
— Winn DM MMMM jp
yon Hee Dc a

i}
|
{
|
tii

i |

5 soo. ft

ee as - _ |

= =a FF) /////) WM ff, ———— ae
é Yf Uff
esTTT)

Miiiitinzay,|
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 Coquille, by Du-

perrey.
i 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 hori-
| zontal shading that of the encircling barrier-reef; from the smallness
of the scale, the lagoon-channel could not be represented.

A A—Outer edge of the coral-reefs, where the sea breaks.
q B B—The shore of the encircled islands.

F

66 BARRIER REEFS. On ii.

(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 ig
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,—tor
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
isa great apparent difficulty—-how were the basalt 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 1s 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 Rev. D. Tyerman and Mr. Bennett (Journal of Voyage and
Travels, vol. 1. p. 215) have briefly suggested this explanation of the
origin of the encircling reefs of the Society Islands.

ex. I. 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 Il. 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 ike a wall, spring up at a distance of two, three,
or more miles from the shore, leaving a channel often
between 200 and 300 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, on a
foundation slowly formed by the accumulation of its 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
F 2

68 BARRIER REEFS. ‘ Cu. II.

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? We shall hereafter see, that there is one, and
I believe only one solution of this difficulty.

_ One other supposition to account for the position
of encircling 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 every one. New
Caledonia, moreover, besides its size, is composed of
primitive formations, as are some of the Comoro
Tslands;! and Aitutaki consists of calcareous rock.
We must, therefore, reject these several explanations,
and conclude that the vertical thickness of barrier-
reefs, from their outer edges to the foundation on
which they rest (from AA in the 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.

1T have been informed that this is the case by Dr. Allen of Forres,
who has visited this group.

69

CHAPTER ITT.

FRINGING OR SHORE REEFS.

Reefs of Mauritius—Shallow channel within the reef—Its slow filling
up—Currents of water formed within tt~—Upraised reefs—Narrow
Jringing-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 bottom of the sea, but not forming reefs.

FRINGING-REEFS, or, as they have been called by some
voyagers, shore-reefs, whether skirting an island or
1 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 a l’Isle de France,’ undertaken in 1768.
According to Captain Carmichael (Hooker’s Bot. Misc., vol. 1i. p. 316), on
one part of the coast there is a space of sixteen miles without a reef.

70 FRINGING REEFS. Geary:

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 8.E. 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
hes 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. Even in this last case, as the -

coast-land is gently inclined from the foot of the
mountains to the sea-beach, and as the soundings
outside the reef indicate an equally gentle slope
beneath the water, there is no reason for supposing
that the basis of the reef, formed by the prolongation
of the strata of the island, lies at a greater depth than
that at which the polypifers could begin constructing
the reef. Some allowance, however, must be made for
the outward extension of 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 —

d . iss ey, A es ™~
4 q sso =~
ia ays
, “¢ ee
: a x ‘
¢ '

g

q 7
¥

|

Gx. IIT. FRINGING REEFS. Tl

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. III.

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 erow up, not
attached to the actual beach, but at some little distance
from it ; and the corals on the outer margin would be
the most vigorous. A shallow channel would thus be
formed within the reef; and this channel could be filled
up only very slowly with sediment, for the breakers
cannot act on the shores of the island, and they do
not often tear up and cast inside fragments from the
outer edge of the reef, whilst every streamlet carries
away its mud in a straight line through breaches in
the reef. But a beach of sand and of fragments of
the smaller kinds of coral seems, in the case of Mauri-
tius, to be slowly encroaching on the shallow channel.
On many shelving and sandy coasts, the breakers tend
to form a bar of sand a little way from the beach, with
a slight increase of depth within it: for instance, Captain
Grey! states that the west coast of Australia, in lat.
24°, is fronted by a sand bar about 200 yards in width,
on which there is only two feet of water; but within
it the depth increases to two fathoms. Similar bars,

1 Captain Grey’s Journal of Two Expeditions, vol. i. p. 369.

” JURE ” ’
yay
ame “ ‘
¥ ad -
q yt
Bis
{ A

Cu. III. FRINGING REEFS. Vig)

more or less perfect, occur on other coasts. In these
eases I suspect that the shallow channel, (which no
doubt during storms is occasionally obliterated,) is
scooped out by the flowing away of the water thrown
beyond the line on which the waves break with the
ereatest force. At Pernambuco the bar of hard sand-
stone, before alluded to, has the same external form
and height as a coral reef, and extends nearly parallel to
the coast; within this bar currents, apparently caused
by the water thrown over it during the greater part of
each tide, run strongly, and are wearing away its inner
wall. From these facts it can hardly be doubted that
within most fringing reefs, especially within those
lying some distance from the land, a return stream

_ must carry away the water thrown over the outer edge;

and the current thus produced would tend to prevent
the channel being filled up with sediment, and might
even deepen it under certain circumstances. To
this latter belief I am led, by finding that channels are
almost universally present within the fringing reefs of
those islands which have undergone recent elevatory
movements; and this could hardly have been the case
if the conversion of the very shallow channel into land
had not been counteracted to a certain extent.

A fringing reef, if elevated in a perfect condition
above the level of the sea, would present the singular
appearance of a broad dry moat bounded by a low wall
or mound. The author! of an interesting pedestrian

1 Voyage a VIsle de France, par un Officier du Roi, Part 1. pp.
192, 200.

Ai
74 FRINGING REEFS. ey |
|i
tour round the Mauritius, seems to have met witha — |
structure of this kind: he says, ‘ J’observai que la, ot
la mer étale indépendamment des rescifs du large,
il y a & terre wne espéce deffoncement, ou chemin
couvert naturel. On y pourrait mettre du canon,’ &e.
In another place he adds, ‘Avant de passer le
Cap, on remarque un gros banc de corail élevé
de plus de quinze pieds: c’est une espece de res-
cif, que la mer a abandonné: il régne au pied
une longue flaque d’eau, dont on pourrait faire un
bassin pour de petits vaisseaux.’ But the margin of
the reef, although the highest and most perfect part,
from being most exposed to the surf, would generally
during a slow rise of the land be either partially or
entirely worn down to that level at which corals could
renew their growth on its upper edge. On some parts
of the coast-land of Mauritius there are little hillocks
of coral-rock, which are either the last remnants of a
continuous reef, or of low islands formed on it. I
observed two such hillocks between Tamarin Bay and
the Great Black River; they were nearly 20 feet
high, about 200 yards from the present beach, and
about 30 feet above its level. They rose abruptly
from a smooth surface, strewn with worn fragments
of coral. ‘They consisted in their lower part of hard
calcareous sandstone, and in their upper of great
blocks of several species of Astrzea and Madrepora,
loosely aggregated; they were divided into irregular
beds, dipping seaward, in one hillock at an angle of 8°,

Cu. TIL. FRINGING REEFS. Wo

=<

and in the other at 18°. The upraised reefs round
this island have been much less worn and modified by
the action of the sea, than in most other cases.

Many islands! are fringed by reefs quite similar to
those of Mauritius: but on coasts where the sea
deepens very suddenly, the reefs are much narrower,

and their iimited extension seems evidently to depend

on the high inclination of the submarine slope ;—a
relation which, as we have seen, does not exist in reefs
of the barrier class. The fringing-reefs on steep coasts
are frequently not more than from 50 to 100 yards in
width: they have a nearly smooth, hard surface,
scarcely uncovered at low-water, and without any
interior shoal channel like that within those fringing-

‘reefs which lie at a ereater distance from the land.

The fragments torn up during gales from the outer
margin, are thrown over the reef on the shores of the
island. I may give as instances, Wateeo, where the
reef is described by Cook as being 100 yards wide;
and Mauti and Elizabeth? Islands, where it is only
50 yards in width: the sea round these islands is very

deep.

1 IT may give Cuba, as another instance; Mr. Taylor (Loudon’s Mag.
of Nat. Hist. vol. ix. p. 449) has described a reef several miles in
length between Gibara and Vjaro, which extends parallel to the shore
at the distance of between half and the third part of a mile, and en-
closes a space of shallow water, with a sandy bottom and tufts of coral.
Outside the edge of the reef, which is formed of great branching corals,
the depth is six and seven fathoms. This coast has been upheaved at
no very distant geological period.

2 Mauti is described by Lord Byron in the voyage of H.M.S. Blonde,
and Elizabeth Island by Captain Beechey.

76 FRINGING REEFS,

Fringing-reefs, like barrier-reefs, surround islands,
and front the shores of continents. In the charts
of the eastern coast of Africa, by Captain Owen,
many extensive fringing-reefs are laid down ;—thus,
for a space of nearly 40 miles, from lat. 1° 5’ to
1° 45’ §., a reef fringes the shore at an average
distance of rather more than one mile, and therefore
at a greater distance than is usual in reefs of this
class; but as the coast-land is not high, and as the
bottom shoals vary gradually, (the depth being only
from 8 to 14 fathoms at a mile and a-half outside the
reef), its extension thus far from the land offers no
difficulty. The external margin of this reef is de-
scribed as formed of projecting points; and within it
there is a channel from six to twelve feet deep,
with patches of living coral. At Mukdeesha (lat.
2° 1’ N.) ‘the port is formed, it is said5')< joy alonom
reef extending eastward four or five miles, within
which there is a narrow channel, with ten to twelve
feet water at low spring tides:’ it les at the distance
of a quarter of a mile from the shore. Again, in the
plan of Mombas (lat. 4° 8S.) a reef extends for thirty-
six miles, at the distance of from half a mile to one
mile and a-quarter from the shore; within it, there is
a channel navigable ‘for canoes and small craft,’
between six and fifteen feet deep: outside the reef the
depth is about 30 fathoms at the distance of nearly

1 Owen’s Africa, vol. 1. p. 357, from which work the foregoing facts
are likewise taken.

a
4

Cu. EE, FRINGING REEFS. a7

halfa mile. Part of this reef is very symmetrical, and
has a uniform breadth of 200 yards.

The coast of Brazil is in many parts fringed by reefs.
Of these, some are not of coral formation ; for instance,
those near Bahia and in front of Pernambuco; but a ~
few miles south of this latter city, the reef follows! every
turn of the shore so closely, that I can hardly doubt it
is of coral. It runs at the distance of three-quarters
of a mile from the land, and within it the depth is
from ten to fifteen feet. I was assured by an intell-
gent pilot, that at Ports Frances and Maceio, the outer
part of the reef consists of living corai, and the inner
of a white stone full of large irregular cavities com-
municating with the sea. The bottom of the sea off
the coast of Brazil shoals gradually to between thirty
and forty fathoms, at the distance of between nine and
ten leagues from the land.

From the description now given, we may conclude
that the dimensions and structure of fringing-reefs -
depend entirely on the greater or less inclination of the
submarine slope, conjoined with the fact, that reef-
building polypifers can exist only at limited depths.
It follows from this, that where the sea is very shallow,
as in the Persian Gulf and in parts of the East Indian
Archipelago, the reefs lose their fringing character,
and appear as separate and irregularly scattered
patches often of considerable area. As the conditions

- 1 Baron Roussin’s Pilote du Brésil, and the pocorn hydro-

eraphical memoir. See also the supplement to this volume on a Bar
of Sandstone off Pernambuco.

78 FRINGING REEFS. _ Cx IIL mt

are less favourable in several respects on the inner
side of these patches, the growth of the coral is more ©
vigorous on the outside; thus causing the reefs to be
generally higher and more perfect in their marginal
than in their central parts. Hence these reefs some- |
times assume (and this circumstance ought not to be
overlooked) the appearance of atolls; but as they are
based on a shallow foundation, and as their central
expanse is much less deep and their form less defined,
this resemblance is easily seen to be merely superticial.
On the other hand, when, in a deep sea, banks of sedi-
ment have accumulated round islands or submerged.
rocks, and they become fringed with reefs, they are dis-
tinguished with difficulty from encircling barrier-reefs
or atolls. In the West Indies there are reefs, which I
should probably have arranged under these two classes,
if the existence of large and level banks, lying a little
beneath the surface and ready to serve as the basis for
the attachment of coral, had not been present; the
formation of such banks through the accumulation of
sediment being sufficiently evident. Fringing-reefs
sometimes coat, and thus protect the foundations of
islands, which have been worn down by the surf to the
level of the sea. According to Ehrenberg, this has been
extensively the case with the islands in the Red Sea,
which formerly ranged parallel to the shores of the
mainland, with deep water within them: hence the
reefs now coating their bases, are situated relatively
to the land lke barrier-reefs, although not belonging
to that class;—but there are, as I believe, in the Red

Cu. III. FRINGING REEFS. ) 79

Sea some true barrier-reefs. The reefs of this sea and
of the West Indies will be described in the Appendix.
In some cases, fringing-reefs appear to be considerably
modified in outline by the course of the prevailing
currents; Dr. J. Allan informs me that on the east
coast of Madagascar, almost every headland and low
point of sand has a coral-reef extending from it in
a S.W. and N.E. line, parallel to the currents on that
shore. I should think the influence of the currents
chiefly consisted in causing an extension, in a certain
direction, of a proper foundation for the attachment of
the coral. Round many intertropical islands, for

instance the Abrolhos on the coast of Brazil sur-

veyed by Captain FitzRoy, and, as I am informed by
Mr. Cuming, round the Philippines, the bottom of the
sea is entirely coated by irregular masses of coral,
which although often of large size, do not reach the
surface and form proper reefs. This must be owing
either to insufficient growth, or to the absence of those
kinds of corals which can withstand the breaking of

the waves.
The three classes, atoll-formed, barrier, and fringing

reefs, together with the modifications just described of
the latter, include all the most remarkable coral-forma-
tions anywhere existing. At the commencement of
the last chapter in the volume, where I detail the

principles on which the map (Plate III.) is coloured,
the exceptional cases will be enumerated.

80

CHAPTER IV.
ON THE GROWTH OF CORAL-REEFS.

In this chapter I will give all the facts, which I have
collected, relating to the distribution of coral-reefs,—
to the conditions favourable to their increase,—to the

rate of their growth,—and to the depth at which they

are formed.

These subjects have an important bearing on the
theory. of the origin of the different classes of coral-
-reefs.

Section I.

On the Distribution of Coral-Reefs, and on the Conditions favourable to
their Increase.

Wir regard to the limits of latitude over which coral-
reefs extend, I have nothing new to add. The Ber-
muda Islands in 32° 15’ N., is the point furthest re-
moved from the equator in which they appear to exist;
and their extension here so far northward no doubt is
due to the warmth of the Gulf stream. In the Pacific,
the Loo Choo islands, in lat. 27° N., have reefs on their
shores, and there is an atoll in.28° 30’, situated N.W.

|
a

Secr. I. DISTRIBUTION OF CORAL-REEFS. SI

of the Sandwich Archipelago. In the Red Sea there
are coral-reefs in lat. 30°. In the southern hemisphere
coral-reefs do not extend so far from the equatorial sea.
In the Southern Pacific there are only a few reefs
beyond the line of the tropic, but Houtmans Abrolhos,
on the western shores of Australia, in lat. 29° S., are of
coral-formation.

The proximity of volcanic land, owing to the lime
generally evolved from it, has been thought to be
favourable to the increase of coral-reefs. There is,
however, no foundation for this view; for nowhere
are coral-reefs more extensive than on the shores of

New Caledonia and of north-eastern Australia, which
consist of primary formations; and the Maldiva,
Chagos, Marshall, Gilbert, and Low Archipelagoes,
the largest groups of atolls in the world, are formed
exclusively of coral.

The entire absence of coral reefs in certain large
areas within the tropical seas, is a remarkable fact.
Thus no coral-reefs were observed during the survey-
ing voyages of the Beagle on the west coast of South
America south of the equator, or round the Galapagos
Islands. It appears, also, that there are none! on this

coast north of the equator; Mr. Lloyd, who surveyed
the isthmus of Panama, remarked to me, that although
he had seen corals living in the Bay of Panama, yet he
had never observed any reefs formed by them. I at first
attributed this absence of reefs on the coasts of Peru and

1 I have been informed that this is the case, by Lieut. Ryder, R.N.,

and others who have had ample opportunities for observation.
a

82 DISTRIBUTION OF CORAL-REEFS. Cu: IV.

of the Galapagos Islands,' to the coldness of the cur-
rents from the south, but the Gulf of Panama is one
of the hottest pelagic districts in the world.? In the
central parts of the Pacitic there are islands entirely
free from reefs ; and in some of these cases this appears
to be due to recent volcanic action: but the existence
of reefs, though scantily developed, and according to
Dana, confined to one part of Hawaii (one of the Sand-
wich Islands), shows that recent voleanic action does
not absolutely prevent their growth.

In the last chapter I stated that the bottom of the
sea round some islands is thickly coated with living
corals, which nevertheless do not form reefs, either
from insufficient growth, or from the species not being
adapted to contend with the breaking waves.

I have been assured by several navigators that
there are no coral-reefs on the west coast of Africa,? or

1 The mean temperature of the surface sea from observations made
by the direction of Captain FitzRoy on the shores of the Galapagos
Islands, between the 16th of September and the 20th of October, 1835,
was 68° Fahr. The lowest temperature observed was 58° °5 at the
S.W. end of Albemarle Island; and on the west coast of this island, it
was several times 62° and 63°. The mean temperature of the sea in
the Low Archipelago of atolls, and near Tahiti, from similar observa-
tions made on board the Beagle, was (although further from the
equator) 77° -5, the lowest any day being 76° °5. Therefore we have
here a difference of 9° °5 in mean temperature, and 18° in extremes ; a
difference doubtless quite sufficient to affect the distribution of organic
beings in the two areas.

2 Humboldt’s Personal Narrative, vol. vii. p..484.

3 It might be concluded, from a paper by Captain Owen (Geograph.
Journ. vol. ii. p. 89), that the reefs off Cape St. Anne and the Sherboro’
Islands were of coral, although the author states that they are not
purely coralline. But I have been assured by Lieut. Holland, R.N.,

that these reefs are not of coral, or at least that they do not at all re-

semble those in the West Indies.

2)
|
4
q

SP ——
a

.

re

‘Secr. I. DISTRIBUTION OF CORAL-REEFS. | 83

s.

round the islands in the Gulf of Guinea. This perhaps

may be attributed to the sediment brought down by
the many rivers debouching on that coast, and to the
extensive mud-banks which line great part of it. But
the islands of St. Helena, Ascension, the Cape Verdes,
St. Paul’s, and Fernando Noronha, are, also, entirely
destitute of reefs, although they lie far out at sea, are
composed of the same ancient volcanic rocks, and have
the same general form with those islands in the Pacific,
the shores of which are surrounded by gigantic walls of
eoral-rock. With the exception of Bermuda, there is
not a single coral-reef in the central expanse of the
Atlantic ‘ocean. It will, perhaps, be suggested that
the quantity of carbonate of lime in different parts of
the sea may regulate the presence of reefs. But this
cannot be the case, for at Ascension, the waves, charged

_ to excess, precipitate a thick layer of calcareous matter
_ on the tidal rocks; and at St. Jago in the Cape Verdes,

carbonate of lime not only is abundant on the shores,
but it forms the chief part of some upraised post-
tertiary strata. The apparently capricious distribution,
therefore, of coral-reefs, cannot be fully explained by
any of the above obvious causes; but, as the study of
the terrestrial and better known half of the world,
must convince every one that no station capable of
supporting life is lost,—nay more, that there is a
struggle for each station between different organisms,
—we may conclude that in those parts of the inter-
tropical sea in which there are no coral-reefs, there
are other organic beings, supplying the place of the
G 2

84 CONDITIONS FAVOURABLE TO Cu. IW.

.reef-building polypifers. It has been shown in the .~

chapter on Keeling atoll that there are some species of
large fish, and the whole tribe of Holothuriz, which
prey on the tenderer parts of the corals. On the other
hand, the polypifers in their turn must prey on other
organic beings; and they would suffer by the diminu-
tion of their prey through any cause. The relations,
therefore, which determine the formation of reefs on
any shore, by the vigorous growth of the efficient kinds
of coral, must be very complex, and with our imperfect
knowledge inexplicable. From these considerations,
we may infer that changes in the condition of the sea,
not obvious to our senses, might destroy all the coral-
reefs in one area, and cause them to appear in another:
thus, the Pacific or Indian ocean might become as
barren of coral-reefs as now is the Atlantic, without our
being able to assign any adequate cause for such a
change.! |

It has been a question with some naturalists, which
part of a reef is most favourable to the growth of coral.
The great mounds of living Porites and of Millepora
round Keeling atoll occur exclusively on the extreme
verge of the reef, which is washed by a constant suc-

1 T have left the foregoing paragraphs nearly as they stood in the
first edition; but, as stated in the Preface to the present work, Dana
has shown that I have undervalued the importance of the mean tempe-
vature of the sea during the coldest season of the year, on the distribu-
tion of coral-reefs, as well as perhaps the injurious effects of recent
volcanic action. But I cannot see that the absence of coral-reefs round
certain islands in the Atlantic, for instance Ascension, St. Paul’s Rock,
and Fernando Noronha, or from the shores of the Gulf of Panama, is
explicable through any known cause.

Fence. i THE GROWTH OF CORAL-REEFS. 85

cession of breakers; and living coral nowhere else
forms solid masses. At the Marshall islands the larger
kinds of corals (chiefly a species of Astra, a genus
closely allied to Porites), ‘which form rocks measuring
several fathoms in thickness,’ prefer, according to
Chamisso,' the most violent surf. I have stated
that the outer margin of the Maldiva atolls consists of
living corals, (some of which, if not all, are of the same
species with those at Keeling atoll), and here the surf is
so tremendous, that even large ships have been thrown,
by a single heave of the sea, high and dry on the reef,
all on board thus escaping with their lives.

Ehrenberg? remarks, that in the Red Sea the
strongest corals live on the outer reefs, and appear to
love the surf; he adds, that the more branched kinds
abound a little way within, but that these in still
more protected places become smaller. Many other
facts having a similar tendency might be adduced.* It
has, however, been doubted by MM. Quoy and Gaimard,
whether any kind of coral can even withstand, much
less flourish in, the breakers of an open sea‘; they
affirm that the saxigenous lithophytes flourish only

1 Kotzebue’s First Voyage (Eng. Transl.), vol. iii. pp. 142, 143, 331.

* Ehrenberg, uber die Natur und Bildung der Corallen Banke im
rothen Meere, p. 49.

$In the West Indies, as I am.informed by Captain Bird Allen, R.N.,
it is the common belief of those who are best acquainted with the reefs,
that the coral flourishes most where freely exposed to the swell of ‘the
open sea.

4 Annales des Sciences Naturelles, tom. vi. pp. 276, 278.—‘ La ot
les ondes sont agitées, les Lytophytes ne peuvent travailler, parce
qwelles détruiraient leurs fragiles édifices,’ &c.

86 CONDITIONS FAVOURARLE TO Cx. iV.

where the water is tranquil, and the heat intense.
This statement has passed from one geological work to
another ; nevertheless, the protection of the whole reef is
undoubtedly due to those kinds of coral, which cannot
even exist in the situations thought by these naturalists
to be most favourable to them. For should the outer —
and living margin perish, of any one of the many low
coral-islands, round which a line of great breakers is
incessantly foaming, the whole, it is scarcely possible
to doubt, would be washed away and destroyed in less
than half a century. But the vital energies of the
corals conquer the mechanical power of the waves;
and the large fragments of reef torn up by every
storm, are replaced by the slow but steady growth
of the innumerable polypifers which form the living
zone on its outer edge.

From these facts, it is certain, that the strongest
and most massive corals flourish where most exposed.
The less perfect state of the reef of most atolls on the
leeward and less exposed side, compared with its state
to windward; and the analogous case of the greater
number of breaches on the near sides of those atolls
in the Maldiva Archipelago which afford some pro-
tection to each other, are obviously explained by this
circumstance. If the question had been, under what
conditions the greater number of species of coral, not
regarding their bulk and strength, were developed, I
should answer,—probably in the situations described
by MM. Quoy and Gaimard, where the water is
tranquil and the heat intense. The total number of |

Sscr. I. THE GROWTH OF CORAL-REEFS.. 87

species of coral in the circumtropical seas must be
very great; in the Red Sea alone, 120 kinds, accord-
ing to Ehrenberg,’ have been observed.

The same author has observed that the recoil of
the sea from a steep shore is injurious to the growth
of coral, although waves breaking over a bank are
not so. Ehrenberg also states, that where there is
much sediment, placed so as to be liable to be moved
by the waves, there is little or no coral; and a col-
lection of living specimens placed by him on a sandy
shore died in the course of a few days.” An experi-
ment, however, will presently be related, in which
some large masses of living coral increased rapidly in
size, after having been secured by stakes on a sand-
bank. That loose sediment should be injurious to
the living polypifers, appears at first sight probable ;
and in sounding off Keeling atoll and Mauritius, the
arming of the lead invariably came up clean, where
the coral was growing vigorously. A strange belief,
which, according to Captain Owen,’ is general amongst
the inhabitants of the Maldiva atolls, namely, that
corals have roots, and therefore grow up again if
merely broken down to the surface, but if rooted
out, are permanently destroyed,—I am inclined to
believe arises from the fact that loose sand injures
the polypifers. For it is probable that sand would

1 Ehrenberg tiber die Natur, &c. &c. p. 46.

2 Thid. p. 49.

3 Captain Owen on the Geography of the Maldiva Islands, Geograph.
Journal, vol. ii. p. 88.

88 CONDITIONS FAVOURABLE TO Ca aEveene

accumulate in the hollows formed by tearing out the
corals, but not on the broken and projecting stumps;
and therefore, in the former case, the fresh growth
of the coral would be prevented. By this means
the inhabitants keep their harbours clear; and thus
the French Governor of St. Mary’s, in Madagascar,
‘cleared out and made a beautiful little port at that
place.’

In the last chapter I remarked, that fringing-
reefs are almost universally breached where streams
enter the sea.! Most authors have attributed this fact
to the injurious effects of the fresh water, even where
it enters the sea only in small quantity and during a
part of the year. No doubt brackish water would pre-
vent or retard the growth of coral; but I believe that
the mud and sand, which is deposited, even by small
rivulets when flooded, is a much more efficient check.
The reef on each side of the channel leading into Port
Louis at Mauritius, ends abruptly in a wall, at the
foot of which I sounded, and found a bed of thick mud.
This steepness of the sides appears to be a general
character in such breaches: Cook,? speaking of one at
Raiatea, says, ‘lke all the rest, it is very steep on
both sides.’ Now, if it were the fresh water mingling

1 Lieut. Wellstead and others have remarked that this is the case in
the Red Sea: Dr. Ruppell (Reise. in Abyss. Band. 1..s. 142) says that
there are pear-shaped .harbours in the upraised coral-coast, into which
periodical streams enter. From this circumstance, I presume, we must
infer that before the upheaval of the strata now forming the coast-land,
fresh water and sediment entered the sea at these points; and the coral
being thus prevented growing, the pear-shaped harbours were produced.

? Cook’s First Voyage, vol. 11. p. 271. (Hawkesworth’s edit.)

D Suen. T. THE GROWTH OF CORAL-REEFS. 89

with the salt, which prevented the growth of coral, the
reef certainly would not terminate abruptly; but as
the polypifers nearest the impure stream would grow
less vigorously than those farther off, so would the
reef gradually thin away. On the other hand, the
sediment brought down from the land would only
prevent the growth of the coral in the line of its
deposition, but would not check it on the side, so that
the reefs might increase till they overhung the bed of
the channel. The breaches are much fewer in number, |
and front only the larger valleys in reefs of the en-

circling barrier class. They probably are kept open

in the same manner as those into the lagoon of an
atoll, namely, by the force of the currents and the
drifting outwards of fine sediment. Their position in
front of valleys, although often separated from the
land by deep-water lagoon-channels, which it might
be thought would entirely remove the injurious effects
both of the fresh water and the sediment, will receive
a simple explanation when we discuss the origin of
barrier-reefs.

In the vegetable kingdom every different station
has its peculiar group of plants, and similar relations
appear to prevail with corals. We have already de-
scribed the great difference between the corals within
the lagoon of an atoll and those on its outer margin.
The corals, also, on the margin of Keeling Island occurred
in zones: thus the Porites and Millepora complanata

' grow to a large size, only where they are washed by a

heavy sea, and are killed by a short exposure to the

90 CONDITIONS FAVOURABLE TO Cu, TV. / #

air; whereas, three species of Nullipora also live amidst
the breakers, but are able to survive uncovered for a
part of each tide: at greater depths, a strong Ma-
drepora and Millepora alcicornis are the commonest
kinds ; the former appearing to be confined to this part :
beneath the zone of massive corals, minute encrusting
corallines and other organic bodies hve. If we com-
pare the external margin of the reef at Keeling atoll
with that on the leeward side of Mauritius, which are
very differently circumstanced, we shall find a corres-
ponding difference in the appearance of the corals. At
the latter place, the genus Madrepora is preponderant
over every other kind ; and beneath the zone of massive
corals there are large beds of Seriatopora. ‘There is
also a marked difference, according to Captain Moresby,}
between the great branching corals of the Red Sea, and
those on the reefs of the Maldiva atolls.

These facts, which in themselves are deserving of
notice, bear, perhaps, not very remotely on a remarkable
circumstance which has been pointed out to me by
Captain Moresby, namely, that with very few exceptions,
none of the coral-knolls within the lagoons of Peros
Banhos, Diego Garcia, and the Great Chagos Bank (all
situated in the Chagos group), rise to the surface of the
water ; whereas, with equally few exceptions, all those
within Solomon and Egmont atolls in the same group,
and likewise those within the large southern Maldiva
atolls, reach the surface. I make these statements, after

1 Captain Moresby on the Northern Maldiva Atolls, Geograph. Journ.
vol, v. p. 401.

Scr. I. THE GROWTH OF CORAL-REEFS. 91

having examined the charts of each atoll. In the lagoon
of Peros Banhos, which is nearly twenty miles across,
there is only one single reef which rises to the surface:
in Diego Garcia there are seven, but several of these lie
close to the margin of the lagoon, and need scarcely
have been reckoned: in the great Chagos Bank there is
not one. On the other hand, in the lagoons of some of
the great southern Maldiva atolls, although thickly
studded with reefs, every one without exception rises to
the surface; and on an average there are less than two
submerged reefs in each atoll: in the northern atolls,
however, the submerged lagoon-reefs are not quite so
rare. The submerged reefs in the Chagos atolls gener-
ally have from one to seven fathoms water on them, but
some have from seven to ten. Most of them are small
with very steep sides ;! at Peros Banhos they rise from
a depth of about thirty fathoms, and some of them in
the Great Chagos Bank from above forty fathoms: they
are covered, Captain Moresby informs me, with living
and healthy coral two and three feet high, consisting
of several species. Why then have not these lagoon-
reefs reached the surface, like the innumerable ones in
the atolls above named? If we attempt to assign any
difference in their external conditions, as the cause of
this diversity, we are at once baffled: the lagoon of
Diego Garcia is not deep, and is almost wholly sur-
rounded by its reef; Peros Ranhos is very deep, much

1 Some of these statements were not communicated to me verbally
by Captain Moresby, but are taken from the MS. account, before alluded
to, of the Chagos Group.

92 CONDITIONS FAVOURABLE TO Ca. Iv |) ee

larger, with many wide passages communicating with
the open sea. On the other hand, of those atolls in
which all, or nearly all the lagoon-reefs have reached
the surface, some are small, others large, some shallow,
others deep, some well enclosed, and others open.
Captain Moresby informs me that he has seen a
French chart of Diego Garcia made eighty years before
his survey, and apparently very accurate ; and from it he
infers, that during this interval there has not been the
smallest change in the depth on any of the knolls within
the lagoon. It is, also, known that during the last fifty-
one years, the eastern channel into the lagoon has
neither become narrower, nor decreased in depth ; and
as there are numerous small knolls of living coral within
it, some change might have been anticipated. Moreover,
as the whole reef round the lagoon of this atoll has been
converted into land—an unparalleled case, I believe, in
an atoll of such large size,—and as the strip of land is for
considerable spaces more than half a mile wide—also a
very unusual circumstance,—we have the best possible
evidence that Diego Garcia has remained at its present
level for a very long period. With this fact, and with
the knowledge that no sensible change has taken place
during eighty years in the coral knolls, and considering
that every single reef has reached the surface in other
atolls, which. do not present the smallest appearance of
being older than Diego Garcia and Peros Banhos, and
which are placed under the same external conditions
with them, one is led to conclude that these submerged
reefs, although covered with luxuriant coral, have no

Sucr. I. THE GROWTH OF CORAL-REEFS. 93

tendency to grow upwards, and that they would remain
at their present levels for an indefinite period.

_ From the number of these knolls, from their posi-
tion, size, and form,—many of them being only one or
two hundred yards across, with a rounded outline and
precipitous sides,—it is indisputable that they have been
formed by the growth of coral; and this makes the case
much more remarkable. In Peros Banhos and in the
Great Chagos bank, some of these almost columnar
masses are 200 feet high, and their summits lie only from
two to eight fathoms beneath the surface; therefore,
a little greater proportional amount of growth would
cause them to attain the surface, like those numerous
knolls which rise from an equally great depth within
the Maldiva atolls. We can hardly suppose that time
has been wanting for the upward growth of the coral ;
as in Diego Garcia, the broad annular strip of land,
formed by the continued accumulation of detritus, shows
how long this atoll has remained at its present level.
We must look to some other cause than the rate of
growth; and I suspect it will be found in the reefs being
formed of different species of corals, adapted to live at
different depths.

The Great Chagos bank is situated in the centre of
the Chagos group, and the Pitt and Speaker banks at
its two extreme points. These banks resemble atolls,
except in their external rim being about eight fathoms
submerged, and in being formed of dead rock, with very
little living coral on it: « portion nine miles long of
the annular reef of Peros Banhos atoll is in the same

94 CONDITIONS OF GROWTH. Cx 1lv. =

condition. These facts, as will hereafter be shown, © 4

render it probable that the whole group at some
former period subsided seven or eight fathoms; and
that the corals perished on the outer margins of those
atolls which are now submerged, but that it continued
alive and grew up to the surface on the others now
perfect. If all these atolls did formerly subside, and if
from the suddenness of the movement or from any other
cause, those species of corals which are best adapted
to live at a certain depth, once got possession of the
knolls, supplanting their former occupants, they would
have little or no power to grow upwards. To illustrate
this, I may observe that if the corals of the upper
zone on the outer edge of Keeling atoll were to perish,
it isimprobable that those of the lower zone would grow
to the surface, and thus become exposed to conditions
for which they do not appear to be adapted. The con-
jecture that the corals on the submerged knolls within
the Chagos atolls have analogous habits with those of
the lower zone outside Keeling atoll, receives some sup-
port from a remark by Captain Moresby, namely, that
they have a different appearance from those on the reefs
in the Maldiva atolls, which, as we have seen, all rise to
the surface: he compares the kind of difference to that
of the vegetation under different climates. I have
entered at considerable length into this case, although
unable to throw much light on it, in order to show that
coral-reefs situated in different places or at different
depths, whether forming the ring of an atoll or the
knolls within a lagoon, need not all be supposed to

* 1. able Salle is malice talons Sei eae i al piemmens

- Scr. IT. RATE OF GROWTH. 95

have an equal tendency to upward growth. The infer-
ence, therefore, that one reef could not grow to the
surface within a given time, because another, not
known to be covered with the same species of corals,
and not known to be placed under exactly the same
conditions, has not within the same time reached the
surface, is unsound.

SECTION SECOND.
On the Rate of Growth of Coral-reefs.

Tue remark made at the close of the last section,
naturally leads to this division of our subject, which
has not, I think, hitherto been considered under a
right point of view. Ehrenberg’ has stated that in
the Red Sea, the corals only coat other rocks in a
layer from one to two feet in thickness, or at most to a
fathom and a-half; and he disbelieves that, in any
ease, they form by their own proper growth, great
masses, stratum over stratum. A nearly similar obser-
vation has been made by MM. Quoy and Gaimard,?
with respect to the thickness of some upraised beds of
coral, which they examined at Timor and some other
places. Ehrenberg? saw certain large massive corals in
the Red Sea, which he imagines to be of such vast
antiquity, that they might have been beheld by
1 Ehrenberg, as before cited, pp. 39, 46, and 50.

* Annales des Sciences Nat., tom. vi. p. 28.
* Ehrenberg, ut sup. p. 42.

96 RATE OF GROWTH. Cu. IV.

Pharaoh; and according to Sir C. Lyell! there are

certain corals at Bermuda, which are known by tra-
dition to have been living for centuries.2 To show
how slowly coral-reefs grow upwards, Captain Beechey®
has adduced the case of the Dolphin Reef off Tahiti,
which has remained at the same depth beneath the
surface, namely, about two fathoms and a-half, for a
period of sixty-seven years. There are reefs in the
Red Sea, which certainly do not appear‘ to have in-
creased in dimensions during the last half century, and
from the comparison of old charts with recent surveys,
probably not during the last two hundred years.
These, and other similar facts, have so strongly im-
pressed many with the belief of the extreme slowness
of the growth of corals, that ,they have even doubted
the possibility of islands in the great oceans having
been formed by their agency. Others again, who have
not been overwhelmed by this difficulty, have ad-
mitted that it would require thousands, and tens of
thousands of years, to form a mass even of incon-

? Lyell’s Principles of Geology, book iu. ch. xviii.

* Since the preceding pages (of the first edit.) have been printed off,
I have received from Sir C. Lyell an interesting pamphlet, entitled
Remarks upon Coral-Formations, &c., by J. Couthouy, Boston, United
States, 1642. A statement (p. 6) is here given on the authority of the
Rey. J. Williams, corroborating the above remarks on the antiquity of
certain individual corals, namely, that at Upolu, one of the Navigator
islands, ‘ particular clumps of coral are known-to the fishermen by
name, derived from either some particular configuration or tradition
attached to them, and handed down from time immemorial.’

3 Beechey’s Voyage to the Pacific, eh. viii.

+ Ehrenberg, ut sup. p. 43,

Srcr. IT. RATE OF GROWTH. 97

siderable thickness: but the subject has not, I believe,
been viewed in the proper light.

That masses of considerable thickness have been
formed by the growth of coral, may be inferred with
certainty from the following facts. In the deep
lagoons of Peros Banhos and of the Great Chagos
bank, there are, as already described, small steep-
sided knolls covered with living coral. There are
similar knolls in the southern Maldiva atolls, some of
which, as Captain Moresby assures me, are less than a
hundred yards in diameter, and rise to the surface
from a depth of between 250 and 300 feet. Con-
sidering their number, form, and position, it would be
preposterous to suppose that they are based on pin-
nacles of rock, or on isolated cones of sediment. As
no kind of living coral grows above the height of a
_ few feet, we are compelled to suppose that these knolls
have been formed by the successive growth and death
of many individuals,—first one being broken off or
killed by some accident, and then another, and one set
of species being replaced by another set with different
habits, as the reef rose nearer the surface, or as other
changes supervened. The spaces between the corals
would become filled up with fragments and sand, and
such matter would probably soon be consolidated, for
we learn from Lieut. Nelson’s ! observations at Bermuda
that a process of this kind takes place beneath water,
without the aid of evaporation. In reefs, also, of the
barrier class, we may feel sure, as I have shown, that

1 Geological Transactions, vol. v. p. 113.

H

98 _ RATE OF GROWIE. CH. 1 2

masses of great thickness have been formed by the — q

erowth of coral. In the case of Vanikoro, judging
only from the depth of the moat between the land
and the reef, the wall of coral-rock must be at least
300 feet in vertical thickness. 2
So again some of the upraised islands in the Pacific
show what thick masses of coral-rock have been
formed. Dana! states that Metia, in the Paumotu
or Low Archipelago, consists of white solid limestone
‘with some disseminated corals; and this island once
existed as an atoll, though now surrounded by cliffs —
250 feet in height. The cliffs round Elizabeth Island
in the same archipelago are 80 feet high, and are
composed, according to Beechey, of homogeneous coral-
rock. Mangaia in the Hervey Group, and Rurutu,
appear both to have once existed as encircled islands,
and their barrier-reefs are now in parts 300 feet above
the level of the sea.”
Some attempts have been made, with but little
success, to ascertain by boring the thickness of coral
formations. At Bow Island, in the Low Archipelago,
Sir E. Belcher? bored to a depth of 45 feet, and
below the first 20 found only coral-sand. During
Wilke’s Expedition’, in a boring of 21 feet in depth

1 Corals and Coral Islands, 1872, p. 193. See also Mr. Couthouy’s
pamphlet above referred to.

2 Dana, Corals and Coral Islands, p. 336. Also Forster's Voyage
round the World with Cook, vol. ii. pp. 163, 167. Williams’s Narrative
of Missionary Enterprise, pp. 30, 48, and 249.

3 Voyage Round the World, vol. i. 1843, p. 369.

4 Narrative U.S. Exploring Expedition, vol. iv. p. 268. Dana,
Corals and Coral Islands, p. 184.

Eee ss = 7
% -

Secr. IT. RATE OF GROWTH. 99

on one of the islands in the same archipelago, coral-
sand was passed through for the first 10 or 11 feet,
and then solid reef rock. On one of the Maldiva
atolls in the Indian Ocean, Captain Moresby bored to
a depth of 26 feet, when his augur broke. He gave
me the matter brought up, and it was perfectly white
like finely triturated coral-rock.

