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Zhe Camelot Series, 

Edited by Ernest Rhys. 



DARWIN'S STRUCTURE AND DISTRIBUTION 
OF CORAL REEFS. 



SHEWING THE DISTRIBUTION -OF THE DIFFERENT KINDS OK COHAI, REEFS. TOGETHER WITH THE POSITION OF 'till-: ACTIVE VOLCANO S IN THE MAP. {SEE NOTE ffl LEFT HAND CORNER.) 




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Note 

Bright blue. Atolls or lagoon. Is??. 

! Pale blue. Barrier Reefs 

I 

Red. Fringiltg Re^s 

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■ Verrmhon- spots k streaks active volcanoes 

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S.B. Forjurther particulars sec bey uuii iig of Chap.TI. and Appendix 



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ON THE STRUCTURE AND 
__ DISTRIBUTION OF CORAL 
REEFS/ 7 BY CHARLES DARWIN. 
EDITED, WITH AN INTRODUC- 
TION, BY JOSEPH W. WILLIAMS. 




LONDON: 

WALTER SCOTT, 24 WARWICK LANE. 

NEW YORK : 3 EAST 14th STREET. 



THE WALTER SCOTT PRESS, NEWCASTLE-ON-TYNE. 






PREFATORY NOTE. 



In the following pages, first published in 1842, as a companion 
volume to his Journal of Researches^ Charles Darwin divided 
coral-reefs into three great classes, each of which is, however, 
formed upon the same type, and each succeeds the other in 
general plan of formation 'in much the same way, as childhood 
passes into youth, and youth into manhood.' The first class, or 
'fringing-reef (Fig. 1), generally surrounds islands, or skirts 




Fig. 1.— Fringing-Reef. 

great masses of continent, and has a channel of shallow water 
between it and the shore and a gently sloping sea-bed on its 
ocean side. Examples are numerous : — The Sandwich Islands, 
the Seychelle Islands, the Solomon Isles, the Friendly Isles, 
the Navigator Isles, the New Hebrides, and Mauritius, are 
margined with this kind of reef; they are also common in the 
Red Sea, on both its African and Arabian shores, and they 
form a prolongation from the southern extremity of the promon- 
tory of Florida. They surround the Nicobar Islands, and skirt 
nearly the whole of the islands of the West Indies. The reefs 
on the Florida coast are extending inland, and from the rate of 
their encroachment upon the shore Louis Agassiz 1 has tried 

1 Natural History Studies, 



vi PRE FA TOR Y NO TE. 

to determine their period of growth, with a result that he 
considers that it would take 1,000 or 1,200 years for coral to 
grow upwards from a depth of seven fathoms to the surface 
of the sea; this estimate cannot, however, be universally 
applicable, since their rate of growth differs in different seas, 
and varies according to the species. Thus, for example, on 
a ship, which was sunk for twenty months in the Persian 
Gulf, there was an incrustation of coral to a depth of two feet 
on her copper bottom; while in the case of the sunken ship 
Shannon, which was found crusted with coral, it was reckoned 
that, even supposing the coral to have commenced growing 
as soon as the ship reached the bottom, the growth could not 
have been more than three inches in a year. Again, some 
corals have been planted on the Madagascar coast, and these 
were observed to have grown to a height of nearly three feet 
in no less than six months. 

The second class, or 'barrier-reef (Fig. 2), differs from the first 
class in that it is situated at a greater distance from the shore, 
and that, as a consequence of this, the depth of the water on 
both its littoral and seaward sides is much greater. They 
occur in the middle of the Red Sea; they are common in the 
Pacific, where they form the great barrier-reef on the north-east 
coast of Australia, and extend around the Society Islands, the 
Fijis, and New Caledonia; and they surround islands like the 
Pelew Islands, and the Comora Isles in the Mozambique 
Channel. Some of them are very large; — that surrounding 
New Caledonia is four hundred miles long, and about ten miles 
distant from the shore; and the one off the north-east coast 
of Australia is from ten to ninety miles broad, about 1,250 
statute miles in length, and rises from the ocean bed on its 
seaward side from a depth which often exceeds 1,800 feet, 




Fig. 2.— Barrier-Reef. 



PREFA TOR Y NOTE. 



VI 1 



This last example of reef has been compared by Jukes 1 to 
'a great submarine wall or terrace, fronting the whole north-east 
coast of Australia, resting at each end on shallow water, but 
rising from very great depths about the centre ; its upper 
surface forming a plateau covered by 10 to 30 fathoms of 
water, but studded all over with steep-sided block-like masses 
which rise up to low water-level. These masses are especially 
numerous, and most linear along the edge of the great bank 
on which they rest; the passage between them being often very 
narrow, like regular embrasures opened here and there through 
the parapet wall of a fortress. These 'individual reefs' 
running along the outer edge protect the comparatively shallow 
water inside, and with the numerous inner reefs that are 
scattered over its space make it one great natural harbour.' 

The third and last class, or 'atoll' (Fig. 3), is an elliptical, oval 
or roundish ring of coral, with here and there a break in its 




Fig. 3.— Stewart Atoll or Sikiana (lat. 8° 22' S. ; long. 162° 58' E.). 
C, Reef Channel ; F, Faule Island. 

continuity, and with a central lake-like expanse of water, known 
as the lagoon. The outside water is generally very deep, and 
the inside shallow ; thus off the Cocos-Keeling Atoll the 

1 Manual of Geology ; p. 131 ; Voyage of H.M.S. Fly, vol. i. 
chap. xiii. 



viii PREFA TOR Y NOTE. 

sounding-lead, at a distance of 2,200 yards from the reefs edge, 
sinks to a depth of 1,200 fathoms, while the lagoon is only from 
two to seven fathoms deep. ' Such a basin with its deep, clear 
channels through the reef — affording (as many of them do) 
room enough for all the navies of Christendom to ride at 
anchor — supplies the very perfection of harbour accommodation, 
even though the surrounding reefs are so low that during storms 
the breakers outside will dash over the massed wall of coral. In 
this lagoon marine animals of all sorts — including fishes, and 
pre-eminently sharks — swarm : but it is seldom that the wind 
disturbs the water with its smooth, glassy expanse, and 
curiously enough the openings in the reef are always on the 
leeward side, that is, in the one least exposed to the prevailing 
winds, so that while a ship has no difficulty in getting out to 
sea, it sometimes happens that it does not find an escape from 
the storm so easy. 5 The commonest localities for atolls are the 
Indian and Pacific Oceans. Stewart's atoll (Fig. 3) in the 
Solomon Islands, and the Menschikoff Island may be taken as 
typical examples. 

Before Darwin wrote, it was universally believed that these 
atolls were formed by the coral polypes growing upon sub- 
merged volcanic craters. This theory finds expression and 
support in the second volume of Lyell's Principles of Geology, 
a bonk which was published in 1832, and which Darwin read. 
It is known that he procured the first volume (just then issued) 
at the suggestion of Henslow previous to his setting out on 
the voyage of the Beagle, which started from Devonport on 
December 27, 1831, and probably, too, that kindly, sagacious, 
sympathetic friend and teacher forwarded him the second 
volume as soon as published. But whatever the case 
may be, it is evident that Darwin was acquainted with 
the prevailing idea, and that his acute and penetrating mind 
discerned at once its weaknesses, for he tells us in his 
Autobiography that the main features of his theory were 
conceived while on the voyage, and thai even previous to seeing 
' a true coral-reef.' ' No other work of mine, 5 he says, 1 ' was 
1 Life and Letters of Charles Darwin, 1888, vol. i. p. 70. 



PREFA TOR Y NOTE. ix 

begun in so deductive a spirit as this, for the whole theory was 
thought out on the west coast of South America, before I had 
seen a true coral-reef. I had only to verify and extend my views 
by a careful examination of living reefs. But it should be 
observed that I had during the two previous years been inces- 
santly attending to the effects on the shores of South America of 
the intermittent elevation of the land, together with denudation 
and the deposition of sediment. This necessarily led me to 
reflect much on the effects of subsidence, and it was easy to 
replace in imagination the continued deposition of sediment by 
the upward growth of corals. To do this was to form my theory 
of the formation of barrier-reefs and atolls.' Coupled with this, 
Darwin, as is manifested by his work, also saw that, in forming 
any theory of the genesis and development of coral-reefs, not 
only must the nature of the platform on which the corals build 
be taken into account, but that other factors, of as equally great 
importance, come into play and must be reckoned, — notably, 
the peculiar conditions of the life of the coral-polypes themselves, 
and the peculiar, and, then, inexplicable distribution of the reefs 
and atolls. He saw more clearly than his precursors had 
done the validity of the dictum of Johannes Miiller, in this and 
indeed in all his works, that the most important truths in 
Natural Science are to be discovered, neither by the mere 
analysis of philosophical ideas, nor by simple experience, but by 
reflective experience, which distinguishes the essential from the 
accidental in the phenomena observed, and thus finds principles 
from which many experiences can be derived. 

The conditions necessary for the proper continuance of the 
organic life of these polypes seem to be a temperature which is 
not lower than 68°F., the presence of clear water, and a depth not 
exceeding twenty fathoms. They also cannot survive exposure 
to the sun and air, and thus are unable to flourish unless the top 
of the reef be below the mark of the lowest tides. Thus reefs 
are absent from the West Coast of America because it is washed 
by a cold extra-tropical current ; they are not found in the 
South Atlantic because that ocean is not within their special 
isotherm; and they are not present on the shores of Trinidad 

b 



x PRE FA TOR Y NO TE. 

or the north-eastern coast of North America, since these are in 
the neighbourhood of the mouths of large rivers which bring 
down a large amount of mud and other suspended matters from 
the land. 

Reasoning on these facts, and fresh with the evidences of 
subsidence, obtained by reading and observation on the South 
American coast, Darwin conceived and nurtured the theory 
which is set forth in the succeeding pages. Briefl}', this theory- 
is as follows : — That — as the polypes cannot live below a 
depth of ioo feet, and are killed by exposure to sunshine 
and air, and could not therefore have grown upward from 
those vast depths to which the coral-masses extend — each atoll 
began as a fringing-reef, then became a barrier-reef, and at last 
appeared as a ring of coral with a central lagoon, owing to 
a slow but progressive subsidence of the site on which the 
polypes first began to build. If, on this view, a fringing-reef be 
formed round an island (Fig. 4, 1st period) between the sea- 
level and the 20-fathom line, and then the island gradually 




Fig. 4.— Illustrating Darwin's theory of formation of the three kinds of Coral 
Reefs. 