In my description of Keeling atoll, I have ad-
vanced some facts showing that the reef has probably
grown outwards; and { found, just within the outer
margin, the great mounds of Porites and of Millepora,
with their summits lately killed, and their sides sub-
sequently thickened by the growth of the coral: a
layer, also, of Nullipora had already coated the dead
surface. As the external slope of the reef is the same
round the whole of this and many other atolls, the
angle of inclination must result from an adaptation
between the growing powers of the coral and the
force of the breakers, and their action on the loose
sediment. The reef, therefore, could not inerease out-
wards without a nearly equal addition to every part
of the slope, so that the original inclination might be
preserved, and this would require a large amount of
sediment, all derived from the wear of corals and
shells, to be added to the lower part. Moreover, at
Keeling atoll and probably in many other cases, the
different kinds of coral would have to encroach on each
other; thus the Nulliporz cannot increase outwards
without encroaching on the Porites and Millepora

H 2

100 RATE OF GROWTH. ) Cn. Ve. aa

complanata, as is now taking place; nor these latter.
without encroaching on the strongly branched Madre- |
pora, the Millepora alcicornis, and some Astreeas ; nor
these again without a foundation being formed for
them within the requisite depth, by the accumulation
of sediment. How slow, then, must be the ordinary
lateral or outward growth of such reefs! But off
Christmas atoll, where the sea is much more shallow
than is usual, we have good reason to believe that,
within a period not very remote, the reef has in-
creased considerably in width. The land has the
extraordinary breadth of three miles; it consists of —
parallel ridges of shells and broken corals, which
furnish ‘an incontestable proof, as observed by Cook,!
‘that the island has been produced by accessions from
the sea, and is:in a state of increase.’ The land is
fronted by a coral-reef, and from the manner in which
islets are known to be formed, we may feel confident
that the reef was not three miles wide when the first,
or most inward ridge, was thrown up; and, there-
fore, we must conclude that the reef has grown out-
wards during the accumulation of the successive ridges.
Here then, a wall of coral-rock of very considerable
breadth has been formed by the cutward growth of
_ the living margin, within a period, during which
ridges of shells and corals, lying on the bare surface,
have not decayed. There can be little doubt, from the
account given by Captain Beechey, that Matilda atoll in
the Low Archipelago has been converted in the space

1 Cook’s Third Voyage, book iii. ch. x.

Szor. IT. RATE OF GROWTH. 101

of thirty-four years, from being, as described by the
erew of a wrecked whaling vessel, a ‘reef of rocks,’
into a lagoon-island fourteen miles in length, with
‘one of its sides covered nearly the whole way with
high trees.! The islets, also, on Keeling atoll, it has
been shown, have increased in length, and since the
construction of an old chart, several of them have
become united into one long islet: but in this case,
and in that of Matilda atoll, we have no proof that
the foundation of the islets, namely the reef, has
increased in breadth, although it must be allowed that
this is probable. :

I think, therefore, in regard to the possible rate of
outward growth of coral-reefs, but little importance
need be attached to the fact that certain reefs in the
Red Sea have not increased during a long interval of
time, or to other similar cases, such as that of Ouluthy
atoll in the Caroline group, where every islet, described
a hundred years before by Cantova, was found in the
same state by Lutké.2 For it cannot be shown that,
in these cases, the conditions were favourable to the
vigorous and unopposed growth of the corals living
in the different zones of depth, and that a proper basis
for the extension of the reef was present. These
conditions must depend on many contingencies, and
a basis within the requisite depth can rarely be pre-

1 Beechey’s Voyage to the Pacific, ch. vii. and vill.

2 F. Lutké’s Voyage autour du Monde. In the group Elato, how-
ever, it appears that what is now the islet Falipi, is called in Cantova’s

Chart, the Bane de Falipi. It is not stated whether this has been
caused by the growth of coral, or by the accumulation of sand.

102 RATE OF GROWTH. Po

sent in the deep oceans where coral formations most
abound.
Nor do I think, when we consider the rate of the
upward growth of reefs under favourable circumstances,
that we should be influenced by the fact that certain
submerged reefs, such as those off Tahiti or those within
Diego Garcia, are not now nearer the surface than they
were many years ago. For it has been shown that all
the reefs have grown to the surface in some of the
Chagos atolls, but that in neighbouring atolls which
appear to be of equal antiquity and to be exposed to
the same external conditions, every reef remains sub-
merged; we are, therefore, almost driven to attribute
this to a difference, not in the rate of growth, but in
the habits of the corals in the two cases.

In an old-standing reef, the corals, which greatly
differ in kind on different parts of it, are probably
all adapted to the stations they occupy, and hold their
places, like other organic beings, by a struggle one
with another and with external nature; hence we may
infer that their growth would be slow except under
peculiarly favourable circumstances. Almost the only
natural condition, allowing a quick upward growth of
the whole surface of a reef, would be a slow subsidence
of the area in which it stood ;—-if, for instance, Keeling
atoll were to subside two or three feet, can we doubt
that the projecting margin of live coral, about half an
inch in thickness, which surrounds the dead upper sur-
faces of the mounds of Porites, would in this case form
a concentric layer over them, and the reef thus increase

Sucr. II. RATE OF GROWTH. 103

upwards, instead of, as at present, outwards? The
Nullipore are now encroaching on the Porites and
-Millepora, but in this case might we not confidently
expect that the latter would, in their turn, encroach on
the Nullipore ? After a subsidence of this kind, the
sea would gain on the islets, and the great fields of dead
but upright corals in the lagoon would be covered by a
sheet of clear water; and might we not then expect
that these reefs would rise to the surface, as they an-
ciently did when the lagoon was less confined by islets,
and as they did within a period of ten years in the
schooner-channel cut by the inhabitants. In one of the

Maldiva atolls, a reef, which within a very few years
existed as an islet bearing cocoa-nut trees, was found by
Lieut. Prentice ‘ entirely covered with live coral and
Madrepore. The natives believe that the islet was
washed away by a change in the currents, but if, in-
stead of this, it had quietly subsided, surely every part
of the island which offered a solid foundation, would in
a like manner have become coated with living coral.
Through steps such as these, any thickness of rock
composed of a singular intermixture of various kinds
of corals, shells, and calcareous sediment, might be

| formed; but without subsidence, the thickness would
| necessarily be determined by the depth at which the
reef-building polypifers can exist. If it be asked, at
what rate in years I suppose a reef of coral favourably

| circumstanced could grow up froma given depth; I

must answer that we have no precise evidence on
this head. It will, however, be hereafter shown that

104 7 RATE OF GROWTH.

in certain large areas where subsidence has probably:

been long in progress, the growth of the corals has — 4

been sufficient to keep the reefs up to the surface ; and
this is a much more important standard of comparison
than any cycle of years.

It may, however, be inferred from the follows
facts, that the rate under favourable circumstances
would be far from slow. Dr. Allan of Forres has given,
in his MS. Thesis deposited in the library of the
Edinburgh University, the following account of some
experiments, which he tried during his travels in
the years 1830 to 1832 on the East Coast of
Madagascar. ‘To ascertain the rise and progress
of the coral family, and fix the number of species |
met with at Foul Point (lat. 17° 40’), twenty spe-
cies of coral were taken off the reef and planted ~
apart on a sand-bank three feet deep at low water.
Hach portion weighed ten pounds, and was kept in
its place by stakes. Similar quantities were placed
in a clump and secured as the rest. . This was done in
December 1830. In July following, each detached
mass was nearly level with the sea at low-water, quite
immovable, and several feet long, stretching, like the
parent reef, in the line of the coast-current from north.
to south. The masses accumulated in a clump were
found equally increased, but some of the species in
such unequal ratios as to be growing over each other.’ !
The loss of Dr. Allan’s magnificent collection by

1T owe the above extract to the kindness of Dr. Malcolmson.

Scr. II. RATE OF GROWTH. 105

shipwreck, unfortunately prevents its being known

to what’ genera these corals belonged; but from the
numbers experimented on, it is certain that all the
more conspicuous kinds must have been included.
Dr. Allan informs me, in a letter, that he believes
it was a Madrepora which grew most vigorously.
One may be permitted to suspect that the level of the
sea might possibly have been somewhat different at the
two stated periods; nevertheless, it is quite evident
that the growth of the ten-pound masses, during the
six or seven months at the end of which they were
found to be immovably fixed! and several feet in
length, must have been very great. The fact of the
different kinds of coral, when placed in one clump,
having increased in extremely unequal ratios, is very

interesting, as it shows the manner in which a reef,

supporting many species of coral, would probably be
affected by a change in the external conditions
favouring one kind more than another. The growth
of the masses of coral in N. and S. lines parallel to
the prevailing currents, whether due to the drift-
ing of sediment or to the simple movement of the
water, is, also, an interesting circumstance.

Lieut. Wellstead, I.N., informed me that in the
Persian Gulf a ship had her copper bottom en-
crusted in the course of twenty months with a layer
_ ‘It is stated by Mr. De la Beche (Geological Manual, p. 148), on
the authority of Mr. Lloyd, who surveyed the Isthmus of Panama, that
some specimens of Polypifers, placed by him in a sheltered pool of

water, were found in the course of a few days firmly fixed by the secre-
tion of a stony matter, to the bottom.

106 RATE OF GROWTH. Cu. IV.

of coral two feet in thickness, which it required great
force to remove when the vessel was docked: it was
not ascertained to what order this coral belonged.'
This fact in some degree corroborates the result of
Dr. Allan’s experiments. The case of the schooner-
channel, choked up with coral in an interval of less
than ten years, in the lagoon of Keeling atoll, should
be here borne in mind. We may also infer, from
the trouble which the inhabitants of the Maldiva
atolls take to root out, as they express it, the coral-

1Mr. Stutchbury (West of England Journal, No. I. p. 50) has de-
scribed a specimen of Agaricia, ‘ weighing 2 lbs. 9 oz., which surrounds
a species of oyster, whose age could not be more than two years, and
yet is completely enveloped by this dense coral.’ I presume that the
oyster was living when the specimen was procured ; otherwise the fact
tells nothing. Mr. Stutchbury also mentions an anchor, which had
become entirely encrusted with coral in fifty years; other cases, how-
ever, are recorded of anchors having long remained amidst coral-reefs
without having become coated. The anchor of the Beagle, in 1882,
after having been down exactly one month at Rio de Janeiro, was so
thickly coated by two species of Tubularia, that large spaces of the iron
were entirely concealed; the tufts of this horny zoophyte were between
two and three inches in length. Spallanzani states (Travels, Eng.
Translat. vol. iv. p. 313) that in the Mediterranean, the red coral of
commerce is usually dredged every ten years, during which time it
erows to a height of one foot. It grows, however, at different
rates in different places. It has been erroneously attempted to
compute the rate of growth of a reef, from the fact mentioned by
Captain Beechey of the Chama gigas being embedded in coral rock.
But it should be remembered, that some species of this genus invariably
live, both whilst young and old, in cavities, which the animal has the
power of enlarging with its growth. I saw many of these shells thus
embedded in the outer ‘flat’ of Keeling atoll, which is composed of
dead rock; and therefore the cavities in this case had no relation
whatever to the growth of coral. M. Lesson, also, speaking of this
shell (Partie Zoolog., Voyage de la Coquille). has remarked, ‘ que con-
stamment ses valves étaient engagés complétement dans la masse des
Madrepores.’

Secr. IIT. DEPTH. 107

_knolls from their harbours, that their growth can
hardly be very slow. I may add, that M. Duchassaing
broke off all the Madrepores growing on a marked
place in a bay at Guadaloupe; and in the course of
two months he found there a greater number of corals
than before.!

From the facts given in this section, it may be
concluded, first, that considerable thicknesses of rock
have certainly been formed within the present geo-
logical era by the growth of corals and the accumu-
lation of their detritus; and, secondly, that the
increase of individual corals and of reefs, both out-
wards or horizontally, and upwards or vertically, under
conditions favourable to such increase, is not slow,
when referred either to the standard of the average
oscillations of level in the earth’s crust, or to the more
precise but less important one of a cycle of years.

SECTION THIRD.
On the Depths at which Reef-building Corals live.

I HAve already described in detail the nature of the
bottom of the sea immediately surrounding Keeling
atoll; and I will here describe with almost equal

care the soundings off the fringing-reefs of Mauritius.
I sounded with the wide bell-shaped lead which Captain
FitzRoy used at Keeling Island. My examination
of the bottom was confined to a few miles of coast

1 LInstitut, 1846, p. 111.

a 0, a ee
‘ Pay vg |
“Th

108 DEPTH AT WHICH Cu, IV.

(between Port Louis and Tomb Bay) on the leeward
side of the island. The edge of the reef is formed
of great shapeless masses of branching Madrepores,
which chiefly consist of two species, — apparently
M. corymbosa and pocillifera,—mingled with a few
other kinds of coral. These masses are separated from
each other by the most irregular gullies and cavities,
into which the lead sinks many feet. Outside this
irregular border of Madrepores, the water deepens —
gradually to 20 fathoms, which depth generally is
found at the distance of from half to three-quarters of
a mile from the reef. A little further out the depth
is 30 fathoms, and thence the bank slopes rapidly
into the depths of the ocean. ‘This inclination is very
gentle compared with that outside Keeling and other
atolls, but compared with most coasts it is steep. The
water was so clear outside the reef, that I could
distinguish every object forming the rugged bottom.
In this part, and to a depth of 8 fathoms, I
sounded repeatedly, and at each cast pounded the
bottom with the broad lead; nevertheless the arming
invariably came up perfectly clean, but deeply in-
dented. From 8 to 15 fathoms a little calca-
reous sand was occasionally brought up, but more
frequently the arming was simply indented. In all
this space the two Madrepores above mentioned, and
two species of Astrea with rather large stars, seemed
the commonest kinds; and it must be noticed that
twice at the depth of 15 fathoms, the arming was
marked with a clean impression of an Astrea,

Szor. IIT. REEF-BUILDING CORALS LIVE. 109

Besides these lithophytes, some fragments of the
Millepora alcicornis which occurs in the same relative
position at Keeling Island, were brought up; and in
the deeper parts there were large beds of a Seriato-
pora, different from S. subulata, but closely allied to
it. On the beach within the reef, the rolled fragments
consisted chiefly of the corals just mentioned, and of
a massive Porites like that at Keeling atoll, of a
Meandrina, Pocillopora verrucosa, and of numerous

fragments of Nullipora. From 15 to 20 fathoms the

bottom was, with few exceptions, either formed of
sand, or thickly covered with Seriatopora: this delicate
coral seems to form at these depths extensive beds
unmingled with any other kind. At 20 fathoms, one
sounding brought up a fragment of Madrepora, ap-
parently WM. pocillifera, and I believe it to be the
same species as that which mainly forms the upper
margin of the reef; if so, it grows in depths varying
from 0 to 20 fathoms. Between 20 and 23 fathoms I
obtained several soundings, and they all showed a sandy
bottom, with one exception at 30 fathoms, when the
arming came up scooped out as if by the margin of a
large Caryophyllia. Beyond 33 fathoms I sounded
only once ; and from 86 fathoms, at the distance of one
mile and a third from the edge of the reef, the arming
brought up calcareous sand with a pebble of volcanic
rock. The circumstance of the arming having in-
variably come up quite clean, when sounding within a
certain number of fathoms off the reefs of Mauritius
and Keeling atoll (8 fathoms in the former case,

110U DEPTH AT WHICH Cu. IV.

and 12 in the latter), and of its having always
come up (with one exception) smoothed and covered
with sand when the depth exceeded 20 fathoms,
probably indicates a criterion, by which the limits
of the vigorous growth of coral might in all cases be
readily ascertained. I do not, however, suppose that
if a vast number of soundings were obtained round
these islands, the limit above assigned would be found
never to vary, but I conceive the facts are sufficient to
show that the exceptions would be few. The cireum-
stance of a gradual change, in the two cases, from a
field of clean coral to a smooth sandy bottom, is far
more important in indicating the depth at which the
larger kinds of coral flourish, than almost any number
of separate observations on the depth at which certain
species have been dredged up. For we can understand
the gradation only as a prolonged struggle against
unfavourable conditions. If a person were to find the
soil clothed with turf on the banks of a stream of
water, but on going to some distance on one side of it,
he observed the blades of grass growing thinner and
thinner with intervening patches of sand, until he
entered a desert of sand, he would safely conclude,
especially if changes of the same kind were noticed in
other places, that the presence of the water was abso-
lutely necessary to the formation of a thick bed of
turf: so may we conclude, with the same feeling of
certainty, that thick beds of coral are formed only at
small depths beneath the surface of the sea.

I have endeavoured to collect every fact which

Srcr. III. REEF-BUILDING CORALS LIVE. Itt

might either invalidate or corroborate this conclusion.
Capiain Moresby, whose opportunities for observation
during his survey of the Maldiva and Chagos Archi-
pelagoes were unrivalled, informs me, that the upper
part or zone of the steep-sided reefs on the inner and
outer coasts of the atolls in both groups, invariably
consisted of coral, and the lower parts of sand. At
7 or 8 fathoms depth, the bottom is formed, as could
be seen through the clear water, of great living
masses of coral, which at about 10 fathoms generally
stand some way apart from each other, with patches of
white sand between them, and at a little greater depth
these patches become united into a smooth steep slope
‘without any coral. Captain Moresby, also, informs me
in support of the above statement, that he only found
decayed coral on the Padua Bank (northern part of the
Laccadive group), which has an average depth of 25
to 35 fathoms; but that on some other banks in the
same group, with a depth of only 10 or 12 fathoms,
(for instance, the Tillacapeni bank) the coral was
living. ,

Professor Dana likewise states that during the various
and extensive surveys in the Pacific Ocean, made during
the United States Exploring Expedition, no evidence
was found of corals growing beyond the depth of 20
fathoms.!_ I may here add that Sir E. Belcher, though
he does not state to what depth hving corals extended,
says that many soundings were taken off Bow atoll, at

1 Corals and Coral Islands, 1872, p, 116.

12 DEPTH AT ' WHICH Cu. IV"

depths ranging from 50 to 960 fathoms, and that the

bottom always consisted of coral sand.!
With regard to the coral-reefs in the Red Sea,
Ehrenberg: has the following passage. ‘The lving

corals do not descend there into great depths. On the |

edges of islets and near reefs, where the depth was
small, very many lived; but we found no more even at
six fathoms. The pearlfishers at Yemen and Massaua
asserted that there was no coral near the pearl-banks at
nine fathoms depth, but only sand. We were not able
to institute any more special researches.’? I am, how-
ever, assured both by Captain Moresby and Lieut. Well-
stead, that in the more northern parts of the Red Sea.
there are extensive beds of living coral at a depth of 25
fathoms, in which the anchors of their vessels were
frequently entangled. Captain Moresby attributes the
less depth at which the corals are able to live in the
- places mentioned by Ehrenberg, to the greater quantity
of sediment there; the situations, where they were
flourishing at the depth of 25 fathoms, were protected,
and the water was extraordinarily limpid. On the
leeward side of Mauritius, where I found the coral
growing at a somewhat greater depth than at Keeling
atoll, the sea, owing apparently to its tranquil state,
was likewise very clear. Within the lagoons of some
of the Marshall atolls, where the water can be but little
agitated, there are, according to Kotzebue, living beds
of coral in 25 fathoms. From these several facts, and

1 Voyage Round the World, 1843, p. 379, vol. i.
* Ehrenberg uber die Natur, &e. p. 50.

ee ee Jude wikia eh 08 :

a j
Scr. III. REEF-BUILDING CORALS LIVE. 113

considering the manner in which the beds of clean coral

off Mauritius, Keeling Island, the Maldiva and Chagos

atolls, graduated into a sandy slope, it appears very
probable that the depth at which reef-building poly-
pifers can exist, is partly determined by the extent of
inclined surface which the currents of the sea and the
recoiling waves have the power to keep free from
sediment.

MM. Quoy and Gaimard!' believe that the growth
of coral is confined within very limited depths; and
they state that they never found any fragment of an
Astrea (the genus they consider most efficient in form-
ing reefs) at a depth above 25 or 30 feet. But we
have seen that in several places the bottom of the sea
is paved with massive corals at more than twice this
depth ; and at 15 fathoms (or thrice this depth) off the
reefs of Mauritius, the arming was marked with the
distinct impression of a living Astrea. Millepora
alcicornis lives in from 0 to 12 fathoms, and the genera
Madrepora and Seriatopora from 0 to 20 fathoms.
Captain Moresby has given me a specimen of Sideropora
scabra (Porites of Lamarck) brought up alive from 17
fathoms. Mr. Couthouy? states that on the Bahama
banks he dredged up considerable masses of Mean-
drina from 16 fathoms, and has seen this coral growing
in 20 fathoms.

Captain Beechey informs me that branches of pink

1 Annales des Sci. Nat. tom. vi.
? Remarks on Coral Formations, p. 12.

I

114 DEPTH AT WHICH Cav:

and yellow coral were frequently brought up from be-
tween 20 and 25 fathoms off the Low atolls; and Lieut.
Stokes, writing to me from the N.W. coast of Australia,
says that a strongly branched coral was procured there
from 30 fathoms: unfortunately it is not known to what
genera these corals belong.

Although the limit of depth, at which each particular
kind of coral ceases to exist, is thus far from being
accurately known; yet when we bear in mind the
manner in which the clumps of coral gradually became
infrequent at about the same depth, and wholly dis-
appeared at a greater depth than 20 fathoms~on the
slope round Keeling atoll, off the reefs in the Pacific
(according to Dana), on the leeward side of the Mauri-
tius, and at rather less depth both within and without
the atolls of the Maldiva and Chagos Archipelagoes ;
and when we know that the reefs round these islands do
not differ from other coral formations in their form and
structure, we may, I think, conclude that in ordinary
cases, reef-building polypifers do not flourish at greater
depths than between 20 and 30 fathoms, and rarely at
above 15 fathoms.

It has been argued,!' that reefs may possibly rise
from very great depths through the means of small
corals first making a platform for the growth of the
stronger kinds. This, however, is an arbitrary supposi-
tion: it is not always remembered, that in such cases
there is an antagonistic power at work, namely, the decay
of organic bodies when not protected by a covering of

1 Journal of the Royal Geograph. Soc. 1831, p. 218.

yt I AN! a ll A te _

Sect. III. REEF-BUILDING CORALS LIVE. 15

sediment or by their own rapid growth. We have,
- moreover, no right to calculate on unlimited time for
the accumulation of small organic bodies into great
masses. Every fact in geology proclaims that neither
the dry land nor the bed of the sea retains the same level
for indefinite periods. As well might it be imagined
that the British Seas would in time become choked up
with beds of oysters, or that the numerous small coral-
lines off the inhospitable shores of Tierra del Fuego
would in time form a solid and extensive coral-reef.!

1] will here record the few facts which I have been able to collect
as to the depths, both within and without the tropics, inhabited by
those corals and corallines which we have no reason to suppose ever
materially aid in the construction of a reef. Mr. Stokes also showed

Depth
NAME OF ZOOPHYTE. | Cee P| ammo.
Sertularia . : : .| +40 | Cape Horn 66°} [Where none is
Cellaria . : : : . | ditto ditto given the ov-
;» A minute scarlet en- servation 1s
crusting species, found my own. |
living 5 , . | 190 |Keeling At. 129
», An allied, small stony
subgeneric form. .| 48 |S. Cruz Riv. 50°
A coral allied to Vincularia,
with eight rows of cells .| 40 | Cape Horn
Tubulipora, near to T. patina . | ditto ditto
Do. do. : . | 94 |Kast Chiloe 439
Cellepora, several species and
allied sub-generic forms .{ 40 | Cape Horn
40 -
oo | and pees |
57 45
J
Ditto . f : : wt (48) 1S. Cruz. 50°
Eschara 30 | { eee el
3 h s ; ; Fuego 53°
Ditto . : : : ers 428) os. Cruz 1) 502
Retepora : - : . | 40 | Cape Horn.
{ Quoy andGai-
C.Good Hope|} mard, Ann.
ey | 00 oP} Gen Nuts
t. vi. p. 284.

ite

116 DEPTH.

COUNTRY AND

Depth
bes S. LATITUDE.

i
fathoms.

NAME OF ZOOPHYTE.

”

in ote rep i wh he ila aa! Po apa

Millepora, a strong coral with

cylindrical branches, of a o4 | © Chileomee
pink colour, about twol| 294 | Tierra ag
inches high, resembling in / 30 Fueso see ;
the form of its orifices M. | 8 j
aspera of Lamarck } i
| Peyssonel, in {
alt 3 aper read to i
Coralium ult te : .| 120 | Barbary 33°N. Bee Society. :
May, 1752.
Antipathes ¢--.. »« .| 16 s| Chonosiee

Albrothos,on | | Capt. Beechey
: informed me

of this fact
in a letter.

Gorgonia’ (or’an allied form) .| 160 the coast of
| Brazil, 18°

me a Caryophyllia which was dredged up alive by Captain King
from a depth of 80 fathoms off Juan Fernandez, in lat. 33° S. Ellis
(Nat. Hist. of Coralline, p. 96) states that Ombellularia was pro-
cured in lat. 79° N. sticking to a line from the depth of 286 fathoms ;
hence this coral either must have been floating loose, or was entangled
in a stray line at the bottom. Off Keeling atoll a compound Ascidia
(Sigillina) was brought up from 39 fathoms, and a piece of sponge,
apparently living, from 70, and a fragment of Nullipora, also appa-
rently living, from 92 fathoms. At a greater depth than 90 fathoms
the bottom was thickly strewed with joints of a Halimeda and small
fragments of other Nullipore, but all dead. Captain B. Allen, R.N.,
informs me that in the survey of the West Indies it was noticed,
that between the depth of 10 and 200 fathoms, the sounding-lead very
generally came up coated with the dead joints of a Halimeda, of which
he showed me specimens. Off Pernambuco, in Brazil, in about 12
fathoms, the bottom was covered with fragments, dead and alive, of a
dull red Nullipora, and I infer from Roussin’s chart, that a bottom
of this kind extends over a wide area. On the beach, within the
coral-reets of Mauritius, vast quantities of fragments of Nullipore

were piled up. From these facts, it appears that these simply or- :
ganised bodies, belonging to the vegetable kingdom, are amongst the
most abundant productions of the sea. ia

CHAPTER V.

THEORY OF THE FORMATION OF THE DIFFERENT CLASSES
OF CORAL-REEFS.

The atolls of the larger archipelagoes not formed on submerged craters,
or on banks of sediment—Immense areas interspersed with atollk—
Their subsidence—The effects of storms and earthquakes on atolls—
Recent changes in their state—The origin of barrier-reefs and of
atolls— Their relative forms—The step-formed ledges and wall8 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 anni-
lar reefs—The disseverment of large atolls—The union of atolls by linear
reefs—The great Chagos Bunk— Objections considered arising from the
area and amount of subsidence required by the theory—The probable
composition of the lower parts of atolls.

Tue naturalists who have visited the Pacific, seem to
have had their attention riveted by the lagoon-islands
or atolls,—those singular rings of coral-land which rise
abruptly out of the unfathomable ocean,—and have
passed over, almost unnoticed, the scarcely less won-
derful encircling barrier-reefs. The theory most
generally received on the formation of atolls, is that
they are based on submarine craters: but where can we
find a crater of the shape of Bow atoll, which is five
times as long as it is broad (Plate I. fig. 4); or like
that of Menchicoff Island (Plate II. fig. 3), with its

118 THEORY OF THE FORMATION . © Cu, VY.

three loops, together sixty miles in length; or like
Rimsky Korsacoff, narrow, crooked, and fifty-four miles

long; or like the northern Maldiva atolls, made up of.

numerous ring-formed reefs, placed on the margin of a
disk,—one of which disks is eighty-eight miles in

length, and only from ten to twenty in breadth? A

further difficulty on this theory of the origin of atolls
arises from the necessary assumption of so large a
number of immense craters crowded together beneath
the sea. But, as we shall presently see, a greater diffi-
eulty is involved, namely, that all these craters must
lie within nearly the same level beneath the sea.
Nevertheless, if the rim of a crater afforded a basis
at the proper depth, I am far from denying that a
reef like a perfectly characterized atoll might not be
formed on it. Some such, perhaps, now exist; but
it is incredible that the greater number could have
thus originated.

An earlier and better theory was proposed by
Chamisso :! he supposes that as the more massive kinds
of corals prefer the surf, the outer portions of a reef will
first reach the surface and consequently form a ring.
I remarked in the third chapter that a reef, growing on
a detached bank, would tend to assume an atoll-like
structure; if, therefore, corals were to grow up from a
bank some fathoms submerged in a deep sea, having
steep sides and a level surface, a reef not to be dis-
tinguished from an atoll might be formed; and I

1 Kotzebue’s First Voyage, vol. iii. p. 331.

ame \
: V9 et it

Gz. V.- OF CORAL-REEFS. 119

believe some such exist in the West Indies. But on
this view it must be assumed, that in every case the
basis consists of a flat bank; for if it were conically
formed like a mountainous mass, we can see no reason
why the corals should spring up from the flanks instead
of from the central and highest parts. As the lagoons
of atolls are sometimes even more than 40 fathoms
deep, it must, also, be assumed on this view, that at a.
depth at which the waves do not break, the coral grows
more vigorously on the edges of a bank than on its
centra] part: and this is an assumption without any
evidence. If we consider, moreover, the number of
the atolls in the midst of the Pacific and Indian
Oceans, this assumption of so many submerged banks
is in itself very improbable.

No theory worthy of notice has been advanced to
account for those barrier-reefs which encircle islands
of moderate dimensions. The great reef which fronts

_ the coast of Australia has been supposed, but without
any evidence, to rest on the edge of a submarine
precipice parallel to the shore. The origin of the
third class, or of fringing-reefs, presents, I believe,
searcely any difficulty, and arises simply from the poly-
pifers growing in moderate depths, and not flourishing
close to gently shelving beaches where the water is
often turbid. ‘

What cause, then, has given to atolls and barrier-
reefs their characteristic forms? Let us see whether
an important deduction does not follow from the
following facts,—first, that reef-building corals only

flourish at a very limited depth,—and secondly, that
throughout areas of vast dimensions, none of the
coral-reefs and coral-islets rise to a greater height
above the level of the sea than that attained by matter

thrown up by the waves and winds. I do not make)

this latter statement vaguely ; I have carefully sought
for descriptions of every island in the inter-tropical
seas; and my task has been in some degree facili-
tated by a map of the Pacific, corrected in 1834 by
MM. D’Urville and Lottin, in which the low islands
are distinguished from the high ones (even from those
much less than a hundred feet in height) by being
written without a capital letter.! I have also ascer-
tained, chiefly from the writings of Cook, Kotzebue,
Bellinghausen, Duperrey, Beechey, and Lutké regard-
ing the Pacific; and from Moresby ? with respect to the
Indian Ocean, that in the following cases the term
‘low island’ strictly means land of the height com-
monly attained by matter thrown up by the winds
and the waves of an open sea. If we draw a line
joining the external atolls of that part of the Low

1T have detected a few errors in this map, respecting the heights of
some of the islands, which will be noticed in the Appendix, where I
treat of coral-formations in geographical order. To the Appendix, also,
I must refer for a more particular account of the data on which the
following statements are grounded.

2 See also Captain Owen’s and Lieut. Wood’s papers in the Geogra-
phical Journal, on the Maldiva and Laccadive Archipelagoes. These
officers particularly refer to the lowness of the islets; but I chiefly
ground my assertion respecting these two groups, and the Chagos
group, from information communicated to me by Captain Moresby.

=
im
2

120 THEORY OF THE FORMATION ~ gee®

-
oa

‘sa
i" ' /
| ‘Cu. V. OF CORAL-REEFS. ea

_ Archipelago in which the islands are numerous—the
plan always adopted—the figure will be a pointed
ellipse (reaching from Hood to Lazaref Island), of
which the longer axis is 840 geographical miles, and
the shorter 420 miles: in this space,! none of the
innumerable islets, united into great rings, rise above
the stated level. The Gilbert group is very narrow,
and 300 miles in length. Ina prolonged line from
this group, at the distance of 240 miles, is the
Marshall archipelago, the figure of which is an
irregular square, one end being broader than the
other; its length is 520 miles with an average width
of 240: these two groups together are 1,040 miles in
length, and all their islets are low. Between the
southern end of the Gilbert and the northern end of
Low Archipelago, the ocean is thinly strewed with

islands, all of which, as far as I have been able to
ascertain, are low: so that from nearly the southern
end of the Low Archipelago to the northern end of
the Marshall Archipelago, there is a narrow band of
ocean more than 4,000 miles in length, containing a
vast number of islands, ali of which are low. In the
Western part of the Caroline Archipelago, there is a
space of 480 miles in length, and about 100 in breadth,

1 Metia or Aurora Island has been upraised; but it lies N.E. of
Tahiti, and in the map appended to this volume is close without
the line bounding the space here referred to. I shall have occasion
hereafter to make some remarks on the supposed slight elevation (of
about three feet) of the atolls of the Low Archipelago, subsequently to

| their original formation.

122 THEORY OF THE FORMATION Cu. V.

thinly interspersed with low islands. Lastly, in the
Indian Ocean, the archipelago of the Maldivas is 470
miles in length, and 60 in breadth; that of the Lac-
cadives is 150 by 100 miles: as there is a low island
between these two groups, they may be considered as
one group of a thousand miles in length. To this
may be added the Chagos group of low islands,
situated 280 miles distant, in a line prolonged from —
the southern extremity of the Maldivas. This group,
including the submerged banks, is 170 miles in length
and 80 in breath. So striking is the uniformity in
direction of these three archipelagoes, all the islands
of which are low, that Captain Moresby, in one of his
papers, speaks of them as parts of one great chain
neatly 1,500 miles long. I am, then, fully justified
in repeating that immense spaces both in the
Pacific and Indian Oceans, are interspersed wit
. islands, of which none rise above the height to which
the waves and winds in an open sea can heap up
matter.

On what foundations, then, have these reefs and
islets of coral been constructed? A foundation must
originally have been present beneath each atoll, at
that limited depth which is indispensable for the
first growth of the reef-building polypifers. A con-
jecture will perhaps be hazarded, that the requisite
bases may have been afforded by the accumulation of
great banks of sediment, which did not quite reach |
the surface owing to the action of superficial currents,
aided possibly by the undulatory movement of the

Cu. V. OF CORAL-REEFS. — 1s

sea. This appears actually to have been the case in
some parts of the West Indian Sea. But in the form
and disposition of the groups of atolls, there is nothing
to countenance this notion; and the assumption that
a number of immense piles of sediment have been
heaped on the floor of the great Pacific and Indian
Oceans in their central parts, far remote from land,
where the dark blue colour of the limpid water
bespeaks its purity, cannot for one moment be
admitted.

The many widely scattered atolls must, therefore,
rest on rocky bases. But we cannot believe that a
broad mountain summit lies buried at the depth of
a few fathoms beneath every atoll, and nevertheless
that throughout the immense areas. above-named, not
one point of rock projects above the level of the sea.
For we may judge of mountains beneath the sea by
those on the land; and where can we find a single
chain, much less several such chains, many hundred
miles in length and of considerable breadth, with
broad summits attaining the same height, from within
120 to 180 feet? Even if it be assumed without any
evidence that the reef-building corals can flourish at a
depth of 100 fathoms, yet the weight of the above
argument is but little diminished ; for it is very im-
probable, that as many submarine mountains, as there.
are low islands in the several great and widely-separated
areas above-specified, should all rise within 600 feet of
the surface of the sea and not one above it, as that they
should be of the same height within the smaller limit

es THEORY OF THE FORMATION Cu. Vy

5 a

of one or two hundred feet. So highly improbable is q
this supposition, that we are compelled to believe, that —
the rocky foundations of the many atolls did never at —

any one period all lie submerged within the depth of a
few fathoms beneath the surface, but that they were
brought into the requisite position or level, some at one
period and some at, another, through movements in the
earth’s crust. But this could not have been effected
by elevation ; for the belief that points so numerous and
so widely-separated were successively uplifted to a cer-
tain level, but that not one point was raised above that
_ level, is quite as improbable as the former supposition,

and indeed differs little from it. It will probably occur

to those who have read Ehrenberg’s account of the reefs
of the Red Sea, that many points in these great areas
may have been elevated, but that as soon as raised, the
protuberant parts were cut off by the destroying action
of the waves: a moment’s reflection, however, on the
basin-like form of the atolls, will show that this is
impossible; for the upheaval and subsequent abrasion
of an island would leave a flat disk, which might become
coated with coral, but not a deeply concave surface ;
moreover, we should expect to see, at least in some
parts, the rock of the foundation brought to the surface.
If, then, the foundations of the many atolls were not
uplifted into the requisite position, they must of neces-
sity have subsided into it; and this at once solves every
difficulty,! for we may safely infer from the facts given

1 The additional difficulty on the crater hypothesis before alluded to,

will now be evident; for on this view the volcanic action must be sup-.

en Vv. OF CORAL-REEFS. 125

in the last chapter, that during a gradual subsidence
the corals would be favourably cireumstanced for build-
ing up their solid frameworks and reaching the surface,
as island after island slowly disappeared. Thus areas
of immense extent in the central and most profound
parts of the great oceans might become interspersed
with coral-islets, none of which would rise to a greater
height than that attained by detritus heaped up by the
sea, and nevertheless they might all have been formed
by corals, which absolutely require for their growth a
solid foundation within a few fathoms of the surface.
It would be out of place here to do more than
allude to the many facts, showing that the supposition
of a gradual subsidence over large areas is by no means
improbable. We have the clearest proof that a move-
ment of this kind is possible, in the upright trees

posed to have formed within the areas specified a vast uumber of
craters, all rising within a few fathoms of the surface, and not one
above it. The supposition that the craters were at different times up-
raised above the surface, and were there abraded by the surf and sub- .
sequently coated by corals, is subjected to nearly the same objections
with those given at the bottom of the last page; but I consider it
superfluous to detail all the arguments opposed to such a notion. Cha-
‘tisso’s theory, from assuming the existence of so many banks, all lying
at the proper depth beneath the water, is also vitally defective. The
same observation applies to an hypothesis of Lieut. Nelson’s (Geolog.
Trans. vol. v. p. 122), who supposes that the ring-formed structure is
caused by a greater number of germs of corals becoming attached to
the declivity, than to the central plateau of a submarine bank; it like-
wise applies to the notion formerly entertained (Forster’s Observ. p.
151), that lagoon-islands owe their peculiar form to the instinctive ten-
dencies of the polypifers. According to this latter view, the corals on
the outer margin of the reef instinctively oppose themselves to the surf
in order to afford protection to corals living in the lagoon which belong
to other genera and to other families!

126 THEORY OF THE FORMATION Ca, V9

buried under strata many thousand feet in thickness ;
we have also every reason for believing that there are
now large areas gradually sinking, in the same manner
as others are rising. And when we consider how many
parts of the surface of the globe have been elevated
within recent geological periods, we must admit that
there have been subsidences on a corresponding scale,
for otherwise the whole globe would have swollen. It
is very remarkable that Sir C. Lyell,! even in the first
edition of his Principles of Geology, inferred that the
amount of subsidence in the Pacific must have exceeded
that of elevation, from the area of land being very
small relatively to the agents there tending to form it,
namely, the growth of coral and voleanic action. But,
although subsidence may explain a phenomenon other-
wise inexplicable, it may be asked, are there any direct
proofs of a subsiding movement in these areas? This,
however, can hardly be expected, for it must ever be
most difficult, excepting in countries long civilized, to
detect a movement the tendency of which is to conceal
the part affected. In barbarous and semi-civilized
nations how long might not a slow movement, even of
elevation such as that now affecting Scandinavia, have
escaped attention !

Mr. Williams? insists strongly that the traditions of

the natives, which he has taken much pains in collect-_

ing, do not indicate the appearance of any new islands:
but on the theory of a gradual subsidence, all that would

1 Principles of Geology, sixth edition, vol. ii1. p. 386.
* Williams’s Narrative of Missionary Enterprise, p. 31.

Cu. V. OF CORAL- REEFS. | 127

be apparent would be, the water sometimes encroaching
_ slowly on the land, and the land again recovering by
| the accumulation of detritus its former extent, and
| perhaps sometimes the conversion of an atoll with coral
| islets on it, into a bare or into a sunken annular reef.
| Such changes would naturally take place at the periods
|

when the sea rose above its usual limits during a gale

| of more than ordinary strength; and the effects of the
| two causes would be hardly distinguishable. In Kotze-
| bue’s Voyage there are accounts of islands, both in the
| Caroline and Marshall Archipelagoes, which have been
partly washed away during hurricanes; and Kadu, the
native who was on board one of the Russian vessels,
said ‘he saw the sea at Radack rise to the feet of the
cocoa-nut trees ; but it was conjured in time.’! A storm
lately entirely swept away two of the Caroline Islands
and converted them into shoals; it also partly destroyed ~
two other islands. According to a tradition which was
communicated to Captain FitzRoy, it is believed in the
Low Archipelago, that the arrival of the first ship caused
a great inundation which destroyed many lives. Mr.
Stutchbury relates that in 1825, the western side of
Chain Atoll in the same group, was completely de-
vastated by a hurricane, and not less than 300 lives
lost: ‘im this instance it was evident, even to the
natives, that the hurricane alone was not sufficient to
account for the violent agitation of the ocean.’? That

1 Kotzebue’s First Voyage, vol. ili. p. 168.
* M. Desmoulins in Comptes Rendus, 1840, p. 837.
3 West of England Journal, No, 1, p. 35.

\

128 THEORY OF THE FORMATION

considerable changes have taken place recently in some » |
of the atolls in the Low Archipelago, appears certain 4
from the case of Matilda Island given in the last chapter. —
With respect to Whitsunday and Gloucester Islands in ~

this same group, we must either attribute great inac-
curacy to their discoverer, the famous cireumnavigator _

Wallis, or believe that they have undergone a consider-
able change in the period of fifty-nine years between his _
voyage and that of Captain Beechey’s. Whitsunday —

Island is described by Wallis as ‘about four miles long,

and three wide,’ now it is only one mile and a-half |

long. The appearance of Gloucester Island, in Captain —
Beechey’s words,' ‘ has been accurately described by its

discoverer, but its present form and extent differ mate-

_rially.’ Blenheim reef in the Chagos group, consists of a 1 i}

water-washed annular reef thirteen miles in circumfer-
ence, surrounding a lagoon ten fathoms deep ; on its sur-
face there are a few worn patches of conglomerate coral-
rock of about the size of hovels; and these Captain
Moresby considers as being, without doubt, the last
remnants of islets; so that here an atoll has been con-

verted into an atoll-formed reef. The inhabitants of 4

the Maldiva Archipelago, as long ago as 1605, declared,
‘that the high tides and violent currents were always
diminishing the number of the islands:’? and I have
already shown, on the authority of Captain Moresby, that

1 Beechey’s Voyage to the Pacific, chap. vii., and Wallis’s Voyage in
the Dolphin, chap. iv.

2 See an extract from Pyrard’s Voyage in Captain Owen’s paper on
the Maldiva Archipelago, in the Geographical Journal, vol. ii. p. 84.

Gn-V. 7 OF CORAL-REEFS. 129

the work of destruction is still in progress; but that on
the other hand the first formation of some islets is
known to the present inhabitants. In such cases, it
would be exceedingly difficult to detect a gradual sub-
sidence of the foundation on which these mutable
structures rest.

Some of the archipelagoes of low coral-islands are
subject to earthquakes: Captain Moresby informs me
that they are frequent, though not very strong, in the
Chagos group, which occupies a central position in the
Indian Ocean, and is far from any land not of coral
formation. One of the islands in this group was
formerly covered by a bed of mould, which disap-
peared after an earthquake, and was believed by the
residents to have been washed by the rain into the
underlying fractured rock: the island was thus ren-
dered unproductive. Chamisso! states that earth-
quakes are felt in the Marshall atolls, which are far
from any high land, and lkewise in the islands of
the Caroline Archipelago. On Oulleay atoll one of
the latter, Admiral Lutké informs me that he ob-
served several straight fissures about a foot in width,
running for some hundred yards obliquely across the
whole width of the reef. Fissures indicate a stretching
of the earth’s crust, and, therefore, probably changes
in its level; but these coral-islands, which have been
shaken and fissured, certainly have not been elevated,
and, therefore, probably have subsided. We shall

1 See Chamisso, in Kotzebue’s First Voyage, vol. iii. p. 182 and 186,
K

130 THEORY OF THE FORMATION Ca.

hereafter see that the position of certain ancient build-
ings in the Caroline Archipelago clearly indicates recent

subsidence. In the chapter on Keeling atoll, I have .

also attempted to show by direct evidence, that the
island subsided during the earthquakes lately felt there.

The facts then stand as follows :—there are many
large spaces of ocean without any high land, inter-
spersed with reefs and islets formed by the growth
of those kinds of coral which cannot live at great
depths; and the existence of these reefs and low
islets in such numbers and at such distant points,
is inexplicable, excepting on the theory that their
rocky bases slowly and successively sank beneath the
level of the sea, whilst the corals continued to grow
upwards. No positive facts are opposed to this view,
and some direct evidence, as well as general considera-
tions, render it probable. There is also evidence of
change in form, whether or not from subsidence, on
some of these coral-islands; and there is evidence of
subterranean disturbances beneath them. Will then
the theory, to which we have thus. been led, solve the
curious problem,—what has given to each class of reef
its peculiar form ? |

Let us in imagination place within a subsiding area,
an island surrounded by a ‘fringing-reef’—that kind
of which the origin alone offers no difficulty. Let the
unbroken lines in the woodcut (No. 5) represent a
vertical section through the land and water; and the
horizontal shading a section through the reef. Now, as
the island sinks down, either a few feet ata time or quite

12 aw eS al

Wifi fif’ y
Lift)

Cu. V. OF CORAL-REEFS. E3i

insensibly, we may infer from what we know of the
conditions favourable to the growth of coral, that the
living masses bathed by the surf on the margin of the
reef, will soon regain the surface. The water, however,
will encroach little by little on the shore, the island
becoming lower and smaller, and the space between the

edge of the reef and the beach proportionally broader.

ee aanee nen en ween cennen= = a rr a

Level af Sea

re, /
| mi

A A—Outer edge of the reef at the level of the sea.

B B—Shores of the island.

A’ A’—Outer edge of the reef, after its upward growth during a period
of subsidence.

C C—The lagoon-channel between the reef and the shores of the now

encircled land.

B’ B/—The shores of the encircled island.
N.B. In this, and the following woodcut, the subsidence of the land

could only be represented by an apparent rise in the level of the sea.

A section of the reef and island in this state, after a
subsidence of several hundred feet, is given by the
dotted lines: coral-islets are supposed to have been
formed on the new reef, and a ship is anchored in
the lagoon-channel. This section is in every respect
that of an encircling barrier-reef, and is, in fact,

kK 2

132 THEORY OF THE FORMATION Cu. V.

taken E. and W. through the highest point of the
encircled island of Bolabola,! of which a plan is given
in Plate I. fig. 5. The same section is more clearly
shown in the following woodcut (No. 6) by the un-
broken lines. The width of the reef and its slope
both on the outer and inner side, will have been
determined by the growing powers of the coral, under
different conditions, for instance, of the force of the
breakers and currents to which it has been exposed ;
and the lagoon-channel will be deeper or shallower, in
proportion to the growth of the delicately branched
corals within the reef, and to the accumulation of
sediment; relatively, also, to the rate of subsidence and
the length of the intervening stationary periods.

It is evident in this section, that a line drawn per-
pendicularly down from the outer edge of the new reef
to the foundation of solid rock, exceeds, by as many
feet as there have been feet of subsidence, that small
limit of depth at which the effective polypifers can
live,—the corals having grown up, as the whole sank
down, from a basis formed of other corals and their con-
solidated fragments. Thus the difficulty on this head,
which before seemed so great, disappears. |

As the space between the reef and the subsiding
shore continued to increase in breadth and depth, and
as the injurious effects of the sediment and fresh water

! The section has been made from the chart given in the Atlas of the
Voyage of the Coquille. The scale is ‘57 of an inch to a mile. The
height of the island, according to M. Lesson, is 4,026 feet. ‘The deepest
part of the lagoon-channel is 162 feet; its depth is exaggerated in the
woodcut for the sake of clearness.

Cr Ve OF CORAL-REEFS. 133

borne down from the land were consequently lessened,
the greater number of the channels with which the reef
in its fringing state must have been breached, especially
those which fronted the smaller streams, will have
become choked up by the growth of coral: on the wind-
ward side of the reef where the coral grows most
vigorously, the breaches will probably have first been
closed. In barrier-reefs, therefore, the breaches kept
open by draining the tidal waters of the lagoon-channel,
will generally be placed on the leeward side, and they
will still face the mouths of the larger streams, although
removed beyond the influence of their sediment and
fresh-water ;—and this, it has been shown, is commonly
the case.

Referring to the following diagram, in which the

| i

A’ A’—Outer edges of the barrier-reef at the level of the sea. The
cocoa-nut trees represent coral islets formed on the reef.

C C—The lagoon channel.

B’ B’—The shores of the island, generally formed of low alluvial land
and of coral detritus from the lagoon-channel.

A” A”—The outer edges of the reef now forming an atoll.

C’—The lagoon of the newly-formed atoll. According to the scale, the
depth of the lagoon and of the lagoon channel is exaggerated.

134 THEORY OF THE FORMATION Cx: V..,

newly-formed barrier-reef is represented by unbroken

lines, instead of by dots as in the former woodcut, let
the work of subsidence go on, and the doubly-pointed
hill will form two small islands included within one
annular reef. Let the island continue to subside, and
the coral-reef will continue growing up on its own

foundation, whilst the water gains inch by inch on the -

land, until the last and highest pinnacle is covered, and
there remains a perfect atoll. A vertical section of this
atoll is shown in the woodcut by the dotted lhnes;—a
ship is anchored in its lagoon, but islets are not supposed
yet to have been formed on the reef. The depth of the
lagoon and the width and slope of the reef, will depend
on the different circumstances to which it has been
exposed, as just stated with respect to barrier-reefs.
Any further subsidence will produce no change in the
atoll, except a diminution in its size, from the reef not
growing vertically upwards. I may here observe, that
a bank either of rock or of hardened sediment, level
with the surface of the sea and fringed with living
coral, would be immediately converted by subsidence
into an atoll, without passing, as in the case of a reef
fringing the shore of an island, through the inter-
mediate form of a barrier-reef. As before remarked, if
such a bank lay a few fathoms submerged, the simple
erowth of the coral, without the aid of subsidence, would
produce a structure scarcely to be distinguished from a
true atoll; for the corals on the outer margin, from being
freely exposed to the open sea, would grow vigorously

and tend to form a continuous ring, whilst the growth |

icc Sl oa adi sn nna aii ate pe itlatciog

anv. - OF CORAL-REEFS. 135

of the less massive kinds on the central expanse, would
be checked by the sediment formed there, and by that
washed inwards by the breakers; and as the space be-
came shallower, their growth would also be checked by
the impurities of the water, and probably by the small
amount of food brought to them by the enfeebled cur-
rents. The subsidence of a reef based on a bank of this
kind, would give depth to the central expanse or lagoon,
steepness to the flanks, and through the free growth of
the coral, symmetry to the whole outline; but, as we
have seen, the larger groups of atolls in the Pacific and
Indian Oceans cannot have been formed on banks of
this nature.

If, instead of an island, as in the diagram, the shore
of a continent fringed by a reef were to subside, a great
barrier-reef like that on the N.E. coast of Australia,
would be the necessary result; and it would be separated
from the main land by a deep-water channel, broad in
proportion to the amount of subsidence, and to the less
or greater inclination of the bed of the sea. The effect
of the continued subsidence of a barrier-reef, and its
probable conversion into a chain of separate atolls, will
be considered when we discuss the progressive dissever-
ment of the larger Maldiva atolls.

We now are able to perceive that the close similarity
in form, dimensions, structure, and relative position
between fringing and encircling barrier-reefs, and be-
tween these latter reefs and atolls, is the necessary result
of the transformation, during subsidence, of the one
class into the other. On this view, the three classes of

136 THEORY OF THE FORMATION Ca.Vean

reefs ought to graduate into each other. Reefs having —
an intermediate character between those of the fring-
ing and barrier classes do exist; for instance, on the
S.W. coast of Madagascar, a reef extends for several
miles, within which there is a broad channel from
7 to 8 fathoms deep, but the sea does not deepen
abruptly outside the reef. Such cases, however, are
open to doubt, for an old fringing-reef which had
extended itself on a basis of its own formation, would
hardly be distinguishable from a barrier-reef produced
by a small amount of subsidence, and with its lagoon-
channel nearly filled up with sediment during a long
stationary period. Between barrier-reefs, encircling
either a single lofty island or several small low ones,
and atolls including a mere expanse of water, a striking
series can be shown; and in proof of this, I need only
refer to Plate I., which speaks more plainly to the eye,
than any description to the ear. The authorities from
which the figures have been copied, together with some
remarks on them, are given on a separate page descrip-
tive of the plate. At New Caledonia (Plate II. fig. 5)
the barrier-reefs extend for 150 miles on each side of
the submarine prolongation of the island; and at the
northern extremity these reefs appear broken up and
converted into a vast atoll supporting a few low coral-
islets. We may imagine that we see in New Caledonia
the effects of subsidence actually in progress,—the
water always encroaching on the northern end of the
island, towards which the mountains slope down, and
the reefs steadily building up their massive fabrics in
the line of their ancient growth.

Cu, V.. OF CORAL-REEFS. 137

We have as yet only considered barrier-reefs and
atolls in their simplest form; but there remain some
peculiarities in structure and some special cases, which

| were described in the two first chapters, to be accounted
-for by our theory. These consist, firstly, in the presence

of an inclined ledge terminated by a wall, and some-
times succeeded by a second ledge with a wall, round
the shores of certain lagoons and lagoon-channels; for
this structure cannot be explained by the mere growth
of the corals ;—secondly, in the ring or bason-like form
of the central‘and circumferential reefs of the northern
Maldiva atolls,—thirdly, in the disseverment of some
of the Maldiva atolls,—fourthly, in the existence of
irregularly formed atolls, some tied together by linear
reefs, and others with spurs projecting from them,—
fifthly, in the submerged condition of the whole, or of
parts of certain barrier and atoll-formed reefs, and in
the submerged parts being generally to leeward,—
and, lastly, in the structure and origin of the Great
Chagos bank.

Step-formed ledges round certaim lagoons.—If we
suppose an atoll to subside at an extremely slow rate,
the living corals would grow up on the outer margin
and on the deeper parts of the bare and hard surface
of the annular reef. Detritus would soon accumulate
and become agelomerated on this surface, after a time
forming islets. Consequently the whole atoll before long
would recover its former structure and appearance. If,
however, an atoll were to subside somewhat suddenly to
the depth of a few fathoms, the whole annular reef,
consisting of solid rock, would constitute an excellent

138 THEORY OF THE FORMATION Cx. Vi 9

basis for the attachment and subsequent upward growth |
of a great bed of living corals. But the corals would
not be able to grow up from the sandy shores of the old
lagoon; consequently the new annular reef would be
separated from the new lagoon by an abrupt edge or wall.
As the corals would grow upwards much more vigorously
on the outer side, and more detritus would be accumu-
lated there, the surface of the new annular reef would
slope gently inwards. Hence the summit of the new
annular reef on the inner side would probably never
rise above the level of the new lagoon, and in this case —
would be covered with sand. If now a second and

®&

somewhat sudden subsidence were to occur, the same
results would follow as during the previous and similar
subsiding movement. Consequently the new lagoon
would be surrounded by two inwardly sloping ledges,
which once existed as the summits of two successive
annular reefs, both terminated on the inner side by
vertical walls or cliffs.! oe
The ring or bason-formed reefs of the northern
Maldiva atolls.—I must first observe that small reefs
within large lagoons or within broad lagoon-channels,
would grow up during subsidence; and therefore such

1 According to Mr. Couthouy (p. 26) the external slope round many
atolls descends by a succession of ledges or terraces. He attempts, but
I doubt whether successfully, to explain this structure somewhat in the
same manner as I have attempted, with respect to the internal ledges
round the lagoons of certain atolls. More facts are wanted regarding
the nature both of the interior and exterior step-like ledges. Are all
the ledges, or only the upper ones, covered with living coral? If they
are all so covered, do the species differ on the different ledges? Do
ledges occur on the inside and outside round the same atolls? &c.

Cu. V. _ OF CORAL-REEFS. . 139

| yeefs would sometimes be found rising abruptly from
a greater depth than that at which the efficient poly-
_ pifers can flourish. We see this well exemplified in
the small abruptly-sided reefs with which the deep

lagoons of the Chagos and Southern Maldiva atolls are
| studded. With respect to the ring or bason-formed reefs
i of the Northern Maldiva atolls (see Plate II. fig. 4),
| it is evident from the perfectly continuous series which
exists, that the rings on the margin, although broader
than the exterior or bounding reef of an ordinary atoll,
| are only modified portions of such a reef; it is also
evident that the central rings, although broader than
the knolls or reefs which commonly occur within
lagoons, occupy the same relative position. The ring-
like structure has been shown to be contingent on
the breaches into the lagoon being wide and numerous,
thus causing the inner side of the marginal reef and
the central reefs to be placed under nearly the same
conditions with the outside of an ordinary atoll which
is exposed to the opensea. Hence the margins of these
reefs have been favourably circumstanced for growing
outwards and increasing beyond their usual breadth ;

and the conditions have likewise been favourable for
their growing vigorously upwards, during that subsiding
movement to which by our theory. the whole archi-
pelago has been subjected; and subsidence together
with the upward growth of the margin would convert

the central space of each little reef into a small
lagoon. This, however, could only take place with
reefs which had increased in breadth sufficiently to

140 THEORY OF THE FORMATION

diately filled up with the sand and detritus driven iM
inwards by the waves from all sides. We can thus |
understand how it is that few reefs less than half |
a mile in diameter, even in the atolls where perfect |
ring-formed reefs are found, include lagoons, This |
remark, I may add, applies to all coral-formations. |
The bason-formed reefs of the Maldiva Archipelago |
may, in fact, be briefly described as small atolls
formed during subsidence over separate portions of a
large and broken atoll, in the same manner as the
latter was originally formed over a reef encircling one
or more mountainous islands.
The disseverment of the larger Maldiva atolls —
The apparent progressive disseverment of large atolls
into smaller ones in the Maldiva Archipelago, de-
mands an explanation. The graduated series which
marks, as I believe, this process, can be observed only in
the northern half of the group, where the atolls have
imperfect margins consisting of detached bason-formed —
reefs. The currents of the sea flow across these atolls
with considerable force, as I am informed by Captain
Moresby, and drift the sediment from side to side
during the monsoons, transporting much of it seaward;
yet the currents sweep with greater force round their
flanks. It is historically known that these atolls have
long existed in their present state; it is intelligible
that they might thus remain, even during a slow sub-
sidence, owing to the continued growth of the corals,
and to the lagoon being kept at nearly its original depth

|
|
|

Cu, V. OF CORAL-REEFS. 141

: by the accumulation of sediment. But during the

action of such nicely balanced forces, it would be strange

if the currents of the sea had never made a direct

passage across some of these atolls, through the many
wide breaches in their margins. As soon as this occurred
the channels would be deepened by the removal of the
finer sediment, and by the check to its further accumu-
lation. The sides also of the channels would soon be
worn into a slope lke that on the outer coasts, from
being exposed to the same force of the currents. In
fact, a channel like that bifurcating one which divides
Mahlos Mahdoo (Plate II. fig. 4) would almost neces-

sarily be formed. The scattered reefs situated near the

borders of the new channel, from being favourably
placed for the growth of coral, would, by their exten-
sion, tend to produce fresh margins to the dissevered
portions: and a tendency of this kind is evident in the
elongated reefs which border the two channels inter-
secting Mahlos Mahdoo. Such channels would become
deeper with continued subsidence, and, from the reefs
on both sides not growing up perpendicularly, some-
what broader. In this case, and more especially if the
channels had been originally formed of considerable
breadth, the dissevered: portions would soon become per-
fect and distinct atolls like Ariand Ross atolls (Plate II.
fig. 6), or like the two Nillandoo atolls, which must be
considered as distinct, although plainly related to each
other in form and position, and separated only by mode-
rately deep channels. Further subsidence would render

‘such channels unfathomable, and the dissevered portions

142 THEORY OF THE FORMATION

would then resemble Phaleedoo and Moluque atolls, or |
Mahlos Mahdoo and Horsburgh atolls (Plate II. fig. 4), |

which are related to each other only in proximity and ©

position. Hence, on the theory of subsidence, the dis- |

severment of large atolls which are exposed to strong —

currents and which have impertfect margins, (for other-

wise their disseverment would be scarcely possible), is |

far from being an improbable event; and the several

stages, from a close connection to the entire isolation |
of some of the atolls in the Maldiva Archipelago, are °

readily explicable.

It is even probable that the Maldiva Archipelago
originally existed as a barrier-reef of nearly the same
dimensions as that of New Caledonia (Plate II. fig. 5):
for if we complete in imagination the subsidence of
this great island, we may infer from the broken condi-

tion of the northern portion of the reef, and from the ~

almost entire absence of reefs on the eastern coast, that
the present barrier, after repeated subsidences, would
become, during its subsequent upward growth, separated
into distinct portions ; and these portions would tend
to assume an atoll-like structure, owing to the corals
growing with vigour where freely exposed to the open sea.
Assome large islands have subsided to a certain amount

and are partly encircled by barrier-reefs, so our theory

makes it probable that there should be other large
islands wholly submerged; and these, as we can now see,
would be surmounted, not by one enormous atoll, but
by several large ones like the atolls of the Maldiva
eroup; and these again, during long periods of sub-

— = =

i

OF CORAL-REEFS. 143

sidence, would sometimes become dissevered into
smaller ones. In the Marshall and Caroline Archipela-

goes, there are atolls standing close together which

have an evident relationship in form; and we may sup-
pose that either two or more encircled islands originally
stood close together and afforded bases for two or more
atolls, or that one large atoll has been dissevered. But
from the position as well as the forms of three atolls
in the Caroline Archipelago (the Namourrek and Elato
groups), which are placed in an irregular circle, I am
strongly inclined to believe that they owe their origin
to the disseverment of a single large atoll.!
Trregularly-formed Atolls.—In the Marshall group,
Musquillo atoll consists of two loops united by a single. .
point ; and Menchicoff atoll is formed of three loops,
two of which (as may be seen in fig. 3, Plate II.) are
connected by a mere ribbon-shaped reef; the three
together being 60 milesinlength. In the Gilbert group.

some of the atolls have narrow reefs like spurs, project-

ing from them. Linear and straight, or crescent-formed
reefs with their extremities more or less curled in-
wards, may sometimes be found standing by themselves.
in the open ocean. All these irregular forms would

1 The same remark is, perhaps, applicable to the islands of Ollap,
Fanadik, and Tamatam in the Caroline Archipelago, of which charts:
are given in the atlas of Duperrey’s voyage; a line drawn through the
linear reefs and lagoons of these three islands form a semicircle.
Consult also the atlas of Lutké’s voyage; and for the Marshall group:
that of Kotzebue; for the Gilbert group (which is referred to in the
ensuing page) consult the atlas of Duperrey’s voyage. Most of the
points here referred to may, however, be seen in’ Krusenstern’s general
Atlas of the Pacific.

144 THEORY OF THE FORMATION

naturally follow from continued subsidence, combined

with the upward growth of reefs fronting one side alone -

of a high island, the reefs on the opposite side having
perished or never having existed.

Submerged and Dead Reefs.—In the second sec
tion of the first chapter, I have shown that there
sometimes exist in the neighbourhood of atolls, deeply
submerged banks with level surfaces; that there are
others, less deeply but yet wholly submerged, having
all the characters of a perfect atoll, but consisting
merely of dead coral-rock; that there are barrier-reefs
and atolls with only a portion of the reef, generally

on the leeward side, submerged; and that such ©

portions either retain their perfect outline, or appear
to be more or less completely effaced, their former

place being marked only by a bank, conforming in

general outline with that part of the reef which remains
perfect. These several cases are, I believe, intimately
related, and can all be explained by the same agency of
subsidence.

We see that in those parts of the ocean where —

coral-reefs are most abundant, one island is fringed and
another neighbouring one is not fringed, and that in
the same archipelago, all the reefs are more perfect

in one part than in another,—for instance, in the

southern compared with the northern half of the
Maldiva Archipelago, and likewise on the outer as
compared with the inner coasts of the double row of
atolls in this same archipelago. The existence of the
innumerable polypifers forming a reef, depends on

a |

Ox. V. . OF CORAL-REEFS. 145

their finding sustenance, and we know that they are
preyed on by other organic beings, and that some
inorganic causes are highly injurious to their growth.
Can it, therefore, be expected that the reef-building
polypifers should keep alive for perpetuity in any one
place, during the round of change to which earth,
air, and water are subjected; and still less can this
be expected during progressive subsidence, to which
by our theory these reefs and islands have been liable ?
Should such subsidence be at any time greater than
the rate of upward growth of the polypifers, the death
of the reef must ensue, and it would have been strange
had we found no evidence of this. It is, then, not
at all improbable that the corals should sometimes

perish either on the whole or on part of a reef.
If only on a part, the dead portion, after a small

amount of subsidence, would still retain its proper
outline and position beneath the water. After a more
prolonged subsidence, it would form, owing to the

accumulation of sediment, a more or less level bank

marking the limits of the former lagoon. Such dead
portions of a reef would generally lie on the leeward
side,' for the impure water and fine sediment are

1 Sir C. Lyell, in the first edition of his Principles of Geology, offered
a somewhat different explanation of this structure. He supposes that
there has been subsidence; but he was not aware that the submerged
portions of reef were in most cases, if not in all, dead; and he attributes
the difference in height in the two sides of most atolls, chiefly to the
greater accumulation of detritus to windward than to leeward. But as
matter is accumulated only on the backward part of the reef, the front

L

|
?

146 THEORY OF THE FORMATION Cn..¥; 9

driven out from the lagoon over this side of the reef,
where the force of the breakers is less than to wind-
ward, and where the corals are, in consequence, less
vigorous and less able to resist any destroying agency.
It is owing to this same cause, that reefs are fre-
quently breached to leeward by channels ‘which serve
as ship-channels. If the corals perished entirely, or
on the greater part of the circumference of an atoll,
the result would be an atoll-shaped bank of dead
rock more or less entirely submerged; and further
subsidence, together with the accumulation of sedi-
ment, would obliterate its atoll-like structure, and
leave only a bank with a nearly level surface.

We meet with all these cases in the Chagos group
of atolls. Here within an area of 160 miles by 60,

there are two atoll-formed banks of dead rock (besides
another very imperfect one) entirely submerged; a
third bank with merely two or three small pieces of
living reef which rise to the surface; and a fourth,
namely, Peros Banhos (Plate I. fig. 9), with a por-
tion nine miles in length dead and submerged. As
by our theory this area has subsided, and as there is
nothing improbable in the death of the corals on por-
tions or over the whole surface of a reef, either from

part would remain of the same height on both sides. I may here
observe that in most cases (for instance at Peros Banhos, the Gambier
group and the Great Chagos bank), and I suspect in all cases, the dead }
and submerged portions do not blend or slope into the living and perfect
parts, but are separated from them by an abrupt line. In some in-
stances small patches of living reef rise to the surface from the middle |
of the submerged and dead parts.

Gu. V. OF CORAL-REEFS. 147

changes in the state of the surrounding sea or from the
subsidence being great or sudden, these Chagos banks
present no difficulty. So far, indeed, are any of the above-
mentioned cases of dead submerged reefs from offering
any difficulty, that their occurrence might have been
anticipated on our theory; and as fresh atolls are sup-
posed to be in progressive formation by the subsidence
of encircling barrier-reefs, a weighty objection might
even have been raised, namely that atolls must increase
indefinitely in number, unless proofs of their occasional
destruction could have been adduced.

The Great Chagos Bank.—I have already shown
that the submerged condition of the Great Chagos
bank (Plate II. fig. 1, with its section, fig. 2) and of
_ some other banks in the Chagos group, may in all pro- —
bability be attributed to the corals having perished
during an unusually rapid or sudden subsidence. The
external rim or upper ledge (shaded in the chart) con-
sists of dead coral-rock thinly covered with sand; it
lies at an average depth of between 5 and 8 fathoms,
and perfectly resembles in form the annular reef of an
atoll. The banks of the second level, the boundaries
of which are marked by dotted lines in the chart, lie
from about 15 to 20 fathoms beneath the surface;
they are several miles in breadth, and terminate in a
- very steep slope round the central expanse. This
central expanse consists of a level muddy flat between
30 and 40 fathoms deep. The banks of the second
level appear at first sight to resemble the internal step-

L 2

?
:

148 THEORY OF THE FORMATION Gia Ve

like ledges of dead coral-rock which border the lagoons
of certain atolls, but their much greater width, and
their being formed of sand, are points of essential dif-
ference. On the eastern side of the atoll some of the
banks are linear and parallel, like islets in a great river,
and they point directly towards a great breach on the
opposite side of the atoll: these are best seen in the
large published chart. I inferred from this circum-
stance, that strong currents sometimes set directly
across this great bank ; and I hear from Captain Moresby
that this is the case. I observed, also, that the channels,
or breaches through the rim, were all of the same depth —
as the central expanse into which they lead; whereas
the channels into the other atolls of the Chagos group,
and as I believe into most other large atolls, are not
nearly as deep as the lagoons. For instance at Peros
Banhos, the channels as well.as the bottom of the lagoon
for a space about a mile and a-half round its shores, are
only between 10 and 20 fathoms in depth, whilst the
central expanse is from 35 to 40 fathoms deep. Now,
if an atoll during a gradual subsidence once became
entirely submerged like the Great Chagos bank, and
therefore no longer exposed to the surf, very little
sediment could any longer be formed from it; conse- |
quently the channels leading into the lagoon would be
no longer filled up with drifted sand and coral detritus,
and would continue increasing in depth, as the whole
sank down. In this case we might expect that the
currents of the open sea, instead of sweeping as at

“—”
i
\

Cu. V. OF CORAL-REEFS. 149

first round the submarine flanks, would, as the many
breaches in the reef increased, flow directly across
the lagoon, thus removing the finer sediment from
the channels, and preventing its further accumulation.
The submerged reef would thus ultimately consist of an
upper and narrow broken rim of rock, surrounded on
the inner side by banks, the remnants of the sandy
bed of the old lagoon, now intersected by many deep
channels; these channels, with their sides worn steep by
the oceanic currents, uniting in the centre and forming
the central deep expanse. By such means the Great
Chagos bank—the most anomalous structure which I
have met with—appears to have originated.

If this bank should continue to subside, a mere
wreck of an atoll would be left; for the corals are
almost everywhere dead. Pitt’s bank, situated not far
southward, appears to be in this actual condition: it
consists of a moderately level, oblong bank of sand,
lying from 10 to 20 fathoms beneath the surface, with
two sides protected by a narrow ledge of rock submerged
between 5 and 8 fathoms. A little to the south of this
ledge, at about the same distance as the southern rim of
the Great Chagos bank les from the northern rim, there
are two other small banks with from 10 to 20 fathoms
on them; and not far eastward, soundings were struck
on a sandy bottom with between 110 and 145 fathoms.
The northern portion of Pitt’s bank with its ledge-like
margin, thus closely resembles any one segment of the
Great Chagos bank between two of the deep-water

150 THEORY OF THE FORMATION Cz. Vv. |

channels, and the scattered banks southward and east- |
ward appear to be the last wreck of the less perfect
portions of one great and now ruined atoll. |
» Ihave examined with care the charts of the Indian
and Pacific Oceans, and have now laid before the reader —
all the cases which I have met with, of reefs differing
from the class to which they belong; and I think it has
been shown that they are all included in our theory,
modified by occasional accidents, such as might have
been anticipated. We have thus seen, that in the lapse
of ages encircling barrier-reefs are converted into atolls,
—the term atoll being applicable as soon as the last
pinnacle of encircled land sinks beneath the surface of
the sea. We have seen that large atolls, during the pro-
eressive subsidence of the areas in which they stand,
sometimes become dissevered into smaller ones. At other
times, when the reef-building polypifers perish, atolls
are converted into atoll-formed banks of dead rock; and
these again, through further subsidence and the accu- —
mulation of sediment, pass into level banks with scarcely
any distinguishing character. Thus may the history of an
atoll be followed from its birth, through the occasional ac-
cidents of its existence, to its death and final obliteration.
Objections to our theory of the formation of Atolls
and Barrier-reefs.—The vast amount of subsidence
both in area and depth, necessary to have submerged
every mountain, even the highest, throughout the
immense spaces of ocean now interspersed with atolls,
will probably strike most persons as a formidable objec-
tion to the theory. But as continents, as large as the

MO alll

Cu. V. OF CORAL-REEFS. 151

spaces supposed to have subsided, have been raised
above the level of the sea,—as whole regions are now
rising, for instance, in Scandinavia and South America,
—and as no reason can be assigned why subsidence
should not have occurred in some parts of the earth’s
crust on as great a scale as elevation, this objection has
little force. The remarkable point is, that a subsiding
movement to such an extent and amount should have
taken place within a period, during which the corals
have continued to add matter to the same reefs. An-
other and less obvious objection to the theory may
perhaps be advanced, namely, that, although atolls and
barrier-reefs are supposed to have gone on subsiding
for a long period, yet that their lagoons and lagoon-
channels have only rarely come to exceed 40 and
never 60 fathoms in depth. But if our theory is worth
consideration, we already admit that the rate of sub-
_sidence has not ordinarily exceeded that of the upward
growth of the massive corals which live on the margins
of the reefs, so that we have only further to suppose
that the rate has never exceeded that at which lagoons
and lagoon-channels are filled up by the growth of the
delicate corals which live there, and by the accumula-
tion of sediment. As the filling-up process, in the case
of barrier-reefs lying far from the land, and of the larger
atolls, must be an extremely slow one, we are led to
conclude that the subsiding movement has always been
equally slow. And this conclusion accords well with
what is known of the rate of recent movements of
elevation.

152 THEORY OF THE FORMATION Ca. Vv. @

It has, I think, been shown in this chapter, that
subsidence explains both the normal structure and
the less reguiar forms of those two great classes of
reefs which have justly excited the astonishment of all
the naturalists who have sailed through the Pacific and
Indian Oceans. The necessity, also, that a foundation
should have existed at the proper depth for the growth
of the corals over certain large areas, almost compels
us to accept this theory. But further to test its truth,
a crowd of questions may be asked. Do the different
kinds of reefs which have been produced by the
same kind of movement, generally lie within the
same or closely adjoining areas? How are such reefs

related to each other in form and_ position,—for .

instance, do neighbouring groups of atolls, and the
separate atolls in each group, bear the same relation
to each other as do ordinary islands? Although coral-
reefs which have just begun to re-grow, after having
been killed by too rapid a subsidence, would at first
belong to the fringing class, yet, as a general rule,
reefs of this class indicate that the land has either
long remained at a stationary level, or has been up-
raised. Of a stationary level it is hardly possible
to find any evidence except of a negative kind; but
of recent elevation, upraised marine remains afford a
sure proof: it may therefore be asked, do fringed
coasts often afford such evidence? Do the areas
which have subsided, as shown by the presence of
atolls and barrier-reefs, and the areas which have
either remained stationary or have been upraised,

oe

Cu. V. OF CORAL-REEFS. 153

as indicated by fringing-reefs, bear any determinate
relation to each other? Is there any relation between
|| the areas of recent subsidence or elevation, and the
i presence of active volcanic vents? These several
questions will be considered in the following chapter.’

1] may take this opportunity of briefly considering the appearance
which would probably be presented by a vertical and deep section
across a coral formation (referring chiefly to an atoll) formed by the
upward growth of coral during successive subsidences. This is a
subject worthy of attention, as a means of comparison with ancient
coral strata. The circumferential parts would consist of massive spe-
cies in a vertical position, with their interstices filled up with detritus;
but this would be the part most subject to subsequent denudation and
removal. It is useless to speculate how large a proportion of the
exterior annular reef would consist of upright coral, and how much of
fragmentary rock, for this would depend on many contingencies,—such
as on the rate of subsidence occasionally allowing a fresh growth of
coral to cover the whole surface, and on the breakers having force suffi-
cient to throw fragments over this same space. The conglomerate which
composes the base of the islets, would (if not removed by denudation
together with the exterior reef on which it rests) be conspicuous from
the size of the fragments,—the different degrees in which they have
been rounded,—the presence of fragments of conglomerate torn up
rounded and re-cemented,—and from the oblique stratification. The
corals which lived in the lagoon-reefs at each successive level, would
-be preserved upright, and they would consist of many kinds, generally
much branched. In this part, however, a very large proportion of
the rock, and in some cases nearly all of it, would be formed of sedi-
mentary matter, being in an excessively fine or moderately coarse
state, with the particles almost blended together. The conglome-
rate which was formed of rounded pieces of the branched corals on
the shores of the lagoon, would differ from that formed on the islets
and derived from the outer coast; although both might have been ac- |
cumulated very near each other. The stratification, taken as a whole,
would be horizontal: but the conglomerate beds resting on the exterior
reef, and the beds of sandstone on the shores of the lagoon and on
the external flanks of the reef, would probably be divided (as at Keeling
atoll and at Mauritius) by numerous layers dipping at considerable angles
in different directions. The calcareous sandstone and coral rock would
almost necessarily contain innumerable shells, echini, and the bones of

154 THEORY OF THE FORMATION Cx, Vi

fish, turtle, and perhaps of birds: possibly, also, the bones of small -
saurians, as these animals find their way to islands far remote from
any continent. The large shells of some species of Tridacna would |
be found vertically imbedded in the solid rock, in the position in which :
they lived. We might expect, also, to find a mixture of the remains of
pelagic and littoral animals in the strata formed in the lagoon, for
pumice and the seeds of plants are floated from distant countries into |
the lagoons of many atolls; on the outer coast of Keeling atoll near 4
the mouth of the lagoon, the shell of a pelagic Pteropodous animal was
brought up on the arming of the sounding lead. All the loose blocks
of coral on Keeling atoll were burrowed by vermiform animals; and as
every cavity, no doubt, ultimately becomes filled with spathose lime-
stone, slabs of the rock would, if polished, probably exhibit the exca-
vations of such burrowing animals. The conglomerate and fine-grained
beds of coral-rock would be hard, sonorous, white, and composed of
nearly pure calcareous matter; in some few parts, judging from the
specimens at Keeling atoll, they would probably contain a small
quantity of iron. I have seen a conglomerate now forming on the
shores of the Maldiva atolls, resembling conglomerate limestone from
Devonshire. Floating pumice and scoriz, and occasionally stones trans-
ported in the roots of trees (see my Journal of Researches, p. 549)
appear the only sources through which foreign matter is brought to coral- —
formations standing in the open ocean. ‘The area over which sediment
is transported from coral-reefs must be considerable ; Captain Moresby
informs me that during the change of monsoons, the sea is discoloured
to a considerable distance off the Maldiva and Chagos atolls. The sedi-
ment off fringing and barrier coral-reefs must be mingled with the
mud which is brought down from the land, and is transported seaward.
through the breaches which occur in front of almost every valley. If
the bed of the ocean were to be upraised and converted into land, the
‘atolls of the larger archipelagoes would form flat-topped mountains,
varying in diameter from a few to sixty miles—for the smallest atolls
would probably be worn quite away; and from being horizontally
stratified and of similar composition, they would, as Sir C. Lyell has
remarked, falsely appear as if they had originally been united into one
vast continuous mass. Such great strata of coral-rock would rarely be
associated with erupted volcanic matter, for this could only take place,
as may be inferred from what follows in the next chapter, when the
area in which they were situated, commenced to rise, or at least
ceased to subside. During the enormous period necessary to effect an
elevation of the kind just alluded to, the surface would necessarily be
greatly denuded; hence it is highly improbable that any fringing-

ier-reef, at least those encircling small islands,
to a distant period. From this same cause, the

er-reefs, and which must consist in a large part of
atter, would more often be preserved to future ages, than

§ sok d reef go nese of massive corals in an upright posi-

156 ee ||

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— I

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 distri-
bution of the land—Areas of subsidence elongated—Their intersection
and alternation with those of elevation—Amount, and slow rate of the
requisite subsidence—Recapitulation.

It will be convenient first to give a short account of

the appended map of the Pacific and Indian Oceans ~

(Plate III.); a fuller one, with the data for colouring
each spot, is reserved for the Appendix, and every
place there referred to may be found in the Index.

A larger chart would have been desirable; but, small

as the adjoined one is, it is the result of many months’
labour. I have consulted, as far as I was able, every
original voyage and map; and the colours were first
laid down on charts on a large scale. The same blue

colour, ‘with merely a difference in the tint, is used
for atolls or lagoon-islands, and for barrier-reefs;

On. Vi. DISTRIBUTION OF CORAL-REEFS. 157
| these being in all essential respects closely related.
Fringing-reefs, on the other hand, have been coloured
dull red, for there is an important distinction between
them and barrier-reefs and atolls with respect to the
| depth beneath the surface, at which, as we must believe,
their foundations he. The two distinct colours, there-
fore, mark two great types of structure.

The dark blue colour represents atolls and sub-
merged annular reefs with deep water in their centres.
I have coloured a few low and small coral-islands
as if they had been atolls, although not including a
lagoon; but this has been done only when it clearly
appeared that they had originally contained one.
When no such evidence exists they have been left
uncoloured.

The pale blue colour represents barrier-reefs. The
most obvious character of reefs of this class is the
broad and deep-water moat within the reef; but this,
like the lagoon of a small atoll, is liable to become
| filled up with detritus and with reefs of delicately-
branched corals. When, therefore, a reef round the
entire circumference of an island extends far into a

profoundly deep sea, so that it can hardly be con-
founded with a fringing-reef which must rest on a
foundation of rock within a small depth, it has been

. coloured pale blue, although it does not now include
| a deep-water moat. But this has been rarely done,
and each case is distinctly mentioned in the Appendix.
The red colour represents reefs which fringe the
land closely where the sea is deep, and extend to

158 - DISTRIBUTION OF CORAL-REEFS. = Cz. VI.

a moderate distance from it where the bottom is
gently inclined; but they never include a deep-water
moat or lagoon-like channel running parallel to the
shore. It must, however, be remembered that fringing-
reefs are frequently breached by deep-water channels,
where mud has been deposited in front of rivers and
valleys. |

In all cases, a space of 30 miles in width has
been coloured round or in front of the reefs of each
class, in order that the colours might be made con-
spicuous in a map on so small a scale. iat

The vermilion spots and streaks represent vol-
canos now in action, or historically known to have
been so. They are laid down chiefly from Von Buch’s
work on the Canary Islands; and my reasons for —
making a few alterations are given in the note below.!