First period, the Fringing-Reef ; second period, the Barrier-Reef; third 
period, the Atoll. 

sink deeper into the sea, it (z.e., the island) will have become 
smaller, and the channel between it and the reef wider ; the 
fringing-reef will in time have become changed into a barrier- 
reef (Fig. 4, 2nd period), provided that the polypes grow 
upward at a rate which keeps pace with the depression. Again, 
another gradual subsidence of the island taking place, and the 
coral growing upward as fast as the base sinks downward, 
there would at last result a more or less ring-shaped reef 



PREFA TOR Y NOTE. xi 

with a central expanse of water (Fig. 4, 3rd period). The 
barrier-reef has become an atoll. On the outer margins of a 
reef thus formed, the waves dash and break off pieces of coral, 
and heap the broken masses upon its surface, so that its edge 
appears above the low-tide level. The majority of the polypes 
then die; 'but the waves continue to pile up on the reef, sand, 
pebbles, and broken masses of coral, some of the masses being 
two to three hundred cubic feet in size, and a field of rough 
rocks begins to appear above the waves. Next a beach is 
formed; and the bank of coral debris, now mostly above the 
salt-water, becomes planted by the waves with sea-borne seeds. 
Trailing shrubs spring up; and afterwards, as the soil deepens, 
palms and other trees rise into forests, and the coral-island 
or atoll comes forth finished.' 

For many years geologists universally accepted the general 
validity of Darwin's theory. The first note of dissent seems to 
have been sounded in 1863 when Professor Semper published an 
article 1 on the Pelew Islands, which are situated at the western 
extremity of the Caroline Archipelago, and which appeared to 
him to show evidence of elevation, rather than of subsidence. 
It is a significant fact that at the southern end of these islands, 
there are raised coral-reefs from 400-500 feet in height, and 
also an island which is entirely destitute of reefs, while at the 
northern extremity, only 60 miles distant, there are true 
atolls. Darwin, however, in the Appendix to his second 
edition, published in 1874, replied that he did not think 
these conditions were insuperable by his theory, and that 
they might be explained on the supposition that the whole 
group had originally subsided, then was upraised, — ' probably 
at the time when the volcanic rocks to the north were erupted' 
— and afterwards again depressed. ' The existence of atolls and 
barrier-reefs in close proximity is manifestly 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 
1 Zeitschr. f, Wissensch, Zoologie., 1863, Bd. xiii. p. 558. 



Xll 



PRE FA TOR Y NOTE. 



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 
remain undistinguishable from fringing-reefs. Now we know 
that the submarine flanks of most atolls are very steep; and if 




Fig. 5. —Madrepore (Goniopora columna, Dana). Natural size. 

an atoll after upheaval and before the sea had eaten deeply 
into the land, and had formed a broad flat surface, were again 
to subside, the reefs which grew to the surface during the sub- 
siding movement would still closely skirt the coast.' In this con- 
nection, Darwin's letter to the same observer is also interesting 
and instructive ; he had received from Professor Semper the 



PREFA TOR Y NOTE. xiii 

portion of the proof-sheets of his book on Animal Life which 
related to corals. 1 (This book was afterwards translated in the 
International Scientific Series.) The letter 2 is dated 'Down, 
October 2, 1879,' and runs thus : — 'My dear Professor Semper, 
— I thank you for your extremely kind letter of the 19th, and 
for the proof-sheets. I believe that I understand all, excepting 
one or two sentences, where my imperfect knowledge of German 
has interfered. This is my sole and poor excuse for the mistake 
which I made in the second edition of my Coral book. Your 
account of the Pelew Islands is a fine addition to our knowledge 
on coral-reefs. I have very little to say on the subject, even if I 
had formerly read your account and seen your maps, but had 
known nothing of the proofs of recent elevation, and of your 
belief that the islands have not since subsided, I have no 
doubt that I should have considered them as formed during 
subsidence. But I should have been much troubled in my 
mind by the sea not being so deep as it usually is round 
atolls, and by the reef on one side sloping so gradually 
beneath the sea ; for this latter fact, as far as my memory 
serves me, is a very unusual and almost unparalleled case. 
I always foresaw that a bank at the proper depth beneath the 
surface would give rise to a reef which could not be distin- 
guished from an atoll formed during subsidence. I must still 
adhere to my opinion, that the atolls and barrier-reefs in the 
middle of the Pacific and Indian Oceans indicate subsidence, 
but I fully agree with you that such cases as that of the Pelew 
Islands, if of at all frequent occurrence would make my general 
conclusions of very little value. Future observers must decide 
between us. It will be a strange fact if there has not been 

1 In the original edition of this book, Professor Semper refers to the 
subject of coral-reefs in the following words : — " Es scheint mir als ob 
er in der zweiten Ausgabe seines allgemein bekannten Werks liber 
Korallenriffe einen Irrthume iiber meine Beobachtungen zum Opfer 
gefallen ist, indem er die Angaben, die ich allerdings bisher immer 
nur sehr kurz gehalten hatte, vollstandig falsch wiedergegeben hat." 

2 Life and Letters of Charles Darwin, vol. iii. p. 182. 



xi v PREFA TOR Y NO TE. 

subsidence of the beds of the great oceans, and if this has not 
affected the forms of the coral-reefs.' The second note of 
dissent was sounded in 1870, when J. J. Rein 1 published some 
observations on the Bermuda Islands, and considered that they 
could be explained on the grounds of an extension upwards of 
accumulations of calcareous sediment from the sea-bottom ; 
this contribution to the subject Darwin did not mention in his 
second edition, and, probably, it escaped him : it has, however, 
been contradicted by the more recent observations of Professor 
Rice in 1884, and Professor Heilprin in 1889. 

The greatest contribution to the controversy has been 
rendered by Dr. J. Murray, who, after his return from the 
Challenger Expedition (to which he acted as naturalist), read, 
on April 5, 1880, a paper" before the Royal Society of Edinburgh 
which has entirely revolutionised the scientific concepts of 
coral-reef formation, and modified to no small degree in the 
minds of thinking geologists the theory which Darwin pro- 
mulgated of the polypes building reefs on areas of subsidence. 
He has pointed out that barrier-reefs do not by themselves 
prove depression, since their bases may be and are formed of a 
talus of their own debris produced by wave-action, and that, 
where such a condition obtains, they appear at first sight to 
consist of a solid, calcareous, coral-like substance which had been 
secreted by the polypes in the exact locality where they are 
now found, and on a bed which had undergone depression. 
He has further shown that those islands which are fringed by 
reefs do not give any evidences of gradual subsidence, and he, 
moreover, states that, in his opinion, were the platforms, on 
which the reefs are built, remnants of a pre-existent continent 
which has been submerged beneath the waves, then it would 
be expected to find traces of strata, other than volcanic, on 
their flanks ; and this it is known is not the case, since 
the only rocks found are lavas and tufas. Again, it is a 
well-known fact that volcanic action takes place on the 

1 Senckenburg, Naiurf. Gesellsch. Wiirzburg, 1869-70, p. 157. 

2 Proc. Roy. Soc. Edin., 1880, p. 505; and Article "Pacific" in 
Encyclopedia Britannica, voL xviii. pp. 128, 129. 



PREFA TOR Y NOTE. xv 

sea-bed, as well as on land, and that sometimes new islands 
are erupted, and sometimes submarine peaks. Thus islands 
like Ascension, St. Paul, Amsterdam, and Reunion were, it 
is evident from their petrological texture, formed in this way, 
and, indeed, in one of them — Reunion — there are still two 
volcanic vents which from time to time throw out molten rock 
and cinders. 1 Etna and Vesuvius, it is also believed, originated 
as submarine volcanoes on a sea-bed which was afterwards 
elevated ; 2 and in the case of the Islands of Santorin and 
Thracia, their structure consists of trass, scoriae, and lava- 
sheets overlying marbles and schists. Indeed these two 
islands form the rim of a vast volcanic crater which descends 
for 1,278 feet below the level of the sea. They are the ' outward 
and visible signs' of an immense submarine volcanic peak 
which, geologically speaking, has been elevated during later 
times, since, on them, Von Fritsch has found in several places, 
up to an altitude of nearly 600 feet above the sea-level, marine 
shells belonging to species which are now living in the sur- 
rounding ocean. Fouque, who has studied these islands more 
closely than any other observer, has arrived at the conclusion 
that ' the volcano formed at one time a large island with 
wooded slopes, and a somewhat civilised human population, 
cultivating a fertile valley in the south-western district, and that 
in prehistoric times the tremendous explosion occurred whereby 
the centre of the island was blown out.' 3 Many more examples 
may be found in our geological text-books. 4 

It is on such platforms as these that Dr. Murray would have 

1 Drasche in Bericht der K. K. Geol. Reichsanstadt, 1875- 1876; 
also Velain, Les Volcans, 1884. 

2 Sartorius von Waltershausen and A. von Lasaulx, Der Aetna, 4to. 
Leipzig, 1880, vol. ii. p. 327. 

3 Compare on the subject: Fritsch, Z. Deutsch. Geol. Ges., xxiii., 
1 87 1, pp. 125-213 ; Fouque s Santorin et ses Eruptions, Paris 5 1880; 
Geikie, Text- Book of Geology, London, 1885, p. 235. 

4 Such as Jukes-Browne's Physical Geology, Prestwich's Manual 
of Geology, Geikie's Text-Book of Geology, and Philips' Manual of 
Geology. 