1I have also made considerable use of the geological part of Berg-
haus’ Physical Atlas. Beginning at the eastern side of the Pacific, 1
have added to the number of the voleanos in the southern part of the
Cordillera, and have coloured Juan Fernandez according to observations
collected during the voyage of the Beagle (Geol. Trans. vol. v. p. 601).
I have added a volcano to Albemarle Island, one of the Galapagos
Archipelago (see my Journal of Researches, p. 457). In the Sandwich
group there are no active volcanos, except at Hawaii; but the Rey. W.
Ellis informs me there are streams of lava apparently modern on
Maui, having a very recent appearance, which can be traced to the
craters whence they flowed. The same gentleman informs me that
there is no reason to believe that any active volcano exists in the
Society Archipelago; nor are there any known in the Samoa or Nayi-
gator group, although some of the streams of lava and craters there
-appear recent. In the Friendly group, the Rev. J. Williams says (Nar-
rative of Missionary Enterprise, p. 29) that Toofoa and Proby Islands
are active voleanos. I infer from Hamilton’s Voyage in the Pandora
(p. 95), that Proby Island is synonymous with Onouafou, but I have not
ventured to colour it. There can be no doubt respecting Toofoa; and

et Be OLE:

re =

—————
——- 2

| Cs. VI. DISTRIBUTION OF CORAL-REEFS. -159

| The uncolowred parts consist, first and chiefly, of
_ coasts where no coral-reefs, or quite insignificant ones,

| Captain Edwards (Von Buch, p. 386) found the lava of a recent erup-
tion at Amargura still smoking. Berghaus marks four active voleanos
actually within the Friendly group; but I do not know on what autho-
rity ; I may mention that Maurelle describes Latte as having a burnt- -up

_ appearance ; I have marked only Toofoa and Armagura. South of the
New Hebrides lies Matthews Rock, which is described as an active crater
in the voyage of the Astrolabe. Between it and the volcano on the
eastern side of New Zealand lies Brimstone Island, which from the
high temperature of the water in the crater may be ranked as active
(Berghaus Vorbemerk, IJ. Lief. S. 56). Malte Brun, vol. xii. p. 231,
says that there is a volcano near Port St. Vincent, in New Caledonia: I
believe this to be an error, arising from smoke seen on the opposite
coast by Cook (2nd voyage, vol. ii. p. 23), which smoke went out at
night. The Mariana Islands, especially the northern ones, contain
many craters (see Freycinet’s Hydrog. Descript.) which are not active.
Von Buch, however, states (p. 462) on the authority of La Peyrouse,
that there are no less than seven volcanos between these islands and
Japan. Gemelli Careri (Churchill’s Collect. vol. iv. p. 458) says there.
are two active volcanos in lat. 23° 30’ and in lat. 24°; but I have not
coloured them. From the statements in Beechey’s eq (p. 518, 4to
edit.) I have coloured one in the northern part of the Bonin group.
M.S. Julien has clearly made ont from Chinese manuscripts not very
ancient (Comptes Rendus, 1840, p. 832), that there are two active vol-
eanos on the eastern side of Formosa. Inthe map appended to the first
edition I marked an active volcano in Torres Straits, and gave my
authority; but Mr. Jukes informs me that there certainly is no volcano
there; a wooded island on fire having been mistaken for one. Mr.
MW Clelland (Report of Committee for Investigating Coal in India, p. 39)
has shown that the volcanic band which passes through Barren Island
must be extended northwards. It appears by an old chart, that Che-
duba was once an active volcano (see also Silliman’s North American
Journal, vol. xxxviiil. p. 385). In Berghaus’ Phys. Atlas, 1840 (No. 7
of Geological Part) a volcano on the coast of Pondicherry is said to
have burst forth in 1757. Ordinaire (Hist. Nat. des Volcans, p. 218) says
that there is one at the mouth of the Persian Gulf, but I have not

_ eoloured it, as he gives no particulars. A volcano in Amsterdam, or

St. Paul’s, in the southern part of the Indian Ocean, has been seen
(Naut. Mag. 1838, p. 842) in action. Dr. J. Allan, of Forres, informs
me in a letter, that when he was at Joanna, he saw flames at night,

160 DISTRIBUTION OF CORAL-REEFS. Cu. VIX |

exist. Secondly, of coasts where the sea is extremely
shallow; and the reefs in this case generally lie far

from the land, and are very irregular, so that they
cannot always be classed. Thirdly, reefs which appear

merely to coat submerged banks of rock or of sedi-

ment; for such reefs differ in some essential respects

from those which owe their whole thickness to the ,
growth of corals. Fourthly, in the Red Sea, and —

within some parts of the East Indian Archipelago

(if the imperfect charts of the latter can be trusted),
there are many scattered reefs of small size, repre- '
sented by mere dots, which rise out of deep water;
and these have likewise béen left uncoloured. In the |

Red Sea, however, some such reefs seem once to have

formed parts of a continuous barrier. There exist,

also, scattered in the open ocean, some linear and
irregularly-formed reefs which are probably, as shown

in the last chapter, remnants of atolls; but as they |

eannot safely be placed in this class, they have not
been coloured; they are, however, few in number, and
of insignificant dimensions. Lastly, some reefs have
been left uncoloured from the want of sufficient in-
formation; and some because they are intermediate
in character between barrier and fringing-reefs. The

value of the map is lessened, in proportion to the
number of reefs which I have thus been obliged to .

apparently volcanic, issuing from the Chief Comoro Island, and that the
Arabs assured him that they were volcanic, adding that the voleano
burnt more during the wet season: I have marked this as a volcano,
though with some hesitation, as the flames may have arisen from
gaseous sources,

Cz. VI. _ DISTRIBUTION OF CORAL-REEFS. 161

leave uncoloured; but their number is not very great,
as will be seen by comparing the map with the state-
ments in the Appendix. I have experienced more
difficulty in colouring fringing-reefs than in colouring
barrier-reefs, as the former, from their small size,
have not much attracted the attention of navigators.
As I have had to seek my information from all kinds
of sources, I do not venture to hope that the map is
free from errors. Nevertheless, I trust it will give
an approximately correct view of the general distri-
bution of the coral-reefs throughout the world, (with
the exception of some fringing-reefs on the coast of
Brazil, not included within the limits of the map,) and
of their arrangement into the three great classes which,
though necessarily ill-defined from. the nature of the
objects classified, have been adopted by most voyagers.
I may further remark, that the dark blue colour repre-
sents land entirely composed of coral-rock; the pale
blue, land with a wide and thick border of coral-rock ;
and the red, land with a mere narrow fringe of coral-rock.
Looking now at the map under a theoretical point

of view, the two blue tints signify that the foundations
of the reefs thus coloured have largely subsided, and
that the rate of subsidence has been less than the up-
ward growth of the corals. It is also probable that in
many cases the foundations are still subsiding. The red
signifies that the shores thus coloured support fringing-
reefs; and they have not, as a general rule, recently
subsided, at least to any considerable amount, for the
effects of subsidence on a small scale would hardly be

M

162 DISTRIBUTION OF CORAL-REEFS. Cu. VI. — ||

distinguishable. Such shores must either haveremained
stationary since the period when they were first fringed; |
or they may have been repeatedly upraised, with new
lines of reefs successively formed round them. If, bow-
ever, coral-reefs became attached for the first time to a
shore which was subsiding, or if a barrier-reef was de-
stroyed and submerged with a new reef re-attached
to the shore, this would necessarily belong at first
to the fringing class, and would be coloured red,
although the land was sinking. So it would be with
a subsiding shore, if it plunged at avery high angle
beneath the sea, for in this case the reef would remain +
closely attached to the land as it grew upwards, and |
would resemble in all respects a fringing-reef. This |
source of doubt applies especially to atolls which
have been upraised (such as Metia and Elizabeth
Islands), for from the steepness of their sub-marine
flanks, a reef growing up during a subsequent
period of subsidence round them, would still continue
closely to skirt the land, and would therefore be
coloured red. Well-characterised atolls or encircling
reefs, where several occur together in a group, or a
single barrier-reef if of large dimensions, clearly indicate
a movement of subsidence. The evidence from a single

atoll, or from a single encircling-reef, must be received

with caution, for the former may be based upon a sub-
merged crater or bank, and the latter on a submerged
margin of sediment or of worn-down rock.

On the distribution of the different classes of reefs.
—Having made the foregoing preliminary remarks, I

- (Gz, VI,

oo
oe

DISTRIBUTION OF CORAL-REEFS. 163

will now consider how far the distribution of the dif-
ferent kinds of coral-islands and reefs corroborates our

| _ theory. A glance at the map shows that the reefs

which are coloured blue and red, and which are believed
to owe their origin either to widely different move-
ments, or in the case of the red to a stationary condition,
are not indiscriminately mingled together. Atolls and
barrier-reefs, as may be seen by the two blue tints,
generally lie near each other; and this would be the
natural result of both having been produced by the same

movement of subsidence. Thus all the Society Islands

are encircled by barrier-reefs, and to the N.W. and
S.E. there are several scattered atolls. To the eastward
lies the great Paumotu or Low Archipelago consisting

entirely of atolls; and still further to the N.E., we meet

with the Mendana or Marquesas Islands, which, from
their abrupt and deeply indented shores, Dana! be-
lieves have probably subsided ; though hardly any coral-
reefs exist there, which might have afforded additional

evidence of subsidence. In the midst of the Caroline
atolls, there are three fine encircled islands. The
northern point of the barrier-reef of New Caledonia
apparently forms, as before remarked, a great atoll.
The Australian barrier is described as including both
atolls and small encircled islands. Captain King? men-
tions many atoll-formed and encircling coral-reefs,
some of which lie within the barrier, and others may be

1 Corals and Coral Islands, 1872, p. 325.
* Sailing Directions, appended to vol. ii. of his Surveying Voyage to
Australia.

m 2

164. DISTRIBUTION OF CORAL-REEFS. Cu. VI.

said (for instance between lat. 16° and 13°) to form
part of it. Flinders! has described an atoll-formed
reef in lat. 10°, seven miles long and from one to three
broad, resembling a boot in shape, and apparently in-
cluding a deep lagoon. Eight miles westward of this,
and forming part of the barrier, lie the Murray Islands,
which are high and are encircled. In the Corallian sea,
between the two great barrier-reefs of Australia and
New Caledonia, there are many low islets and coral-
reefs, some of which are annular, or like a horse-shoe.
Bearing in mind the smallness of the scale of our map

(the lines of latitude being 900 miles apart), we seethat |

none of the larger groups of reefs and islands which are
coloured blue, and which are supposed to have been
produced by long-continued subsidence, lie near exten-
sive lines of coast coloured red; these latter having
either long remained stationary, or having been upraised
with new reefs re-formed on them. Where red and blue
circles do occur near each other, I am able, in several
instances, to show that there have been oscillations of
level; subsidence having preceded the elevation of the
red spots; and elevation having preceded the subsidence
of the blue spots; and in this case the juxtaposition of
reefs belonging to the two great types of structure is

little surprising. We find, therefore, that atolls and

barrier-reefs, which both owe their origin to subsidence,
lie near together and are asa general rule separated
from fringing-reefs, which show that the land is sta-

tionary or rising; and all this holds good to the full {

1 Voyage to Terra Australis, vol. 11. p. 336.

Gu. VI. DISTRIBUTION OF CORAL-REEFS. 165

extent which might have been anticipated by our
theory.

As atolls have been formed during the sinking of
the land by the upward growth of the reefs which pri-
marily fringed the shores of ordinary islands; so we
might expect that these rings of coral, like so many
rude outline charts, would still retain traces of the
general form, or at least of the general range, of the
islands round which they were first modelled. That this
is the case with the atolls in the Southern Pacific, as
far as their range is concerned, seems highly probable,
when we observe that the principal groups are directed
in nearly N.W. and S.E. lines, and that nearly all the
mountainous islands and shores in the 8. Pacific range
in this same direction; namely, N. Eastern Australia,
New Caledonia, the northern half of New Zealand, the
New Hebrides, Saloman, Navigator, Society, Marquesas,
and Austral archipelagoes. In the Northern Pacific, the
Caroline atolls almost abut against the N.W. line of
the Marshall atolls, much in the same manner as the E
and W. line of islands extending from Ceram to New
Britain abuts against New Ireland. In the Indian
Ocean the Laccadive and Maldiva atolls extend nearly
parallel to the western mountains of India. There is
also a close resemblance between atolls and ordinary
islands in the manner in which they are grouped, as
well as in their shapes. Thus the outline of all the
larger groups of atolls is elongated; and the atolls
themselves are generally elongated in the same direc-~
tion with the group. The Chagos group is less elon-

166 DISTRIBUTION OF CORAL-REEFS. Cx. VI. |

gated than is usual, and the individual atolls in it are
likewise but little elongated ; this is strikingly seen by
comparing them with the neighbouring Maldiva atolls.
In the Marshall and Maldiva archipelagoes, the atolls
are ranged in two parallel lines, like a great double
mountain-chain. Some of the atolis in the larger
archipelagoes stand so near to each other, and have such
an evident relationship, that they compose little sub-—
groups ; in the Caroline Archipelago, one such sub-group
consists of Pouynipéte, a lofty island encircled by a
barrier-reef, and separated by a channel only four miles
and a half in width from Andeema atoll, with a second
atoll a little further removed.

On the direct evidence of the blue spaces vn the map
having subsided during the wpward growth of the
reefs thus coloured, and of the red spaces having
remained stationary, or having been wpraised.—With
respect to subsidence, we cannot expect to obtain in
semi-civilized countries proofs of a movement which
tends to conceal its own evidence. But on coral-islands
we see plain signs of a round of decay and renovation—
on some, the last vestiges of land—its first commence-
ment on others: we hear of storms washing away and
desolating the islets to an extent which astonished the
inhabitants ; we know by the great fissures with which —
some of these islands are traversed, and by the earth-
quakes felt under others, that subterranean disturbances
are in progress. All these appearances accord well with
the belief that these islands have recently subsided ;
though not proving the fact. At Keeling atoll, however, —

Cx. VI. DISTRIBUTION OF CORAL-REEFS. 167

I have described certain appearances, which seem
directly to show that the surface subsided there during
the late earthquakes. In the Caroline Archipelago,
the island of Pouynipéte (Plate I. fig. 7), from being en-
circled by a great barrier-reef, must have subsided, in
accordance with our theory; and in the New South
Wales Lit. Advert. Feb. 1835, there is an account of

this island, (subsequently confirmed by Mr. Campbell,)

in which it is said, ‘ At the N.E. end, at a place called
Tamen, there are ruins of a town, now only accessible
by boats, the waves reaching to the steps of the houses.’
Hence it would appear that the island must have sub-
sided since these houses were built. Mr. Hales also states,
from information acquired during the U. 8. Exploring
Expedition, that certain buildings on this island are
now in the water: ‘what were once paths are now
passages for canoes, and when the walls are broken down
the water enters the enclosures.”! Vanikoro, according
to the Chevalier Dillon, is often violently shaken by
earthquakes, and there, the unusual depth of the channel
between the shore and the reef, the wall-like structure
on the inner side of the reef, the small quantity of low
alluvial land at the foot of the mountains, and the almost
entire absence of islets on the reef, all seem to show
that this island has not remained long at its present
level.2 At the Society Archipelago, on the other hand,

1 Professor Dana also concludes from these facts that the island
subsiding ; see Corals and Coral Islands, 1872, p. 330.

2 See Captain Dillon’s Voyage in search of La Peyrouse. M. Cordier,
in his Report on the Voyage of the Astrolabe (vol. i. p. cxi.), speaking of
Vanikoro, says the shores are surrounded by reefs of madrepore, ‘ gwon
assure étre de formation tout-a-fait moderne.’.

168 DISTRIBUTION OF CORAL-REEFS. Cu. VIB

where a slight tremor is only rarely felt, the shoalness of |
the lagoon-channels round some of the islands, the |
number of islets formed on the reefs of others, and the
broad belt of low land at the foot of the mountains, all -
indicate that these islands have not undergone for a long
period, any movement of subsidence, although their
encircling reefs must on our theory have been originally
produced through subsidence.!

Although Dana admits that atolls and barrier-reefs
must have been originally formed by the subsidence of
their foundations, he believes that a large number of |
atolls, situated between the Paumotu or Low group to the —
east and the Feejees to the west, and northward nearly
as far as the equator, have recently been uplifted to the
height of a very few feet.2 Mr. Couthouy came to a
similar conclusion during the same expedition with
respect to many of the Paumotu atolls. These observers
ground their belief chiefly from having found the great
shells of the Tridacna vertically embedded in coral-rock,,
at a height at which they cannot now exist. Mr.
Couthouy also states that he found corals standing on
the shores and in the midst of the lagoons, from 12 to
30 inches above the sea-level, with the tips of their
branches dead. He also refers to masses of coral-rock |

1 Mr. Couthouy states (Remarks, p. 44), that at Tahiti and Eimeo
the space between the reef and the shore has been nearly filled up by |
the extension of coral-reefs of the kind which within most barrier-reefs
merely fringe the land. From this circumstance, he arrives at the same
conclusion as I have done, namely, that the Society Islands have
remained stationary during a long period.

? Corals and Coral Islands, 1872, pp. 199, 345. See also Mr. Cou-
thouy’s Remarks on Coral Formations, o

® zy. VI. DISTRIBUTION OF CORAL-REEFS. 169

which he thinks could not have been carried into their —
present positions and subsequently been water-worn,
whilst the land stood at its present level. Nevertheless
it might, I think, have been anticipated that many
atolls would have presented the above appearance, if
they had long remained at a stationary level. The sea,
after the land had at some former period subsided a few
feet, would have continued for a long time breaking
over the whole reef, even after the living corals had
grown up to their full height on the outer margin. The
waters of the lagoon would thus have been disturbed and
raised, so that shells and corals, from being bathed by
the troubled waters, could have existed at a greater
height than that at which they could exist after the

_ reef had been raised by the agglutination of fragments

and sand, and after islets had been formed on its sur-
face. Even the mere outward growth of a reef, and the
consequent increase of its breadth, by checking the
inward rush of the breakers, would tend to lower the
level in the lagoon at which corals and shells can live.

We have seen that at the Keeling Islands there are
fields of rotten coral with the tips of their branches pro-
jecting above the surface of the lagoon,—the result of
the tides not rising so high as formerly (as is said to be
the case by the inhabitants), from the closing of the
channels between the islets on the outer reef, and from
the lagoon being partially choked up by the growth
of the corals. Here, so far from there having been
any recent elevation of the land, we have reason to
believe that there has been subsidence. Messrs. Dana

170 DISTRIBUTION OF CORAL-REEFS. Cu. VI.

and Couthouy’s observations relate chiefly to the
Paumotu atolls, and here again some facts indicate
recent subsidence rather than elevation: I refer to the
manner in which Chain atoll suffered during a storm,
and to Sir E. Belcher’s statement,! that after an interval
of fourteen years, a well-known islet had disappeared,
and the lagoon at a particular spot had become deeper
than it was before. :

There are other causes of change which might, as it
appears to me, easily lead to a mistaken belief in the
recent elevation of low coral formations. We must re-
member that the outer and living margin of the reef
grows up to a height determined by the constant break-
ing of the waves. Outside this margin there is a sloping
surface also covered with living corals, but belonging to
species which do not grow to the surface; and beyond
this, there is a much steeper slope consisting of- coral-
sand. Now after a somewhat rapid subsidence of, for
instance, one or two fathoms, we may feel almost sure
that the corals on the outer margin would grow up
quickly to the surface and form a nearly vertical wall.
This would be succeeded outside by a steeply sloping
surface of living corals, which would likewise sooner or
later grow up in their former level; but outside this, —
the much steeper slope, formed by the slow accumula-
tion of fine detritus, would not recover for a very long
time its former angle relatively to the upper bank of
living corals. Now it seems highly probable that a

1 Voyage Round the World, vol. i. 1848, p. 382.

Cu. VI. DISTRIBUTION OF CORAL-REEFS. ayia

change of any kind in the outer submarine slope of an
| island would influence the height to which the living
corals on the margin would be constantly bathed by the
surf, and to which they would consequently be able to
grow. Again, it seems possible that if during one season
of the year the currents of the sea and the prevalent
winds coincided in direction, the waves would then reach
to a higher level and the corals grow higher, than at

another season when the currents and the winds did not

coincide in direction. The result would be that the
_ corals which during the one season had grown to their
| fall height, would at the other season expose their dead
- summits, and give the appearance of the land having
been slightly elevated. I have referred to these possi-
bilities merely to show how difficult it must ever be to
judge whether low coral formations have really been
raised to a height of only two or three feet, as Dana
believes to have been the case with several groups of
atolls. To me it seems more probable that all the above-
mentioned appearances merely indicate that the atolls
in question have long remained at the same level. If,
however, the conclusion arrived at by so excellent an
observer as Professor Dana, should hereafter be con-
firmed, the question will arise, seeing how immense an
area has been thus affected, whether those geologists are.
not right who believe that the level of the ocean is
subject to secular changes from astronomical causes.

172 DISTRIBUTION OF CORAL-REEFS. Cx. Via

Evidence that many coasts fringed with coral-reefs
and colowred red on the map, have been recently
elevated.—As the areas which have slowly subsided with-
in the period of existing corals are many and large,
we might have expected that such movements would
have been counterbalanced by the recent elevation of

other equally large areas; and this, as we shall see, |

apparently holds good. Corals attached to a rising
coast would necessarily form a fringing-reef; and this
reef would be upraised at each successive elevation, with

a new one formed on the coast at a lower level. Such |

reefs would differ only by their smaller breadth from
those attached to a shore which had long remained
stationary; for they would not have had sufficient time
to form a foundation of their own detritus and grow far

outwards. J ringing-reefs indicate as a general rule»

that the land to which they are attached has not re-
cently subsided. But they do not tell us whether
the land is rising or stationary. Nevertheless, the
crust of the earth seems liable to such incessant changes
of level that a long-continued stationary condition ap-
parently is rare. We may infer that this is so from

the number of cases, within the limits of our map, in ;

which upraised corals or other organic remains have
been found on the shores which are fringed with reefs,
and are, therefore, coloured red. It may be mentioned
as bearing on this subject, that I was much surprised

on first reading a memoir on coral formations by

it

ip

Gx. Vi. DISTRIBUTION OF CORAL-REEFS. 173
L MM. Quoy and Gaimard,! by finding that their descrip-
i tions applied only to reefs of the fringing class, for I
| knew that they had crossed both the Pacific and Indian
| Oceans ; but my surprise ended in satisfaction, when I
I discovered that all the islands which they had visited,
| though several in number—namely, Mauritius, Timor,
New Guinea, the Mariana and Sandwich Archipelagoes
-—_-could be shown by their own statements to have
been elevated within a recent geological period.

I will now enter on some details, showing how many of
the islands and coasts which from being fringed with reefs are
coloured red on our map, have been recently upraised.

Sandwich Islands.—Several of these islands are fringed
| with reefs, though Dana found very few corals at Hawaii ;
and almost every naturalist who has visited them has there
observed upraised corals and shells, apparently identical with
living species. The Rev. W. Ellis informs me that he
noticed round several parts of Hawaii, beds of coral detritus,
about twenty feet above the level of the sea, and where the
coast is low they extend far inland. Upraised coral-rock
forms a considerable part of the borders of Oahu; and at
Elizabeth Island? it composes three strata, each about ten
feet thick. Nihau, which forms the northern, as Hawaii does
the southern end of the group (350 miles in length), likewise
seems to consist of coral and volcanic rocks. Mr. Couthouy?
has lately described several upraised beaches and ancient reefs
with their surfaces perfectly preserved, as well as beds of
recent shells and corals, at the Islands of Maui, Morokai,

1 Annales des Sciences Nat. tom. vi. p. 279, &c.

2 Zoology of Captain Beechey’s Voyage, p. 176. See also MM.
Quoy and Gaimard in Annales des Sciences Nat. tom. vi. |

3 Remarks on Coral Formations, p. 51.

174 DISTRIBUTION OF CORAL-REEFS.. Cx. vi.
Oahu, and Tauai (or Kauai), all in this group. Mr. Pierce, |
an intelligent resident at Oahu, is convinced, from changes |
which have taken place within his memory during the last
sixteen years, ‘that the elevation is at present going forward
at a very perceptible rate.’ The natives at Kauai state
that the land is there gaining rapidly on the sea; and
Mr. Couthouy has no doubt, from the nature of the strata,
that this is the result of elevation.

Hlizabeth Island, in the southern part of the Low or
Paumotu Archipelago, and Metia in the northern part,
consist of upraised coral-rock, closely fringed by living reefs.!
In cases like these, where islands have the appearance which
one of the smaller surrounding atolls with a shallow lagoon
would present if elevated, we are led to conclude that the
elevation has taken place at an epoch not geologically re-
mote; for it 1s improbable that such small and low fabrics
should have resisted for an immense period all the many
destroying agents of nature. When the surface of an ordi-
nary island is strewed with marine remains, from the beach
to a certain height, and not above that height, it is exceed-
ingly improbable that these remains, although they may not |
have been specifically examined, should belong to any very
ancient period. It is necessary to bear these remarks in
mind, in considering the evidence of the elevatory move-
ments in the Pacific and Indian Oceans, as it does not often
rest on specific determinations, and therefore should be re-
ceived with caution. Six of the Cook and Austral Islands
(S.W. of the Society group) are fringed; of these, five
were described to me by the Rev. J. Williams, as formed of |
coral-rock (associated with some basalt in Mangaia), and the
sixth as lofty and basaltic. Mangaia is nearly 300 feet high

1 Beechey’s Voyage in the Pacific, p. 46,4to edit. Dana, Corals and
Coral Islands, p. 198, Wilkes, U. 8. Exploring Expedition, vol. i. p.
337.

Gu. VI, DISTRIBUTION OF CORAL-REEFS. 175

with a level summit; and, according to Mr. S. Wilson,! is an
upraised reef; ‘and there are in the central hollow, formerly

the bed of the lagoon, many scattered patches of coral-rock,

some of them raised to a height of forty feet.’ These knolls
_ of coral-rock were evidently once reefs within the lagoon of
an atoll. Mr, Martens, at Sydney, informed me that this
_ island is surrounded by a terrace-like plain at about the
height of 100 feet, which probably marks a pause in its
elevation. From these facts we may infer that the Cook and
Austral Islands have been upheaved at a not very remote
period.

Savage Island (S.E. of the Friendly group) is according
to Forster about 40 feet in height, and according to Williams
about 100 feet. Forster? describes the plants as already
growing out of the dead but still upright and spreading trees
of coral; and the younger Forster? believes that an ancient
lagoon is now represented by a central plain: here we cannot
doubt that the elevatory forces have recently acted. The
same conclusion may be extended to the islands of the
Friendly Group, which have been well described in the
second and third voyages of Cook, and recently by Dana.
The surface of Tongatabou is low and level, but with parts
50 or 60 feet high; the whole consists of coral-rock, ‘ which
yet shews the cavities and irregularities worn into it by the
action of the tides.4 On Houa the same appearances were
noticed at an elevation of between 200 and 800 feet. Vavao,
also, at the opposite or northern end of the group, consists,
according to the Rev. J. Williams, of coral-rock. Tonya-
tabou, with its northern extensive reefs, resembles either an
upraised atoll with one half originally imperfect, or one un-
equably elevated; and Anamouka, an atoll equably elevated,

1 Couthouy’s Remarks, p. 34.

? Observations made during Voyage Round the World, p. 147.
3 Voyage, vol. ii. p. 163. ;

4 Cook’s Third Voyage (4to edit.), vol. i. p. 314.

176 DISTRIBUTION OF CORAL-REEFS. Cu. VI.

This latter island contains! in its centre a salt-water lake, ; {
about a mile and a half in diameter, without any communi. |
cation with the sea, and around it the land rises gradually ‘
like a bank: the highest part is only between twenty and

thirty feet; but on this part, as well as on the rest of the

Jand, (which, as Cook observes, rises above the height of a — !

true lagoon-island,) coral-rock like that on the beach was
found. In the Navigator or Samoan Archipelago, Mr.

Couthouy? found on Manua many large fragments of coral at |

the height of eighty feet, ‘on a steep hill-side, rising half a
mile inland from a low sandy plain abounding in marine
remains. The fragments were embedded in a mixture of
‘decomposed lava and sand. It is not stated whether they

were accompanied by shells, or whether the corals resembled

recent species; as these remains were embedded, they
possibly may belong to a remote epoch; but I presume
this was not the opinion of Mr. Couthouy. On the other

hand, Mr. Dana says expressly in one place, that ‘no

satisfactory evidences of elevation were detected about these

islands ;’ and in another place he says (p. 326) that some of ©

the islands have probably subsided. THarthquakes are very
frequent in this archipelago.

Still proceeding westward we come to the New Hebrides.
On these islands, Mr. G. Bennett (author of Wanderings

in New South Wales) informs me that he found much coral |
ata great altitude, which he considered of recent origin. The

Loyalty Islands are situated west of the New Hebrides, and

not far from New Caledonia; and one of these islands has —
been clearly shown by the Rev. W. B. Clarke (Journal of

Geolog. Soc. 1847, p. 61) to consist wholly of coral-rock, and
to have been raised within a recent period by at least two
’ distinct elevations to the height ‘of 250 feet. The shores are
now fringed by reefs. Respecting Santa Cruz and the

1 Cook’s Third Voyage (4to edit.), vol. i. p. ey
2 Remarks on Coral Formations, p. 50.

Pe ine? of ate

Ss

‘ per “ 8

Cx. VI.

DISTRIBUTION OF CORAL-REEFS. 177

Saloman Archipelago, I have no information; but at New
Treland, which forms the northern point of the latter chain,
both Labillardiere and Lesson have described large beds of
an apparently very modern madreporitic rock, with the form
‘of the corals little altered. The latter author! states that this
formation composes a newer line of coast, modelled round
an ancient one. ‘There only remains to be described in the
Pacific, that curved line of fringed islands, of which the
Marianas form the main part. Of these Guam, Rota, Tinian,
Saypan, and some islets farther north, are described by Quoy
and Gaimard,” and Chamisso,? as chiefly composed of madre-
poritic limestone, which attains a considerable elevation, and
is in several cases worn into successively rising cliffs: the two
former naturalists seem to have compared the corals and shells
with the existing ones, and state that they are of recent species..
Peel Island, one of the Bonin or Arzobispo group, between
the Marianas and Japan, has fringing-reefs; and it has clearly
been upraised to a height of at least 50 feet, as shown by the
ridges of corals and shells extending uniformly at this level.‘
Fais, which lies in the prolonged line of the Marianas,
between this group and the Pellews, is fringed by reefs; it
is 90 feet high, and consists entirely of madreporitic rock.®

In the Last Indian Archipelago, many authors have re-
corded proofs of recent elevation. M. Lesson® states that
near Port Dory, on the north coast of New Guinea, the shores
are flanked, to the height of 150 feet, by madreporitic strata
of a modern date. He mentions similar formations at Waigiou,
Amboina, Bourou, Ceram, Sonda, and Timor: at this latter place,
MM. Quoy and Gaimard’ have likewise described the primi-

1 Voyage dela Coquille, Part. Zoolog.

* Freycinet’s Voyage autourdu Monde. See also the Hydrographical
Memoir, p. 215. 3 Kotzebue’s First Voyage.

4P. W. Graves, Journal of Geological Soc. 1855, p. 532.

5 Lutké’s Voyage, vol. ii. p. 304.

6 Partie Zoolog. Voyage de la Coquille.

7 Ann. des Scien. Nat., tom. vi. p. 281.

N

178 DISTRIBUTION OF CORAL-REEFS. Cz. VI.

tive rocks, as coated to a considerable height with coral.
Some small islets eastward of Timor are said in Kolff’s Voyage!
to resemble small coral islets upraised some feet above the sea.
Dr. Macolmson informs me that Dr. Hardie found in Javaan
extensive formation, containing an abundance of shells, of
which the greater part appear to be of existing species. Dr.
Jack? has described some upraised shells and corals, apparently _
recent, on Pulo Nias off Sumatra; and Marsden relates in his _
history of this great island, that the names of many promon-
tories show that they were originally islands. On part.of the —
west coast of Borneo and at the Sooloo Islands, the form of the
land, the nature of the soil, and the water-washed rocks,
present appearances? (although it is doubtful whether such
vague evidence is worthy of mention) of having recently been __
covered by the sea; and the inhabitants of the Sooloo Islands
believe that this has been the case. Mr. Cuming, who has
lately investigated with so much success the mollusca of the
Phillippines, found near Cabagan, in Luzon, about 50 feet
above the Isvel of the R. Cagayan and 70 miles from its
mouth, a large bed of fossil shells: these, ashe informs me, _ .
are certainly of the same species with those now existing on
the shores of the neighbouring islands. From the accounts
given by Captain Basil Hall and Captain Beechey?* of the lines

1 Translated by Windsor Harl, chaps. vi. and vii.

* Geolog. Transact. 2nd series, vol. 1. p. 408. On the Peninsula of
Malacea, in front of Penang, 5° 30’ N., Dr. Ward collected some shells
which Dr. Malcolmson informs me, although not compared with exist-
ing species, had a recent appearance. Dr. ,Ward describes in this —
neighbourhood (Trans. Asiat. Soc. vol. xviil., part 2, p. 166) asingle
water-worn rock, with a conglomerate of sea-shells at its base, situated
six miles inJand, which, according to the traditions of the natives, was
once surrounded by the sea. Captain Low has also described (ibid. Part i.
p. 181) mounds of shells lying two miles inland on this line of coast.

8 Notices of the East Indian Arch., Singapore, 1828, fp. 6, and
Append. p. 43. a

4 Captain B. Hall, Voyage to Loo Choo, Append. pp. xxi. and xxy.
Captain Beechey’s Voyage, p. 496.

j if

Cx. VIL DISTRIBUTION OF CORAL-REEFS. 179

of inland reefs, and walls of coral rock worn into caves, above

- the present reach of the waves, at the Loo Choo Islands, there

ean be little doubt that they have been upraised at no very
remote period. |

Dr. Davy! describes the northern province of Ceylon as
being very low, and composed of a limestone with shells and
corals of very recent origin ; he adds, that it does not admit of
a doubt that the sea has retired from this district even within
the memory of man. There is also some reason for believing
that the eastern shores of India, north of Ceylon, have been
upraised within the recent period.” On the opposite side of
the Gulf of Bengal, Captain Halstead everywhere found during
his survey of the Burmese coast (as he informed Sir C. Lyell),
proofs of recent elevation in upraised beaches and beds of
shells and corals. In the Indian Ocean Mauritius has been
recently upraised, as I have shown in the chapter on fringing-
reefs. The northern extremity of Madagascar is described by
Captain Owen? as formed of madreporitic rock, as likewise are
the shores and outlying islands along an immense space of
Eastern Africa, from a little north of the equator for 900
miles southward. Nothing can be more vague than the ex-

1 Travels in Ceylon, p. 138. This madreporitic formation is men-
tioned by M. Cordier in his report to the Institute (May 4, 1839) on
the voyage of the Cheuretie, as one of immense extent, and belonging to
the latest tertiary period.

2 Dr. Benza, in his Journey through the N. Circars (the Madras Lit.
and Scient. Journal, vol. v.), has described a formation with recent
freshwater and marine shells, occurring at the distance of three or four
miles from the present shore. Dr. Benza, in conversation with me,
attributed their position to a rise of the land. Dr. Malcolmson, how-
ever, (and there cannot be a higher authority on the geology of India,)

_ informs me that he suspects that these beds may have been formed by

the mere action of the waves and currents accumulating sediment. From
analogy I should much incline to Dr. Benza’s opinion.
$ Owen’s Africa, vol. 11. p. 37, for Madagascar ; and forS. Africa, vol. i.

| pp. 412 and 426. Lieut. Boteler’s narrative contains fuller particulars.
|| regarding the coral rock, vol. i. p. 174, and vol. ii. pp. 41 and 54. See
|| also Ruschenberger’s Voyage round the World, vol. i. p. 60.

N 2

PSOie DISTRIBUTION OF CORAL~REEFS.

pression ‘madreporitic rock;’ but at the same time it is,I —
think, scarcely possible to look at the chart of the linear islets ©

running in front of the coast from the equator far southward,
and rising to a greater height than can be accounted for by
the growth of coral, without feeling convinced that a line of fring-
ing-reefs has been elevated at a period so recent, that no great
changes have since taken place on the surface of this part ot

the globe. Some, also, of the higher islands of madreporitic

rock on this coast, for instance Pemba, are singularly shaped,
apparently showing the combined effect of the growth of coral
on submerged banks, together with their subsequent upheaval.
Dr. Allan informs me that he never observed any elevated

organic remains on the Seychelles, which come under our

fringed class.
The nature of the formations round the shores of the Red

Sea, as described by several authors, proves that the whole of —

this large area has been elevated within a very recent tertiary
epoch. A part of this space in the appended map is coloured
blue, indicating the presence of barrier-reefs ; on which cir-
cumstance I shall presently make some remarks. Ruppell!
states that the tertiary formation, of which he has examined
the organic remains, forms a fringe along the shores with a
uniform height of from 30 to 40 feet, from the mouth of the

Gulf of Suez to about lat. 26°; but that south of 26°, the

beds attain only the height of from 12 to 15 feet. This, how-
ever, can hardly be quite accurate; although possibly there
may be a decrease in the elevation of the shores in the middle
parts of the Red Sea, for Dr. Malcolmson informs me that he

collected shells and corals, apparently recent, from the cliffs —

of Camaran Island (lat. 15° 30'S.) at a height of between 30
to 40 feet; and Mr. Salt (Travels in Abyssinia) describes a
similar formation a little southward on the opposite shore at
Amphila. Moreover, near the mouth of the Gulf of Suez,
although on the coast opposite to that on which Dr. Riippell

1 Rippell, Reise in Abyssinien, Band i. s. 141.

Cu. vie I

Cu. VI. ‘ DISTRIBUTION OF CORAL-REEFS. 181]
a
iY says that the modern beds attain a height of only 30 to
AO feet, Mr. Burton! found a deposit replete with existing
_. species of shells, at the height of 200 feet. In an admirable
_ series of drawings by Captain Moresby, I could see how continu-
ously the cliff-bounded, low, tertiary plains extended with a
nearly equable height, both on the eastern and western shores.
The southern coast of Arabia seems to have been subjected to
the same elevatory movement, for Dr. Malcolmson found at
_ Sahar low cliffs containing shells and corals apparently of
recent species.

The Persian Gulf abounds with coral-reefs; but as in
this shallow sea it is difficult to distinguish reefs from sand-
banks, I have coloured only some near the mouth. Towards
the head of the gulf Mr. Ainsworth? says that the land is
worn into terraces, and that the strata contain organic remains
of existing forms.

' The West Indian Archipelago of ‘fringed islands’ alone
_ remains to be mentioned: evidence of an elevation within a
late tertiary epoch of nearly the whole of this great area, may
be found in the works of almost all the geologists who have
visited it.. I will give some of the principal references in a
note.?
_ * Lyell’s Principles of Geology, 5th edition, vol. iv. p. 25.
2 Ainsworth’s Assyria and Babylon, p. 217.

$ These references only relate to works published before 1842, the
_ date of the first edition of this book. On Florida and the north shores
of the Gulf of Mexico, Rogers’ Report to Brit. Assoc. vol. ili. p. 14.—
On the shores of Mexico, Humboldt, Polit. Essay on New Spain, vol. 1.
p. 62. (I have.also some corroborative facts with respect to the shores
of Mexico.)—Honduras and the Antilles, Lyell’s Principles, 5th ed.
vol. iv. p. 22.—Santa Cruz and Barbadoes, Prof. Hovey, Silliman’s Journ.
vol. xxxy. p. 74.—St. Domingo, Courrojolles Jour. de Phys. tom. liv.
p- 106.—Bahamas United Service Journ. No. lxxi. pp. 218 and 224.—
Jamaica, De la Beche, Geol. Man. p. 142.—Cuba, Taylor in Lond. and
Edin. Phil. Mag. vol. xi.p.17. Dr. Daubeney also at a meeting of the
Geolog. Soc. orally described some very moderna beds lying on the N.W.
parts of Cuba. I might have added many other less important

_ references.

182 DISTRIBUTION OF CORAL-REEFS. Ca. VI: @

On reviewing the above details it is impossible not
to be struck with the number of cases in which upraised

organic remains, apparently belonging to the recent —

period, have been found on the shores now fringed by
reefs, and which are coloured red onour map. It may,
however, be thought that similar proofs of elevation
could be found on the coasts coloured blue, and which
we have good reason to believe have recently subsided ;
but such proofs cannot be found, with the few follow-
ing and doubtful exceptions. :

The entire area of the Red Sea appears to have been
upraised within a late tertiary period; nevertheless I

have been compelled, though on unsatisfactory evidence

(given in the Appendix), to class the reefs in the middle

part of the coast, not as fringing, but as barrier-reefs.

y <* rh

If, however, the statements should prove accurate re- —

specting the less height of the tertiary beds in the
middle, compared with the northern and southern
districts, we might well suspect that the former had
‘subsided subsequently to a general elevation by which
the whole area had previously been upraised. Several
authors’ have observed shells and corals high up on the

1 Ellis, in his Polynesian Researches, was the first to call attention

to these remains (vol. i. p. 88), and the tradition of the natives concern-

ing them. See also Williams, Nar. of Miss. Enterprise, p. 21; also
Tyerman and G. Bennett, Journ. of Voyage, vol. i, p. 218; also Mr.
Couthouy’s Remarks, p. 51; but his principal fact, namely, that there
is a mass of upraised coral on the narrow peninsula of Tiarubu, is from
hearsay evidence ; also Mr. Stutchbury, West of England Journ. No. 1,
p. 54. There is a passage in Von Zach, Corres. Astronom. vol. x.
p. 266, inferring an uprising at Tahiti, from a footpath now used, which
was formerly impassable: but I particularly enquired from several
native chiefs, whether they knew of any change of this kind, and they
were unanimous in giving me an answer in the negative.

ee, nS st aa

: Cu. VI. DISTRIBUTION OF CORAL-REEFS. 183

mountains of the Society Islands,—a group of islands

encircled by barrier-reefs, and which, therefore, must

have recently subsided. ‘Thus at Tahiti, Mr. Stutchbury

found. on the summit of one of the highest mountains,
between 5,000 and 7,000 feet above the level of the sea,
‘a distinct and regular stratum of semi-fossil coral ;’
but we cannot infer from such evidence as this that the
island has been elevated within the recent period; and
on the other hand, several naturalists, including Mr.

Dana and myself, have in vain searched near the coast
for upraised shells and corals, where if present they

could not have been overlooked. Two of the Harvey
Islands, namely, Aitutaki and Manouai, are formed of
upraised coral rocks, and have probably been elevated
within a recent period; nevertheless they are encircled

by reefs extending so far from the land, that I have

coloured them blue, though with much hesitation, as
the space within the reef is shallow, and the encircled
land is not abrupt. If these reefs really belong to the
barrier class, we have here another instance of subsi-
dence having followed elevation, both movements having
been effected apparently within the recent period. There
are also many cases of coral-formations, such as Elizabeth
Island, Metia, Mangaia, several of the Friendly and one of
the Loyalty Islands, which it can hardly be doubted once
existed as atolls, and were originally formed during sub-
sidence, but have since been elevated, and are now sur-

rounded. by fringing-reefs. We have, however, no

reason to feel surprise at occasional or even frequent
alternations of level of the above two kinds.

/

184 DISTRIBUTION OF CORAL-REEFS. Cx. VI BY

On the absence of active Volcanos im the areas of
subsidence, and on their frequent presence in the areas
of elevation.'—The absence of active volcanos through-
out the great areas of subsidence on our map, as
shown by the pale and dark blue tints,—namely, in
the central parts of the Indian Ocean, in the China
Sea, in the sea between the barriers of Australia and
New Caledonia, in the Caroline, Marshall, Gilbert, and
Low Archipelagoes,—is a very striking fact. So is the
presence of active volcanic vents and chains on or near
many of the shores coloured red on our map, and which
are fringed with reefs; for, as we have just seen, these
fringed coasts have been recently upheaved in a large
number of cases. Active volcanos likewise coincide with
proofs of recent elevation on or near several other
long lines of coast within the limits of our map,
where there are no reefs of living corals, and which
consequently are not coloured red. It must be here
remarked with regard to the proofs of both subsidence
and elevation, that I do not judge by the absence or
presence or nature of the coral-reefs round the volcanos
themselves; for, as Dana repeatedly insists, the corals
may have been there destroyed or injured by the heat
or exhalations. Nor have I taken into account the
presence of upraised organic remains on the flanks of
the voleanos themselves. I judge from the position of

‘It may be well here to state that all the reefs on the map were
coloured either red or blue before the vermilion spots and streaks,
showing the position of the active volcanos and volcanic chains, were

added ; and indeed before I knew of the existence of several of them. » e

zt
a

is

See. VI. “DISTRIBUTION OF CORAL-REEFS. 185

“the active volcanic vents in relation to neighbouring
islands and coasts, situated at too great a distance for

any corals growing there to be injured by the eruptions ;
and where, from the presence of atoll-formed or barrier-

reefs, or of upraised marine remains, we have reason

to believe. that either subsidence or elevation has
occurred within a recent period.