XVI 



PREFA TOR Y NOTE. 



us believe that coral-reefs are built. 'Whether built up 
sufficiently high to rise above the surface of the sea and thus 
form islands, or brought up only to varying heights below the 
sea-level, these volcanic eminences tend to become platforms on 
which coral-reefs may be formed.' Thus he conceives that if 
the volcanic peak be above the surface, it will be brought down 
to the lower limit of breaker action by the force of the waves, — as 




Fig. 6.— Section of Red Coral showing the Polypes. 

was the case, for example, with Graham's Island in the Mediter- 
ranean 1 which arose on July 18th, 1831, as an active volcanic 
crater, about thirty miles off the south-western shores of Sicily, 
but which was soon demolished by the waves, until a shoal of 

1 Phil. Trans., 1832; Prevost, Mem. Soc. Geol. France, ii. p. 91 ; and 
Ann. des Sci. Nat. , vol. xxiv. 



PREFA TOR Y NOTE. xvii 

scoriae alone remained to mark its former site — or if submarine 
and more than ioo feet below the surface, then it would tend to 
reach the bathymetrical zone at which the polypes live by 
accumulation on its summit of the dead shells of foraminifera, 
molluscs and other testaceous organisms. Then, on such a 
peak, it is evident that the coral-polypes, growing upwards, 
would assume the shape of an atoll. The windward side of a 
reef thus formed grows faster than the lagoon-side, because it is 
on that side that the currents bring food to the polypes ; and as 
the atoll grows outward so the lagoon enlarges, owing to its 
water containing carbonic acid, derived from the decay of the 
polypes and the sea-weed brought in by the tides, which 
dissolves the dead coral and removes in solution the calcium 
carbonate of which it consists. The size of the lagoon can then 
be taken as a general index of the age of the reef. Similarly a 
fringing-reef may be formed round an island which has not 
undergone, or is not undergoing, subsidence and become con- 
verted by extension outwards, on a tains of its own debris, into 
a barrier-reef; provided that, pari passu with the outward 
growth, the littoral side of the reef has its channel widened by 
the solvent action of the carbonic acid in the water obtained 
by the disintegration of the dead polypes. Darwin did not 
live to bring out a third edition of his book, and was 
therefore unable to criticise this theory which had been 
advanced by Murray. In fact, his only contribution to the 
controversy was a letter 1 which he wrote on May 5, 1881, to 
Alexander Agassiz, and which, as it shows in a great measure 
that either his theory had been misrepresented or his work 
had not been given the justice which it deserved, must be 
reproduced here. The letter is as follows: — 'You will have 
seen Mr. Murray's views on the formation of atolls and barrier- 
reefs. Before publishing my book, I thought long over the 
same view, but only as far as ordinary marine organisms are 
concerned, for at that time little was known of the multitude 
of minute oceanic organisms. I rejected this view, as from 
the few dredgings made in the Beagle^ in the south temperate 
1 Life and Letters of Charles Darwin, vol. iii. p. 183. 



xviii PRE FA TOR Y NO TE. 

regions, I concluded that shells, the smaller corals, etc., 
decayed, and were dissolved, when not protected by the 
deposition of sediment, and sediment could not accumulate 
in the open ocean. Certainly, shells, etc., were in several 
places completely rotten, and crumbled into mud between my 
fingers ; but you will know well whether this is in any degree 
common. I have expressly said that a bank at the proper 
depth would give rise to an atoll, which could not be distin- 
guished from one formed during subsidence. I can, however, 
hardly believe in the former presence of as many banks (there 
having been no subsidence) as there are atolls in the great 
oceans, within a reasonable depth, on which minute oceanic 
organisms could have accumulated to the thickness of many 
hundred feet. . . . Pray forgive me for troubling you at such 
length, but it has occurred [to me] that you might be disposed 
to give, after your wide experience, your judgment. If I am 
wrong, the sooner I am knocked on the head and annihilated 
so much the better. It still seems to me a marvellous thing 
that there should not have been much, and long continued 
subsidence in the beds of the great oceans. I wish that 
some doubly rich millionaire would take it into his head to 
have borings made in some of the Pacific and Indian atolls, 
and bring home cores for slicing from a depth of 500 or 600 
feet.' 

Stimulated, perhaps, by this letter from Darwin, Agassiz went 
to work on the Florida reefs, and, in the next year, published 
a paper 1 which contained the gist of his researches. In this 
paper, he considered that these reefs could not be explained 
by the theory of subsidence ; but that the polypes have grown, 
under the most favourable conditions of food, temperature, and 
oceanic currents, on banks which have been brought into their 
bathymetrical zone by the accumulation of calcareous detritus. 
'This explanation,' he says, 'tested as it has been by pene- 
trating into the thickness of the beds underlying the coral-reefs, 

1 Mem. Amer. Acad. Arts and Set., vol. xi. p. 107; see also the 
"Three Cruises of the Blake" {Bull. Mus, Comp. Zool. Harvard 
Univ.) vol. xiv. iSSS). 



PREFA TOR Y NOTE. xix 

seems a more natural one, for many of the phenomena at 
least, than that of the subsidence of the foundation to which 
the great vertical thickness of barrier-reefs has been hitherto 
referred.' He, however, acknowledges that it is 'difficult 
to account for the great depth of some of the lagoons — forty 
fathoms — on any other theory than that of subsidence.' 
This explanation, however, appears to be negatived in some 
measure by the observations of some American geologists, 
among whom the names of W. H. Dall and A. Heilprin may 
be specially mentioned. Thus, the former states 1 that 'the 
coral formation observed by Agassiz in the region in the keys 
must be of very limited scope, as it has not been identified 
from the mainland of Florida by any modern geologist ;' and 
the latter also notes 2 that ' no observed facts sustain the coral 
theory of formation as propounded by Agassiz. They prove, 
on the contrary, that the coral tract of Florida is confined to 
a border region on the south and south-east, and there are 
no tertiary reefs whatever.' 

More recent still than the observations of Agassiz are those 
of Guppy, 3 who has spent several years among the islands of the 
Solomon Archipelago. These observations, as far as they con- 
cern the theories in question, may be briefly summarised. The 
islands, by the masses of coral limestone which have been 
found on them, indicate elevation ; these upheaved reefs are 
situated upon a basis of volcanic mud having the same character 
as that dredged up by the Challenger Expedition from around 
volcanic islands ; and this mud envelops ' anciently sub- 
merged volcanic peaks.' He also states that corals thrive 
best in the breaker-wash, and do not flourish in the 'break of 
the tide-swell;' that the de:ached reefs, which are submerged 
round these islands, represent the earliest stage in reef-forma- 
tion, and that when they have in their growth upward reached 
a height of from 4 to 8 fathoms distant from the surface, 

1 Amer. Jotirn. Set., 3rd ser., xxxiv. p. 161. 

2 Trans. Wagner. Inst. Set., May, 1887. 

3 Trans. Roy. Soc. Edinb., xxxii. p. 545; Proc. Roy. Soc. Edinb., 
xiii. p. 857. 



xx PREFA TOR Y NOTE. 

they are unable to extend higher, without the help of elevation ; 
that the islands north of St. Christoval, called the "Three 
Sisters," commenced their growth as two flat-topped and sub- 
merged reefs ; and that coral-reefs may grow from a depth 
greater than 25 fathoms, — the conditions necessary being the 
state of the water, and more particularly as to whether it carries 
suspended mud, which is often fatal to the life of the polypes. 
And in a recent letter to Dr. Murray, which has been published 
in Nature (vol. xxxix. p. 236), the same observer states that, in 
his opinion, many features of importance were overlooked by 
Darwin when examining the Keeling atoll, and that these 
give no support whatever to the theory of subsidence. 

Lastly, Mr. G. C. Bourne has printed a very interesting 
paper 1 on the Chagos group, in which he arrives at the con- 
clusion that the majority of the reefs in the Indian Ocean show 
evidences of elevation 'rather than of rest' ; and that 'certainly 
they are not evidences of subsidence. 5 He challenges the two 
chief features in Murray's theory — the shape and character of 
lagoons depending on the more vigorous growth of the polypes 
on the periphery of the reef owing to ocean-currents, and the 
solution of its interior by the carbonic acid in the water ; and 
states that it must be realised 'that the laws governing the 
formation of coral-reefs are exceedingly complex, and that 
many circumstances have to be taken into account before any 
perfect explanation of their structure can be obtained. 5 ' That 
sea-water exercises a solvent action upon carbonate of lime 
does not admit of a doubt, and that the scour of tides, com- 
bined with the solvent action of the water, does affect the 
extent and depth of a lagoon is obvious. But I challenge the 
statement that the destructive agencies within an atoll or a 
submerged bank are in excess of the constructive. It would 
be nearer the mark to say that they nearly balance one another. 
In the first place, the carbonate of lime held in solution by 
sea-water is deposited as crystalline limestone in the interstices 
of dead corals or coral debris. Any one who is acquainted with 
the structure of coralline rock, knows how such a porous mass 
1 Proc. Roy. Soc, xliii. p. 440, 1888. 



PREFA TOR Y NOTE. xxi 

as a mceandrina head becomes perfectly solid by the deposition 
of lime within its mass. This deposition can only be effected 
by the infiltration of sea-water. In reckoning the solvent 
action of sea-water, therefore, account must be taken of the 
fact that a not inconsiderable proportion of the carbonate of 
lime held in solution is re-deposited in the form of crystalline 
limestone. Of this, it seems, Mr. Murray has not taken 
sufficient account, and has, therefore, overstated the destructive 
agency of the sea. Secondly, the growth of corals, and the 
consequent formation of coral-rock within the lagoon, is 
generally overlooked. 

'Whilst diving for corals at Diego Garcia, I had abundant 
opportunities of studying the formation of coral-rock within 
the lagoon, in depths under 2 fathoms. The layers of tolerably 
compact rock thus formed are of no mean extent or thickness ; 
they soon become covered with sand, and are thus protected 
from the solvent action of the water. I have found it impossible 
to reconcile Mr. Murray's views with what I saw of coral growth 
within a lagoon. Not only do the more delicate branching 
species of the madreporatia flourish in considerable numbers, 
but true reef-building species, porites, mceandrina, pocillopora y 
and various stout species of madrepora are found there. It 
is a mistake to suppose that certain species of corals are 
restricted to the external shores, others to the lagoon. My 
collections proved that many of the species growing in the 
lagoon at distances of five miles and upwards from its outlet 
are identical with those growing on the outer reef. In addition 
to them are numerous species, such as Seriatopora strida, 
Mussa corymbosa, Favia lobata, Fungia dentata, and many 
others that are not found on the outside. The reason is that 
the last-named are either free forms such as fungia, or are 
attached by such slender and fragile stems to their supports that 
they could not possibly obtain a foothold and maintain them- 
selves among the powerful currents and waves of the open ocean. 