The following cases offer a few partial exceptions to
the rule that active volcanos are situated at a distance
from the areas of subsidence. The Great Comoro
Island probably contains a volcano, and it is only
twenty miles distant from the barrier-reef of Mohila.
Ambil volcano, in the Phillippine Archipelago, is dis-
tant only a little more than sixty miles from the atoll-
formed Appoo reef: and there are two other volcanos
on the map within ninety miles of circles coloured
blue. But there is not a single active volcano within
several hundred miles of a group, ever a small group, of
atolls ; and it is clear that a group of atolls, surmount-
ing a number of islands now all sunk beneath the level
of the sea, implies a much greater amount of subsi-
dence, than does a single atoll or a single encircling
barrier-reef. It isa striking fact that two volcanos are
known to have been in recent action in the Friendly
Archipelago; and the islands have here been formed
by the recent elevation of a group of atolls. Again,
extinct craters and well-preserved streams of lava occur
on many of the encircled islands in the Pacific, and
these by our theory have subsided at no very remote
period ; but although thus plainly formed of volcanic

186 DISTRIBUTION OF CORAL-REEFS. Cu. VEN 3

matter, they do not offer a single active volcano. In

these cases the voleanos seem to have come into action
or to have been extinguished, in accordance with the
latest movements of elevation or subsidence.

Within the limits of our map, active volcanos occur —

on or near other coasts besides those which are fringed

with reefs and coloured red; and some of these coasts

are known to have been upraised within the recent
period. Thus I have shown in my Geological Observa-
tions on 8. America (1846) that the whole western shore
of this great continent, for a space of between 2,000 and

3,000 miles south of the equator, has undergone an up- —
ward movement during the period of existing marine —

shells; and the Andes here form the grandest volcanic
chain in the world. The islands on the north-western
side of the Pacific, forming the second grandest volcanic
chain, are very imperfectly known; but Luzon, in the
Phillippines, and the Loo Choo islands, have been re-
cently elevated; and at Kamtschatka! there are exten-

sive tertiary beds of modern date. The co-existence

in other parts of the world, of active volcanos with

upraised beds of a modern origin, will occur to every

geologist. Nevertheless, until it could be shown that
volcanos were absent or inactive in subsiding areas, the
conclusion that their distribution depended on the

nature of the subterranean movements in progress, ~

would have been hazardous. But now, viewing the

1 Namely, at Sedanka, in lat. 58° N. (Von Buch’s Descript. des Isles

Canaries, p. 455).

.
re

Cu. VI.

DISTRIBUTION OF CORAL-REEFS. 187

appended map, it may, I think, be considered as
almost established, that volcanos are often present in

the areas which have lately risen or are still rising,
and are invariably absent in those which have lately
subsided or are still subsiding; and this, I think, is
the most important generalisation to which the study

of coral-reefs has indirectly led me.!

On the dimensions and relative positions of the
subsiding areas on our map, as indicated by the pre-
sence of atolls and barrier-reefs ; and of the rising or
stationary areas, as known by upraised organic re-
mais, or inferred from the presence of fringing-reefs.
—The immense surfaces seen on the map, which ac-
cording to our theory, or from the plain evidence of
upraised remains, have undergone either a downwards

or upwards change of level within a geologically late

period, is a highly remarkable fact. The existence of
continents shows that the areas which have been up-
raised are immense. With respect to South America
we may feel sure, and with respect to the western shores
of the Indian Ocean we have reason to suspect, that
this rising is either now actually in progress, or has
taken place quite recently. By our theory, it may
safely be inferred that the areas which have lately sub-
sided are likewise immense; or, judging from the earth-
quakes now occasionally felt there, and from other ap-

1 We may infer from this rule, that at any place where an old formation
contains interstratified beds of erupted matter, the surface of the land
or the bed of the sea, formed, at the period of eruption, a rising, at
least not a subsiding area.

pearances, are still subsiding. The smallness of the
scale of our map should not be overlooked; each square
on it containing 810,000 square miles. If we take the
space of ocean from near the southern end of the Low
Archipelago to the northern end of the Marshall Archi-
pelago,—a length of 4,500 miles, we see that, as far as
known, every island, excepting Metia, is atoll-formed.
The eastern and western boundaries of our map are

continents, and they are rising: the central areas of

the great Indian and Pacific Oceans, are mostly sub-

188 DISTRIBUTION OF CORAL-REEFS. Cu, Vi, |

> 7
7

siding; between them, north of Australia, lies the most |

broken land on the globe, and there the rising parts
are surrounded and penetrated by areas of subsidence ;?
so that the prevailing movements now in progress, seem

to accord with the present state of the great terrestrial —

and oceanic divisions of the world.

The blue spaces on the map are nearly all elongated;
such as the great north and south line of atolls in the
Indian Ocean, the space between the barrier-reefs of
Australia and New Caledonia, the Caroline Archi-
pelago, &c. Whether adjoining elongated spaces, run-
ning in different directions, have subsided by one com-
mon movement, or independently of each other, we do
not know. In the case of the Caroline and Marshall
Archipelagos, situated near each other, but extending
in different directions, it seems probable that they have

1 I suspect that the Arru and Timor-laut Islands present an included
small area of subsidence, like that of the China Sea; but I have not
ventured to colour them blue, owing to the want of sufficient informa-
tion. See Appendix.

ig
a
i x »
; Cua. Wi.
fe

‘

DISTRIBUTION OF CORAL-REEFS. 189

: Babsided independently of each other; for the McAskill
—Islands,! lying towards the eastern end of the Caroline

Archipelago, are. formed of upraised coral-rock ; and we
thus see that the above two areas of subsidence have
been at one time interrupted by an area of upheaval.
The curved line of elevation formed by the Mariana
Islands, seems to cross a former line of subsidence pro-
longed from the Caroline Archipelago; for the island
of Fais, apparently an upraised atoll, is situated nearly
at the point of intersection of the two lines. The
Sandwich Archipelago is 530 miles in length, from
Hawaii to the westernmost rocky islet, but is pro-
longed by numerous reefs to a point 2,000 miles dis-
tant from Hawaii. The south-eastern end of this long
line is one of elevation and of volcanic activity ;
whereas the north-western end, judging from the
structure of the reefs, though these are imperfectly
known, is one of subsidence.” So that here we ap-
parently have opposite movements in progress towards
the two extremities of the same long line. The com-
Mmonest case seems to be a tendency to alternation
between the areas of subsidence and elevation, as if
the sinking of one had counterbalanced the rising of

another.

The existence in many parts of the world of lofty

_table-land, proves that large surfaces have been upraised

in mass to a great height above the level of the sea;
although in almost every country the highest points

1 Dana, Corals and Coral Islands, p. 306.
2 Dana, Corals and Coral Islands, p. 807, 355. See alsomy Appendix.

190 DISTRIBUTION OF CORAL-REEFS. Cu. Vi" |

consist of upturned strata, or of erupted matter: and
from the wide spaces over which atolls are scattered,
although not one pinnacle of land now remains above
the level of the sea, we may conclude that immense
areas have subsided to an amount sufficient to bury not

only any formerly existing lofty table-land, but even the ©

heights formed by fractured strata and erupted matter.
The effects left on the land by the later elevatory

movements, namely, successively rising cliffs, succes- .

sive lines of erosion, and great beds of shells and
pebbles, all requiring time for their production, prove
that these movements have been extremely slow. And
with respect to the whole amount of subsidence neces-
sary to have produced the many atolls widely scattered
over immense spaces, the movement, as already shown,
must either have been uniform and exceedingly slow,
or effected by small steps separated from each other by
long intervals of time, so as to have allowed the reef-con-
structing polypifers to bring up their solid frame-works
to the surface; and this is one of the most interesting
conclusions to which we are led by the study of coral-
formations. We have little means of judging whether
many considerable oscillations of level have usually
occurred during the elevation of large areas; but we
know from clear geological evidence, such as trees still
standing upright at successive levels and covered by
marine strata, that this has frequently been the case;
and we have seen on our map, that some of the same

islands after having subsided, have been upraised ; and —

that others after having been uplifted, have subsided.

2 eae See

RECAPITULATION. 191

We may therefore conclude that the subterranean

changes which cause some areas to rise and others to
sink, have generally acted in a closely similar manner.

_ Recapitulation.—In the three first chapters, the
principal kinds of coral-reefs were described in detail,
and they were found to differ little, as far as relates to
the actual surface of the reef. An atoll differs from
an encircling barrier-reef only in the absence of land
within its central expanse; and a barrier-reef differs
from a fringing-reef only in being placed, relatively
to the probable inclination of its submarine foundation,
at a much greater distance from the land, and in
the presence of a deep lagoon-hke space within the
reef. In the fourth chapter the growing powers of the
reef-constructing polypifers were discussed ; and it was
shown that they cannot flourish beneath a very limited
depth. In accordance with this limit, there is no diffi-
eulty respecting the foundation on which a fringing-
reef is based ; whereas, with barrier-reefs and atolls,
there is the greatest difficulty on this head;—in bar-
rier-reefs from the improbability of rock or banks of
sediment having extended, in every instance, so far
seaward within the required depth;—and in atolls,
from the immensity of the spaces over which they are
interspersed, and the apparent necessity for believing
that they are all based on mountain-summits, which,
although rising very near to the surface of the sea, in
no one instance rise above it. To escape this latter
admission, which implies the existence of submarine

192 RECAPITULATION. Cat VI

chains of mountains of almost exactly the same height q |
extending over many thousand square miles, there is ; |

but one alternative ; namely, the prolonged subsidence |

of the foundations on which the atolls first became i

attached, together with the upward growth of the

reef-constructing corals. On this view every difficulty
vanishes: fringing-reefs are thus easily converted into |

barrier-reefs ; and barrier-reefs into atolls, as soon as

the last pinnacle of land sinks beneath the surface of :

the sea.
The wall-like structure on the inner sides of atolls
and barrier-reefs—the bason or ring-like shape of the

marginal and central reefs in the Maldiva atolls—the |

union of some atolls as if by a ribbon—the apparent
disseverment of others—the ordinary outline of groups —
of atolls and their forms—are all thus explained. We
thus understand the occurrence in both atolls and
barrier-reefs of portions, or of the whole, in a dead and
submerged condition, though still retaining the outline
of a living reef. The existence of breaches through
barrier-reefs in front of valleys, though separated from
them by wide spaces of deep water, can be simiiarly |
explained. It confirms our theory that we find the
two kinds of reefs formed through subsidence generally
situated near each other and at a distance from the spaces —
where fringing-reefs abound. On searching for other

evidence of the movements assumed by the theory, —

we find marks of change in atolls and in barrier-reefs,
and of subterranean disturbances beneath them; but
from the nature of things, it is scarcely possible to

Pex Vi. - RECAPITULATION. 193

find direct proofs of subsidence, although some appear-

anees are strongly in favour of it. On the fringed
coasts, however, the frequent presence of upraised
marine remains belonging to a recent epoch, plainly
shows that these coasts have been lately elevated.
Finally, when the two great types of structure,
namely barrier-reefs and atolls on the one hand,
and fringing-reefs on the other, are laid down on a
map, they offer a grand and harmonious picture of
the movements which the crust of the earth has
undergone within a late period. We there see vast
areas rising, with volcanic matter every now and then
bursting forth. We see other wide spaces sinking with-
out any volcanic outbursts; and we may feel sure that
the movement has been so slow as to have allowed the
corals to grow up to the surface, and so widely extended
as to have buried over the broad face of the ocean

_ every one of those mountains, above which the atolls

now stand like monuments, marking the place of their
burial.

APPENDIX. |

197

APPENDIX:

CONTAINING

A DETAILED DESCRIPTION OF THE REEFS AND ISLANDS
IN THE COLOURED MAP, Ptatz II.

Iy the beginning of the last chapter I stated the principles on
which the map has been coloured. There only remains to be
said, that it is an exact copy of one by M. C. Gressier, pub-
lished by the Dépot Général de la Marine, in 1835. The
names have been altered into English, and the longitude has
been reduced to that of Greenwich. The colours were first
laid down on accurate charts, on a large scale. The data, on
which the volcanos historically known to have been in action,
have been marked with vermilion, were given in a note to the
last chapter. I will commence my description on the eastern
side of the map, and will describe each group of islands con-
secutively, proceeding westward across the Pacific and Indian
Oceans, and ending with the West Indies.

The WESTERN SHORES OF AMERICA appear to be entirely
without coral-reefs: south of the equator the survey of the
Beagie, and north of it the published charts show that this is
the case. Even in the Bay of Panama, where corals flourish,
there are no true coral-reefs, as I have been informed by
Mr. Lloyd. There are no coral-reefs in the Galapagos archi-
pelago, as I know from personal inspection; and I believe
there are none on the Cocos, Revilla-gigedo, and other neigh-
bouring islands. Clipperton rock, 10° N., 109° W., from a
drawing appended toa MS. plan in the Admiralty, does not

198 APPENDIX.

appear to be an atoll, but Sir H. Belcher (Voyage round the
World, vol. i. 1843, p. 255) speaks of it as of coral-forma-
tion, with deep water within the lagoon; left uncoloured.
The eastern part of the Pacific presents an enormous area
without any islands, except Haster and Gomez, which do not
appear to be surrounded by reefs.

THE Low or Paumotu ARCHIPELAGO.—This group consists
of about 80 atolls: it would be quite superfluous to refer to’
descriptions of each. In D’Urville and Lottin’s chart, one
island (Wolchonsky) is written with a capital letter, signifying,
as explained in a former chapter, that it is a high island; but
this must be a mistake, as the original chart by Bellinghausen
shows that it is a true atoll. Captain Beechey says of the 32
groups which he examined (of the greater number of which I
have seen beautiful MS. charts in the Admiralty), that 29
now contain lagoons, and he believes the other three originally
did so. Bellinghausen (see an account of this Russian voyage,
in the Biblioth. des Voyages, 1834, p. 443) says that the 17
islands which he discovered resembled each other in structure,
and he has given charts on a large scale of all of them. Kot-
zebue has given plans of several; Cook and Bligh mention
others; a few were seen during the voyage of the Beagle;
and notices of other atolls are scattered through several pub-
lications. ‘The Acton group in this archipelago has lately
been discovered (Geograph. Journ., vol. vil. p. 454); it
consists of three small and low islets, one of which has a
lagoon. Another lagoon-island has been discovered (Naut.
Mag. 1839, p. 770), in 22° 4’ S. and 136° 20’ W. Dana,
in his work on Corals and Coral Islands, gives a full
account of this archipelago. Towards the S.E. there are
some islands of a different nature: Elizabeth Island is
described by Beechey (p. 46, 4to. edit.) as fringed by reefs,
at the distance of between two and three hundred yards;
coloured red. Pitcairn Island, in the immediate neighbourhood,
according to the same authority, has no reefs of any kind,

PACIFIC OCEAN. 199

although numerous pieces of coral are thrown up on the beach;
the sea close to its shore is very deep (see Zool. of Beechey’s
Voyage, p. 164); left uncoloured. Gambier Islands (see Plate I.
fig. 8) are encircled by a barrier-reef: the greatest depth
within is 38 fathoms; coloured pale blue. Metia or Aurora
Island les N.E. of Tahiti, close to the large space coloured
dark blue in the map; it has been already described as an
upraised atoll; as it is said by Captain Wilkes (Narrative of
U.S. Exploring Expedition, vol. 1. p. 387).to be surrounded
by fringing-reefs, in one part 500 feet in width, it has been
coloured red. But I must remind the reader of the discussion
in the sixth chapter, showing that if an upraised atoll were to
subside again, the reef would probably retain for a long time
or for ever, its fringing character, owing to the steepness of
the submarine flanks.

The Society ARCHIPELAGO is separated by a narrow space
from the Low Archipelago ; and in their parallel direction they
manifest some relation to each other. I have already described
the general character of the reefs of these encircled islands.
In the atlas of the Coquille’s Voyage there is a good general
chart of the group, and separate plans of some of the islands.
Tahiti, the largest island in the group, is almost surrounded,
as seen in Cook’s chart, by a reef from half a mile to a mile
and a half from the shore, with from 10 to 30 fathoms within
it. Some considerable submerged reefs, lying parallel to the
shore, with a broad and deep space within, have lately been
discovered on the N.E. coast of the island, (Naut. Mag. 1836,
p- 264,) where none are laid down by Cook. At Evmeo the
reef, ‘ which like a ring surrounds it, is in some places one or
two miles distant from tne shore, in others united to the
beach’ (Ellis, Polynesian Researches, vol. i. p. 18, 12mo.
edit.). Cook found deep water (20 fathoms) in some of the
harbours within the reef. Mr. Couthouy, however, states
(Remarks, p. 45) that both at Tahiti and Eimeo, the space
between the barrier-reef and the shore has been almost filled

200 APPENDIX.

up,—‘a nearly continuous fringing- reef surrounding the island,
and varying from a few yards to rather more than a mile in
width, the lagoons merely forming canals between this and
the sea-reef, that is the barrier-reef. Tapamanoa is sur-
rounded by a reef at a considerable distance from the shore ;
from the island being small, it 1s breached, as I am informed
by the Rev. W. Ellis, only by a narrow and crooked boat-
channel. This is the lowest island in the group, its height
probably not exceeding 500 feet. A little way north of Tahiti,
the low coral islets of Teturoa are situated ; from the descrip-
tion of them given me by the Rev. J. Williams (the author of
the Narrative of Missionary Enterprise), I should have thought
that they formed a small atoll, and likewise from the descrip-
tion given by the Rev. D. Tyerman and G. Bennett (Journ. of
Voy. and Travels, vol. 1. p. 183), who say that ten low coral
islets ‘are comprehended within one general reef, and sepa-
rated from each other by interjacent lagoons;’ but as Mr.
Stutchbury (West of England Journal, vol. 1. p. 54) describes
it as consisting of a mere narrow ridge, I have left it un-
coloured. Maitea, eastward of the group, is classed by Forster
as a high encircled island; but from the account given by
the Rev. D. Tyerman and G. Bennett (vol. 1. p. 97) iu

appears to be an exceedingly abrupt cone rising from the sea

without any reef; left uncoloured. It would be superfluous
to describe the northern islands in this group, as they may be
well seen in the chart accompanying the 4to. edition of Cook’s
Voyages, and in the atlas of the Coquille’s Voyage. Maurua
is the only one of the northern islands in which the water
within the reef is not deep, being only 44 fathoms; but the
ereat width of the reef, stretching three miles and a half south-
ward of the land (which is represented in the drawing in the
atlas of the Coguille’s Voyage as descending abruptly to the
water), shows, on the principle explained in the beginuing of
the last chapter, that it belongs to the barrier class. I may
here mention, from information communicated to me by the

“

PACIFIC OCEAN. 201

Rev. W. Ellis, that on the N.E, side of Huaheine there isa
bank of sand, about a quarter of a mile wide, extending
parallel to the shore, and separated from it by an extensive

and deep lagoon : this bank of sand rests on coral-rock, which
undoubtedly was once a living reef. North of Bolabola
lies the atoll of Youbat (Motou-iti of the Coquille’s atlas),
which is coloured dark blue; all the islands which are sur-
rounded by barrier-reefs are coloured pale blue: three of

them are represented in figures 3, 4, and 5, in Plate I. There

are three low coral-groups lying a little W. of the Society
_ Archipelago, and almost forming part of it, namely, Belling-
_ hausen, which is said by Kotzebue (Second Voyage, vol. i. p.
955) to be a lagoon-island; Mopeha, which from Cook’s descrip-
tion (Second Voyage, book iii. chap. i.) no doubt is an atoll;

and the Sczlly Islands, which are said by Wallis (Voyage,

chap. ix.) to form a group of low islets and shoals, and which,
thérefore, probably compose an atoll: the two former have
been coloured blue, but not the latter.

Menpana oR Marquesas Grour.—These islands are almost
entirely destitute of reefs, as may be seen in Krusenstern’s

_ Atlas, making a remarkable contrast with the adjacent group

of the Society’s Islands. Mr. F. D. Bennett has given some
account of this group, in the seventh volume of the Geograph.
Journ. He informs me that all the islands have the same
general character, and that the water is very deep close to
their shores. He visited three of them, namely, Dominicana,
Christiana, and Roapoa, their beaches are strewed with
rcunded masses of coral, and although no regular reefs exist,
yet the shcre is in many places lined by coral rock, so that a
boat grounds on this formation. Hence these islands ought
perhaps to come within the class of fringed islands and be
coloured red; but as I am determined to err on the cautious
side, I have left them uncoloured. Dana infers (Corals and
Coral Islands. p. 325), from their steepness and deeply in-
dented outline, that they have subsided.

202 APPENDIX.

Cook or Harvey anp Austrat Istanps.—Palmerston Island

is minutely described as an atoll by Captain Cook during his |

voyage in 1774; itis coloured blue. Aztutaki was partially

‘surveyed by the Beagle (see map accompanying Voyages of —

Adventure and Beagle); the land is hilly, sloping gently to the

beach; the highest point is 360 feet; on the southern side, —

the reef projects five miles from the land: off this point the
Beagle found no bottom with 270 fathoms: the reef is sur-
mounted by many low coral-islets. JI am informed by the
Rev. J. Williams, that within the reef the water is exceed-
ingly shallow, not being more than a few feet deep; never-
theless, from the great extension of the reef into a profoundly
deep ocean, this island probably belongs, on the principle
lately adverted to, to the barrier class, and I have coloured it
pale blue, although with much hesitation. Manouaz or Harvey

Island: the highest point is about 50 feet: the Rev. J. Wil-

liams informs me that although the reef lies far from the shore, —

it is less distant than at Aitutaki, but the water within the reef
is rather deeper: I have likewise coloured this island pale

blue, but with many doubts.—Round MMitvaro Island, as lam ~

informed by Mr. Williams, the reef is attached to the shore;
coloured red.—-Mauki, or Maouti: the reef round this island
(under the name of Parry Island in the Voyage of H.M.S.
Blonde, p. 209) is described as a coral flat, only 50 yards
‘wide, and two feet under water. This statement has been
corroborated by Mr. Williams, who calls the reef attached ;
coloured red.—Atiu, or Wateeo : a moderately elevated, hilly
island, like the others of the group; the reef is described in

Cook’s Voyage as attached to the shore, and about 100 yards

wide; coloured red.—/’enowa-it? : Cook describes this island as
very low, not more than six or seven feet in height (vol. 1.

book ii. chap. 111. 1777); in the chart published in the Co-

quille’s atlas, a reefis engraved close to the shore: this island —

is not mentioned in the list given by Mr. Williams (p. 16) in
the Narrative of Missionary Enterprise ; nature doubtful; but.

PACIFIC OCEAN. 203

_as it lies so near Atiu, it has been unavoidably coloured red.—
Rarotonga: Mr. Williams informs me that this is a lofty
basaltic island, with an attached reef; coloured red.—There are
three other islands, Roureuti, Roxburgh, and Hull, of which I
have not been able to obtain any account, and have left them
uncoloured. Aull Island, in the French chart, is written with
small letters as being low.—Mangaia: height about 300 feet ;
‘the surrounding reef joins the shore’ (Williams’s Narrative,
p- 18); coloured red.—Rimetara: Mr. Williams informs me
that the reef is rather close to the shore; but, from informa-
tion given me by Mr. Ellis, the reef does not appear to be
_ quite so closely attached to it as in the foregoing cases: the

island is about 300 feet high (Naut. Mag. 1839, p. 738);
coloured red.—Rurutu: Mr. Williams and Mr. Ellis inform
me that this island has an attached reef; coloured red. It is
described by Cook under the name of Oheteroa: he says it is
not surrounded like the neighbouring islands, by a reef; but he
must mean a distant reef.i— Toubouai : in Cook’s chart (Second
Voyage, vol. ii. p. 2) the reef is laid down in a part at the dis-
tance of one mile, and in another part at the distance of two
miles from the shore; Mr. Hllis (Polynes. Res. vol. 11. p. 381)
says the low land round the base of the island is very extensive ;
and this gentleman informs me that the water within the
reef appears deep; coloured blue.—Ravvaivaz, or Vivitao:
Mr. Williams informs me that the reef is here distant from
the shore; Mr. Ellis, however, says that this is certainly
not the case on one side of the island; and he believes that
the water within the reef is not deep; hence I have left it
uncoloured.— Lancaster Reef, described in Naut. Mag. 1833
(p. 693), as an extensive crescent-formed coral-reef, has not
been coloured.—Rapa, or Oparree: from the accounts given
of it by Ellis and Vancouver, there does not appear to be
any reef.—J. de Bass is an adjoining island, of which I can-
not find any account.—Kemun Island: Krusenstern seems hardly
to know its position, and gives no further particulars.

204 APPENDIX.

IsLANDS BETWEEN the Low and Gilbert Archipelagoes.

Caroline Island (10° S., 150° W.) is described by Mr. F. D.
Bennett (Geograph. Journ. vol. vii. p. 225) as containing a fine
lagoon; coloured blue. Westward of Caroline Island, a small

lagoon-island is described in the U.S. Exploring Expedition |

in lat. 10° 8. and 152° 22’ W. long.; coloured blue.—Flint
Island (11° 8., 151° W.): Krusenstern believes that it is the
same with Peregrino, which is described by Quiros (Burney’s
Chron. Hist. vol. i. p. 283) as ‘a cluster of small islands
connected by a reef, and forming a lagoon in the middle;’
coloured blue—— Wostock is an island a little more than half a
mile in diameter, and apparently quite flat and low, discovered
by Bellinghausen; it is situated a little west of Caroline Island,
but it is not placed on the French charts; I have not coloured
it, although I entertain little doubt from the chart of Bell-

inghausen, that it originally contained a small lagoon.—Pen-_

rhyn Island (9° S., 158° W.): a plan in the atlas of the First
Voyage of Kotzebue, shows that it is an atoll, which accord-
ing to Wilkes (U.S. Exploring Expedition, vol. iv. p. 277) is
nine miles in length ; coloured blue.—=Starbuck Island (5° S..,
156° W.) is described in Byron’s Voyage in the Blonde
(p. 206) as formed of a flat coral-rock, with no trees; the
height not given; not coloured.—Malden Island (4° §., 154°
W.): in the same Voyage (p. 205) this island is said to be of
coral formation, and no part above 40 feet high; I have not
ventured to colour it, although from being of coral formation,

it is probably fringed; in which case it should be red.—

Jarvis, or Bunker Island (0° 20’ S., 160° W.) is described by
Mr. F. D. Bennett (Geograph. Journ. vol. vii. p. 227) as a
narrow, low strip of coral formation; not coloured.—Brook is
a small, low island between the two latter; its position, and
perhaps even existence is doubtful; not coloured.—Pescado
and Humphrey Islands: I can find out nothing about these

islands, except that the latter appears to be small and low;

mm -. Vat. ea

iY a

r ai
My, ‘
be

PACIFIC OCEAN. 205

not coloured.—Rearson, or Grand Duke Alexander’s (10° S.,
161° W.): an atoll, of which a plan is given by Belling-
hausen; blue.—Souvorof Islands (13° §., 168° W.): Ad-
miral Krusenstern, in the most obliging manner, obtained for
me an account of these islands from Admiral Lazareff, who
discovered them. ‘They consist of five very low islands of
coral formation, two of which are connected by a reef, with
deep water close to it. They do not surround a lagoon, but
are so placed that a line drawn through them includes an oval
space, part of which is shallow ; these islets, therefore, probably
once (as is the case with some of the islands in the Caroline
Archipelago) formeda single atoll; but I have not coloured them.
—Danger Island (10° S., 166° W.): described as low by Com-
modore Byron, and more lately surveyed by Bellinghausen ; it
isa small atoll with three islets on it; blue.—Clarence Island
(9°S., 172° W.): discovered in the Pandora (G. Hamilton’s
Voyage, p. 75): it is said, ‘In running along the land, we saw
several canoes crossing the lagoons ;’ as this island is in the
close vicinity of other low islands, and as it is said that the
natives make reservoirs of water in old cocoa-nut trees (which
shows the nature of the land), I have no doubt it is an atoll,
and have coloured it blue.—VYork Island (8° S., 172° W.) is
described by Commodore Byron (chap. x. of his Voyage) as
an atoll; blue-—Sydney Island (4° S., 172° W.) is about three
miles in diameter, with its interior occupied by alagoon (Captain
Tromelin, Annal. Marit. 1829, p. 297); coloured blue.—Aull
Island is situated 60 miles to the west of Sydney Island, and
is described by Wilkes (U. S. Exploring Expedition, vol. iii., p.
369) as a lagoon-island; coloured blue.-—Pheniz Island (4° S.,
171° W.) is nearly circular, low, sandy, not more than two
miles in diameter, and very steep outside (Tromelin Annal.
Marit. 1829, p. 297): it may be inferred that this island
originally contained a lagoon, but I have not coloured it.—
New Nantucket (0° 15’ N., 174° W.): from the French chart
it must be a low island; I can find nothing more about it, or

206 APPENDIX.

about M ary Island ; both uncoloured.— Gardner Island (5° 5
174° W. ), from its position, is certainly the same as Kamin i
Island, and is described (Krusenstern, p. 455. Appen. to Mem.
published 1827) as having a lagoon in its centre; coloured blue.

Istanps soutH of the Sandwich Archipelago.

Christmas Island (2° N., 157° W.): Captain Cook, in his
Third Voyage (vol. ii. chap. x.), has given a detailed account
of this atoll. The breadth of the isiets on the reef is un-
usually great, and the sea near it does not deepen so suddenly
as is generally the case. It has more lately been visited by
Mr. F. D. Bennett (Geograph. Journ. vol. vii. p. 226); and |
he assures me that it is low and of coral formation: I par- |
ticularly mention this, because it is engraved with a capital
letter, signifying a high island, in D’Urville and Lottin’s chart.
Mr. Couthouy, also, has given some account of it (Remarks,
p. 46) from the Hawaiian Spectator; he believes it has lately
undergone a small elevation, but his evidence does not appear
to me satisfactory ; the deepest part of the lagoon is said to
be only ten feet; nevertheless, I have coloured it blue.—Fan-
ning Island (4° N., 158° W.), according to Captain Tromelin,
(Ann. Maritim. 1829, p. 283), is an atoll: his account, as
observed by Krusenstern, differs from that given in Fanning’s
Voyage (p. 224), which, however, is far from clear; coloured
blue.— Washington Island (4° N., 159° W.) is engraved as a
low island in D’Urville’s chart, but is described by Fanning
(p. 226) as having a much greater elevation than Fanning
Island, and hence I presume it is not an atoll; not coloured.—
Palmyra Island (6° N., 162° W.) is an atoll divided into ‘two
parts (Krusenstern’s Mem. Suppl. p. 50, also Fanning’s
Voyage, p. 2383); blue.—Smyth’s, or Johnston’s Islands (17°
N., 170° W.): Captain Smyth, R.N., has had the kindness to
inform me that they consist of two very low small islands,
with a dangerous reef off the east end of them; Captain

PACIFIC OCEAN. 207

Smyth does not recollect whether these islets, together with
the reef, surrounded a lagoon; uncoloured.

SanDWICH ARCHIPELAGO.—Hawaz: in the chart in Frey-
cinet’s Atlas, small portions of the coast are fringed by reefs ;
and in the accompanying Hydrog. Memoir, reefs are mentioned
in several places, and the coral is said to injure the cables; but
Dana saw hardly any reefs here. On one side of the islet of

' Kohaihai there is a bank of sand and coral with five feet

of water on it, running parallel to the shore, and leaving a
channel of about fifteen feet deep within. I have coloured
this island red, but it is very much less perfectly fringed than
others of the group—ZM€aui: in Freycinet’s chart of the
anchorage of Raheina, two or three miles of coast are seen to
be fringed; and in the Hydrog. Memoir ‘banks cf coral
along shore’ are spoken of. Mr. F. D. Bennett informs me
that the reefs, on an average, extend about a quarter of a
mile from the beach; the land is not very steep, and out-
side the reefs the sea does not become suddenly deep;
coloured red.—WMorotoi, I presume, is fringed: Freycinet
speaks of the breakers extending along the shore at a little
distance from it. From the chart, I believe it is fringed;
coloured red.—Oahu: Freycinet, in his Hydrog. Memoir,
mentions some reefs. Mr. F. D. Bennett informs me that
the shore is skirted for forty or fifty miles in length. There
is even a harbour for ships formed by the reefs, but it is at
the mouth of a valley ; red.—Atooz, in La Peyrouse’s charts,
is represented as fringed by a reef, in the same manner as
Oahu and Morotoi; and this, [ am informed by Mr. Ellis,
is of coral-formation on part at least of the shore: the
reef does not leave a deep channel within ; red.—Oneehow:
Mr. Hllis believes that this island is also fringed by a coral-
reef: considering its close proximity to the other islands, I
have ventured to colour it red. I have in vain consulted the
works of Cook, Vancouver, La Peyrouse, and Lisiansky, for
any satisfactory account of the small islands and reefs which

4 ‘

208 APPENDIX. 5

lie scattered in a N.W. line prolonged for agreat distance from
the Sandwich group, and hence have left them uncoloured,
with one exception; for Iam indebted to Mr. F. D. Bennett
for informing me of an atoll-formed reef, in lat. 28° 22’, —
long. 178° 30’ W., on which the Giledstanes was wrecked in
1837. It is apparently of large size, and extends in a N.W.
and 8.H. line: very few islets have been formed on it. The
lagoon seems to be shallow; at least, the deepest part which —
was surveyed was only three fathoms. Mr. Couthouy (Re- —
marks, p. 38) describes this island under the name of Ocean
Island. Considerable doubts should be entertained regarding
the nature of a reef of this kind, with a very shallow lagoon,

and standing far from any other atoll, on account of the possi-
bility of a crater or flat bank of rock lying at the proper depth
beneath the surface of the water, having afforded a founda-
tion for a ring-formed coral-reef. I have, however, thought —
myself compelled, from its large size and symmetrical outline,
to colour it blue. Some information and references are
given by Dana (Corals and Coral Islands, p. 324, 365) with
respect to the reefs and islets extending for 2,000 miles in a
N.W. line from Hawaii.

- Samoa or Navicator Group.—Kotzebue, in his Second
Voyage, contrasts these islands with many others in the
Pacific, in not having harbours for ships, formed by distant
coral-reefs. The Rev. J. Williams, however, informs me
that coral-reefs do occur in irregular patches on the shores;

but that they do not form a continuous band, as round Mangaia,
and other such perfect cases of fringed islands. From the charts
accompanying La Peyrouse’s Voyage, it appears that the north —
shore of Savaiu, Maouna, Orosenga, and Manua are fringed by
reefs. La Peyrouse, speaking of Maouna (p. 126), says that
the coral-reef surrounding its shores almost touches the
beach, and is breached in front of the little coves and
streams, forming passages for canoes, and probably even for
boats. Further on (p. 159) he extends the same observation

PACIFIC OCEAN. 209

to all the islands which he visited—Mr. Williams in his
Narrative, speaks of a reef going rounda small island attached
to Oyolava, and returning again to it: all these islands have -
been coloured red.—A chart of Rose Island, at the extreme
west end of the group, is given by Freycinet, from which I
should have thought that it had been an atoll; but according
to Mr. Couthouy (Remarks, p. 43) it consists of a reef, only a
league in circuit, surmounted by avery few low islets: the
lagoon is very shallow, and is strewed with numerous large
boulders of volcanic rock. This island, therefore, probably
consists of a bank of rock, a few feet submerged, with the
outer margin fringed with reefs; hence it cannot be properly
classed with atolls, in which, as we have reason to believe, the
foundations always lie at a depth greater than that at which
the reef-constructing polypifers can live ; not coloured.
Beveridge Reef, 20° 8., 167° W., is described in the Naut.
Mag. (May 1833, p. 442) as ten miles long inaN. and 6S. line,

‘and eight wide; ‘in the inside of the reef, there appears deep

water; ’ there is a passage near the 8.W. corner: this there-
fore seems to be a submerged atoll, and is coloured blue.

Savage Island, 19° S., 170° W., has been described by Cook
and Forster. The younger Forster (vol. ii. p. 163) says it
is about 40 feet high: he suspects that it contains a low
plain, which formerly was the lagoon. The Rev. J. Williams
gives 100 feet as its height, and he informs me that the reef
fringing its shores, resembles that round Mangaia; coloured
red,

FRIENDLY ARCHIPELAGO.—Pylstaart Island: judging from
the chart in Freycinet’s Atlas, I should have supposed that ithad
been regularly fringed; but as nothing is said in the Hydrog.
Memoir (or in the Voyage of Tasman, the discoverer) about
coral-reefs, I have left it uncoloured.—Tongatabou : in the
atlas of the Voyage of the Astrolabe, the whole south side of
the island is represented as narrowly fringed by the same reef
which forms an extensive platform on the northern side. The

P

210 APPENDIX.

origin of this latter reef, which might have been mistaken for
a barrier-reef, has already been attempted to be explained,
when giving the proofs of the recent elevation of this island.
—In Cook’s charts the little outlying island of Hoaigee is
represented as fringed; coloured red.—Houwa : I cannot make
out from Captain Cook’s charts and descriptions, that this island
has any reef, although the bottom of the neighbouring sea
seems to be covered with corals, and the island itself is formed
of coral-rock. Forster, however, distinctly (Observations,
p- 14) classes it with the high islands having reefs, but it
certainly is not encircled by a barrier-reef; and the younger
Forster (Voyage, vol. i. p. 426) says, that ‘a bed of coral
rocks surrounded the coast towards the landing-place.’ I
have therefore classed it with the fringed islands, and coloured
it red. Dana also shows (Corals and Coral Islands, p. 337)
that most of the islands of this group are formed of upraised
coral-rock. The several islands lymg N.W. of Tongatabou,
namely Anamouka, Komango, Kotou, Lefouga, Foa, &e., are’
seen in Captain Cook’s chart to be fringed by reefs, and several
of them are connected together. From the various statements
in the first volume of Cook’s Third Voyage, and especially
in Chapters IV. and VI., it appears that these reefs are
of coral, and certainly do not belong to the barrier class ;
coloured red.—TZoufoa and Kao, forming the western part of
the group, according to Forster, have no reefs; the former
is an active volcano. Vavao: there is a chart of this
singularly-formed island, by Espinoza: according to Mr.
Williams it consists of coral-rock: the Chevalier Dillon
informs me that it is not fringed; not coloured. Nor are
the islands of Latte and Amargura coloured, for I have not
seen plans of them on a large scale, and I do not know whether
they are fringed: Amargura is said (Atheneum, 1848, p. 40)
to have been lately in violent eruption.

Niouha, 16° 8., 174° W., or Keppel Island of Wallis, or
Cecos Island: from a view and chart of this island, given in

<= ,

PACIFIC OCEAN. ANG

Wallis’s Voyage, (4to. edit.) it is evidently encircled by a

reef; coloured blue. It is, however, remarkable that Bos-
cawen Island, immediately adjoining, has no reef of any kind;
uncoloured.

Wallis Island, 18° §., 176° W.: a chart and view of this
island in Wallis’s Voyage (4to. edit.) shows that it is encircled.
A view of it in the Naut. Mag. July 1833, p. 376, shows the
same fact. Nine islands, most of them high, are said in

_ Wilkes U. 8. Exploring Expedition (vol. ii. p. 157) to be

enclosed within the same reef, through which, it is asserted,

ships can enter; coloured blue.

Alloufatou, or Horn Island, Onouafu, or Proby Island,
and Hunter Islands, lie between the Navigator and Fidji
sroups. I can find no distinct accounts of them.

Fipsi or Frersee or Vit1 Group.—Until lately the best
chart of the numerous islands of this group, was that in

_ the Atlas of the Astrolabe’s Voyage; but now the islands

have been surveyed during the U. 8. Exploring Expedition,
and full information respecting them and the reefs has been
given by Dana. Many of the islands are bold and mountain-
ous, and are surrounded by reefs, lying far from the land, and
outside the ocean appears very deep. The Astrolabe sounded
with 90 fathoms in several places about a mile from the reefs,
and found no bottom. It is evident that the water within
many of the encircling reefs is deep : as indeed I was formerly
assured was the case by Dillon. Beyond the high and en-
circled islands there are numerous atoll-formed reefs. Hence
the whole group has been coloured blue. In the S.E. part
lies Batoa, or Turtle Island of Cook (Second Voyage, vol. i1.
p: 28, and chart; 4to. edit.), surrounded by a coral-reef, ‘which
in some places extends two miles from the shore ;’ within the
reef the water appears to be deep, and outside it is unfathom-
able; coloured pale blue. At the distance of a few miles,
Captain Cook (ibid. p. 24) found a circular coral-reef, four or
five leagues in circuit, with deep water within; ‘in short,
Pp 2

212 | _ APPENDIX.

the bank wants only a few little islets to make it exactly like
one of the half-drowned isles so often mentioned,—namely,
atolls. South of Batoa lies the high island of Ono, which
appears in Bellinghausen’s Atlas to be encircled; as do some
other small islands to the south; coloured pale blue: near
Ono, there is an annular reef, quite similar to the one just
described in the words of Captain Cook; coloured dark blue.

Rotoumah, 13° §., 179° E.—From the chart in Duperrey’s —

Atlas, I thought that this island was encircled, but the
Chevalier Dillon assures me that the reef is ony a shore or
fringing one ; coloured red.

iesendenter Island, 10° S., 179° E., is described by Mr. G.
. Bennett (United Service Journ. 1831, part ii. p. 197) as a
low island of coral formation; it is small, and does not
appear to contain a lagoon, although an opening through
the reef is referred to. A lagoon probably once existed, and
has since been filled up; left uncoloured.

ELLicE Group.—Oscar, Peyster, and Hilice Islands are
figured in Arrowsmith’s Chart of the Pacific (corrected to
1832) as atolls, and are said to be very low; blue.—WNeder-
landisch Island: I am greatly indebted to the kindness of Admi-
ral Krusenstern, for sending me the original documents con-
cerning this island. From the plans given by Captains Eeg
and Khremtshenko, and from the detailed account given by
the former, it appears that it is a narrow coral-island, about
two miles long, containing a small lagoon. The sea is very
deep close to the shore, which is fronted by sharp coral-rocks.
Captain Heg compares the lagoon with that of other coral-
islands; and he distinctly says, the land is ‘very low.’ I
have therefore coloured it blue. Admiral Krusenstern —
(Memoir on the Pacific, Append. 1835) states that its
shores are 80 feet high; this probably arose from the height
of the cocoa-nut trees, with which it is covered, being
mistaken for land.—Grand Cocal is said in Krusenstern’s
Memoir to be low, and to be surrounded by a reef; it is

\

: PACIFIC OCEAN. 913

small, and therefore probably once contained a lagoon; un-

-coloured.— St. Augustin: from a chart and view of it,

given in the Atlas of the Coquille’s Voyage, it appears to
be a small atoll, with its lagoon partly filled up; coloured
blue. .

Gitzert Group.—The chart of this group, given in the
Atlas of the Coquille’s Voyage, shows that it is composed of
ter well-characterized, but very irregularly shaped atolls.
In D’Urville and Lottin’s chart, Sydenham is written with
a capital letter, signifying that it is high; but this certainly
is not the case, for it is a perfectly characterized atoll, and a
sketch, showing how low it is, is given in the Coquzile’s Atlas.
Some narrow strip-like reefs project from the southern side of
Drummond atoll, and render it irregular. The southern island
of the group is called Chase (in some charts, Rotches); of this
I can find no account, but Mr. F. D. Bennett discovered
(Geograph. Journ. vol. vii. p. 229) a low extensive island in
nearly the same latitude, about three degrees westward of the
longitude assigned to Rotches; and this probably is the same ©
island. Mr. Bennett informs me that the man at the mast-
head reported an appearance of lagoon-water in the centre;
and, therefore, considering its position, I have coloured it blue.
—Pitt Island, at the extreme northern point of the group, is
left uncoloured, as neither its exact position or nature is
known.— Byron Island, which lies a little to the eastward, does
not appear to have been visited since Commodore Byron’s
Voyage, and it was then seen only from a distance of 18
miles: it is said to be low; uncoloured.

Ocean, Pleasant, and Atlantic Islands all lie considerably
to the west of the Gilbert group: I have been unable to find
any distinct account of them. Ocean Island is written with
small letters in the French chart, but in Krusenstern’s Memoir
it is said to be high.

MarsHALL Group.—We are well acquainted with this
group from the excellent charts of the separate islands, made

214 APPENDIX.

during the two voyages of Kotzebue: a reduced one of the
whole group may be seen in Krusenstern’s Atlas, and in
Kotzebue’s Second Voyage. The group consists (with the ex-
ception of two /ittle islands which probably have had their
lagoon filled up) of a double row of 23 large and well-charac-
terized atolls, from the examination of which Chamisso drew
up his well-known account of coral formations. I include in
this group Gaspar-Rico, or Cornwallis Island, which is de-
scribed by Chamisso (Kotzebue’s First Voyage, vol. ii. p. 179)
‘as a low sickle-formed group, with mould only on the wind-
ward side.’ Gaspard Island is considered by some geographers —
as a distinct island lying N.E. of the group, but it is not en-
tered in the chart by Krusenstern ; left uncoloured. In the ©
S.W. part of this group lies Baring Island, of which little is
known (see Krusenstern’s Appendix, 1835, p. 149). I have
left it uncoloured; but Boston Island I have coloured blue, as
it is described (ibid.) as consisting of 14 small islands, which,
no doubt, inclose a lagoon, as represented in a chart in the
Coquille’s Atlas.—Two islands, Aur AKawen and Gaspar Rico,
are written in the French chart with capital letters; but this
is an error, for from the account given by Chamisso in Kotze-
bue’s First Voyage, they are certainly low. The nature, posi-
tion, and even existence, of the shoals and small islands north
of the Marshall group are doubtful.

New Hesripes.—Any chart, on even a small scale, of these
islands will show that their shores are almost without reefs,
presenting a remarkable contrast with those of New Caledonia
on the one hand, and the Fidji group on the other. Never-
theless, I have been assured by Mr. G. Bennett, that coral
grows vigorously on their shores; as, indeed, will be further
shown in some of the following notices. As, therefore, these
islands are not encircled, and as coral grows vigorously on their
shores, we might almost conclude, without further evidence,
that they are fringed, and hence I have applied the red colour
with rathergreater freedom than in other instances.—Matthew’s _

PACIFIC OCEAN. 215

_ Rock, an active volcano, someway south of the group (of which
a plan is given in Atlas of the Astrolabe’s Voyage) does not
appear to have reefs of any kind about it—Annatom, the
southernmost of the Hebrides: from a rough woodcut given
in the United Service Journal (1831, part iii. p. 190), accom-
panying a paper by Mr. Bennett, it appears that the shore is
fringed; coloured red.—Tanna: Forster, in his Observations
(p. 22), says Tanna has on its shores coral-rock and madre-
pores ; and the younger Forster, in his account (vol. 11. p. 269),
speaking of the harbour, says the whole 8.E. side consists of
coral-reefs, which are overflowed at high water: part of the
southern shore in Cook’s chart is represented as fringed;
coloured red.—Jmmer is described (United Service Journ.
1831, part i. p. 192) by Mr. Bennett as being of moderate
elevation, with cliffs appearing like sandstone; coral grows in
patches on its shore, but I have not coloured it; and I mention
these facts because Immer might have been thought, from
Forster’s classification (Observations, p. 14), to have been a
low island, or even an atoll. Hrromango Island: Cook (Second
Voyage, vol. i. p. 45, 4to. edit.) speaks of rocks everywhere
lining the coast, and the natives offered to haul his boat over the
breakers to the sandy beach: Mr. Bennett, in a letter to the
editor of the Singapore Chron., alludes to the reefs on its shores.
It may, I think, be safely inferred from these passages that the
shore is fringed in parts by coral-reefs ; coloured red.—Sand-
wich Island: the east coast is said (Cook’s Second Voyage,
vol. 11. p.41) to be low, and to be guarded by a chain of breakers.
In the accompanying chart it is seen to be fringed by a reef;
coloured red.— Mallicollo : Forster speaks of the reef-bounded
shore: the reef is about 30 yards wide, and so shallow that
a boat cannot pass over it. Forster, also, (Observat. p. 23,)
says that the rocks of the sea-shore consist of madrepore. In
the plan of Sandwich harbour, the headlands are represented
as fringed ; coloured red.—Awurora and Pentecost Islands, ac-
cording to Bougainville, apparently have no reefs; nor has the

216 APPENDIX.

large island of S. Espiritu, nor Bligh Island, nor Banks’ Islands,
which latter lie to the N.E. of the Hebrides. But in none of
these cases have I met with any detailed account of their
shores, or seen plans on a large scale; and it will be evident
that a fringing-reef of only thirty, or even a few hundred yards
in width, is of so little importance to navigation, that it will
seldom be noticed, excepting by chance; and hence I do not
doubt that several of these islands, now left uncoloured, ought
to be red. . | ;
Santa-Cruz Grour.—Vanikoro (Fig. 1, Plate I.) offers a
striking example of a barrier-reef: it was first described by
the Chevalier Dillon, in his Voyage, and was surveyed in the
Astrolabe; coloured pale blue.—Tikopia and Fataka Islands
appear, from the descriptions of Dillon and D’Urville, to have
no reefs: Anouda is a low, flat island, surrounded by cliffs,
(Astrolabe. Hydrog. and Krusenstern Mem. vol. ii. p. 482);
these are uncoloured.—Toupoua (Oiooboa of Dillon) is stated
by Captain Tromelin (Annales Marit. 1829, p. 289) to be al-
most entirely included in a reef, lying at the distance of two
miles from the shore. There is a space of three miles without
any reef, which, although indented with bays, offers no anchor-
age from the extreme depth of the water close to the shore:
Captain Dillon also speaks of the reefs fronting this island ;
coloured blue.—Santa Cruz: I have carefully examined the
works of Carteret, Dentrecasteaux, Wilson, and Tromelin, and
I cannot discover any mention of reefs on its shores; left un-
coloured.—Tinakoro is a constantly active volcano without
reefs.—Mendana Isles (mentioned by Dillon under the name
of Mammee, &c.) are said by Krusenstern to be low, and in-
tertwined with reefs. I do not believe they include a lagoon ;
I have left them uncoloured.—Duf’s Islands compose a small
group directed in a N.W. and S8.E. band; they are described
by Wilson (p. 296, Miss. Voy. 4to. edit.) as formed by bold
peaked land, with the islands surrounded by coral-reefs, ex-
tending about half a mile from the shore: at the distance of

PACIFIC OCEAN. 217

a mile from the reefs he found only seven fathoms. AsI have
no reason for supposing there is deep water within these reefs,

I have coloured them red.—Kennedy Island, N.E. of Duff's:
I have been unable to find any account of it.
New Catzponta.—The great barrier-reefs on the shores of

‘this island have already been described (Fig. 5, Plate II.).

They have been visited by Labillardiére, Cook, and the
northern point by D’Urville; this latter part so closely resem-
bles an atoll that I have coloured it dark blue. The Loyalty
group is situated to the east of New Caledonia; some at least
of the islands are formed of upraised coral-rock, and are
fringed with living reefs; see Rev. W. B. Clarke, in Journal
of Geolog. Soc. 1847, p. 61; coloured red. North of this
group there are some extensive low reefs (called Astrolabe
and Beaupré), which do not seem to be atoll-formed: these
are left uncoloured.

AUSTRALIAN BarrieR-REEF.—This great reef, which has
already been described, has been coloured from the charts of
Flinders and King. Jukes has given many details respecting
it in the Voyage of H.M.S. Fly (vol. 1. 1847, chap. xiii.). In
the northern parts, an atoll-formed reef, lymg outside the
barrier, hasbeen described by Bligh, and is coloured dark

blue. In the space between Australia and New Caledonia,

called by Flinders the Corallian Sea, there are numerous reefs.

Of these, some are represented in Krusenstern’s Atlas as having

an atoll-like structure; namely, Bampton Shoal, Frederic,
Vine or Horse-shoe, and Alert Reefs ; these have been coloured
dark blue.
Loutstape.—The dangerous reefs which front and surround
the western, southern, and northern coasts of this so-called
peninsula and archipelago, seem evidently to ‘belong to the
barrier class. The land is lofty, with a low fringe on the
coast ; the reefs are distant, and the sea outside them pro-
foundly deep. Nearly all that is known of this group is de-
rived from the labours of Dentrecasteaux and Bougainville:

218 APPENDIX.

the latter has represented one continuous reef 90 miles |

long, parallel to the shore, and in places as much as 10 miles
from it; coloured pale blue. A little distance northward, we
have the Zaughlan Islands, the reefs round which are engraved _
in the Atlas of the Voyage of the Astrolabe, in the same man- _
ner as round the encircled islands of the Caroline Archipelago :
the reef is, in parts, a mile and a half from the shore, to which
it does not appear to be attached; coloured blue. At some
little distance from the extremity of the Louisiade lies Wells
Reef, described in G. Hamilton’s Voyage in H.M.S. Pandora
{p. 100): it is said, ‘We found we had got embayed ina
double reef, which will soon be an island.’ As this statement
is only intelligible on the supposition of the reef being crescent
or horse-shoe formed, like so many other submerged annular
reefs, I have ventured to colour it blue.

SaLtomMaN ARCHIPELAGO.—The chart in Krusenstern’s Atlas
shows that these islands are not encircled; and as coral appears, —
from the works of Surville, Bougainville, and Labillardiére, to
grow on their shores, this circumstance, as in the case of the
New Hebrides, is a presumption that they are fringed. I
cannot find out anything from Dentrecasteaux’s Voyage, re-
garding the southern islands of the group, so have left them
uncoloured.—Malayta Island, in a rough MS. chart in the Ad-
miralty, has its northern shore fringed.— Ysabel Island: the N.E.
part of this island, as shown in the same chart, is also fringed :
Mendana (Burney, vol. i. p. 280), speaking of an islet adjoin-
ing the northern coast, says it is surrounded by reefs: the

shores, also, of Port Praslin appear regularly fringed. Choiseul _

Island: parts of the shores are fringed by coral-reefs, in Bou-
gainville’s chart of Choiseul Bay.— Bougainville Island: accord-
ing to Dentrecasteaux, the western shore abounds with coral-
reefs, and the smaller islands are said to be attached to the
larger ones by reefs; all the above-mentioned islands have been
coloured red.—Bouka Islands: Captain Duperrey has kindly
informed me in a letter, that he passed close round the northern —

PACIFIC OCEAN. 219

side of this island (of which a plan is given in his Atlas of the
Coquille’s Voyage), and that it was ‘ garnie d’une bande de
récifs a fleur d’eau adhérentes au rivage ;’ and he infers, from _ .
the abundance of coral on the islands north and south of

-Bouka, that the reef probably is of coral; coloured red.

Off the north coast of the Saloman Archipelago there are
several small groups which are littie known: they appear to
be low, and of coral formation; and some of them probably
have an atoll-like structure: the Chevalier Dillon, however,

* informs me this is not the case with the B. de Candelaria.—

Outong Java, according to the Spanish navigator, Maurelle, is
thus characterized; but this is the only one, which I have
ventured to colour blue.

New IreLanp.—The shores of the $.W. point of this
island and some adjoining islets, are fringed by reefs, as may
be seen in the Atlases of the Voyages of the Coquille and
Astrolabe. M. Lesson observes that the reefs are open in
front of each streamlet. The Duke of York’s Island is also
fringed; but with regard to the other parts of New Ireland,
New Hanover, and the small islands lying northward, I have
been unable to obtain any information. I will only add that
no part of New Ireland appears to be fronted by distant reefs.
I have coloured red only the above specified portions.

New Britain AND THE NORTHERN SHORE OF New GUINEA.—
From the charts in the Voyage of the Astrolabe, and from the
Hydrog. Memoir, it appears that these coasts are entirely
without reefs, as are the Schouton Islands, lying close to the
northern shore of New Guinea. The western and south-
western parts of New Guinea will be treated of when we
come to the islands of the East Indian Archipelago.

ADMIRALTY GRouP.—From the accounts given by Bougain-
ville, Maurelle, Dentrecasteaux, and the scattered notices col-
lected by Horsburgh, it appears that some of the many islands
composing it are high, with a bold outline; and others are
low, small, and interlaced with reefs. All the high islands

220 APPENDIX.

appear to be fronted by distant reefs rising abruptly from the
sea, and within some of which, there is reason to believe that
the water is deep. I have therefore little doubt that they
belong to the barrier class. In the southern part of the group,
we have Elisabeth Island, which is surrounded by a reef at the
distance of a mile ; and two miles eastward of it (Krusenstern,
Append. 1835, p. 42) there is a little island containing a
lagoon. Near here, also, lies Circular Reef (Horsburgh
Direct. vol. i. p. 691, 4th edit.), ‘three or four miles in dia-
meter, having deep water inside with an opening at the
N.N.W. part, and on the outside steep to.’ I have from these
data, coloured the group pale biue, and Circular Reef dark
blue-—The Anachorites, Echequier, and Hermites consist of
innumerable low islands of coral formation, which probably
are atolls; but not being able to ascertain this, I have not
coloured them, nor Durour Island, which is described by Car-
teret as low.

The Carotine ARCHIPELAGO is now well known, chiefly
from the hydrographical labours of Lutké: it contains about
forty groups of atolls, and three encircled islands, two of which
are engraved in Figs, 2and 7, Plate I. Commencing with the
eastern part, the encircling reef round Ualan appears to be
only about half a mile from the shore ; but as the land is low,
and covered with mangroves (Voyage autour du Monde, par
F. Lutké, vol. i. p. 3839), its margin has not probably been
ascertained. The extreme depth in one of the harbours within
the reef is 33 fathoms (see charts in Atlas of Coquille’s
. Voyage), and outside at half a mile distance from the reef,
no bottom was obtained with 250 fathoms. The reef is sur-
mounted by many islets, and the lagoon-like channel within
is mostly shallow, and appears to have been much encroached
on by the low land surrounding the central mountains; these
facts show that time has allowed much detritus to accumulate ;
coloured pale blue.—Pouynipéte or Seniavine. In the greater
part of the circumference of this island, the reef is about one

PACIFIC OCEAN. 231

mile and three quarters from the shore; but on the north side
it is five miles distant from the included high islets. The
reef is broken in several places; and just within it, the depth
in one place is 30 fathoms, and in another, 28, beyond which,
to all appearance, there was ‘un port vaste et sir’ (Lutké,
vol. 11. p. 4). Coloured pale blue—AHogoleu or Roug. This
wonderful group contains at least 62 islands, and its reef is
135 miles in circuit. Of the islands, only a few, about six or
eight (see Hydrog. Description, p. 428, of the Voyage of the
Astroiabe, and the large accompanying chart taken chiefly
from that given by Duperrey) are high, and the rest are all
small, low, and formed on the reef. ‘The depth of the great
interior lake has not been ascertained; but Captain D’Urville
appears to have entertained no doubt about the possibility of
taking in a frigate. The reef lies no less than 14 miles
distant from the northern coasts of the interior high islands;
seven miles from their western sides, and 20 from the
southern: the sea is deep outside. 'This island resembles on
a grand scale the Gambier group in the Low Archipelago. Of
the low! islands forming the chief part of the Caroline Archi-
pelago, all those of larger size (as may be seen in the Atlas by
Captain Lutkée), and some even of the small ones of which plans
are given in the Atlas of the Coquille’s Voyage, are true atolls.
There are, however, some low, small islands of coral formation,
namely, Ollap, Tamatam, Bigali, Satahoual, which do not
contain lagoons; but it is probable that lagoons originally
_ existed, but have since filled up: Lutké (vol. il. p. 304) seems
to have thought that all the low islands, with only one excep-
tion, contained lagoons. The most southern island in the
group, namely, Pzguirum, is not coloured, because I have found
no account of it. ougouor, or Monte Verdison, which was not

?In D’Urville and Lottin’s chart, Pescrare is written with capital
letters ; but this evidently is an error, for it is one of the low islets on
the reef of Namonouyto (see Lutké’s charts), which is a regular atoll.

‘

222 APPENDIX.

visited by Lutké, is described and figured by Mr. Bennett
(United Service Journal, Jan. 1832) as an atoll. All the
before-mentioned islands have been coloured blue. It must,

however, be stated that between Nalan and Pouynipéte, the

three McAskill Islands rise to a height of from 40 to 100 feet,
and consist, according to Dana (Corals and Coral Islands,
p- 806), of coral-rock; whether they are encircled or fringed by
coral-reefs does not seem to be known.

WESTERN PART OF THE CAROLINE ARCHIPELAGO.—Fais

Island is 90 feet high, and is surrounded, as I have been in- -

formed by Admiral Lutké, by a narrow reef of living coral,
of which the broadest part, as represented in the charts, is
only 150 yards; coloured red.—Philip Island, I believe, is
low; but Hunter, in his Historical Journal, gives no clear
account of it; uncoloured. ivi: from the manner in which
the islets on the reefs are engraved in the Atlas of the Astro-
labe’s Voyage, I should have thought they were above the
ordinary height; but Admiral Lutké assures me that this is
not the case: they form a regular atoll; coloured blue. Gouap
(Eap of Chamisso) is a high island with a reef (see Chart in
Voyage of Astrolabe) in most parts more than a mile distant

from the shore, and two miles in one part. Captain D’Urville

thinks that there would be anchorage (Hydrog. Descript.
Astrolabe Voyage, p. 486) for ships within the reef, if a
passage could be found; coloured pale blue——Goulou, from
the chart in the Astrolabe’s atlas, appears to be an atoll:
D’Urville (Hydrog. Descript. p. 487) speaks of low islets on
the reef; coloured dark blue.

PrLew IsLanps.—Krusenstern speaks of some of the islands
being mountainous; the reefs are distant from the shore, and
there are spaces within them, not opposite to any valley, from
10 to 15 fathoms deep. According to a MS. chart of the
group by Lieut. Hlmer in the Admiralty, there is a large space
within the reef with deepish water: although the high land

does not hold a central position with respect to the reefs, as is _

PACIFIC OCEAN. 2235

"generally the case, I have little doubt that the reefs of the

Pelew islands ought to be ranked in the barrier class, and I

have coloured them pale blue. In Lieut. Elmer’s chart there

is a horse-shoe-formed shoal, 13 miles N.W. of Pelew,
with 15 fathoms within the reef, and some dry banks on
it; coloured dark blue.—wSpanish, Martires, Sanserat, Pulo
Anna and Mariere Islands are not coloured, because I know
nothing about them, excepting that according to Krusenstern,
the second, third, and fourth mentioned, are low, placed on
coral-reefs, and therefore perhaps include a lagoon; but Pulo

Mariere is a little higher. Since the above remarks were

written Prof. Semper has published an interesting article
(Zeitschr. f. Wissensch. Zoologie. Bd. xiii. 1863, p. 558) on
these islands. He states that the southern islands consist of
coral-rock, upraised to the height of from 400 to 500 feet ;

~ and some of them, before their upheaval, seem to have existed

as atolls. ‘They are now merely fringed by living reefs. The
northern islands are volcanic, deeply indented by bays, and

are fronted by barrier-reefs. To the north there are three

true atolls. Prof. Semper doubts whether the whole group
has subsided, partly from the fact of the southern islands being

_ formed of upraised coral-rock ; but there seems to me no im-

probability in their having originally subsided, then having
been upraised (probably at the time when the volcanic rocks
to the north were erupted), and again having subsided. The
existence of atolls and barrier-reefs in close proximity is mani-
festly not opposed to my views. On the other hand, the
presence of reefs fringing the southern islands is opposed to
my views, as such reefs generally indicate that the land has
either long remained stationary, or has been upraised. It
must, however, be borne in mind (as remarked in our sixth
chapter) that when the land is prolonged beneath the sea in
an extremely steep slope, reefs formed there during subsidence
will remain closely attached to the shore, and will be undis-
tinguishable from fringing-reefs. Now we know that the

924 APPENDIX.

submarine flanks of most atolls are very steep; and if an atoll F
after upheaval and before the sea had eaten deeply into the 3
land, and had formed a broad flat surface, were again to sub-
side, the reefs which grew to the surface during the subsiding —

movement, would still closely skirt the coast. After some
hesitation, I have thought myself justified in leaving these

islands coloured blue.

Mariana ARCHIPELAGO, OR LapRoNES.—Guahan: almost

the whole of this island is fringed by reefs, which extend in
most parts about a third of a mile from the land. Hven where
the reefs are most extensive, the water within them is shallow.
In several parts there is a navigable channel for boats and
canoes within the reefs. In Freycinet’s Hydrog. Mem. there
is an account of these reefs, and in the atlas, a map on a large
scale; coloured red.—fota: ‘L’ile est presque entierement
entourée des récifs’ (p. 212, Freycinet’s Hydrog. Mem.).
These reefs project about a quarter of a mile from the shore ;
coloured red.—Tinian : the eastern coast is precipitous, and
is without reefs; but the western side is fringed like the last
island; coloured red.—Saypan: the N.E. coast, and like-
wise the western shores appear to be fringed; but there isa
great, irregular, horn-like reef projecting far from this side : |
coloured red.—Larallon de Medinilla appears so regularly
and closely-fringed in Freycinet’s charts, that I have ventured
to colour it red, although nothing is said about reefs in the
Hydrographical Memoir. The several islands which form the
northern part of the group are volcanic (with the exception
perhaps of Torres, which resembles in form the madreporitic
island of Medinilla), and appear to be without reefs.—Mangs,
however, is described (by Freycinet, p. 219, Hydrog.) from
some Spanish charts, as formed of small islands placed ‘au
milieu des nombreux récifs;’ and as these reefs in the general
chart of the group do not project so much as a mile; and as
there is no appearance from a double line, of the existence of
deep water within, I have ventured, although with much

PACIFIC OCEAN. - 995

hesitation, to colour them red. Respecting olger and Mar-
shall Islands, which lie some way east of the Marianas, I can
find out nothing, excepting that they are probably low. Kru-
senstern says this of Marshall Island; and Folger Island is
written with small letters in D’Urville’s chart; uncoloured.
Bonin or Arzopispo Group.—Peel Island has been ex-
amined by Captain Beechey, to whose kindness I am much
indebted for giving me information regarding it: ‘at Port
Lloyd there is a great deal of coral; and the inner harbour is
entirely formed by coral-reefs, which extend outside the port
along the coast.’ Captain Beechey, in another part of his letter
to me, alludes to the reefs fringing the island in all directions;
but at the same time it must be observed that the surf washes
the volcanic rocks of the coast in the greater part of its cir-
ceumference. This island has certainly been elevated at least
50 feet within the recent period (see Journal of Geolog. Soc.

1855, p- 982). I do not know whether the other islands of

the archipelago are fringed; I have coloured Peel island red.
—Grampus Island, to the eastward, does not appear (Meare’s
Voyage, p. 95) to have any reefs, nor does Rosario Island
(from Lutké’s chart), which lies to the westward. Respect-
ing the few other islands in this part of the sea, namely
the Sulphur Islands, with an active volcano, and those lying
between Bonin and Japan (situated near the extreme limit
in latitude at which reefs can grow), I have not been able
to find any clear account.

West Enp or New Guinea.—Port Dory: from the
charts in the Voyage of the Coquille, it would appear that
the coast in this part is fringed by coral-reefs; M. Lesson,
however, remarks that the corals are sickly; coloured red.
—Waigiou: a considerable portion of the northern shore of
these islands are seen in the charts (on a large scale) in Frey-
cinet’s Atlas to be fringed by coral-reefs. Forrest (p. 21,
Voyage to New Guinea) alludes to the coral-reefs lining the
heads of Piapis Bay; and Horsburgh (vol. uu. p. 599, 4th

Q

226 - APPENDIX.

edit.), speaking of the islands in Dampier Strait, says, ‘ sharp
coral-rocks line their shores;’ coloured red.—In the sea
north of these islands, we have Guedes (or Freewill, or St.
David's), which from the chart given in the 4to. edit. of
Carteret’s Voyage must be an atoll. Krusenstern says the
islets are very low; coloured blue.—Carteret’s Shoals, in
2° 58/ N., are described as circular, with stony points show-
ing all round, with deeper water in the middle; coloured
blue.—Azou: the plan of this group, given in the atlas of the
Voyage of the Astrolabe, shows that it is an atoll; and, from
a chart in Forrest’s Voyage, it appears there is 12 fathoms
within the circular reef; coloured blue-—The 8.W. coast of
New Guinea appears to be low, muddy, and devoid of reefs.
The Arru, Timor-laut and Tenimber Groups have lately been
examined by Captain Kolff, the MS. translation of which, by
Mr. W. Earl, I have been permitted to read, through the
kindness of Captain Washington, R.N. These islands are mostly
rather low, and are surrounded by distant reefs (the Ki
Islands, however, are lofty, and, from Mr. Stanley’s survey,
appear without reefs); the sea in some parts is shallow, in
others profoundly deep, as near Larrat. From the imper-
fection of the published charts, I have been unable to decide
to which class these reefs belong. From the distance to which
they extend from the land where the sea is very deep, I am
strongly inclined to believe they ought to come within the
_. barrier class, and be coloured blue; but I have been forced to
leave them uncoloured.—The last-mentioned groups are con-
nected with the east end of Ceram by a chain of small islands,
of which the small groups of Ceram-laut, Goram, and Keffing
are surrounded by very extensive reefs, projecting into deep
water, which, as in the last case, I strongly suspect belong to
the barrier class; but I have not coloured them. From the
south side of Keffing, the reefs project five miles (Windsor
Earl’s Sailing Direct. for the Arafura Sea, p. 9).

Crram.—In various charts which I have examined, several

EAST-INDIAN ARCHIPELAGO. 227

parts of the coast are represented as fringed by reefs.—Manipa
Island, between Ceram and Bourou, in an old MS. chart in
the Admiralty, is fringed by a very irregular reef, partly dry
at low water, which I do not doubt is of coral formation;
both islands coloured red.—Bourow: parts of this island
appear fringed by coral-reefs, namely, the eastern coast, as
seen in Freycinet’s chart; and Cajelz Bay, which is said by
Horsburgh (vol. ii. p. 630) to be lined by coral-reefs, that
stretch out a little way, and have only a few feet of water on
them. In several charts, portions of the islands forming the

—

Ampoina Group are fringed by reefs; for instance, Noessa,

Harenca, and Ucaster, in Freycinet’s charts. The above-
mentioned islands have been coloured red, although the
evidence is not very satisfactory—North of Bourou the
parallel line of the Xwlla Isles extends: I have not been
able to find out anything about them, excepting that Hors-
burgh (vol. ii. p. 548) says that the northern shore is sur-
rounded by a reef at the distance of two or three miles;
uncoloured.—Mysol Group: the Kanary Islands are said by
Forrest (Voyage, p. 180) to be divided from each other by
deep straits, and are lined with coral-rocks; coloured red.—
Guebe, lying between Waigiou and Gilolo, is engraved as if
fringed; and it is said by Freycinet, that all the soundings
under five fathoms were on coral; coloured red.—Gilolo : in
a chart published by Dalrymple, the numerous islands on the
western, southern (Batchian and the Strait of Patientia),

_ and eastern sides appear fringed by narrow reefs; these reefs,

I suppose, are of coral, for it is said in Malte Brun (vol. xii.
p- 156), ‘sur les cétes (of Batchian), comme dans la plupart
des iles de cet archipel, il y a des rocs de madrepores d’une
beauté et d’une variété infinies.’ Forrest, also (p. 50), says
Seland, near Batchian, is a little island with reefs of coral;
coloured red.—Morty Island (north of Gilolo): Horsburgh
(vol. i. p. 506) says the northern coast is lined by reefs,
projecting one or two miles, and having no soundings close
Q 2

228 APPENDIX.

to them; I have left it uncoloured, although, as in some —
former cases, it ought probably to be pale blue.—Celedes.
The western and northern coasts appear in the charts to be
bold and without reefs. Near the extreme northern point,
however, an islet in the Straits of Zzmbe, and part of the
adjoining shore, appear to be fringed: the east side of
the bay of Manado has deep water, and is fringed by sand
and coral (Astrol. Voyage, Hydrog. Part, p. 4538-4); this
extreme point, therefore, I have coloured red. Captain Keppell,
also, speaks (Expedition to Borneo, vol. i. p. 180) of the
shore being in parts fringed with reefs; he found upraised
coral-reefs at the height of from 80 to 100 feet above the
level of the sea.—Of the islands between the northern point of
Celebes and the Phillippines, I have not been able to find any
account, except of Serangani, which appears surrounded by
narrow reefs; and Forrest (Voyage, p. 164) speaks of coral on
its shores; I have, therefore, coloured this island red. ‘To the
eastward of this chain lie several islands; of which I cannot
find any account, except of Karkalang, which is said by
Horsburgh (vol. ii. p. 504) to be lined by a dangerous reef,
projecting several miles from the northern shore ; not coloured.

IsLANDS NEAR Timor.—The account of the following islands
is taken from Captain D. Kolff’s Voyage in 1825, translated by
Mr. W. Earl from the Dutch.—Lette has ‘ reefs extending
along shore at the distance of half a mile from the land.’-—
Moa has reefs on the 8.W. part.—Lakor has a reef lining its
shore; these islands are coloured red.—Still more eastward,
Tuan, differently from the last-mentioned islands, has an
extensive reef; it is steep outside, and within there is a
depth of 12 feet; from these facts, it is impossible to
decide to which class this island belongs.—K7vssa, off the
point of Timor, has its ‘shore fronted by a reef, steep too
on the outer side, over which small proahs can go at the time
of high water ;’ coloured red.— Timor: most of the points,
and. some considerable spaces of the northern shore, are seen

ie
t EAST-INDIAN ARCHIPELAGO. 229

in Freycinet’s chart to be fringed by coral-reefs; and mention
is made of them in the accompanying Hydrog. Memoir ;
* coloured red.—Savu, S.E. of Timor, appears in Flinders’
chart to be fringed; but I have not coloured it, as I do not
know that the reefs are of coral.—Sandalwood Island has,
according to Horsburgh (vol. ii. p. 607), a reef on its
southern shore, four miles distant from the land; as the
neighbouring sea is deep, and generally bold, this probably
is a barrier-reef, but I have not ventured to colour it.

N.W. Coast or AuSTRALIA.—It appears, in Captain King’s
Sailing Directions (Narrative of Survey, vol. il. pp. 325 to
369), that there are. many extensive coral-reefs skirting,
often at considerable distances, the N.W. shores, and en-
compassing the small adjoining islets. Deep water, in no
instance, is represented in the charts between these reefs
and the land; and, therefore, they probably belong to the
fringing class. But as they extend far into the sea, which
is generally shallow, even in places where the land seems
to be somewhat precipitous, I have not coloured them.
Houtman’s Abrolhos (lat. 28° S. on west coast) have lately
been surveyed by Captain Wickham (as described in Naut.
Mag. 1841, p. 511): they lie on the edge of a steeply-
shelving bank, which extends about 30 miles seaward, along
the whole line of coast. The two southern reefs, or islands,
enclose a lagoon-like space of water, varying in depth from
5 to 15 fathoms, and in one spot with 23 fathoms. The
greater part of the land has been formed on their inland
sides, by the accumulation of fragments of corals; the
seaward face consisting of nearly bare ledges of rock. Some
of the specimens, brought home by Captain Wickham,
contained fragments of marine shells, but others did not;
and these closely resembled a formation at King George’s
Sound, principally due to the action of the wind on calcareous
dust, which I have described in my work on Volcanic Islands.
From the extreme irregularity of these reefs with their lagoons,

230 APPENDIX.

and from their position on a bank, the usual depth of which
is only 30 fathoms, I have not ventured to class them with
atolls, and hence have left them uncoloured.—Rowley Shoals:
these lie some way from the N.W. coast of Australia : accord-
ing to Captain King (Narrative of Survey, vol. i. p. 60), they
are of coral-formation. They rise abruptly from the sea, and
Captain King found no bottom with 170 fathoms close to them.
Three of them are crescent-shaped; ‘a third oval reef of
the same group is entirely submerged’ (Lyell, Principles of
Geolog., book iii. chap. xviii.) ; coloured blue.—WScott’s Reefs,
lying north of Rowley Shoals, are briefly described by Captain
Wickham (Naut. Mag., 1841, p. 440) as of great size, of a
circular form, and ‘with smooth water within, forming pro-
bably a lagoon of great extent.’ There is a break on the
western side, where there probably is an entrance: the water
is very deep off these reefs; coloured blue.

Proceeding westward along the great volcanic chain of the
Kast Indian or Malay Archipelago, Solor Strait is represented
as fringed in a chart published by Dalrymple from a Dutch
MS.; as are parts of flores, Adenara, and Solor.. Horsburgh
speaks of coral growing on these shores, and therefore I have
no doubt that the reefs are of coral, and have coloured them
red. We hear from Horsburgh (vol. 11. p. 602) that a coral
flat bounds the shores of Sapy Bay. From the same autho-
rity it appears (p. 610) that reefs fringe the island of Z%mar-
Young, on the N. shore of Sumbawa; and likewise (p. 600)
that Bally town in Lombock, is fronted by a reef, stretching
along the shore at the distance of a hundred fathoms, with
channels through it for boats; these places, therefore, have
been coloured red.—Bally Island: ina Dutch MS. chart on a
large scale of Java, which was brought from that island by
Dr. Horsfield, who had the kindness to show it me at the
India House, its western, northern, and southern shores ap-
pear regularly fringed by a reef (see also Horsburgh, vol. il.
p- 993); and as coral is found abundantly there, I have no

EAST-INDIAN ARCHIPELAGO. 231

doubt that the reef is of coral, and therefore have coloured
it red.

JAvA.—My information regarding the reefs of this great
island is derived from the chart just mentioned. ‘The greater
part of Madura is represented in it as regularly fringed, and
likewise portions of the coast of Java immediately south of it.
Dr. Horsfield informs me that coral is very abundant near
Sourabaya. The adjoining islets, and parts of the N. coast of
Java, west of Point Buang, or Japara, are fringed by reefs,
said to be of coral. Lubeck, or Bavian Islands, lying at some
distance from the shore of Java, are regularly fringed by coral-
reefs: Curimon Java appears equally so, though it is not
directly said that the reefs are of coral; there is a depth of
between 30 and 40 fathoms round these islands. Parts of the
shore of Sunda Straits, where the water is from 40 to 80
fathoms deep, and the islets near Batavia appear in several
charts to be fringed. In the Dutch chart the southern shore,
in the narrowest part of the island, is in two places fringed by
reefs of coral. West of Segorrowodee Bay, and the extreme
S.E. and E. shores are likewise fringed by coral-reefs; all the
above-mentioned places coloured red.

Macassar Strait: the east coast of Borneo appears, in most
parts, free from reefs, and where they occur, as on the coast
of Pamaroong, the sea is very shallow; hence no part is
coloured. In Macassar Straititself, in about lat. 2° S., there
are many small islands with coral shoals projecting far from
them. There are also (old charts by Dalrymple) numerous
little flats of coral, not rising to the surface of the water, and
shelving suddenly from five fathoms to no bottom with 50
fathoms; they do not appear to have a lagoon-like structure,
There are similar coral-shoals a little farther south ; and in lat.
4° 55’ there are two, which are engraved from modern sur-
veys, in a manner which may represent an annular reef with
deep water inside: Capt. Moresby, however, who was formerly
in this sea, doubts this fact, so that I have left them un-

232 APPENDIX.

coloured : at the same time I may remark, that these two
shoals make a nearer approach to the atoll-like structure than
any other within the E. Indian Archipelago. Southward of these
shoals there are other low islands and irregular coral-reefs ;
and in the space of sea, north of the great volcanic chain, from
Timor to Java, we have other islands, such as the Postillions,
Kalatoa, Tokan-Bessees, &c., which are chiefly low, and are
surrounded by very irregular and distant reefs. From the
imperfect charts I have seen, I have not been able to decide
whether they belong to the atoll or barrier classes, or whether
they merely fringe submarine banks, and gently sloping land.
In the Bay of Bonin, between the two southern arms of Celebes,
there are numerous coral-reefs ; but none of them seem to have
an atoll-like structure. I have, therefore, not coloured any of
the islands in this part of the sea; I think it, however, ex-
_ ceedingly probable that some of them ought to be blue. I
may add that there is a harbour on the 8.H. coast of Bouton,
which, according to an old chart, is formed by a reef, parallel
to the shore, with deep water within; and in the Voyage of
the Coquille, some neighbouring islands are represented with
distant reefs, but I do not know whether with deep water
within. I have not thought the evidence sufficient to permit
me to colour them.

SuMATRA.—Commencing with the west coast and outlying’
islands; Hngano Island is represented in the published chart as
surrounded by anarrow reef, and Napier, in his Sailing Direc-
tions, speaks of the reef being of coral (also Horsburgh, vol. 11.
p- 115); coloured red. at Island (8° 51’ S.) is surrounded
by reefs of coral, partly dry at low water (Horsburgh, vol. ii.
p- 96).—Trieste Island (4° 2’ S.)\: the shore is represented,
in a chart which I saw at the India House, as fringed in such
a manner, that I feel sure the fringe consists of coral; but as
the island is so low that the sea sometimes flows quite over it
(Dampier, Voyage, vol. i. p. 474) I have not coloured it.—
Pulo Dooa (lat. 8°): it is said in an old chart that there are

EAST-INDIAN ARCHIPELAGO. 2as

chasms in the reef round the island, admitting boats to the
watering-place, and that the southern islet consists of a mass
of sand and coral.—Pulo Pisang: Horsburgh (vol. il. p. 86)
says that the rocky coral-bank, which stretches about 40
yards from the shore, is steep all round: in a chart, also,
which I have seen, the island is represented as regularly fringed.
—Pulo Mintao is lined with reefs on its west side (Horsburgh,
vol. ii. p. 107).—Pulo Baniak: the same authority (vol. ii.
p- 105), speaking of a part, says it is faced with coral rocks.—
Minguin (3° 36’ N.): acoral reef fronts this place, and projects
into the sea nearly a quarter of a mile (Notices of the Indian
Archipelago, published at Singapore, p. 105).—Pulo Brassa
(5° 46’ N.): areef surrounds it at a cable’s length (Horsburgh,
vol. ii. p. 60). I have coloured all the above specified points
red. I may here add, that both Horsburgh and Mr. Moor (in
the Notices just alluded to) frequently speak of the numerous
reefs and banks of coral on the west coast of Sumatra; but
they nowhere have the structure of a barrier-reef, and Marsden
(History of Sumatra) states that where the coast js flat, the
fringing-reefs extend far from it. The northern and southern
points, and the greater part of the east coast, are low, and
faced with mud banks, and therefore without coral.

Nicozar Istanps.—The chart represents the islands of this
group as fringed by reefs. With regard to Great Nicobar,
Captain Moresby informs me that itis fringed by reefs of coral,

extending between 200 and 300 yards from the shore. The

Northern Nicobars appear so regularly fringed in the published
charts, that I have no doubt the reefs are of coral. This group,
therefore, is coloured red.

AnDaMAN IsLanps.—From an esuuiiateg of the MS.
chart, on a large scale, of this island, by Captain Arch. Blair, in
the Admiralty, several portions of the coast appear fringed;
and as Horsburgh speaks of coral-reefs being numerous in the
vicinity of these islands, I should have coloured them red, had
not some expressions in a paper in the Asiatic Researches

234 APPENDIX.

(vol. iv. p. 402) led me to doubt the existence of reefs; un-
coloured.

The coast of Malacca, Tanasserim, and the coasts north-
ward, appear in the greater part to be low and muddy: where
reefs occur, as in parts of Malacca Straits, and near Singapore,
they are of the fringing kind; but the water is so shoal, that I

have not colouredthem. In the sea, however, between Malacca _

and the west coast of Borneo, where there is a greater depth
from 40 to 50 fathoms, I have coloured red some of the groups,
which are regularly fringed. The northern Natunas and the

Anambas Islands are represented in the charts on a large scale, .

published in the atlas of the Voyage of the Favourite, as
fringed by reefs of coral, with very shoal water within them.
Tumbelan and Bunoa Islands (1° N.) are represented in the
English charts as surrounded by a very regular fringe.—Sz?.
Barbes (0° 15’ N.) is said by Horsburgh (vol. 1. p. 279) to be
fronted by a reef, over which boats can land only at high
water.—The shore of Borneo, at Tunjong Apee, is aiso fronted
by a reef, extending not far from the land (Horsburgh, vol. 11.
p- 468). These places I have coloured red; although with
some hesitation, as the water is shallow. I might perhaps
have added Pulo Leat, in Gaspar Strait, Lucepara and Cari-
mata; but as the sea is confined and shallow, and the reefs
not very regular, I have left them uncoloured.