' These various species, numbers of which grow close together, 
form knolls and patches within the lagoon, and it cannot be 
doubted that their tendency is to fill it up. Again, in reefs 



xxii PRE FA TOR Y NO TE. 

which do not rise above the surface, or are awash for the 
greater part of their extent at low tides, great quantities of 
debris ) torn from the outer slopes, are constantly carried over 
the rim of the reef and tend to fill it up. Hence it follows that 
in a lagoon entirely surrounded by dry land, or nearly so, as is 
the case at Diego Garcia, the tendency to the accumulation of 
material within the lagoon would be less than in submerged or 
incomplete atolls, for debris cannot be swept over into the 
lagoon, and the only constructive agency is the growth of coral. 
If the power of solution of sea-water is so great, it must be 
supposed that in complete or nearly complete atolls the lagoon 
would be deepening rather than shallowing ; yet at Diego 
Garcia the lagoon is obviously shallowing in many places, and 
has nowhere increased in depth since Captain Moresby's 
survey in 1837. Indeed, the southern part seemed to have 
shoaled a fathom since that time, and this is the more remark- 
able, since the S.E. trade- winds are by far the most constant 
and strongest winds there, and tend to accumulate material at 
the northern rather than the southern end. The fact is, that 
these winds sweep the sand out of the southern part, and thus 
leave an area particularly favourably situated for the growth of 
corals. Mr. Murray points out that larger atolls generally 
have deeper lagoons than small atolls, and urges this fact in 
support of his theory ; but here again the facts in the Chagos 
group are against him. Victory Bank is a submerged atoll, 
the Solomons is an atoll with a large extent of dry land ; in 
each the lagoon attains a depth of 17-18 fathoms, and in Diego 
Garcia the lagoon, although far larger, does not attain a 
greater depth. Peros Banhos is far smaller than the Great 
Chagos Bank, yet in both the lagoons attain nearly the same 
maximum depth, viz., 41 fathoms for Peros Banhos, 44 fathoms 
for the Great Chagos Bank. Speaker's Bank is very little 
larger than Peros Banhos ; its lagoon is far shallower, having 
a maximum depth of 24 fathoms. . . . Corals grow best in 
places where a moderate current flows constantly over them. 
They are killed in still water by the deposition of sediment, and 
they will not grow in places where a strong current sets directly 



PRE FA TOR Y NOTE. xxiii 

against them. I noted at Diego Garcia in many places, but 
particularly at the east end of East Islet, that a strong and 
direct ocean current is most unfavourable to coral growth, and 
that the reef is barren and suffering rapid erosion at such spots 
as allow the whole force of the current to fall directly upon 
them. As the current parts and flows round the obstacle, 
one meets with a reef covered with debris, but barren of 
live coral ; further on, as the current moderates in force, 
one finds a few growing heads of coral ; and, finally, at 
the further end of the reef, where the current has abated 
its force considerably, there is a luxuriant bed of living 
corals and Alcyonaria. This can be seen in perfection on 
the southern reef of East Islet. Dr. Hickson tells me that 
he has observed the same facts at Celebes, that direct and 
strong currents are unfavourable to coral growth, that moderate 
tangential currents are extremely favourable, and sluggish or 
still water again unfavourable. This view, which both of us 
can support by many observations, is much at variance with 
the old accepted saying that corals grow best where the 
breakers are the heaviest. It appeared to me that heavy 
breakers are not favourable to coral growth, because of the quan- 
tity of shingle which they dash against the soft-bodied polypes. 
Some massive forms might withstand the force of breakers 
and violent currents if the polypes could be sufficiently pro- 
tected from the shingle, but the branching madrepores are soon 
broken off and swept away, and even the more massive mcean- 
drina soon follows, for whilst the surface of the colony grows 
the base is dead, is soon riddled by boring sponges, serpulas, 
etc., and is no longer able to bear the strain put upon it. The 
great mass then breaks off, and is rolled along the reef, pound- 
ing other corals in its course.' 

The whole question is still under consideration, and the 
reader must judge for himself which of these theories he will 
accept. But it may be stated that Professor Dana 1 — a dis- 
tinguished authority and student of corals — has advanced his 
opinion that ' all the hypotheses of objection to Darwin's 
1 Am. Journ. Sci., 1885, p. 190. 



xxiv PREFA TOR Y NOTE. 

theory are alike weak, for all have made these processes 
{i.e., solution and abrasion) their chief reliance, whether appeal- 
ing to a calcareous, a volcanic, or a mountain-peak basement 
for the structure. The subsidence which the Darwinian theory 
requires has not been opposed by the mention of any fact at 
variance with it, nor by setting aside Darwin's arguments in its 
favour ; and it has found new support in the soundings off 
Tahiti that have been put in array against it, and strong cor- 
roboration in the facts from the West Indies.' And if at such 
an early stage another impartial opinion may be expressed, it is 
this one : — that many of Darwin's critics have not carefully 
read his work, and that, so far as the controversy has, as yet, 
advanced, the theory of subsidence accounts for the majority, if 
not all, of the features of coral-reef formation. On no other 
theory, indeed, can the African element in the Indian fauna be 
explained than on the supposition that land once stretched 
between Mozambique and the Malabar coast which has become 
depressed, and which is now alone represented by the Chagos 
Bank, the Saya de Malha, and the Laccadive and Maldive 
Islands. Darwin died on April 19, 1882, and, if we may judge 
by his letters, he remained convinced to the last of the general 
truthfulness of his theory. But, no matter how future observa- 
tions may decide, had he written no other work and simply 
rested on his laurels, this book alone by its very inductive 
reasoning and patient marshalling of facts would have remained 
as an everlasting monument of scientific acumen, and would 
have placed him in the front rank of investigators. 

In bringing this book, as far as possible, up to the date of our 
present information on the subject, the Editor has made free 
use of Professor Bonney's Appendix to the third edition of 
Darwin's Coral Reefs (Smith, Elder, & Co.), and of Professor 
Geikie's Address to the Royal Physical Society of Edinburgh 
{Proceedings, vol. viii. p. 1). He has also to thank his friend, 
Mr. A. Paling, for several valuable suggestions, and for kindly 
revising the sheets during the time they were passing through 
the press. 

JOSEPH W. WILLIAMS. 



THE STRUCTURE AND DISTRIBUTION 
OF CORAL-REEFS. , 



866 



PREFACE. 



I shall have occasion, in many parts of the following 
volume, to acknowledge the valuable information I have 
received from several persons ; but I must particularly 
express my obligations to Captain R. Moresby, I.N., who 
conducted the survey of the Red Sea, and of the archi- 
pelagoes of low coral-islands in the Indian Ocean. I beg 
also to be permitted to return my best thanks to Captain 
Beaufort, R.N., for having given me free access to the 
charts in the Admiralty, as well as to Captain Beecher, 
R.N., for most kindly aiding me in consulting them. My 
thanks are likewise especially due to Captain Washington, 
R.N., for his invariable desire to assist me in every possible 
manner. Having in former publications had the pleasure 
of acknowledging how much I owe to Captain Fitzroy, for 
having permitted me to volunteer my services on board 
H.M.S. Beagle, and for his uniform kindness in giving me 
assistance in my researches, I can here only repeat my 
obligations to him. The materials for this volume were 
nearly ready two years ago ; but owing to ill-health its 
publication has been delayed. The two succeeding Parts, — 
one on the volcanic islands visited during the voyage of the 
Beagle, and the other on South America, — will appear as 
soon as they can be prepared. 

2nd May 1842. 



DESCRIPTION OF THE PLATES. 
^ 

PLATE I. 

In the several original surveys, from which the small plans on 
these plates 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 Archi- 
pelago, published by the East Indian Company, from the 
survey by Capt. Moresby and Lieut. Powell. The surface 
of the reef, which dries at low water, is represented 
by a surface with small crosses : the coral-islets on the reef 
are marked by small linear spaces, on which a few cocoa- 
nut trees, out of all proportion too large, have been 
introduced for the sake of clearness. The entire annular 
reef, which when surrounding an open expanse of water, 
forms an ' atoll,' and when surrounding one or more high 
islands, forms an encircling ' barrier-reef,' has a nearly 
uniform structure. The reefs in some of the original 
surveys are represented merely by a single line with 
crosses, so that their breadth is not given ; I have had 
such reefs engraved of the width usually attained by coral- 
reefs. I have not thought it worth while to introduce all 
those small and very numerous reefs, which occur within 
the lagoons of most atolls and within the lagoon-channels 
of most barrier-reefs, and which stand either isolated, or 
are attached to the shores of the reef or land. At Peros 
Banhos none of the lagoon-reefs rise to the surface of the 
water ; a few of them have been introduced, and are 
marked by plain dotted circles. A few of the deepest 
soundings are laid down within each reef; they are in 
fathoms, of six English feet. 



6 DESCRIPTION OF THE PLATES, 

Fig. I. — Vanikoro, situated in the western part of the South 
Pacific ; taken from the survey by Capt. D'Urville in the 
Astrolabej the soundings on the southern side of the 
island, namely, from 30 to 40 fathoms, are given from 
the voyage of the Chev. Dillon ; the other soundings are 
laid down from the survey by D'Urville ; height of the 
summit of the island is 3,032 feet. The principal small 
detached reefs within the lagoon-channel have in this 
instance been represented. The southern shore of the 
island is narrowly fringed by a reef : if the engraver had 
carried this reef entirely round both islands, this figure 
would have served (by leaving out in imagination the 
barrier-reef) as a good specimen of an abruptly-sided 
island, surrounded by a reef of the fringing class. 

Fig. 2. — Menchikoff atoll (or lagoon-island), in the Marshall 
Archipelago, Northern Pacific Ocean; from Krusenstern's 
Atlas of the Pacific; originally surveyed by Capt. Hage- 
meister; the depth within the lagoons is unknown. 

Fig. 3. — Pouynipete, or Seniavine, in the Caroline Archi- 
pelago ; from the survey by Admiral Lutkd. 

PLATE II. 

Fig. 1. — Bolabola, in the Society Archipelago, from the 
survey of Capt Duperrey in the Coquille : the soundings 
in this and the following figures have been altered from 
French feet to English fathoms ; height of highest point of 
the island 4,026 feet. 

Fig. 2. — Keeling, or Cocos atoll (or lagoon-island), in the 
Indian Ocean ; from the survey by Capt. Fitzroy ; the 
lagoon south of the dotted line is very shallow, and is left 
almost bare at low water ; the part north of the line is 
choked up with irregular reefs. The annular reef on the 
north-west side is broken, and blends into a shoal sand- 
bank, on which the sea breaks. 