The water deepens very gradually from the whole west coast

of Borneo ; and I cannot make out that it has any reefs of coral. j

The islands, however, off the northern extremity, and near

the S.W. end of Palawan, are fringed by very distant coral |

reefs: thus the reefs off Balabac are no less than five miles
from the land; but the sea, in the whole of this district, is so
shallow, that the reefs might be expected to extend very far
from the land. I have not, therefore, thought myself autho-
rized to colour them. The N.E. point of Borneo, where the
water is very shoal, is connected with Magindanao by a chain

of islands called the Sooloo Archipelago, about which I have—

been able to obtain very little information; Pangootaran,

EAST-INDIAN ARCHIPELAGO. 235

although ten miles long, entirely consists of a bed of coral-rock
(Notices of E. Indian Arch. p. 58): I believe from Horsburgh
that the island is low; not coloured.—Tahow Bank, in some
old charts, appears like a submerged atoll; not coloured.
Forrest (Voyage, p. 21) states that one of the islands near
Sooloo is surrounded by coral-rocks; but there is no distant
reef. Near the S. end of Basselan, some of the islets in the
chart accompanying Forrest’s Voyage appear fringed with reefs;
hence I have coloured, though unwillingly, parts of the Sooloo
group red. ‘The sea between Sooloo and Palawan, near the
shoal coast of Borneo, is interspersed with irregular reefs and
shoal patches; not coloured: but in the northern part of this sea

there are two low islets, Cagayanes and Cavilli, surrounded

by extensive coral-reefs; the breakers round the latter
(Horsburgh, vol. ii. p. 513) extend five or six miles from a
sand-bank, which forms the only dry part; these breakers are
steep to outside: there appears to be an opening through the
reef on one side, with four or five fathoms within: from this de-
scription, I strongly suspect that Cavilli ought to be considered
an atoll; but, as I have not seen any chart of it, even on a
moderately large scale, I have not coloured it. The islets off
the northern end of Palawan are, like those off the southern

- end, fringed by reefs, some way distant from the shore, but

the water is exceedingly shallow ; uncoloured. The western
shore of Palawan will be treated of under the China Sea.
PHILLIPPINE ARCHIPELAGO.—A chart on a large scale of
Appoo Shoal, which lies near the 8.E. coast of Mindoro, has
been executed by Captain D. Ross: it appears atoll-formed, but
with rather an irregular outline; its diameter is about ten
miles; there are two well-defined passages leading into the
lagoon; close outside and all round the reef, there is no
bottom with 70 fathoms; coloured blue.—Mindoro: the
N.W. coast is represented in several charts as fringed by a
reef; and Luban Island is said by Horsburgh (vol. i. p. 436)
to be ‘ lined by a reef..—Luzon: Mr. Cuming, who has lately

236 | APPENDIX.

investigated with so much success the Natural History of the
Phillippines, informs me that a length of about three miles of
the shore northward of Point St. Jago is fringed by a reef; as
are (Horsburgh, vol. ii. p. 437) the Three Friars off Silanguin
Bay. Between Point Capones and Playa Honda, the coast is
“lined by a coral-reef, stretching out nearly a mile in some
places’ (Horsburgh) ; and Mr. Cuming visited some fringing-
reefs on. other parts of the coast, namely, near Puebla, Iba, and
Mansinglor. In the neighbourhood of Solon-solon Bay, the
shore is lined (Horsburgh, vol.ii. p. 489) by coral-reefs, stretch-
ing out a great way: there are also reefs about the islets off
Solamague; and as I am informed by Mr. Cuming, near St.
Catalina, and a little north of it. The same gentleman informs
me that there are reefson the S.E. point of this island in front
of Samar, extending from Malalabon to Bulusan. ‘These
appear to be the principal fringing-reefs on the coasts of
Luzon; and they have all been coloured red. Mr. Cuming
informs me that none of them have deep water within; al-
though it appears from Horsburgh that.some few extend to a
considerable distance from the land. Within the Phillippine
Archipelago, the shores of the islands do not appear to be
commonly fringed, with the exception of the S. shore of Mas-
bate, and nearly the whole of Bohol; which are both coloured
red. On the S. shore of M/agindanao, Bunwoot Island is sur-
rounded (according to Forrest,’Voyage, p. 253) by a ccral-
reef, which in the chart appears one of the fringing class.
With respect to the eastern coasts of the archipelago, I have
not been able to obtain any account. Prof. Semper has re-
cently published a notice (Zeitschr. f. Wissensch. Zoologie, Bd.
xill. 1863, p. 558) respecting the coral-reefs of this archi-
pelago. It appears that some of them come under the class
of barrier-reefs; but as I have not seen a chart on a large
scale, and know nothing about the depth of the water outside
the reefs, nor about the slope of the encircled land, I cannot
judge whether they properly come under the barrier class.

EAST-INDIAN ARCHIPELAGO. 237

BasuyAan Istanps.—Horsburgh says (vol. ii. p. 442) coral-
reefs line the shores of the harbour in Fuga; and the charts
show there are other reefs about these islands. _ Camiguin has
its shore in parts lined by coral-rock (Horsburgh, p. 443); and
about a mile off shore the depth is between 30 and 35 fathoms.
The plan of Port San Pio Quinto shows that its shores are
fringed with coral; coloured red.—Basuee Istanps: Hors-
burgh, speaking of the southern part of the group (vol. i.
p- 445), says the shores of both islands are fortified by a reef,
and through some of the gaps in it the natives can pass in
their boats in fine weather; the bottom near the land is coral-
rock. From the published charts, it is evident that several
of these islands are regularly fringed; coloured red. The
northern islands are left uncoloured, as I have been unable to
find any account of them.—Formosa: the shores, especially
the western one, seem composed chiefly of mud and sand, and
I cannot make out that they are anywhere lined by reefs, ex-
‘cept in a harbour (Horsburgh, vol. i. p. 449) at the extreme
northern point: hence, of course, the whole of this island is left
uncoloured. The small adjoining islands are in the same case.
—PatcHow, or Mapsixo-sima Groups: Patchuson: Captain
Broughton says (Voyage to the N. Pacific, p. 191) that boats,

_ with some difficulty, can pass through the coral-reefs, which

extend along the coast, nearly half a mile off it. His boats were
well sheltered within the reef; but it does not appear that the
water is deep there. Outside the reef the depth is very irregu-
lar, varying from 5 to 50 fathoms; the form of the land is not
very abrupt; coloured red.—Taypin-san: from the descrip-
tion given by the same author (p. 195) it appears that a
very irregular reef extends from the southern island to the
distance of several miles; but whether it encircles a space of
deep water is not evident; nor, indeed, whether these outly-
ing reefs are connected with those more immediately adjoining
the land; left uncoloured. I may here add that the shore of
Kumi (lying west of Patchow) has a narrow reef attached to

238 APPENDIX.

it in the plan, in La Peyrouse’s Atlas; but in the account of
the voyage it is not stated to be of coral; uncoloured.—Loo
Cuoo: the greater part of the coast of this moderately hilly
island is skirted by reefs, which do not extend far from the
shore, and which do not leave a channel of deep water within
them, as may be seen in the charts accompanying Captain B.
Hall’s Voyage to Loo Choo (see also remarks in Appendix,
p- Xxl. and xxv.) ‘There are, however, some ports with deep
water, formed by reefs in front of the valleys, in the same
manner as happens at Mauritius. Captain Beechey, in a letter
to me, compares these reefs with those encircling the Society
Islands; but there appears to be a marked difference between
them, in the less distance at which the Loo Choo reefs lie from
the land with relation to the probable submarine inclination
of the land, and in the absence of an interior deep-water
channel. Hence I have classed these reefs with fringing-reefs,
and coloured them red.—PrscaporEs (west of Formosa) :
Dampier (vol. i. p. 416) has compared the appearance of
these islands to the southern parts of England ; they are inter-
laced with coral-reefs; but as the water is very shoal, and as
spits of sand and gravel (Horsburgh, vol. 11. p. 400) extend far
out from them, it is impossible to decide whether the reefs are
of coral.

Cuina Sra.—Proceeding from north to south, we first
meet the Pratas Shoal (lat. 20° N.), which, according to Hors-
burgh (vol. ii. p. 385), is composed of coral, is of a circular
form, and has a low islet on it. ‘The reef is on a level with
the water’s edge, and when the sea runs high, there are
breakers nearly all round; ‘the water within seems pretty
deep in some places; although steep in most parts outside,
there appear to be several parts where a ship might find
anchorage outside the breakers ; ’ coloured blue-—The Para-
celis have been accurately surveyed by Captain D. Ross, and
charts on a large scale published: only a few low islets have

been formed on these shoals, and this seems to be a general

—

circumstance in the China Sea; the sea close outside these reefs
is deep; several of them have a lagoon-like structure; in
other cases separate islets (Prattle, Robert, Drummond, &c.)
are so arranged round a moderately shallow space as to ap-
pear as if they had once formed one large atoll.—Bombay
Shoal (one of the Paracells) has the form of an annular reef,
and is ‘apparently deep within ;’ it seems to have an entrance
(Horsburgh, vol. ii. p. 332) on the west side; it is very steep
outside.—Dziscovery Shoal, also, is of an oval form, with a
lagoon-like space within, and three openings leading into it, in
which there is a depth from 2 to 20 fathoms. Outside,
at the distance (Horsburgh, vol. il. p. 333) of only 20
yards from the reef, soundings could not be obtained. The
Paracells are coloured blue.—Macclesfield Bank is a coral
bank of great size, lying east of the Paracells; some parts are
level, with a sandy bottom, but generally the depth is very
irregular, and intersected by deep channels; not coloured.—
Scarborough Shoal: this coral shoal is engraved with a
double row of crosses, forming a circle, as if there was deep
water within the reef: close outside no bottom was found
with a hundred fathoms; coloured blue.—The sea off the
west coast of Palawan and the northern part of Borneo is
strewed with shoals: Swallow Shoal, according to Horsburgh
(vol. il. p. 451), ‘is formed, uke most of the shoals hereabouts,
of a belt of coral-rocks, with a basin of deeper water within.’
—Half-Moon Shoal has a similar structure ; Captain D. Ross
- describes it as.a narrow belt of coral-rock, ‘ with a basin of
deeper water in the centre,’ and deep sea close outside.—Bom-
bay Shoal appears (Horsburgh, vol. ii. p. 482) ‘ to be a basin
___ of smooth water surrounded by breakers.’ I have coloured
* these three shoals blue-—The Paraquas Shoals are of a cir-
cular form, with deep gaps running through them; not
coloured. A bank, gradually shoaling to the depth of 30
fathoms, extends to a distance of about 20 miles from the
__ northern part of Borneo, and to 30 miles from the northern

EAST-INDIAN ARCHIPELAGO. 239

240 APPENDIX.

part of Palawan; near the land this hank appears tolerably
free from danger, but a little further out is thickly studded
with coral-reefs, which do not generally rise to the surface;
some of them are very steep, whilst others have a fringe of
shoal-water round them. I should have thought that these
shoals had level surfaces, had it not been for a statement made
by Horsburgh, ‘ that most of the shoals hereabouts are formed
of a belt of coral:’ I have not coloured them.—The coasts
of China, Tonquin, and Cochin-China, forming the western
boundary of the China Sea, appear to be. without reefs: with
regard to the two last-mentioned coasts, I judge from an ex-
amination of the charts on a large scale in the atlas of the
Voyage of the Favourite.

Inp1AN Ocran.—South Keeling atoll has been specially
described in my first chapter. Nine miles north of it lies
North Keeling, a very small atoll, surveyed by the Beagle,
the lagoon of which is dry at low water.—Christmas Island,
lying to the east, is a high island, without, as I have been
informed, any reef.—CEYLON : a space of about 80 miles
in length on the south-western and southern shores of these
islands has been described by Mr. Twynam (Naut. Mag.
1836, pp. 365 and 518); and parts appear to be regularly
fringed by coral-reefs, which extend from a quarter to half
a mile from the shore. These reefs are in places breached,
and afford safe anchorage for the small trading craft. Outside,
the sea gradually deepens; there is 40 fathoms about six
miles off shore: I have coloured these reefs red. In the
published charts of Ceylon, reefs also appéar to fringe several

parts of the south-eastern shores, coloured red.—At Venloos

Bay the shore is likewise fringed. North of Trincomalee
there are also reefs of the same character. The sea off the
northern part of Ceylon is exceedingly shallow; and there-
fore I have not coloured the reefs which partially fringe por-
tions of the shores, and the adjoining islets, as well as the
Indian promontory of Madura.

INDIAN OCEAN. 241

Cuacos, Matpiva, anD LaccaDIvVe ARCHIPELAGOES.—'T'hese
three great groups of atolls and atoll-formed banks, have been
often referred to in this volume, and are now well known from
the admirable surveys of Captain Moresby and Lieut. Poweli.
Their published charts are worthy of the most attentive ex-
amination. In the Laccadive group, the atoll-like structure
is less evident than in the Maldivas; nevertheless the islands
are all low, not exceeding the usual height of coral formations
(see Lieut. Wood’s account, Geograph. Journ. vol. vi. p. 29),
and most of the reefs are circular; within several of them, as
Iam informed by Captain Moresby, there is deepish water ;
these, therefore, have been coloured blue. Directly north of
the Laccadives, and almost forming part of the same group,
there is a long, narrow, slightly-curved bank, rising out of
the depths of the ocean, composed of sand shells and decayed
coral, with from 23 to 30 fathoms on it. I have no doubt
that it has had the same origin with the other atoll-like
banks; but as it does not deepen towards the centre, I have
not coloured it. J might have referred to other authorities
regarding these three archipelagoes; but after the publication
of the charts by Captain Moresby (to whose personal kind-
ness in giving me much information I am exceedingly in-
debted), this would have been superfluous.

The Sahia de Malha Bank consists of « series of narrow
banks, with from 8 to 16 fathoms on them; they are arranged
in a semi-circular manner, round a space about 40 fathoms
in depth, sloping to the S.E. to unfathomable depths; they
are steep on both sides, but more especially on the ocean-
side. Hence this bank closely resembles in structure, and
I may add from Captain Moresby’s information in composition,

_ Pitt’s Bank in the Chagos group; and Pitt’s Bank must, from

what we know about the great Chagos Bank, be considered

as a sunken, half-destroyed atoll; hence coloured blue.—

Cargados Carajos Bank: its southern portion consists of a

large, curved coral-shoal, with some low islets on the eastern
R

Ei «ion
Fee wih ce ie
Po ae

242 APPENDIX.

edge, and likewise some on the western side, between which
there is a depth of about 12 fathoms: northward, a great
bank extends. I cannot (probably owing to the want of per-
fect charts) refer this reef and bank to any class; therefore
not coloured.—Ile de Sable is a little island, lying west of C.
Carajos, only some toises in height (Voyage of the Favourite,
vol. 1. p. 180); it is surrounded by reefs; but its structure is
unintelligible to me. There are some small banks north of it,
of which I can find no clear account.— Mauritius: the reefs
round this island have been described in the chapter on fring-
ing-reefs; coloured red.—fodriguez: the coral-reefs here
are very extensive; in one part they project even five miles
from the shore. As far as I can make out, there is no deep—
water channel within them; and the sea outside does not
deepen very suddenly. The outline, however, of the land
appears to be (Life of Sir J. Makintosh, vol. ii. p. 165)
hilly and rugged. I am unable to decide whether these
reefs belong to the barrier class, as seems probable from
their great extension, or to the fringing class; uncoloured.
Bourbon: the greater part of the shores of this island are
without reefs; but Captain Carmichael (Hooker’s Bot. Misc.)
states that a portion, 15 miles in length, on the S.H. side,
is imperfectly fringed with coral-reefs: I have not thought
this sufficient evidence for colouring the island.
SEYCHELLES.—The rocky islands of primary formation,
composing this group, rise from a very extensive and toler-
ably level bank, having a depth of between 20 and 40
fathoms. In Captain Owen’s chart, and in that in the atlas
of the Voyage of the Favourite, it appears that the east side
of Mahé and the adjoining islets of St. Anne and Cerf, are
regulariy fringed by coral-reefs. A portion of the S.E. part
of Curieuse Island, the N. and part of the S.W. shore of
Prastin Island, and the whole west side of Digue Island, appear
fringed. From a MS. account of these islands by Captain F.
Moresby, in the Admiralty, it appears that Silhouette is also —

INDIAN OCEAN. 243

fringed ; he states that all these islands are formed of granite
and quartz, that they rise abruptly from the sea, and that
‘coral-reefs have grown round them, and project for some
distance.’ Dr. Allan of Forres, who visited these islands,
informs me that there is no deep water between the reefs and
the shore. The above specified points have been coloured
red. Amrantes Islands: the small islands of this neigh-
bouring group, according to the MS. account of them by
Captain F. Moresby, are situated on an extensive bank; they
consist of the débris of corals and shells; they are only about
20 feet in height, and are environed by reefs, some attached
to the shore, and some rather distant from it.—I have taken

‘pains to procure plans and information regarding the several

islands which he to the S.E. and S.W. of the Seychelles;
from accounts given me by Captain F. Moresby and Dr. Allan,
it appears that the greater number—namely, Platte, Alphonse,
Coetivi, Galega, Providence, St. Pierre, Astova, Assomption,
and Glorioso—are low, formed of sand or coral-rock, and irregu-
larly shaped; they are situated on very extensive banks, and
are in connection with great coral-reefs. Galega is said by
Dr. Allan to be rather higher than the others; and St.
Pierre is described by Captain F. Moresby as being cavernous
throughout, and as not consisting of either limestone or
eranite. “These islands, as well as the Amirantes, certainly
are not atoll-formed, and they seem to differ from all other
groups; I have not coloured them; but probably the reefs
belong to the fringing class. Their formation is attributed
both by Dr. Allan and Captain I’. Moresby, to the action of the
currents, here exceedingly violent, on banks which no doubt
have had an independent geological origin. They resemble
in many respects some of the islands and. banks in the West
Indies, which owe their origin to a similar agency, in ¢onjunc-
tion with an elevation of the entire area. In close vicinity to
the above several islands, there are three others of an ap-
parently different nature; first, Juan de Nova, which appears

Ree

244 APPENDIX.

from some plans and accounts to be an atoll, but from others
this does not appear to be the case; not coloured. Secondly,
Cosmoledo : ‘ this group consists of a ring of coral, ten leagues
in circumference, and a quarter of a mile broad in some places,
inclosing a magnificent lagoon, into which there did not appear
a single opening’ (Horsburgh, vol. i. p. 151); coloured blue.
Thirdly, Aldabra: consists of three islets, about 25 feet in
height, with red cliffs (Horsburgh, vol. i. p. 176), surround-
ing a very shallow basin or lagoon. The sea is profoundly
deep close to the shore. Viewing this island in a chart, it
would be thought to be an atoll; but the foregoing descrip-
tion shows that there is something different in its nature; Dr.
Allan also states that it is cavernous, and that the coral-rock
has a vitrified appearance. Is it an upheaved atoll, or the
crater of a volcano ?—uncoloured. .
Comoro Grour.—Mayotia, according to Horsburgh (vol. i.
p. 216, 4th edit.), is completely surrounded by a reef, which
runs at the distance of three, four, and in some places even
_ five miles from the land; in an old chart, published by
Dalrymple, a depth in many places of 36 and 38 fathoms is
laid down within the reef. In the same chart, the space of
open water within the reef is in some parts even more than
three miles wide: the land is bold and peaked; this island,
therefore, is encircled by a well-characterized barrier-reef,
and is coloured pale blue.-—Johanna: Horsburgh says (vol. i.
p- 217), this island from the N.W. to the S.W. point, is
bounded by a reef, at the distance of two miles from the
shore; in some parts, however, the reef must be attached,
since Lieut. Boteler (Narr. vol. i. p. 161) describes a passage
through it, within which there is room only for a few boats.
Its height, as I am informed by Dr. Allan, is about 3,500
feet; it is very precipitous, and is composed of granite,
greenstone, and quartz; coloured blue.—Mohilla: on the
S. side of this island there is anchorage between a reef and
the shore in from 30 to 405 fathoms (Horsburgh, vol. 1.

i.
|g

M4

INDIAN OCEAN. 245

p- 214); it appears also encircled in Captain Owen’s chart of
Madagascar; coloured blue-—Great Comoro Island is, as I
am informed by Dr. Allan, about 8,000 feet high, and ap-
parently volcanic; it is not regularly encircled; but reefs
of various shapes and dimensions jut out from every head-
land on the W.,8S., and S.E. coasts, inside of which reefs
there are channels, often parallel with the shore, with deep
water. On the N.W. coasts the reefs appear attached to the
shore. The land near the coast is in some places bold, but
generally speaking it is flat; Horsburgh says (vol. 1.
p- 214), the water is profoundly deep close to the shore, from
which expression I presume some parts are without reefs.
From this description, I apprehend the reef belongs to the
barrier class; but I have not coloured it, as most of the
charts which I have seen represent the reefs round it as very
much less extensive than round the other islands of the group.

Mapacascar.—My information is chiefly derived from the
published charts by Captain Owen, and the accounts given by
him and by Lieut, Boteler. Commencing at the 8.W. ex-
tremity of the island : towards the northern part of Star Bank
(in lat. 25° S.) the coast for ten miles is fringed by a reef;
coloured red. The shore immediately S. of St. Augustin’s
Bay appears fringed; but Tullear Harbour, directly N. of it,
is formed by a narrow reef ten miles long, extending parallel
to the shore, with from 4 to 10 fathoms within it. If this
reef had been more extensive, it must have been classed as a
barrier-reef; but as the line of coast falls inwards here, a
submarine bank perhaps extends parallel to the shore, which
has offered a foundation for the growth of the coral; I have
left this part uncoloured. From lat. 22° 16’ to 21° 37’, the
shore is fringed by coral-reefs (see Lieut. Boteler’s Narrative,
vol. ii. p. 106), less than a mile in width, and with shallow
water within. There are outlying coral shoals in several
parts of the offing, with about 10 fathoms between them and
the shore, and the depth of the sea one mile and a half sea-

246 | APPENDIX.

ward, is only about 30 fathoms. 'The part above specified is |

engraved on a large scale; and as in the charts on rather a
smaller scale the same fringe of reef extends as far as lat.
33° 15’, I have coloured the whole of this part of the coast
red. The islands of Juan de Nova (in lat. 17° S.) appear in
the charts on a large scale to be fringed, but I have not been

able to ascertain whether the reefs are of coral; uncoloured. —

The main part of the west coast appears to be low, with out-
lying sand banks, which Lieut. Boteler (vol. ii. p. 106) says,
‘are faced on the edge of deep water by a line of sharp-pointed
coral-rocks.’ Nevertheless I have not coloured this part, as I
cannot make out by the charts that the coast itself is fringed.
_ The headlands of Narrenda and Passandava Bays (14° 40’)
and the islands in front of Radama harbour are presented in
the plans as regularly fringed, and have accordingly been
coloured red. With respect to the Hast coast of Madagascar,
Dr. Allan informs me, that the whole line of coast, from Tam-
atave in 18° 12’ to C. Amber at the extreme northern point of
the island, is bordered by coral-reefs. ‘The land is low, un-
even, and gradually rises from the coast. From Captain
Owen’s charts, the existence of reefs, which evidently belong to
the fringing class, N. of British Sound and near Ngoncy, might
also have been inferred. Lieut. Boteler (vol. 1. p. 155) speaks
of ‘the reef surrounding the island of St. AZary’s at a small
distance from the shore.’ Ina previous chapter I have de-
scribed, from the information of Dr. Allan, the manner in
which the reefs extend in N.E. lines from the headlands on
this coast, thus sometimes forming rather deep channels within
them: this seems caused by the currents, the reefs springing
up from the submarine prolongations of the sandy headlands.
The above specified portion of the coast is coloured red. The
remaining S.E. portions do not appear in any published chart
to possess reefs of any kind; and the Rev. W. Ellis believes
that there are none.

East Coast or Arrica.—The northern parts appear, for

EAST COAST OF AFRICA. 247

a considerable space, to be without reefs. My information, I
may observe, is derived from the survey by Captain Owen,
together with his Narrative; and that by Lieut. Boteler. At
Mukdeesha (2° 1’ N.) there is a coral-reef extending four or
five miles along the shore (Owen’s Nar. vol. i. p. 357), which
in the chart lies at a distance of a quarter of a mile from the
shore, and has within it from 6 to 10 feet of water: this then
is a fringing-reef and is coloured red. From Juba,.a little S.
of the equator, to Lamoo (in 2° 20’ S.) ‘the coast and islands

_are formed of madrepore’ (Owen’s Narrative, vol. i. p. 863).

The chart of this part (entitled Dundas Islands) presents an
extraordinary appearance; the coast of the mainland is quite
straight, and is fronted at the average distance of two miles,
by exceedingly narrow, straight islets, fringed with reefs.
Within this chain of islets, there are extensive tidal flats and
muddy bays, into which many rivers enter: the depth of these
spaces varies from one to four fathoms—the latter depth not
being common, and about 12 feet the average. Outside
the chain of islets, the sea, at the distance of a mile, varies
in depth from 8 to 15 fathoms. Lieut. Boteler (Nar.
vol. i. p. 369) describes the muddy bay of Patta, which seems
to resemble other parts of the coast, as fronted by small, nar-
row, level islets formed of decomposing coral, the margin of
which is seldom of greater height than 12 feet, overhang-
ing the rocky surface from which the islets rise. Knowing
that the islets are formed of coral, it is I think scarcely possible
to view the coast, and not at once conclude that we here see a
fringing-reef, which has been upraised a few feet: the unusual
depth of from two to four fathoms within some of these islets,
is probably due to the mud of the rivers having prevented the
growth of coral near the shore. As several parts of this line
of coast are undoubtedly fringed by living reefs, I have
coloured it red.—Maleenda (3° 20’ S.): in the plan of the
harbour, the south headland appears fringed; and in Owen’s
chart on a larger scale, the reefs are seen to extend nearly

248 APPENDIX.

30 miles southward; coloured red. Mombas (4° 5’ 8.):
the island which forms the harbour ‘is surrounded by cliffs
of madrepore, capable of being rendered almost impregnable’
(Owen’s Nar. vol. i. p. 412), The shore of the main land,
N. and S. of Mombas, is regularly fringed by a coral-reef at a
distance from half a mile to a mile and a quarter from the
land; within the reef the depth is from 9 to 15 feet;
outside the reef the depth at rather less than half a mile is
30 fathoms. From the charts it appears that a space about
56 miles in length, is here fringed; coloured red.mPemba
(5° 8.) is an island of coral formation, level, and about
200 feet in height (Owen’s Nar. vol. 1. p. 425); it is 85 miles
long, and is separated from the main land by a deep sea. The
outer coast is represented in the charts as regularly fringed ;
coloured red. The mainland in front of Pemba is likewise
fringed.—Zanzibar resembles Pemba in most respects; its
southern half on the western side and the neighbouring islets
are fringed; coloured red. On the main land, a little 5. of
Zanzibar, there are some banks parallel to the coast, which I
- should have thought had been formed of coral, had it not been
said (Boteler’s Nar. vol. ii. p. 89) that they were composed of
sand : not coloured.—Latham’s Bank is a small island fringed
by coral-reefs; but being only 10 feet high it has not been
coloured.—Monfeea is an island of the same character as
Pemba: its outer shore is fringed, and its southern extremity
is connected with Keelwa Point on the main land by a chain
of islands fringed by reefs; coloured red. The four last-men-
tioned islands resemble in many respects some of the islands
in the Red Sea, which will presently be described.—Keelwa :
in a plan of the shore, a space of 20 miles N. and S. of this
place is fringed by reefs, apparently of coral; these reefs are
prolonged still further southward in Owen’s general chart. In
the plans of the rivers Lindy and Monghow (9° 59’ and 10° 7’
S.) the coast seems to have the same structure, coloured red.—
Querimba Islands (from 10° 40’ to 13° S.): a chart on a

PERSIAN GULF. 249

large scale is given of these islands ; they are low and of coral
formation (Boteler’s Nar. vol. ii. p. 54); and generally have
extensive reefs projecting from them, which are dry at low
water, and which on the outside rise abruptly from a deep
sea; on the inside they are separated from the continent by a
channel, or rather a succession of bays, with an average depth
of 10 fathoms. The small headlands on the continent also
have coral banks attached to them; and the Querimba islands
and banks are placed on the line of prolongation of these head-
lands, and are separated from them by very shallow channels.
It is evident that whatever cause, whether the drifting of sedi-
ment or subterranean movements, produced the headlands, like-
wise produced, as might have been expected, submarine prolon-
gations to them; and these towards their outer extremities have
since afforded a favourable basis for the growth of coral-reefs,
and subsequently for the formation of islets. As these reefs
clearly belong to the fringing class, the Querimba Islands
have been coloured red.—AZonabila (18° 32’ 8.) : in the plan
of this harbour, the headlands outside are fringed by reefs
apparently of coral; coloured red.mMozambique (15° 8.) : the
outer part of the island on which the city is built, and the
neighbouring islands are fringed by coral-reefs ; coloured red.
From the description given in Owen’s Nar. (vol. i. p. 162) the
shore from Mozambique to Delagoa Bay appears to be low and
sandy : many of the shoals and islets off this line of coast are
of coral formation ; but from their sma!l size and lowness, it is
not possible, from the charts, te know whether they are truly
fringed. Hence this portion of coast is left uncoloured, as are
likewise those parts more northward, of which no mention has
been made in the foregoing pages, from the want of informa-
tion.

PERSIAN GuULF.—From the charts lately published on a
large scale by the East Indian Company, it appears that several
parts, especially the southern shores, are fringed by coral-reefs;
but as the water is very shallow, and as there are numerous

250 APPENDIX.

sand-banks, which are difficult tc distinguish on the chart from
reefs, I have not coloured the upper part red. ‘Towards the
mouth, however, where the water is rather deeper, the islands
of Ormuz and Larrack, appear so regularly fringed, that I
have coloured them red. There are certainly no atolls in the
Persian Gulf. The shores of Immaum, and of the promontory
forming the southern headland of the Persian Gulf, seem to be
without reefs. The whole S.W. part of Arabia Felix, except
one or two small patches, and the shores of Socotra appear
from the charts and the memoir of Captain Haines (Geograph.
Journ. 1839, p. 125) to be without reefs. I believe there are
no extensive coral-reefs on any part of the coasts of India, ex-
cept on the low promontory of Madura (as already mentioned)
in front of Ceylon.

Rep Sea.—My information is chiefly derived from the
admirable charts published by the East India Company in
1836, from personal communication with Captain Moresby, one
of the surveyors, and from the excellent memoir, ‘ Ueber die
Natur der Corallen-Binken des Rothen Meeres, by Ehren-
berg. The plains immediately bordering the Red Sea seem
to consist chiefly of a sedimentary formation of the newer ter-
tiary period. The shore is, with the exception of a few parts,
fringed by coral-reefs. The water is generally profoundly
deep close to the shore; but this fact, which has attracted the
attention of most voyagers, seems to have no necessary connec-
tion with the presence of reefs; for Captain Moresby particu-
larly observed that, in lat. 24° 10’ on the eastern side, there is
a piece of coast with very deep water close to it, without any
reefs, but not differing in any other respect from the usual
coast line. The most remarkable feature in the Red Sea is
the chain cf submerged banks, reefs, and islands, lying some
way from the shore, chiefly on the eastern side; the space
within being deep enough to admit safe navigation in small
vessels. The banks are generally of an oval form, and some
miles in width; but some of them are very long in proportion

“ opt

RED SEA. 257

to their width. Captain Moresby informs me that any one who
had not made actual plans of them, would be apt to think that
they were much more elongated than they really are. Many
of them rise to the surface, but the greater number lie from
3 to 30 fathoms beneath it, with irregular soundings on them.
They consist of sand and living coral; the latter in most cases,
according to Captain Moresby, covering the greater part of their
surface. They extend parallel to the shore, and are not un-
frequently connected in their middle parts by short transverse
banks with the main land. ‘The sea is generally profoundly
deep quite close to them, as it is near most parts of the coast
of the main land; but this is not universally the case, for be-
tween lat. 15° and 17° the water deepens quite gradually from
the banks, both on the eastern and western shores. In many
parts islands rise from the banks; they are low, flat-topped,
and consist of the same horizontally stratified formation with
that forming the plain-like margin of the main land. Some of
the smaller and lower islands consist of mere sand. Captain
Moresby informs me that small masses of rock, the remnants
of islands, are left on many of the banks where there is now
nodry land. Hhrenberg also asserts that most of the islets,
even the lowest, have a fiat abraded basis, composed of the
same tertiary formation as elsewhere : he believes that as soon
as the surf wears down the protuberant parts of the banks to
just beneath the level of the sea, the surface becomes protected
from further abrasion by the growth of coral, and he thus
accounts for the existence of so many banks standing on a level
with the surface of this sea. It appears that most of the islands
are certainly decreasing in size.

The banks and islands are curiously shaped in the pants
just referred to, namely, from lat. 15° to 17°, where the sea
deepens quite gradually: the Dhalac group, on the western
coast, is surrounded by an intricate archipelago of islets and
shoals ; the main island is irregular in outline, and includes a
bay seven miles long, by four across, in which no bottom was

AS > APPENDIX.

found with 252 feet; there is only one entrance into it, half a

mile wide, and with an island in front. The submerged banks
on the eastern coast, within the same latitudes, round Farsan
Island, are, likewise, penetrated by many narrow creeks of deep
water; one is twelve miles long, in the form of a hatchet, and
close to its broad upper end, soundings were not struck with 360
feet; its entrance is only half a mile wide. In another creek of
the same nature, but even with a more irregular outline, there
was no bottom with 480 feet. The island of Farsan itself,
has as singular a form as any of its surrounding banks. The
bottom of the sea round the Dhalac and Farsan Islands con-
sists chiefly of sand and agglutinated fragments of coral, but,
in the deep and narrow creeks, it consists of mud; the islands
consist of thin, horizontally stratified, modern tertiary beds,
containing but little broken coral;! their shores are fringed
by living coral-reefs.

From the account given by Riippell? of the manner in
which Dhalac is rent by fissures, the opposite sides of which
have been unequally elevated (in one instance to the amount
of 50 feet), it seems probable that this irregular form, as well
as that of Farsan, may have been partly caused by unequal
elevation ; but, considering the general form of the banks, and
of the deep-water creeks, together with the composition of the
land, I think their configuration is more probably due in great
part to currents having drifted sediment over an uneven bot-
tom. Itis almost certain that their form cannot be attributed
to the growth of coral. The greater number of banks on the
eastern side of the Red Sea seems to have originated in nearly
the same manner, whatever this may have been, as the
Dhalac and Farsan archipelagoes. I judge of this from their
similar configuration (in proof of which I may instance a
bank on the east coast in lat. 22°) and from their similar com-
position. The depth, however, within the banks northward of

1 Ruppell, Reise in Abyssinie, band 1. s. 247. * Ibid. s. 246.

— a

RED SEA. . Zoe

Jat. 17° is usually greater, and their outer sides shelve more

abruptly (circumstances which seem to go together) than in
the Dhalac and Farsan archipelagoes ; but this may have been
caused by a stronger action of the currents during their forma-
tion : moreover, the greater abundance of living coral on the
northern banks, tends to give them steeper margins.

From this account, brief and imperfect as it is, we can
see that the great chain of banks on the eastern side of the
Red Sea, and on the western side of the southern portion,
differ greatly from true barrier-reefs, which are wholly formed
by the growth of coral. Ehrenberg also concludes (Ueber die,
&c. pp. 45 and 51) that these banks owe their origin in a
quite secondary manner to the growth of coral. He remarks
that the islands off the coast of Norway, if worn down level
with the sea, and merely coated with living coral, would pre-
sent a nearly similar appearance. It seems, however, from
information given me by Dr. Malcolmson and Captain Moresby,
that Ehrenberg has rather under-rated the influence of
corals on the formation of the tertiary deposits of the Red
Sea.

The West Coast of the Red Sea between Lat. 19° and 22°.
—Reefs exist here, which, if I had known nothing of the
others in the Red Sea, I should unhesitatingly have considered
as barrier-reefs. One of these reefs, in 20° 15’, is twenty .
miles long, less than a mile in width (but expanding at the
northern end into a disk), slightly sinuous, and parallel to the
main land at the distance of five miles from it, with very deep
water inside, so that in one place soundings were not obtained
with 205 fathoms. Some leagues further south, there is
another very narrow reef, ten miles long, with other small
portions of reef, north and south, almost connected with it;
and within this line of reefs (as well as outside) the water is
profoundly deep. There are also some small linear and sickle-
formed reefs, ying a little way out at sea. All these reefs are
covered, as I am informed by Captain Moresby, by living corals.

254 APPENDIX.

Here, then, we have all the characters of reefs of the barrier
class, and some of the outlying reefs partially resemble atolls.
My only source of doubt arises from the narrowness and
straightness of the spits of sand and rock in the Dhalac and
Farsan groups ; one of these spits in the former group is nearly
fifteen miles long, only two broad, and is bordered on each
side with deep water; so that, if worn down by the surf, and
coated with living corals, it would form a reef nearly similar —
to those within the space under consideration. Nevertheless
I cannot believe that the many small, isolated, and sickle-
formed reefs, as well as others long, nearly straight, and very
narrow, with the water unfathomably deep close round them,
could have been formed by corals merely coating banks of sedi-
ment or the abraded surfaces of irregularly shaped islands.
It seems more probable that the foundations of these reefs have
subsided, and that the corals, during their upward growth, have
given to them their present forms. I have, therefore, with
much hesitation coloured this part blue.

The West Coast, from Lat. 22° to 24°.—This part of the
coast (north of the space coloured blue on the map) is fronted
by an irregularly shelving bank, from 10 to 30 fathoms deep ;
numerous little reefs, some of which have the most singular
shapes, rise from this bank. Many of them may have been
formed by the growth of coral on small abraded islets; but
some almost atoll-formed reefs rising from deep water near a
promontory in lat. 24°, are probably allied to the barrier class.
I have not, however, ventured to colour this portion of coast
blue.—On the west coast, from lat. 19° to 17° (south of the
space coloured blue on the map), there are many low islets of
small dimensions, not much elongated, and rising out of great
depths at a distance from the coast: these cannot be classed
either with atolls, or barrier, or fringing-reefs.

Eastern Coast—There are many small outlying coral-
reefs along this whole line of coast; but as the greater number
rise from banks not very deeply submerged, their origin, as —

=; See
ri

WEST INDIES. 255

we have seen, may be due simply to the growth of corals on
an irregular abraded foundation. But between lat. 18° and
20° there are so many linear, elliptic and extremely small
reefs, rising abruptly out of profound depths, that the same
reasons which led me to colour a portion of the west coast
blue, have induced me here to do the same. There are some
small outlying reefs on the east coast, north of lat. 20° (the
northern limit coloured blue), which rise from deep water ;
but as they are not numerous, and as scarcely any of them
are linear, I have left them uncoloured.

In the southern parts of the Red Sea, considerable spaces
of the main land, and some of the Dhalac islands, are skirted
by reefs, which, as I am informed by Captain Moresby, are of
living coral, and have all the characters of the fringing class.
As there are here no outlying linear or sickle-formed reefs,
rising out of unfathomable depths, I have coloured these parts
of the coast red. On similar grounds I have coloured the
northern parts of the western coast (north of lat. 24° 30’)
red, and likewise the shores of the chief part of the Gulf of .
Suez. In the Gulf of Acaba, as I am informed by Captain
Moresby, there are no coral-reefs, and the water is profoundly
deep.

West Inpies.—My information regarding the reefs of this
area is derived from various sources, and from an examina-
tion of numerous charts; especially of those lately executed
during the survey under Captain Owen, R.N. I lie under par-
ticular obligation to Captain Bird Allen, R.N., one of the mem-
bers of the late survey, for many personal communications on
this subject. As in the case of the Red Sea, it is necessary to
make some preliminary remarks on the submerged banks of
the West Indies, which are in some degree connected with
coral-reefs, and cause considerable doubts in their classification.
That large accumulations of sediment are in progress on the
West Indian shores, will be evident to any one who examines
the charts of that sea, especially of the portion north of a line

256 APPENDIX.

joing Yucutan and Florida. ‘The area of deposition seems
less intimately connected with the débouchement of the great
rivers, than with the’ course of the sea-currents; as is evident

‘from the vast extension of the banks from the promontories of

Yucutan and Mosquito. Ne
Besides the coast-banks, there are others of various dimen-
sions which stand isolated; these closely resemble each other ;
they lie from 2 or 3 to 20 or 30 fathoms under water, and are
composed of sand, sometimes firmly agglutinated, with little or
no coral; their surfaces are smooth and nearly level, shelving
very gradually to the amount of a few fathoms all round
towards their edges, where they plunge abruptly into the un-
fathomable sea. This steep inclination of their sides, which is
likewise characteristic of the coast-banks, is very remarkable :
I may give as an instance, the Misteriosa Bank, on the edges
of which the soundings change in 250 fathoms horizontal dis-
tance, from 11 to 210 fathoms; off the northern point of Old
Providence bank, in 200 fathoms horizontal distance, the
change is from 19 to 152 fathoms; off the Great Bahama
Bank, in 160 fathoms horizontal distance, the inclination is in
many places from 10 fathoms to no bottom with 190 fathoms.
In all parts of the world, where sediment is accumulating,
something of the same kind may be observed; the banks
shelving very gently far out to sea, and then terminating
abruptly. The form and composition of the banks in the
middle parts of W. Indian sea, clearly show that their origin
must be chiefly attributed to the accumulation of sediment ;
and the only obvious explanation of their isolated position is
the presence of a nucleus, round which the currents have col-
lected fine drift matter. Any one who will compare the bank —
surrounding the hilly island of Old Providence, with the banks
in its neighbourhood which stand isolated, will scarcely doubt
that they surround submerged mountains. We are led to the
same conclusion by examining the bank called Thunder Knoll,
which is separated from the great Mosquito bank by a channel

WEST INDIES. 257

only seven miles wide, and 145 fathoms deep. There cannot
be any doubt that the Mosquito bank has been formed by the
accumulation of sediment round the promontory of the same
name; and Thunder Knoll resembles the Mosquito bank, in
the state of its surface submerged 20 fathoms, in the inclina-
tion of its sides, in composition, and in every other respect.
I may observe, although the remark is here irrelevant, that
geologists should be cautious in concluding that all the out-
lyers of any formation have once been connected together, for
we here see that deposits, doubtless of exactly the same nature,
may be deposited with large valley-lke spaces between them.