DESCRIPTION OF THE PLATES. 7 

Fig. 3. — HOGOLEU, or ROUG, in the Caroline Archipelago ; 
taken from the Atlas of the Voyage of the Astrolabe^ 
compiled from the surveys of Captains Duperrey and 
D'Urville ; the depth of the immense lagoon-like space 
within the reef is not known. 

Fig. 4. — Raiatea, in the Society Archipelago ; from the map 
given in the quarto edition of Cootfs First Voyage; it is 
probably not accurate. 

Fig. 5.— Gambier Islands, in the southern part of the Low 
Archipelago ; from the survey by Capt. Beechey ; height 
of highest island, 1,246 feet ; the islands are surrounded by 
extensive and irregular reefs ; the reef on the southern side 
is submerged. 

PLATE III. 

Fig. 1. — Maurua, in the Society Archipelago ; from the survey 
by Capt. Duperrey in the Coquille : height of land about 
800 feet 

Fig. 2. — Maldiva Archipelago, in the Indian Ocean ; from 
the survey by Capt. Moresby and Lieut. Powell. 

Fig. 3. — New Caledonia, in the western part of the Pacific; 
from Krusenstern's At/as, compiled from several sur- 
veys; I have slightly altered the northern point of the 
reef, in accordance with the Atlas of the Voyage of the 
Astrolabe. In Krusenstern's Atlas^ the reef is repre- 
sented by a single line with crosses ; I have for the sake 
of uniformity added an interior line. 

Fig. 4.— MAHLOS Mahdoo ATOLL, together with Horsburgh 
atoll, in the Maldiva Archipelago ; from the survey by 
Capt. Moresby and Lieut. Powell ; the white spaces 
in the middle of the separate small reefs, both on the 
margin and in the middle part, are meant to represent 
little lagoons; but it was found not possible to distinguish 
them clearly from the small islets, which have been formed 
on these same small reefs ; many of the smaller reefs could 



8 DESCRIPTION OF THE PLATES. 

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. 

Fig. 5. — Bow, or Heyou ATOLL (or lagoon-island), in the Low 
Archipelago, from the survey by Capt. Beechey, R.N. ; 
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. 

PLATE IV. 

Fig. 1. — Great Chagos Bank, in the Indian Ocean; taken 
from the survey by Capt. Moresby and Lieut. Powell ; 
the parts which are shaded, with the exception of two 
or three islets on the western and northern sides, do 
not rise to the surface, but are submerged from 4 to 
10 fathoms ; the banks bounded by the dotted lines lie 
from 15 to 20 fathoms beneath the surface, and are 
formed of sand ; the central space is of mud, and from 
30 to 50 fathoms deep. 

Fig. 2. — A vertical section, on the same scale, in an E. and W. 
line across the Great Chagos Bank, given for the sake of 
exhibiting more clearly its structure. 

Fig. 3. — Peros Banhos atoll (or lagoon-island), in the 
Chagos group in the Indian Ocean ; from the survey by 
Capt. Moresby and Lieut. Powell ; not nearly all the 
small submerged reefs in the lagoon are represented; the 
annular reef on the southern side is submerged. 

PLATE V. 

The principles on which this map was coloured are explained 
in the beginning of Chapter VI. ; and the authorities for 
each particular spot are detailed in the Appendix. The 
names printed in italics in the Index refer to the Appendix. 



CONTENTS. 



PAGE 

Introduction . . . . . ,15 

CHAPTER I. 

ATOLLS OR LAGOON-ISLANDS. 

Section I. — Description of keeling atoll. 

Corals on the outer margin. — Zone of Nulliporae. — Exterior 
reef. — Islets. — Coral - conglomerate. — Lagoon. — Cal- 
careous sediment. — Scari and Holuthuriae subsisting 
on corals. — Changes in the condition of the reefs and 
islets. — Probable subsidence of the atoll.— -Future state 
of the lagoon . . . . 19 to 36 

Section II.— General description of atolls. 

General form and size of atolls, their reefs and islets. — 
External slope. — Zone of Nulliporae. — Conglomerate. 
— Depth of lagoons. — Sediment. — Reefs submerged 
wholly or in part. — Breaches in the reef. — Ledge- 
formed shores round certain lagoons. — Conversion of 
lagoons into land . . . . . 36 to 50 

Section III. — Atolls of the maldiva archi- 
pelago—great chagos bank. 

Maldiva Archipelago. — Ring-formed reefs marginal and 
central. — Great depth in the lagoons of the southern 



io CONTENTS. 

PAGE 

atolls. — Reefs in the lagoons all rising to the surface. 
— Position of islets, and breaches in the reefs with 
respect to the prevalent winds and action of the waves. 
— Destruction of islets. — Connection in the position 
and submarine foundation of distinct atolls. — The 
apparent disseverment of large atolls. — The Great 
Chagos Bank. — Its submerged condition and extra- 
ordinary structure . . . 50 to 60 



CHAPTER II. 

BARRIER- REEFS. 

Closely resemble in general form and structure atoll-reefs. — 
. Width and depth of the lagoon-channels. — Breaches 
through the reef in front of valleys, and generally on 
the leeward side. — Checks to the filling up of the 
lagoon-channels. — Size and constitution of the encircled 
islands. — Number of islands within the same reef. — 
Barrier-reefs of New Caledonia and Australia, — 
Position of the reef relative to the slope of the adjoining 
land. — Probable great thickness of barrier-reefs . 61 to 72 

CHAPTER III. 

FRINGING OR SHORE-REEFS. 

Reefs of Mauritius. — Shallow channel within the reef. — Its 
slow filling up. — Currents of water formed within it. — 
Upraised reefs. — Narrow fringing-reefs in deep seas. — 
Reefs on the coast of E. Africa and of Brazil. — Fringing- 
reefs in very shallow seas, round banks of sediment 
and on worn-down islands. — Fringing-reefs affected by 
currents of the sea. — Coral coating the bottom of the 
sea, but not forming reefs . . . 73 to 82 



CONTENTS, ii 



CHAPTER IV. 

ON THE DISTRIBUTION AND GROWTH OF CORAL- 
REEFS. 

PAGE 

Section I.— On the distribution of coral-reefs, 

AND ON THE CONDITIONS FAVOURABLE TO THEIR 
INCREASE . . . . '. 83 to 96 

Section II.— On the rate of growth of coral- 
reefs . . . . . . 96 to 106 

Section III.— On the depths at which reef-build- 
ing POLYPIFERS CAN LIVE . . . I06 to II4 



CHAPTER V. 

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

The atolls of the larger archipelagoes are not formed 
on submerged craters, or on banks of sediment. — 
Immense areas interspersed with atolls. — Their sub- 
sidence. — The effects of storms and earthquakes on 
atolls. — Recent changes in their state. — The origin of 
barrier-reefs and of atolls. — Their relative forms. — 
The step-formed ledges and walls round the shores of 
some lagoons. — The ring- formed reefs of the Maldiva 
atolls. — The submerged condition of parts or of the 
whole of some annular reefs. — The disseverment of 
large atolls. — The union of atolls by linear reefs. — 
The Great Chagos Bank. — Objections from the area 
and amount of subsidence required by the theory, 
considered. — The probable composition of the lower 
parts of atolls ..... 11510150 



12 CONTENTS. 

CHAPTER VI. 

ON THE DISTRIBUTION OF CORAL-REEFS WITH REFER- 
ENCE TO THE THEORY OF THEIR FORMATION. 

PAGE 

Description of the coloured map. — Proximity of atolls and 
barrier-reefs. — Relation in form and position of atolls 
with ordinary islands. — Direct evidence of subsidence 
difficult to be detected. — Proofs of recent elevation 
where fringing-reefs occur. — Oscillations of level. — 
Absence of active volcanoes in the areas of sub- 
sidence. — Immensity of the areas which have been 
elevated and have subsided. — Their relation to the 
present distribution of the land. — Areas of subsidence 
elongated, their intersection and alternation with 
those of elevation. — Amount and slow rate of the 
subsidence. — Recapitulation . . 151 to 185 

APPENDIX. 

Containing a detailed description of the reefs and islands 

in Plate V. . . . . . 187 to 264 

General Index . . . . . 265 to 273 



CORAL-REEFS, 



CORAL-REEFS. 



INTRODUCTION 

The object of this volume is to describe from my own 
observation and the works of others, the principal kinds of 
coral-reefs, more especially those occurring in the open 
ocean, and to explain the origin of their peculiar forms. 
I do not here treat of the polypifers, which construct these 
vast works, except so far as relates to their distribution, 
and to the conditions favourable to their vigorous growth. 
Without any distinct intention to classify coral-reefs, most 
voyagers have spoken of them under the following heads : 
'lagoon-islands,' or 'atolls/ 'barrier' or 'encircling 
reefs,' and 'fringing 5 or 'shore -reefs.' The lagoon- 
islands have received much the most attention; and it is 
not surprising, for every one must be struck with astonish- 
ment, when he first beholds one of these vast rings of 
coral-rock, often many leagues in diameter, here and there 
surmounted by a low verdant island with dazzling white 
shores, bathed on the outside by the foaming breakers of 
the ocean, and on the inside surrounding a calm expanse of 
water, which from reflection, is of a bright but pale green 
colour. The naturalist will feel this astonishment more 
deeply after having examined the soft and almost gelatinous 
bodies of these apparently insignificant creatures, and when 
he knows that the solid reef increases only on the outer 
edge, which day and night is lashed by the breakers of an 
ocean never at rest. Well did Francois Pyrard de Laval, 



1 6 CORAL-REEFS. 

in the year 1605, exclaim, "C'est une merueille de voir 
chacun de ces atollons, enuironne d'un grand banc de 
pierre tout autour, n'y ayant point d'artifice humain." The 
accompanying sketch of Whitsunday Island, in the South 
Pacific, taken from Capt. Beechey's admirable Voyage, 
although excellent of its kind, gives but a faint idea of the 
singular aspect of one of these lagoon-islands. 





Whitsunday Island is of small size, and the whole circle 
has been converted into land, which is a comparatively rare 
circumstance. As the reef of a lagoon-island generally 
supports many separate small islands, the word 'island,' 
applied to the whole, is often the cause of confusion; 
hence I have invariably used in this volume the term 
'atoll,' which is the name given to these circular groups of 
coral-islets by their inhabitants in the Indian Ocean, and is 
synonymous with ' lagoon-island.' 

Barrier-reefs v when encircling small islands, have been 
comparatively little noticed by voyagers; but they well 
deserve attention. In their structure they are little less 
marvellous than atolls, and they give a singular and most 
picturesque character to the scenery of the islands they 
surround. In the accompanying sketch, taken from the 
Voyage of the Coquille> the reef is seen from within, 
from one of the high peaks of the island of Bolabola. 1 

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



CORAL-REEFS. 17 

Here, as in Whitsunday Island, the whole of that part of 
the reef which is visible is converted into land. This is a 




circumstance of rare occurrence; more usually a snow- 
white line of great breakers, with here and there an islet 
crowned by cocoa-nut trees, separates the smooth waters of 
the lagoon-like channel from the waves of the open sea. The 
barrier-reefs of Australia and of New Caledonia, owing to 
their enormous dimensions, have excited much attention : 
in structure and form they resemble those encircling many 
of the smaller islands in the Pacific Ocean. 

With respect to fringing, or shore-reefs, there is little in 
their structure which needs explanation ; and their name 
expresses their comparatively small extension. They differ 
from barrier-reefs in not lying so far from the shore, and in 
not having within a broad channel of deep water. Reefs 
also occur around submerged banks of sediment and of 
worn-down rock ; and others are scattered quite irregularly 
where the sea is very shallow ; these in most respects are 
allied to those of the fringing class, but they are of com- 
paratively little interest. 

I have given a separate chapter to each of the above 
classes, and have described some one reef or island, on 
which I possessed most information, as typical ; and have 
afterwards compared it with others of a like kind. Although 
this classification is useful from being obvious, and from 
including most of the coral-reefs existing in the open sea, it 
admits of a more fundamental division into barrier and 
atoll-formed reefs on the one hand, where there is a great 

867 



i8 CORAL-REEFS. 

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 adjoin- 
ing land, there is no such difficulty. The two blue tints 
and the red colour on the map (Plate V.) represent this 
main division, as explained in the beginning of the last 
chapter. In the Appendix, every existing coral-reef, except 
some on the coast of Brazil not included in the map, is 
briefly described in geographical order, as far as I possessed 
information ; and any particular spot may be found by 
consulting the Index. 

Several theories have been advanced to explain the origin 
of atolls, or lagoon-islands, but scarcely one to account for 
barrier-reefs. From the limited depths at which reef-build- 
ing polypifers can flourish, taken into consideration with 
certain other circumstances, we are compelled to conclude, 
as it will be seen, that both in atolls and barrier-reefs, the 
foundation on which the coral was primarily attached, has 
subsided; and that during this downward movement, the 
reefs have grown upwards. This conclusion, it will be 
further seen, explains most satisfactorily the outline and 
general form of atolls and barrier-reefs, and likewise certain 
peculiarities in their structure. The distribution, also, of 
the different kinds of coral reefs, and their position with 
relation to the areas of recent elevation, and to the points 
subject to volcanic eruptions, fully accord with this theory 
of their origin. 1 

1 A brief account of my views on coral formations, now published in 
my Journal of Researches, was read May 31st, 1837, before the Geo- 
logical Society, and an abstract has appeared in the Proceedings. 
(Reprinted at the end of this volume. — Ed.) 



CHAPTER I. 

ATOLLS OR LAGOON-ISLANDS. 



Section First. — Keeling Atoll. 

Corah on the outer margin. — Zone of Nulliporce. — Exteridf reef — Islets. 
— Coral-conglomerate. — Lagoon. — Calcareous sediment, — Scari and 
Holuthurice 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 longitude 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 II., Fig. 2. The greatest 
width of this atoll is nine miles and a half. Its structure 
is in most respects characteristic of the class to which it 
belongs, with the exception of the shallowness of the lagoon. 
The accompanying woodcut represents a vertical section, 
supposed to be drawn at low water from the outer coast 
across one of the low islets (one being taken of average 
dimensions) to within the lagoon. 




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. 



20 CORAL-REEFS. 

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. 

B.— A low projecting ledge of brecciated 1 coral-rock washed by the waves at 
high water. 

E. — A slope of loose fragments, reached by the sea only during gales : the 
upper part, which is from six to twelve feet high, is clothed with vegetation. 
The surface of the islet gently slopes to the lagoon. 

F.— Level of the lagoon at low water. 

The section is true to the scale in a horizontal line, but 
it could not be made so in a vertical one, as the average 
greatest height of the land is only between six and twelve 
feet above high-water mark. I will describe the section, 
commencing with the outer margin, I must first observe 
that the reef-building polypifers, not being tidal animals, 
require to be constantly submerged or washed by the 
breakers. I was assured by Air. Liesk, a very intelligent 
resident on these islands, as well as by some chiefs at 
Tahiti (Otaheite), that an exposure to the rays of the sun 
for a very short time invariably causes their destruction. 
Hence it is possible only under the most favourable circum- 
stances, afforded by an unusually low tide and smooth 
water, to reach the outer margin, where the coral is alive. 
I succeeded only twice in gaining this part, and found it 
almost entirely composed of a living Pontes, which forms 
great irregularly rounded masses (like those of an Astraea, 
but larger) from four to eight feet broad, and little less in 
thickness. These mounds are separated from each other 
by narrow crooked channels, about six feet deep, most of 
which intersect the line of reef at right angles. On the 
furthest mound, which I was able to reach by the aid of a 
leaping-pole, and over which the sea broke with some 

1 That is, sharp angular fragments of coral-rock partially or wholly 
cemented together by calcium carbonate. — Ed. 



CORAL-REEFS. 21 

violence, although the day was quite calm and the tide low, 
the polypifers in the uppermost cells were all dead, but 
between three and four inches lower down on its side they 
were living, and formed a projecting border round the upper 
and dead surface. The coral being thus checked in its 
upward growth, extends laterally, and hence most of the 
masses, especially those a little further inwards, had broad 
flat dead summits. On the other hand I could see, during 
the recoil of the breakers, that a few yards further seaward, 
the whole convex surface of the Porites was alive ; so that 
the point where we were standing was almost on the exact 
upward and shoreward limit of existence of those corals 
which form the outer margin of the reef. We shall presently 
see that there are other organic productions, fitted to bear a 
somewhat longer exposure to the air and sun. 

Next, but much inferior in importance to the Porites, 
is the Millepora comftlanata. 1 It grows in thick vertical 
plates, intersecting each other at various angles, and forms 
an exceedingly strong honeycombed mass, which generally 
affects a circular form, the marginal plates alone being 
alive. Between these plates and in the protected crevices 
on the reef, a multitude of branching zoophytes and other 
productions flourish, but the Porites and Millepora alone 
seem able to resist the fury of the breakers on its upper 
and outer edge : at the depth of a few fathoms other kinds 
of stony corals live. Mr. Liesk, who was intimately 
acquainted with every part of this reef, and likewise with 
that of North Keeling atoll, assured me that these corals 
invariably compose the outer margin. The lagoon is 
inhabited by quite a distinct set of corals, generally brittle 

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



22 CORAL-REEFS. 

and thinly branched; but a Pontes, apparently of the 
same species with that on the outside, is found there, 
although it does not seem to thrive, and certainly does 
not attain the thousandth part in bulk of the masses 
opposed to the breakers. 

The woodcut shows the form of the bottom off the 
reef: the water deepens for a space between one and two 
hundred yards wide, very gradually to 25 fathoms (A in 
section), beyond which the sides plunge into the unfathom- 
able ocean at an angle of 45 . * To the depth of ten or 
twelve fathoms the bottom is exceedingly rugged, and seems 
formed of great masses of living coral, similar to those 
on the margin. The arming of the lead here invariably 
came up quite clean, but deeply indented, and chains 
and anchors which were lowered, in the hopes of tearing 
up the coral, were broken. Many small fragments, how- 
ever, of Millepora alcicornis were brought up; and on 
the arming from an eight-fathom cast, there was a perfect 
impression of an Astraga, apparently alive. I examined 
the rolled fragments cast on the beach during gales, in 
order further to ascertain what corals grew outside the 
reef. The fragments consisted of many kinds, of which 
the Porites already mentioned and a Madrepora, apparently 
the M. cory??ibosa, were the most abundant. As I searched 
in vain in the hollows on the reef and in the lagoon, for 
a living specimen of this Madrepore, I conclude that it 

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 cut off and brought on board for me to examine. The arming 
is a preparation of tallow, placed in the concavity at the bottom of 
the lead. Sand, and even small fragments of rock, will adhere to 
it ; and if the bottom be of rock it brings up an exact impression of 
its surface. 



CORAL-REEFS. 23 

is confined to a zone outside, and beneath the surface, 
where it must be very abundant. Fragments of the Mille- 
pora alcicornis and of an Astrsea were also numerous ; 
the former is found, but not in proportionate numbers, 
in the hollows on the reef; but the Astraea I did not see 
living. Hence we may infer, that these are the kinds of 
coral which form the rugged sloping surface (represented 
in the woodcut by an uneven line), round and beneath 
the external margin. Between 12 and 20 fathoms the 
arming came up an equal number of times smoothed with 
sand, and indented with coral : an anchor and lead were 
lost at the respective depths of 13 and 16 fathoms. 
Out of twenty-five soundings taken at a greater depth 
than 20 fathoms, every one showed the bottom was 
covered with sand; whereas, at a less depth than 12 
fathoms, every sounding showed that it was exceedingly 
rugged, and free from all extraneous particles. Two sound- 
ings 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 zoophytes, but not, as far as I could distinguish, 
of a particle of any lamelliform genus : fragments of shells 
were rare. 

At a distance of 2,200 yards from the breakers, Captain 
Fitzroy found no bottom with a line of 7,200 feet in length ; 
hence the submarine slope of this coral formation is steeper 
than that of any volcanic cone. Off the mouth of the 
lagoon, and likewise off the northern point of the atoll, 
where the currents act violently, the inclination, owing to 
the accumulation of sediment, is less. As the arming of 
the lead from all the greater depths showed a smooth 
sandy bottom, I at first concluded that the whole consisted 
of a vast conical pile of calcareous sand, but the sudden 



24 CORAL-REEFS. 

increase of depth at some points, and the circumstance 
of the line having been cut, as if rubbed, when between 
500 and 600 fathoms were out, indicate the probable 
existence of submarine cliffs. 