Linear coral-reefs and small knolls project from many of
the isolated, as weil as from the coast banks; sometimes they
are irregularly placed, as on the Mosquito bank, but more
generally they form crescents on the windward side, situated
some little distance within the outer edge :—thus on the Ser-
ranilla bank they form an interrupted chain which ranges
‘between two and three miles within the windward margin:
generally they occur, as on Roncador, Courtown and Anegada
banks, nearer the line of deep water. Their occurrence on
the windward side is conformable to the general rule, of the
efficient kinds of corals flourishing best where most exposed ;
but I cannot explain their position some way within the line
of deep water unless it be that a depth somewhat less than
that close to the outer margin is most favourable to their
growth. Where the corals have formed a nearly continuous
rim, close to the windward edge of a bank some fathoms sub-
merged, the reef closely resembles an atoll; and if the bank
surrounds an island (as in the case of Old Providence), the
reef resembles an encircling barrier-reef. I should undoubtedly
have classed some of these fringed banks as imperfect atolls,
or barrier-reefs, if the sedimentary nature of their foundations
had not been evident from the presence of other neighbouring
banks, of similar forms and of similar composition, but with-
out the crescent-like marginal reef. In the third chapter, I

S

258 ; APPENDIX.

remarked that some atoll-like reefs probably did exist, which

had originated in the manner here supposed.

Proofs of elevation within recent tertiary periods abound,
as referred to in the sixth chapter, over nearly the whole area
of the West Indies. Hence itis easy to understand the origin
of the low land near those coasts where sediment is now ac-
cumulating ; for instance, on the northern part of Yucutan, and
on the N.H. part of Mosquito. Hence, also, the origin of the
great Baliama banks, which are bordered on their western and
southern edges by narrow, long, singularly-shaped islands,
formed of sand, shells and coral-rock, some of them being
about a hundred feet in height, is easily explained by the ele-
vation of banks fringed on their windward sides by coral-reefs.
On this view, however, we must suppose either that the great
Bahama sand-banks were all originally deeply submerged, and
were brought up to their present level by the same elevatory
action which formed the linear islands ; or that during the
elevation of the banks, the superficial currents and swell of
the waves wore them down, and kept them at a nearly uniform
level. But this level is not quite uniform; for in proceeding
from the N.W. end of the Bahama group towards the 8.H., the
depth of the banks increases, and the area of land decreases,
in a very gradual and remarkable manner. The view that
these banks have been worn down by the currents and waves
of the sea during their elevation, seems to me the most prob-
able one. This view is also, I believe, applicable to many of
the submerged banks, in widely distant parts of the West
Indian sea; for, on any other view, the elevatory forces must
have acted with astonishing uniformity.

The shore of the Gulf of Mexico, for a space of many
hundred miles, is formed by a chain of lagoons, from 1
to 20 miles in breadth (Columbian Navigator, p. 178,
&c.), containing either fresh or salt water, and separated
from the sea by linear strips of sand. The shores of southern

Brazil, and of the United States from Long Island (as

WEST INDIES. 259

observed by Professor Rogers,) to Florida, have the same
character. Professor Rogers, in his Report to the British
Association (vol. iii. p. 18), speculates on the origin of
these low, sandy, linear islets; he states that the layers of
which they are composed are too homogeneous, and contain
too large a proportion of shelis, to permit the common sup-
position of their formation being simply due to matter thrown
up, where it now lies, by the surf: he considers these islands
as upheaved bars or shoals, which were deposited in lines
where opposed currents met. It is evident that these islands
and spits of sand parallel to the coast, and separated from it
by shallow lagoons, have no necessary connection with coral-
formations. “.

Having now endeavoured to remove some sources of doubt
in classifying the reefs of the West Indies, I will give my
authorities for colouring such portions of coast as I have
thought myself warranted in doing. Captain Bird Allen in-

_ forms me that most of the islands on the Bahama Banks are

fringed, especially on their windward sides, with living reefs ;
and hence I have coloured those, which are thus represented
in Captain Owen’s chart, red. The same officer informs me,
that the islets along the southern part of Florida are similarly

fringed ; coloured red.—Cusa : proceeding along the northern

coast, at the distance of 40 miles from the extreme S.E.
point, the shores are fringed by reefs, which extend westward
for a space of 160 miles, with only a few breaks. Parts of
these reefs are represented in the plans of the harbours on
this coast by Captain Owen; and an excellent description is
given of them by Mr. Taylor (Loudon’s Mag. of Nat. Hist.
vol. ix. p. 449); he states that they enclose a space called the
‘baxo,’ from half to three-quarters of a mile in width, with a
sandy bottom, and a little coral. In most parts people can
wade, at low water, to the reef; but in some parts the depth
is between two and three fathoms. Close outside the reef, the
depth is between six and seven fathoms: these well-charac-
s 2

260 APPENDIX.

terized fringing-reefs are coloured red.—Westward of long.
77° 30’, on the northern side of Cuba, a great bank com-
mences, which extends along the coast for nearly four degrees
of longitude. In its structure, and in the ‘cays,’ or low islands
on its edge, there is a marked correspondence (as observed by
Humboldt, Pers. Narr. vol. vii. p. 88) between it and the great
Bahama and Sal Banks, which lie directly in front. Hence
one is led to attribute the same origin to all these banks;
namely, the accumulation of sediment, conjoined with an
elevatory movement, and the growth of coral on their outer
edges. The parts which are fringed by living reefs are
coloured red.—Westward of these banks, there is a portion
of coast apparently without reefs, except in the harbours, the
shores of which seem in the published plans to be fringed.—
The Colorado Shoals (see Captain Owen’s charts), and the low
land at the western end of Cuba, correspond as closely in
relative position and structure to the banks at the extreme
point of Florida, as the banks above described on the north
side of Cuba, do tothe Bahamas. The depth within the islets
and reefs on the outer edge of the Colorados, is generally be-
tween two and three fathoms, increasing to 12 fathoms in the
southern part, where the bank becomes nearly open, without
islets or coral-reefs ; the portions which arefringed are coloured
red.—The southern shore of Cuba is deeply concave, and the in-
cluded space is filled up with mud and sand banks, low islands
and coral-reefs. Between the mountainous Isle of Pines and the
southern shore of Cuba, the general depth is only between two
and three fathoms; and in this part, small islands, formed
of fragmentary rocks and broken madrepores (Humboldt,
Pers. Narr. vol. vil. pp. 51, 86 to 90, 291, 309, 320), rise
abruptly, and just reach the surface of the sea. From some
expressions used in the Columbian Navigator (vol. i. pt. ii.
p- 94), it appears that considerable spaces along the outer
coast of southern Cuba are bounded by cliffs of coral-rock,

formed probably by the upheaval of coral-reefs and sand-—

WEST INDIES. 261

banks. The charts represent the southern part of the Isle
of Pines as fringed by reefs, which the Columb. Navig. says

extend some way from the coast, but have only from 9 to

12 feet of water on them; these are coloured red.—I have
not been able to procure any detailed description of the large
group of banks and ‘ cays’ further eastward on the southern
side of Cuba; within them there is a large expanse, with a
muddy bottom, from 8 to 12 fathoms deep: although some
parts on this line of coast are represented in the general
charts of the West Indies, as fringed, I have not thought it
prudent to colour them. The remaining portion of the south
coast of Cuba appears to be without coral-reefs.
Yucuran.—The N.E. part: of the promontory appears, in
Captain Owen’s charts, to be fringed; coloured red. The
eastern coast from 20° to 18° is fringed. South of lat. 18°,
there commences the most remarkable reef in the West Indies:
it is about 130 miles in length, ranging in a N. and S. line, at
an average distance of 15 miles from the coast. ‘The islets on
it are all low, as I have been informed by Captain B. Allen;
the water deepens suddenly on the outside of the reef, but not
more abruptly than off many of the sedimentary banks:
within its southern extremity (off Honduras) the depth is
25 fathoms; but in the more northern parts, the depth soon
decreases to 10 fathoms, and within the northernmost part, for
a space of 20 miles, the depth is only from one to two
fathoms. In most of these respects we have the character-
istics of a barrier-reef; nevertheless, from observing, first,
that the channel within the reef is a continuation of a great
irregular bay, which penetrates the main land to the depth of
50 miles; and secondly, that considerably spaces of this
barrier-like reef (for instance, in lat. 16° 45’ and 16° 12’)
are described in the charts as formed of pure sand; and
thirdly, from knowing that sediment is accumulating in many
parts of the West Indies in banks parallel to the shore; I
have not ventured to colour this reef as a barrier. To add

262 APPENDIX.

to my doubts, close outside this barrier-like reef, Turnefe,
Lighthouse, and Glover reefs are situated, and these have so’
completely the form of atolls, that if they had occurred in
the Pacific, I should not have hesitated to colour them blue.
Turneje Reef seems almost entirely filled up with low mud
islets; and the depth within the other two reefs is only from
one to three fathoms. From this circumstance, and from their
similarity in form, structure, and relative position, both to the
bank called Northern Triangles, on which there is an islet
between 70 and 80 feet in height, and to Cozumel Island, the
level surface of which is likewise between 70 and 80 feet
high, it is probable that the three foregoing banks are the
worn-down bases of upheaved shoals, fringed with corals; left
uncoloured. ,

In front of the eastern Mosquito coast there are, between
lat. 12° and 16°, some extensive banks (already mentioned),
with high islands rising from their centres, and others wholly
submerged, both kinds being bordered, near their windward
margins, by crescent-shaped coral-reefs. But it can hardly
be doubted that these banks owe their origin, like the great
bank extending from the Mosquito promontory, almost en-
tirely to the accumulation of sediment, and not to the growth
of corals; hence I have not coloured them.

Cayman Island: this island appears in the charts to be
fringed; and Captain B. Allen informs me that reefs extend
about a mile from the shore, and have only from 5 to 12 feet
of water within them ; coloured red.—Jamaica : judging from
the charts, about 15 miles of the $.H. extremity, and about
twice that length at the S.W. extremity, and some portions
on the S. side near Kingston and Port Royal, are regularly
fringed, and are therefore coloured red. From the plans of
some harbours on the N. side, parts of the coast appear to be
there fringed; but I have not coloured them.—/St. Domingo :
I have not been able to obtain sufficient information, either
from plans of the harbours, or from general charts, to enable

a,

WEST INDIES. 263

me to colour any part of the coast, except 60 miles from Port
de Plata westward, which seems regularly fringed : many other
parts, however, of the coast are probably fringed, especially
towards the eastern end of the island.— Puerto Rico: consider-
able portions of the southern, western, and eastern coasts, aud
some parts of the northern coast, appear in the charts to be
fringed ; coloured red. Some miles in length of the southern
side of the Island of St. Thomas is fringed ; most of the Virgin
Gorda Islands, as I am informed by Sir R. Schomburgk, are
fringed ; the shores of Anegada, as well as the bank on which
it stands, are likewise fringed ; these islands have been coloured
red. The greater part of the southern side of Santa Cruz
appears in the Danish survey to be fringed (see also Professor
Hovey’s account of this island, in Silliman’s Journal, vol.
xxxv. p. 74); the reefs extend along shore for a considerable
space, and project rather more than a mile; the depth within
the reef is three fathoms; coloured red.—The Antilles, as re-
marked by Von Buch (Descrip. Iles Canaries, p. 494), may
be divided into two linear groups, the western row being vol-
canic, and the eastern of modern calcareous origin; my infor-
mation is very defective on the whole group. Of the eastern
islands, Barbuda and the western coasts of Antiqua and
Mariagalante appear to be fringed ; this 1s also the case with

Barbadoes, as I have been informed by a resident; these

islands are coloured red. On the shores of the western An-
tilles, of volcanic origin, very few coral-reefs appear to exist.
The island of Martinique, of which there are beautifully exe~
cuted French charts on a very large scale, alone presents any
appearance worthy of special notice. The south-western,
southern, and eastern coasts, together forming about half
the circumference of the island, are skirted by very irregular
banks, projecting generally rather less than a mile from the
shore, and lying from two to five fathoms submerged. In
front of almost every valley, they are breached by narrow,
crooked, steep-sided passages. ‘The French engineers ascer-

264 APPENDIX.

tained by boring, that these submerged banks consisted of

madreporitic rocks, covered in many parts by thin layers of
mud or sand. From this fact, and especially from the struc-
ture of the narrow breaches, these banks were probably formed
by living reefs, which fringed the shores of the island, and
once reached the surface. From some of these submerged
banks reefs of living coral still rise abruptly, either in small
detached patches, or in lines parallel to, but some way within,
the margin. Besides the above banks which skirt the shores
of the island, there is on the eastern side a range of linear
banks, similarly constituted, 20 miles in length, extending
parallel to the coast-line, and separated from it by a space
between two and four miles in width, and from 8d to 15
fathoms in depth. From this range of detached banks, some
linear reefs of living coral likewise rise abruptly ; and if they
had been of greater length (for they do not front more than a
sixth part of the circumference of the island) they would ne-
cessarily from their position have been coloured as barrier-
reefs; as the case stands, they are left uncoloured.
Fiorma.—An account of the reefs on this coast, together
with references to various authorities, will be found in Pro-
fessor Dana’s work on Corals and Coral Islands, 1872, p. 204.
The Breruupa Istanps have been carefully described by
Lieut. Nelson, in an excellent memoir in the Geol. Transac-
tions (vol. v. parti. p. 103). In the form of the bank or reef,
on one side of which tke islands stand, there is a close general
resemblance to an atoll; but in the following respects there is
a considerable difference,—first, in the margin of the reef not
forming (as I have been informed by Mr. Chaffers, R.N.) a
flat, solid surface, which is laid bare at low water ; secondly,
in the water gradually shoaling for nearly a mile and a haif in
width round the entire reef, as may be seen in Captain Hurd’s
chart ; and thirdly, in the size, height, and extraordinary
form of the islands, which present little resemblance to the
long, narrow, simple islets, seldom exceeding half a mile in

a

BERMUDA ISLANDS. 265

breadth, which surmount the annular reefs of almost all the
atolls in the Indian and Pacific Oceans. Moreover, there are
evident proofs (Nelson, ibid. p. 118) that islands similar to
the existing ones formerly extended over other parts of the
reef. It would, I believe, be difficult to find a true atoll with
land exceeding 30 feet in height; whereas, Mr. Nelson esti-
mates the highest point of the Bermuda Islands at 260 feet ;
if, however, Mr. Nelson’s view, that the whole land consists of
sand drifted by the winds and agglutinated together, is correct,
this difference would be immaterial; but, from his own ac:
count (p. 118), there occur in one place, five or six layers of
red earth, interstratified with the ordinary calcareous rock,
and including stones too heavy for the wind to have moved,
without having at the same time utterly dispersed every grain
of the accompanying drifted matter. Mr. Nelson attributes
the origin of these several layers, with their embedded stones,
to violent catastrophes; but further investigation has generally
succeeded in explaining such phenomena by simpler means.
Finally, I may remark that these islands bear a considerable
resemblance in shape to Barbuda in the West Indies, and to
Pemba on the eastern coast of Africa, which latter island is
about 200 feet in height, and consists of coral-rock. I believe
that the Bermuda Islands, from being fringed by living reefs,
ought to have been coloured red; but I have left them un-
coloured, on account of their general resemblance in external
form to a lagoon-island or atoll. Professor Dana (Corals and

Coral Islands, pp. 218, 269) ranks them in this class.

Supplement on a remarkable Bar of Sandstone off Pernambuco,
on the Coast of Brazil. (Originally published in the
Philosophical Magazine, October 1841, p. 257.)

In entering the harbour of Pernambuco, a vessel passes
close round the point of along reef, which, viewed at high
water when the waves break heavily over it, would naturally

266 - APPENDIX.

be thought to be of coral formation, but when beheld at low
water might be mistaken for an artificial breakwater, erected
by cyclopean workmen. At low tide it shows itself as a
smooth level-topped ridge, from 30 to 60 yards in width,
with even sides, and extending in a perfectly straight line, for
several miles parallel to the shore. Off the town it includes a
shallow lagoon or channel about half a mile m width, which
further south decreases to scarcely more than a hundred
yards. Close within the northern point, ships lie moored to
old guns let into the reef. Here, on the inner side, at low
water spring-tides, a section of about seven feet in height is
exhibited. This consists of hard pale-coloured sandstone —
breaking with a smooth fracture, and formed of siliceous
grains, cemented by calcareous matter. Well-rounded quartz
pebbles, from the size of a bean, rarely to that of an apple, are
embedded in it, together with a very few fragments of shells.
Traces of stratification are obscure, but in one spot there was
an included iayer of stalactitic limestone, an eighth of an inch
in thickness, In another place some false strata, dipping
landwards at an angle of 45°, were capped by a horizontal mass.
On each side of the ridge quadrangular fragments have sub-
sided; and the whole mass is in some places fissured, appa-
rently from the washing out of some soft underlying bed. One
day, at low water, I walked a full mile along this singular,
smooth, and narrow causeway, with water on both sides of me,
and could see that for nearly a mile further south its form re-
mained unaltered: In Baron. Roussin’s beautiful chart of
Pernambuco (Le Pilote du Brésil) it is represented as stretch-
ing on, in an absolutely straight line, for several leagues ; how
far its composition remains the same, I know net; but from
the accounts I received from intelligent native pilots, it seens
to be replaced on some parts of the coast by true coral-reefs.
The upper surface, though it must on a large scale be
called smooth, yet presents, from unequal disintegration, nu-
merous small irregularities. The larger imbedded pebbles
stand out supported on short pedestals of sandstone. ‘There

Si)

BAR OF SANDSTONE. 267

are, also, many sinuous cavities, two or three inches in width
and depth, and from six inches to two feet in length. The
upper edges of these furrows sometimes slightly overhang their
sides; and they end abruptly with a rounded outline. A
furrow occasionally branches into two arms, but generally they
run nearly parallel to each other, in a line transverse to the
sands:one ridge. I know not how to account for their origin
except through the washing to and fro of pebbles in originally
slight depressions, by the waves which break daily over the
bar. Opposed to this notion is the fact that some of these
furrows were lined with numerous small living Actinee. The
exterior surface of the bar is coated with a thin layer of cal-
careous matter; this, on the outer subsided masses, which can
be reached only at low water, between the successively break-
ing waves, is so thick that I could seldom expose the sand-
stone by the aid of a heavy hammer. I procured, however,
some fragments, which were between three and four inches in
thickness, and consisted chiefly of small Serpule, including
some Balani, with a few thin paper-like layers of a Wullipora.
The surface alone is alive, and all within consists of the above
organic bodies, filled up with dirty white calcareous matter.
The layer, though not hard, is tough, and from its rounded
surface resists the breakers. Along the whole external margin
of the bar, I only saw one very small point of sandstone which
was exposed to thesurf. In the Pacific and Indian Oceans the

outer and upper inargin of the coral-reefs are, as we have seen,

protected by a similar coating; but formed almost exclusively
of several species of Nullipore. Lieut. Nelson, in his excel-
lent memoir on the Bermudas (Geol. Trans. vol. v. part 1,
p. 117), says that the reefs there are formed of similar masses
of Serpule ; but I suspect that they are only thus coated.

I enquired from some old pilots at Pernambuco whether
there was any tradition of the bar having undergone any
change during the lapse of time; but they were unanimous in
answering me in the negative. It is astonishing to reflect,
that although waves of turbid water, charged with sediment,

268 APPENDIX.

are driven night and day by the ceaseless trade wind against
the abrupt edges of this natural breakwater, yet that it has
lasted in its present perfect state for centuries, or perhaps for
thousands of years. Seeing that the surface on the inner side
does gradually wear away, as shown by the pebbles on the
little sandstone pedestals, this durability must be entirely due
to the protection afforded by the thin coating of Serpule and
other organic bodies. This is a fine example of what apparently
inefficient means may be effectual.

I believe that similar bars of rock occur in front of sore
of the other bays and rivers on the coast of Brazil: Baron
Roussin states that at Porto Seguro there is a ‘ quay’ similar
to that of Pernambuco. Spaces of several hundred miles in
length on the shores of the Gulf of Mexico, the United States,
and of Southern Brazil are formed by long narrow islands and
spits of sand, including extensive shallow lagoons, some of
which are several leagues in width. ‘The origin of these linear
islets is rather obscure: Professor Rogers (Report to British
Association, vol. iil. p. 13) gives reasons for suspecting that
they have been formed by the upheaval of sand-banks, deposited
where currents formerly met. The bar of sandstone at Per-
nambuco has probably been formed in an analogous manner.
The town stands partly on a low narrow islet and partly on a
long spit of sand, in front of a low shore, bounded in the dis-
tance by a semicircle of hills. By digging at low water near
the town, the sand is found consolidated into sandstone, similar
to that of the bar, but containing many more shells. If, then,
the nucleus of a spit of sand, extending in front of the bay,
had formerly become consolidated, a small change, probably of
level, but perhaps merely in the currents, might have given
rise, by washing away the loose sand, to a structure like that
of the bar in front of Pernambuco and along the coast south-
ward of it; but without the protection afforded by the succes-
sive growth of the above-named organic beings, its duration
would have been short.

269

END a Xe:

The names in italics are all names of places, and refer exclusively to the Appendix; in
well-defined archipelagoes, or groups of islands, the name of each separate island is not

given.

ABR

Abrolhos, Brazil, coated by corals,
79

Abrothos (Australia), 229

Absence of coral-reefs from certain
coasts, 81

Acaba, gulf of, 255

Admiralty group, 219

Africa, east coast, fringing-reef of,
76. Madreporitic rocks of, 179

Africa, east coast, 246

Age of individual corals, 95, 96

‘Aiou, 226

Aitutaki, 202
Aldabra, 244
Alert reef, 217
Alexander Grand Duke, island, 205
Allan, Dr.
on Holuthurie feeding on corals,
20
on quick growth of corals at
Madagascar, 104
on reefs affected by currents, 79
Alloufatou, 211
Alphonse, 243
Amargura, 210
Amboina, 227
America, west coast, 197
Amirantes, 243
Anachorites, 220
Anambas, 284
Anamouka, description of, 175
Anamouke, 210

ATO

Andaman islands, 233
Antilles, 263
Appoo reef, 235
Arabia Felix, 250
Areas,
‘great extent of, interspersed
with low islands, 121
of subsidence and of elevation,
187
of subsidence appear to be
elongated, 188
of subsidence alternating with
areas of elevation, 189
Arru group, 226
Arzobispo, 225
Ascension, no reef at, 83
Ascidia, depth at which found, 116
Assomption, 243
Astova, 243
Atlantic islands, 83, 213
Atolls,
breaches in their reefs, 39, 145
dimensions of, 27
dimensions of groups of, 121
not based on craters, or on
banks of sediment, or on
rock, 118, 123, 124, 125,191
of irregular forms, 28, 143
steepness of their flanks, 29,
162, 224
width of their reef and islets,
28

270
ATO
Atolls,
their lowness, 120
lagoons, 35

general range, 165
with part of their reef sub-
merged, and theory of, 37,
144
whole reef submerged and
‘theory of, 38, 144
Augustin, St., 218
Aurora island, an upraised atoll, 121
Aurora, 199
Austral islands, recently elevated,
174, 183
Austral islands, 202
Australia, N.W. coast, 229
Australian barrier-reef, 63, 163
Australian barrier, 217

- Babuyan group, 237
Bahama banks, 258, 259
Balabac, 234
Bally, 230
Baring, 214
Barrier-reef
of Australia, 63, 163
of New Caledonia, 67
Barrier-reefs,
breaches through, 133
not based on worn-down margin
of rock, 66
on banks of sediment, 67
on submarine craters, 68
steepness of their flanks, 58,
162, 233
their probable vertical thick-
ness, 64, 132
theory of their formation, 131,
135
Bampton shoal, 217
Banks’ islands, 216
Banks in the West Indies, 255
Bashee islands, 237
Buss island, 203
Batoa, 211
Beaupre reef, 217
Beechey, Capt.,
obligations of the author to, 30
on submerged reefs, 37
account of Matilda island, 100

MRT PET SE ese ee
es

INDEX.

CEY

Belcher, Sir E.
on boring through coral-reefs,
98
on changes in Bow atoll, 170
on Clipperton rock, 198
Bellinghausen, 198
Bengal, gulf of, elevation of eastern
shores of, 179
Bermuda islands, 264
Beveridge reef, 209
Bligh, 216
Bolabola, view of, 3
Bombay shoal, 239
Bonin bay, 232
Bonin group, 225
Borings through coral reefs, 98
Borneo, W. coast, recently elevated, -
178
Borneo,
EF. coast, 231
S.W. and W. coast, 234
N. coast, 234
western bank, 289
Boscawen, 211
Boston, 214
Bouka, 218
Bourbon, 242
Bourou, 227
Bouton, 232
Brazil, fringing-reefs on coast of, 77
Breaches through barrier-reefs, 133
Brook, 204
Bunker, 204
Bunoa, 234
Byron, 218

Cagayanes, 235

Candelaria, 219

Cargados Carajos, 241

Caroline island, 204

Caroline archipelago, 220

Carteret shoal, 226

Caryophyllia, depths at which it
lives, 116

Cavilli, 235

Cayman island, 262

Celebes, 228

Ceram, 226

Ceylon, recently elevated, 179

CEY

Ceylon, 240
Chagos, Great bank, description and
theory of, 53, 147
Chagos group, 148
Chagos group, 241
Chama shells embedded in coral
rock, 106
Chamisso on corals preferring the
surf, 85
Changes
in the state of Keeling atoll,
20
of atolls, 128, 168
Channels
leading into the lagoons, 59
of atolls, 39, 146
the Maldiva atolls, 43,
49
through barrier-reefs, 133
Chase, 213
China sea, 238
Christmas atoll, 100
Christmas atoll, 206
Christmas island (Indian ocean), 240
Clarence, 205
Clarke, W. B., on recent elevation of |
the Loyalty islands, 176
Clipperton rock, 197
Cocos, or Keeling atoll, 7
Cocos (or Keeling), 240
Cocos tsland (Pacific), 197

Cochin China, 240
Coetivi, 243
Comoro group, 244
Composition of coral formations, 153
Conglomerate
coral rock on Keeling atoll, 17
on other atolls, 35
coral rock, 153
Cook islands, recently elevated, 175
Cook islands, 202
Coral-rocks bored by vermiform
animals, 19, 154
Coral-reefs,
their distribution and absence
from certain areas, 80
destroyed by loose sediment, 87
Coral-rock,
at Keeling atoll, 16
Mauritius, 74

INDEX. via |

- DAN

Coral-rock,
at Metia, 98
organic remains of, 154

Corals,
dead but upright in Keeling

lagoon, 21
depths at which they live, 113
off Keeling atoll, 11

killed by a short exposure, 8
living in the lagoon of Keeling

atoll, 18

quick orowth of, in Keeling
lagoon, 17

merely coating the bottom of
the sea, 78

standing exposed in the Low
archipelago, 168
Corallian sea, 164
Corallian sea, 217
Cornwallis, 214
Cosmoledo, 244
Couthouy, Mr.
alleged proofs of recent eleva-
tion of the Low archipelago,
168
on external ledges round coral
islands, 138
Crescent-formed reefs, 143
Cuba, 259
Cuming, Mr., on the recent elevation
of the Phillippines, 178

Dana, Prof., on the reef of Hawaii,
82, 173
distribution of coral-reefs as
affected by the temperature
of the sea, 84
upraised coral-rock of Metia, 98
boring through coral-rock, 98
depth at which corals live, 111
subsidence of the Mendina is-
land, 168, 201
subsidence in the Caroline
archipelago, 167
slight recent subsidence of the
Paumotu archipelago, 168
extension of the Hawaii archi-
pelago, 189, 208
Feejee islands, 211
“Dangerous or Low archipelago, 198

272

DAN

Danger islands, 205
Depths,
at which reef-building corals
live, 107
at Mauritius, the Red Sea,
and in the Maldiva ar-
chipelago, 112
at which other corals and coral-
lines can live, 115
* Dhalae group, 251
Diego Garcia, slow growth of reefs,
92

Dimensions of the larger groups of
atolls, 120

Disseverment of the Maldiva atolls,
and theory of, 50, 140

Distribution of coral-reefs, 80

Domingo, St., 262

Dory Port, recently elevated, 177

Dory Port, 225

Duchassaing on rapid growth of
corals, 107

Duff’s islands, 216

Durour, 220

Hap, 222
Karthquakes,
at Keeling atoll. 24
in groups of atolis, 129
in Navigator archipelago, 176
Hast Indian archipelago recently
elevated, 177
Easter, 198
Echequier, 220
Ehrenberg,
on the banks of the Red Sea,
78, 250
on depth at which corals live
in the Red Sea, 112
on corals preferring the surf, 85
on the antiquity of certain
corals, 95
Eimeo, 199
Elevated Reef of Mauritius, 74
Elevations, recent,
proofs of, 173
immense areas of, 187
Elivi, 222
Elizabeth island, 98
recently elevated, 174, 183

INDEX.

FRI

Elizabeth island, 198
Hillice group, 212
Encircled islands,
their height, 62
their geological composition,
63, 68
Koua, description of, 175
Eoua, 210
Erupted matter, probably not asso-
ciated with thick masses of
coral rock, 154

Fais recently elevated, 177, 189
Frais, 222
Fanning, 206
Farallon de Medinilla, 224
Harsan Group, 252
Fataka, 216
Fidji archipelago, 211
Fish,
feeding on corals, 19
killed in Keeling lagoon by
heavy rain, 26
Fissures across coral islands, 129,
252
FitzRoy, Capt.,
on a submerged shed at Keeling
atoll, 24
on an inundation in the Low
archipelago, 127
Flint, 204
Flores, 230, 259
Florida, 264.
Folger, 225
Formosa, 237
Forster, theory of coral formations,
124
Frederic reef, 217
Freewill, 226
Friendly group recently elevated,
175, 183
Friendly archipelago, 209
Fringing-reefs
absent where coast cpeinttone
69
breached in front of streams,
88
described by MM. Quoy and
Gaimard, 173

FRI

Fringing-reefs,

not closely attached to shelving
coasts, 72

of east coast of Africa, 76

of Cuba, 75

of Mauritius, 71

on worn-down banks of rock, 78

on banks of sediment, 78

their appearance when elevated,
74

their growth influenced by cur-

rents, 79
by shallowness of sea, 77

Galapagos archipelago, 197

Galega, 243

Gambier islands, section of, 65

Gambier islands, 199

Gardner, 206

Gaspar-rico, 214

Geological composition of coral
formations, 153

Giloert archipelago, 213

Gilolo, 227

Glorioso, 243

Gloucester island, 128

Glover reef, 262

Gomez, 198

Gouap, 222

Goulou, 222

Grampus, 225

Gran Cocal, 212

Graves, on the recent elevation in
the Bonin archipelago, 177

Great Chagos bank, description and
theory of, 58, 147

Grey, Capt., on sand-bars, 72

distribution of the different

classes of reefs, 162

Guedes, 226

Hales, Mr., on subsidence in the
Caroline archipelago, 167

Hall, Capt. B., on Loo Choo, 178

Halstead, Capt., elevation of eastern
shore of Gulf of Bengal, 17

Harvey islands recently elevated,
183

' INDEX.

KEP

Harvey or Cook islands, 202

Height of encircled islands, 61

Hermites, 220

Hogoleu, 221

Holuthurie feeding on corals, 20

Honduras, reef off; 261

Horn, 211

Houtman’s Abrolhos, 229

Huaheine, 201

Hull island, 205

Humphrey, 204

Hunter, 211

Hurricanes, effects of on coralislands,
127

Immaum, 250

Independence, 212

India, east coast, recently elevated,
179

India, 250

Irregular reefs in shallow seas, 77

Islets of coral-rock, their formation,
15

their destruction in the Mal-

diva atolls, 50

Jamaica, 262

Jarvis, 204

Java recently elevated, 178

Java, 231

Johnston island, 206

Juan de Nova, 243

Juan de Nova (Madagascar), 246

Jukes, Prof., the barrier reef of
Australia, 64

Kalatoa, 232
Kamtschatka, proofs of its recent
elevation, 186
Karkalang, 228
Keeling atoll, section of reef, 7
Keeling,
south atoll, 240
north atoll, 240
Keffing, 226
Kemin, 203, 206
Kennedy, 217
Keppel island, 210

274
KEP
Keppell, Capt., on the reefs and ele-

vation of Celebes, 228
Kumi, 237

Laccadive group, 241
Ladrones or Marianas, recently ele-
vated, 177
Ladrones archipelago, 224
Lagoon of Keeling atoll, 17
Lagoons,
bordered by inclined ledges
and walls, and theory of their
formation, 41, 137
of small atolls filled up with
sediment, 42
Lagoon-channels within
reefs, 59
Lagoon-reefs, all submerged in some
atolls, and all rising to the
surface in others, 90
Lancaster reef, 203
Latte, 210
Lauglan islands, 218
Ledges round certain lagoons, 41,
137
Lette, 228
Lighthouse reef, 262
Lloyd, Mr., on corals refixing them-
selves, 105
Loo Choo, recently elevated, 179
Loo Choo, 238
Louisiade, 217
Low archipelago, alleged proofs of
its recent elevation, 168
Low archipelago, 198
Lowness of coral islands, 120
Loyalty Group, 217
Lucepara, 234
Lutké, Adm., on fissures across
coral-islands, 129
Luzon recently elevated, 178
Luzon, 235
Lyell, Sir C.,
on channels into the lagoons of
atolls, 39
on the lowness of their leeward
sides, 145
on the antiquity of certain
corals, 96

barrier

INDEX.

MAT

Lyell, Sir C.
on the apparent continuity of
distinct coral-islands, 154
on the recently elevated beds of
the Red Sea, 181

MacAskill Islands, formed of up-
raised coral rock, 189, 222
Macassar strait, 231
Macclesfield bank, 239
Madagascar,
quick growth of corals at, 104
madreporitic rock of, 179
Madagascar, 245
Madjiko-sima, 237
Madura (Java), 231
Madura (India), 240
Mahlos Mahdoo, theory of formation,
141
Malacca recently elevated, 178
Malacca, 234
Malcolmson, Dr.,
on recent elevation of E. coast
of India, 179
on recent elevation of Camaran
island, 180
Malden, 204
Maldiva atolls,
and theory of their formation,
43, 138, 140
steepness of their flanks, 30
erowth of coral at, 103
Maldiva archipelago, 241
Mangaia island,
recently elevated, 174, 183
Mangaia, 203
Mangs, 224
Marianas, recently elevated, 177
Mariana archipelago, 224
Mariere, 223
Marquesas, 201
Marquesas, subsidence of, 165, 201
Marshall archipelago, 213
Marshall island, 225
Martinique, 263
Martires, 223
Mary’s, St., in Madagascar, harbour
made in reefs, 88
Mary island, 206
Matilda atoll, 101

INDEX.

MAU

Mauritius,
fringing-reefs of, 69
depths at which corals there
live, 109
recently elevated, 179
Mauritius, 242
Maurua, section of, 65
Maurua, 200
Menchicoff atoll, 28, 148
Mendana isles, 201
Mendana isles, 210
Mendana island, subsidence of, 163,
201
Metia, 98, 121, 183
Mexico, gulf of, 258
Millepora complanata at Keeling
atoll, 9
Mindoro, 235
Mohilla, 244
Molucca islands recently elevated,
ete
Mopeha, 201
Moresby, Capt., on boring through
coral-reefs, 99
Morty, 227
Mosquito coast, 262
Mosquillo atoll, 143
Mysol, 227

Namourrek gronp, 143

Natunas, 234

Navigator archipelago, elevation of,
176

Navigator archipelago, 208
Nederlandisch islands, 212
Nelson, Lieut.,
on the consolidation of coral-
rock, under water, 97
theory of coral-formations, 125
on the Bermuda islands, 264
New Britain, 219
New Caledonia,
steepness of its reef, 57
barrier-reef of, 67, 1386, 142,
163
New Caledonia, 217
New Guinea (E. end), 219
New Guinea (W. end), 225
New Hanover, 219

7 2

275

PEY

New Hebrides, recently elevated, 176
New Hebrides, 214
New Ireland recently elevated, 177
New Ireland, 219
New Nantucket, 205
Nicobar islands, 233
Mouha, 210
Nullipore,
at Keeling atoll, 12
on the reefs of atolls, 33
on barrier reefs, 57
their wide distribution and
abundance, 116

Objections to the theory of subsi-
dence, 151

Ocean islands, 208, 213

Ono, 212

Onouafu, 211

Ormuz, 250

Oscar group, 212

Oscillations of level, 182, 19¢

Oualan or Ualan, 220

Ouluthy atoll, 101

Outong Java, 219

Palawan,
S.W. coast, 234
N.W. coast, 235
western bank, 239
Palmerston, 202
Palmyra, 206
Paracells, 238
Paraquas, 239
Patchow, 237
Pawmotu archipelago, 168, 198
Pelew islands, 222
Pemba island, singular form of, 180
Pemba, 248
Penrhyn, 204
Peregrino, 204
Pernambuco, bar of sandstone. at,
78, 265
Persian gulf, recently elevated, 181
Persian gulf, 249
Pescado, 204
Pescadores, 238
Peyster group, 212

9

276 INDEX. ;

PHI | SED

Philip, 222 Revilla-gigedo, 197
Philippine archipelago, recently ele- | Ring-formed reefs of the Maldiva
vated, 178 atolls, and theory of, 44,138

Philippine archipelago, 235

Pheniz, 208

Piguiram, 221

Pitcairn, 198

Pitt’s bank, 149

Pitt island, 213

Platte, 243

Pleasant, 213

Porites, chief coral on margin of
Keeling atoll, 9

Postilions, 232

Pouynipete, 166

: its probable subsidence, 167

Pouynipéte, 220

Pratas shoal, 238

Proby, 211

Providence, 243

Puerto Rico, 263

Pulo Anno, 223

Pumice floated to coral islands, 154

Pylstaart, 209

Pyrard de Laval, astonishment at the
atolls in the Indian Ocean, 20

Quoy and Gaimard,
depths at which corals live, 113
description of reefs applicable
only to fringing-reefs, 173

Range of atolls, 165

Rapa, 203

Rearson, 205

Red Sea,
banks of rock coated by reefs, 78
proofs ofits recent elevation, 180
supposed subsidence of, 182

Red Sea, 250

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

Rodriguez, 242

Rosario, 225

Rose island, 209

Rotches, 218

Routoumah, 212

Roug, 221

Rowley shoals, 230

Rippell, Dr., on the recent deposits
of Red Sea, 180

Sable, fle de, 242
Sahia de Matha, 241
St. Pierre, 243
Salomon archipelago, 218
Samoan, or Navigator archipelago,
elevation of, 176
Samoa archipelago, 208
Sandbars, parallel to coasts, 73
Sandalwood, 229
Sandwich archipelago, recently ele-
vated, 173
extension of, 189, 208
Sandwich Archipelago, 207
Sanserot, 223
Santa Cruz group, 216
Savage island recently elevated, 175
Savage, 209
Savu, 229
Saya, or Sahia de Mala, 241
Scarborough shoal, 239
Searus feeding on corals, 19
Schouton, 219
Scilly, 201
Scorie floated to coral islands, 154
Scott's reef, 230
Sections,
of islands encircled by barrier-
reefs, 66,131 ~
of Bolabola, 132
Sediment,
in Keeling-lagoon, 19
in other atolls, 35, 47
injurious to corals, 87
transported from coral-islands
far seaward, 154 .

INDEX.

SEM

Semper, Prof.,
on the Pelew islands, 223
on the reef of the Philippine
archipelago, 236
Seniavine, 220
Serangani, 228
Seychelles, 242
Ship-bottom quickly coated with
coral, 105
Smyth island, 206
Society archipelago, 163
stationary condition of, 167
alleged proofs of recent eleva-
tion, 183
Society Archipelago, 199
Socotra, 250
_ Solor, 230
Sooloo islands, recently elevated,
178
Sooloo islands, 284
Souvoroff, 205
Spallanzani
on growth of coral, 106
Spanish, 223
Starbuck, 204
Stones transported in roots of trees,
154
Storms, effects of on coral-islands,
127
Stutchbury, Mr.,
on the growth of an Agaricia,
106
on upraised corals in Society
archipelago, 183
Subsidence
of Keeling atoll, 23
extreme slowness of, 156, 190
areas of, apparently elongated,
188

areas of, immense, 187
great amount of, 190
Suez, quif of, 255
Sulphur islands, 225
Sumatra, recently elevated, 178
Sumatra, 232
Sumbawa, 230
Surf favourable to the growth of
massive corals, 85
Swallow shoal, 239
Sydney island, 205

277

VIT.

Tahiti, alleged proofs of its recent
elevation, 183

Tahiti, 199

Tanasserim, 234

Temperature of the sea at the Gala-
pagos archipelago, 82

Tenimber island, 226

Teturoa, 200

Theories on coral formations, 117,
125

Theory of subsidence, and objec-
tions to, 124, 151

Thickness, vertical, of barrier-reefs,
66, 132

Thomas, St., 263

Tikopia, 216

Timor recently elevated, 177

Timor, 228

Timor-laut, 226

Tokan Bessees, 232

Tongatabou, description of, 175

Tongatabou, 209

Tonquin, 240

Toubai, 201

Toufoa, 210

Toupoua, 216

Traditions of change in coral-islands,
126

Tridacne,

embedded in coral-rock, 154
left exposed in the Low Archi-

pelago, 168

Tubularia,.quick growth of, 106

Tumbelan, 234

Turneffe reef, 262

Turtle, 211

Ualan, 220

Vanikoro,
section of, 64
its state and changes in its reefs,
167
Vanikoro, 216
Vine reef, 217
Virgin Gorda, 263
Viti archipelago, 211

278 INDEX.
VOL ZON
‘Voleanic West Indies,
islands, with living corals on banks of sediment, fringed by
their shores, 81 reefs, 78

matter, probably not associated
with thick masses of coral-
rock, 154

Voleanos,

authorities for their position on
the map, 158

their presence determined by
the movements in progress,
184

absent or extinct in the areas
of subsidence, 185

Waigiou, 225

Wallis island, 211

Washington, 206

Wells’ reef, 218

Wellstead, Lieut., account of a ship
coated with corals, 105

recently elevated, 181
West Indies, 255
Whitsunday island,
view of, 2
changes in its state, 128
Williams, Rev. J.,
on traditions of the natives
regarding coral-islands, 126
on antiquity of certain corals, 96
Wolchonsky, 198
Wostock, 204.

Xulla islands, 227

York island, 205
Yucutan, coast of, 261

Zones of different kinds of corals out-
side the same reefs, 74, 89, 99.

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