On the margin of the reefs, close within the line where 
the upper surface of the Pontes and of the Millepora is 
dead, three species of Nullipora flourish. One grows in 
thin sheets, like a lichen on old trees ; the second in stony 
knobs, as thick as a man's finger, radiating from a common 
centre ; and the third, which is less common, in a moss-like 
reticulation of thin, but perfectly rigid branches. 1 The 
three species occur either separately or mingled together; 
and they form by their successive growth a layer two or 
three feet in thickness, which in some cases is hard, but 
where formed of the lichen-like kind, readily yields an 
impression to the hammer: the surface is of a reddish 
colour. These Nulliporae, although able to exist above 
the limit of true corals, seem to require to be bathed during 
the greater part of each tide by breaking water, for they are 
not found in any abundance in the protected hollows on the 
back part of the reef, where they might be immersed either 
during the whole or an equal proportional time of each tide. 
It is remarkable that organic productions of such extreme 
simplicity, for the Nulliporae 2 undoubtedly belong to one of 

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 knobbe 1 at the extremities. The extremities only are alive and 
brightly coloured. The two other species are of a dirty purplish-white. 
The second species is extremely hard ; its short knob-like, branches are 
cylindrical, and do not grow thicker at their extremities. 

2 The xTullipores belong to the Corallinaceae, a group of the sub-class 
Carpophyccce and the class Algse. Their distinctive characteristic is 
the encrustation of the thalli with calcium carbonate, hence their 
re?emblanc2 to the true corals. — Ed. 



CORAL-REEFS. 25 

the lowest classes of the vegetable kingdom, should be 
limited to a zone so peculiarly circumstanced. Hence the 
layer composed by their growth merely fringes the reef for 
a space of about 20 yards in width, either under the form of 
separate mammillated 1 projections, where the outer masses 
of coral are separate, or, more commonly, where the corals 
are united into a solid margin, as a continuous smooth 
convex mound (B in woodcut), like an artificial breakwater. 
Both the mound and mammillated projections stand about 
three feet higher than any other part of the reef, by which 
term I do not include the islets, formed by the accumula- 
tion of rolled fragments. We shall hereafter see that other 
coral-reefs are protected by a similar thick growth of 
Nulliporse on the outer margin, the part most exposed to the 
breakers, and this must effectually aid in preserving it from 
being worn down. 

The woodcut represents a section across one of the islets 
on the reef, but if all that part which is above the level of C 
were removed, the section would be that of the simple reef, 
as it occurs where no islet has been formed. It is this reef 
which essentially forms the atoll. It is a ring, enclos- 
ing 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 
300 yards; its surface is level, or very slightly inclined 
towards the lagoon, and at high tide the sea breaks 
entirely over it : the water at low tide thrown by the breakers 
on the reef, is carried by the many narrow and shoal gullies 
or channels on its surface, into the lagoon : a return stream 
sets out of the lagoon through the main entrance. The 
most frequent coral in the hollows on the reef is Pocillopora 
verrucosa^ which grows in short sinuous plates, or branches, 
1 Nipple-shaped. — Ed. 



26 CORAL-REEFS. 

and when alive is of a beautiful pale lake-red : a Madre- 
pora, closely allied or identical with M. pociillfera, is also 
common. As soon as an islet is formed, and the waves are 
prevented breaking entirely over the reef, the channels and 
hollows in it become filled up with cemented fragments, 
and its surface is converted into a hard smooth floor (C of 
woodcut), like an artificial one of freestone. This flat sur- 
face varies in width from ioo 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. 
Nothing can be more singular than the appearance at low 
tide of this 'flat' of naked stone, especially where it is 
externally bounded by the smooth convex mound of 
Nulliporae, appearing like a breakwater built to resist the 
waves, which are constantly throwing over it sheets of 
foaming water. The characteristic appearance of this 
'flat' is shown in the foregoing woodcut of Whitsunday 
atoll. 

The islets on the reef are first formed between 200 and 
300 yards from its outer edge, through the accumulation of 
a pile of fragments, thrown together by some unusually 
strong gale. Their ordinary width is under a quarter of a 
mile, and their length varies from a few yards to several 
miles. Those on the S.E. and windward side of the atoll, 
increase solely by the addition of fragments on their outer 
side j hence the loose blocks of coral, of which their sur- 
face is composed, as well as the shells mingled with them, 
almost exclusively consist of those kinds which live on the 



CORAL-REEFS. 27 

outer coast. The highest part of the islets (excepting 
hillocks of blown sand, some of which are 30 feet high) is 
close to the outer beach (E of the woodcut), and averages 
from six to ten feet above ordinary high-water mark. From 
the outer beach the surface slopes gently to the shores of 
the lagoon, which no doubt has been caused by the 
breakers, the further they have rolled over the reef, having 
had less power to throw up fragments. The little waves of 
the lagoon heap up sand and fragments of thinly-branched 
corals on the inner side of the islets on the leeward side of 
the atoll ; and these islets are broader than those to wind- 
ward, some being even 800 yards in width ; but the land 
thus added is very low. The fragments beneath the sur- 
face are cemented into a solid mass, which is exposed as a 
ledge (D of the woodcut), projecting some yards in front of 
the outer shore and from two to four feet high. This ledge 
is just reached by the waves at ordinary high-water : it 
extends in front of all the islets, and everywhere has a 
water- worn and scooped appearance. The fragments of 
coral which are occasionally cast on the 'flat' are during 
gales of unusual violence swept together on the beach, 
where the waves each day at high-water tend to remove 
and gradually wear them down; but the lower fragments 
having become firmly cemented together by the percolation 
of calcareous matter, resist the daily tides longer, and hence 
project as a ledge. The cemented mass is generally of a 
white colour, but in some few parts reddish from ferruginous 
matter; it is very hard, and is sonorous under the hammer; 
it is obscurely divided by seams, dipping at a small angle 
seaward ; it consists of fragments of the corals which grow 
on the outer margin, some quite and others partially 
rounded, some small and others between two and three 
feet across ; and of masses of previously formed 



28 CORAL-REEFS. 

conglomerate, torn up, rounded, and re-cemented ; or it con- 
sists of a calcareous sandstone, entirely composed of rounded 
particles, generally almost blended together, of shells, corals, 
the spines of echini, and other organic bodies ; — rocks, of 
this latter kind, occur on many shores, where there are no 
coral-reefs. The structure of the coral in the conglomerate 
has generally been much obscured by the infiltration 
of spathose 1 calcareous matter ; and I collected a very 
interesting series, beginning with fragments of unaltered 
coral, and ending with others, where it was impossible to 
discover with the naked eye any trace of organic structure. 
In some specimens I was unable, even with the aid of a 
lens, and by wetting them, to distinguish the boundaries of 
the altered coral and spathose limestone. Many even of 
the blocks of coral lying loose on the beach, had their 
central parts altered and infiltrated. 

The lagoon alone remains to be described ; it is much 
shallower than that of most atolls of considerable size. 
The southern part is almost filled up with banks of mud 
and fields of coral, both dead and alive; but there are 
considerable spaces, between three and four fathoms, and 
smaller basins, from eight to ten fathoms deep. Probably 
about half its area consists of sediment, and half of coral- 
reefs. The corals composing these reefs have a very 
different aspect from those on the outside ; they are very 
numerous in kind, and most of them are thinly branched. 
Meandrina, however, lives in the lagoon, and great rounded 
masses of this coral are numerous, lying quite or almost 
loose on the bottom. The other commonest kinds con- 
sist of three closely allied species of true Madrepora in 
thin branches ; of Seriatafiora subulata; two species of 

1 Resembling spar. — Ed. 



PLAT£ 1. 




Bartholomew. .Edin? 



CORAL-REEFS. 2g 

Pontes 1 with cylindrical branches, one of which forms circular 
clumps, with the exterior branches only alive; and lastly, 
a coral something like an Explanaria, but with stars on 
both surfaces, growing in thin, brittle, stony, foliaceous 
expansions, especially in the deeper basins of the lagoon. 
The reefs on which these corals grow are very irregular in 
form, are full of cavities, and have not a solid flat surface 
of dead rock, like that surrounding the lagoon; nor can 
they be nearly so hard, for the inhabitants made with crow- 
bars a channel of considerable length through these reefs, 
in which a schooner, built on the S.E. islet, was floated out. 
It is a very interesting circumstance, pointed out to us by 
Mr. Liesk, that this channel, although made less than ten 
years before our visit, was then, as we saw, almost choked 
up with living coral, so that fresh excavations would 
be absolutely necessary to allow another vessel to pass 
through it. 

The sediment from the deepest parts in the lagoon, when 
wet, appeared chalky, but, when dry, like very fine sand, 
Large soft banks of similar, but even finer grained mud, 
occur on the S.E. shore of the lagoon, affording a thick 
growth of a Fucus, 2 on which turtle feed : this mud, 
although discoloured by vegetable matter, appears from its 
entire solution in acids to be purely calcareous. I have 
seen in the Museum of the Geological Society, a similar 
but more remarkable substance, brought by Lieut. Nelson 
from the reefs of Bermuda, which, when shown to several 

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

2 A genus of sea- weeds. — Ed. 



3© CORAL-REEFS. 

experienced geologists, was mistaken by them for true 
chalk. On the outside of the reef much sediment must be 
formed by the action of the surf on the rolled fragments of 
coral ; but in the calm waters of the lagoon, this can take 
place only in a small degree. There are, however, other 
and unexpected agents at work here : large shoals of two 
species of Scarus, 1 one inhabiting the surf outside the reef 
and the other the lagoon, subsist entirely, as I was assured 
by Mr. Liesk, the intelligent resident before referred to, by 
browsing on the living polypifers. I opened several of 
these fish, which are very numerous and of considerable 
size, and I found their intestines distended by small pieces 
of coral, and firmly ground calcareous matter. This must 
daily pass from them as the finest sediment; much also 
must be produced by the infinitely numerous vermiform 
and molluscous animals, which make cavities in almost 
every block of coral. Dr. J. Allan, of Forres, who has 
enjoyed the best means of observation, informs me in a 
letter that the Holothuriae (a family of Radiata) subsist on 
living coral ; and the singular structure of bone within the 
anterior extremity of their bodies, certainly appears well 
adapted for this purpose. The number of the species of 
Holothuria, and of the individuals which swarm on every 
part of these coral-reefs, is extraordinarily great ; and many 
ship-loads are annually freighted, as is well known, for 
China with the trepang, which is a species of this genus. 
The amount of coral yearly consumed, and ground down 
into the finest mud, by these several creatures, and 
probably by many other kinds, must be immense. These 
facts are, however, of more importance in another point of 
view, as showing us that there are living checks to the 

1 A genus of fish, commonly called Parrot-fish, with fleshy protrusible 
lips, belonging to the Teleostei. — Ed. 



CORAL-REEFS. 31 

growth of coral-reefs, and that the almost universal law of 
''consumed and be consumed," holds good even with the 
polypifers forming those massive bulwarks, which are able 
to withstand the force of the open ocean. 

Considering that Keeling atoll, like other coral forma- 
tions, has been entirely formed by the growth of organic 
beings, and the accumulation of their detritus, one is 
naturally led to inquire how long it has continued, and 
how long it is likely to continue, in its present state. Mr. 
Liesk informed me that he had seen an old chart in which 
the present long island on the S.E. side was divided by 
several channels into as many islets ; and he assures me 
that the channels can still be distinguished by the smaller 
size of the trees on them. On several islets, also, I 
observed that only young cocoa-nut trees were growing on 
the extremities; and that older and taller trees rose in 
regular succession behind them; which shows that these 
islets have very lately increased in length. In the upper 
and south-eastern part of the lagoon, I was much surprised 
by finding an irregular field of at least a mile square of 
branching corals, still upright, but entirely dead. They 
consisted of the species already mentioned ; they were of a 
brown colour, and so rotten, that in trying to stand on 
them I sank halfway up the leg, as if through decayed 
brushwood. The tops of the branches were barely covered 
by water at the time of lowest tide. Several facts having 
led me to disbelieve in any elevation of the whole atoll, I 
was at first unable to imagine what cause could have killed 
so large a field of coral. Upon reflection, however, it 
appeared to me that the closing up of the above-mentioned 
channels would be a sufficient cause; for before this, a 
strong breeze by forcing water through them into the head 
of the lagoon, would tend to raise its level. But now this 



32 CORAL-REEFS. 

cannot happen, and the inhabitants observe that the tide 
rises to a less height, during a high S.E. wind, at the head 
than at the mouth of the lagoon. The corals, which, under 
the former condition of things, had attained the utmost 
possible limit of upward growth, would thus occasionally be 
exposed for a short time to the sun, and be killed. 

Besides the increase of dry land, indicated by the fore- 
going facts, the exterior solid reef appears to have grown 
outwards. On the western side of the atoll, the f flat ' lying 
between the margin of the reef and the beach is very wide ; 
and in front of the regular beach with its conglomerate 
basis, there is, in most parts, a bed of sand and loose 
fragments with trees growing out of it, which apparently is 
not reached even by the spray at high water. It is evident 
some change has taken place since the waves formed the 
inner beach ; that they formerly beat against it with 
violence was evident, from a remarkably thick and water- 
worn point of conglomerate at one spot, now protected by 
vegetation and a bank of sand ; that they beat against it in 
the same peculiar manner in which the swell from windward 
now obliquely curls round the margin of the reef, was 
evident from the conglomerate having been worn into a 
point projecting from the beach in a similarly oblique 
manner. This retreat in the line of action of the breakers 
might result, either from the surface of the reef in front of 
the islets having been submerged at one time, and after- 
ward having grown upwards, or from the mounds of coral 
on the margin having continued to grow outwards. That 
an outward growth of this part is in process, can hardly be 
doubted from the fact already mentioned of the mounds of 
Pontes with their summits apparently lately killed, and their 
sides only three or four inches lower down thickened by a 
fresh layer of living coral. But there is a difficulty on this 



CORAL-REEFS. 33 

supposition which I must not pass over. If the whole, or 
a large part of the 'flat,' had been formed by the outward 
growth of the margin, each successive margin would 
naturally have been coated by the Nulliporae, and so much 
of the surface would have been of equal height with the 
existing zone of living Nulliporse : this is not the case, as 
may be seen in the woodcut. It is, however, evident from 
the abraded state of the ' flat,' with its original inequalities 
filled up, that its surface has been much modified ; and it is 
possible that the hinder portions of the zone of Nulliporse, 
perishing as the reef grows outwards, might be worn down 
by the surf. If this has not taken place, the reef can in no 
part have increased outwards in breadth since its formation, 
or at least since the Nulliporae formed the convex mound 
on its margin ; for the zone thus formed, and which 
stands between two and three feet above the other parts 
of the reef, is nowhere much above twenty yards in 
width. 

Thus far we have considered facts, which indicate, with 
more or less probability, the increase of the atoll in its 
different parts : there are others having an opposite tend- 
ency. On the S.E. side, Lieut. Sulivan, to whose kindness 
I am indebted for many interesting observations, found the 
conglomerate projecting on the reef nearly fifty yards in 
front of the beach : we may infer from what we see in all 
other parts of the atoll, that the conglomerate was not 
originally so much exposed, but formed the base of an islet, 
the front and upper part of which has since been swept 
away. The degree to which the conglomerate, round nearly 
the whole atoll, has been scooped, broken up, and the 
fragments cast on the beach, is certainly very surprising, 
even on the view that it is the office of occasional gales to 
pile up fragments, and of the daily tides to wear them 

868 



34 CORAL-REEFS. 

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 recom- 
menced an attack on this line of coast. Appearances 
indicating a slight encroachment of the water on the land, 
are plainer within the lagoon : I noticed in several places, 
both on its windward and leeward shores, old cocoa-nut 
trees falling with their roots undermined, and the rotten 
stumps of others on the beach, where the inhabitants assured 
us the cocoa-nut could not now grow. Captain Fitzroy 
pointed out to me, near the settlement, the foundation 
posts of a shed, now washed by every tide, but which the 
inhabitants stated, had seven years before stood above high- 
water mark. In the calm waters of the lagoon, directly 
connected with a great, and therefore stable ocean, it seems 
very improbable that a change in the currents, sufficiently 
great to cause the water to eat into the land on all sides, 
should have taken place within a limited period. From 
these considerations I inferred, that probably the atoll had 
lately subsided to a small amount ; and this inference was 
strengthened by the circumstance, that in 1834, two years 
before our visit, the island had been shaken by a severe 
earthquake, and by two slighter ones during the ten 
previous years. If, during these subterranean disturbances, 
the atoll did subside, the downward movement must have 
been very small, as we must conclude from the fields of 
dead coral still lipping the surface of the lagoon, and from 
the breakers on the western shore not having yet regained 
the line of their former action. The subsidence must, also, 
have been preceded by a long period of rest, during which 
the islets extended to their present size, and the living 



CORAL-REEFS. 35 

margin of the reef grew either upwards, or as I believe 
outwards, to its present distance from the beach. 

Whether this view be correct or not, the above facts are 
worthy of attention, as showing how severe a struggle is in 
progress on these low coral formations between the two 
nicely balanced powers of land and water. With respect to 
the future state of Keeling atoll, if left undisturbed, we can 
see that the islets may still extend in length ; but as they 
cannot resist the surf until broken by rolling over a wide 
space, their increase in breadth must depend on the increasing 
breadth of the reef; and this must be limited by the steep- 
ness 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 pro- 
ducing fine sediment, it might be thought that the lagoon 
would necessarily become quickly filled up. Some of this 
sediment, however, is transported into the open sea, as 
appears from the soundings off the mouth of the lagoon, 
instead of being deposited within it. The deposition, 
moreover, of sediment checks the growth of coral-reefs, so 
that these two agencies cannot act together with full effect 
in filling it up. We know so little of the habits of the 
many different species of corals, which form the lagoon- 
reefs, that we' have no more reasons for supposing that their 
whole surface would grow up as quickly as the coral did in 
the schooner-channel, than for supposing that the whole 
surface of a peat-moss would increase as quickly as parts are 
known to do in holes, where the peat has been cut away. 
These agencies, nevertheless, tend to fill up the lagoon ; but 
in proportion as it becomes shallower, so must the polypifers 
be subject to many injurious agencies, such as impure water 
and loss of food. For instance, Mr. Liesk informed me, 



36 CORAL-REEFS. 

that some years before our visit unusually heavy rain killed 
nearly all the fish in the lagoon, and probably the same 
cause would likewise injure the corals. The reefs also, it 
must be remembered, cannot possibly rise above the level 
of the lowest spring-tide, so that the final conversion of the 
lagoon into land must be due to the accumulation of 
sediment ; and in the midst of the clear water of the ocean, 
and with no surrounding high land, this process must be 
exceedingly slow. 

Section Second. 

General form and size of atolls, their reefs and islets. — External slope. — 
Zone ef Nullipara. — Conglomerate. — Depth of lagoons. — Sediment. 
— Reefs submerged wholly or in part. — Breaches in the reef. — 
Ledge-formed shores round certain lagoons. — Conversion of lagoons 
into land. 

I will here give a sketch of the general form and structure 
of the many atolls and atoll-formed reefs which occur in the 
Pacific and Indian Oceans, comparing them with Keeling 
atoll. The Maldiva atoll 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 group, appears to be sixty miles long and twenty 
broad. Most of the atolls in this group are of an elongated 
form ; thus Bow Island is thirty miles in length, and on an 
average only six in width (see Fig. 5, Plate III.), and 
Clermont Tonnere has nearly the same proportions. In 
the Marshall Archipelago (the Ralick and Radack group of 



CORAL-REEFS. 37 

Kotzebue) several of the atolls are more than thirty miles 
in length, and Rimsky Korsacoff is fifty-four long, and 
twenty wide, at the broadest part of its irregular outline. 
Most of the atolls in the Maldiva Archipelago are of great 
size, one of them (which, however, bears a double name) 
measured in a medial and slightly curved line, is no less 
than eighty-eight geographical miles long, its greatest width 
being under twenty, and its least only nine and a half miles. 
Some atolls have spurs projecting from them ; and in the 
Marshall group there are atolls united together by linear 
reefs, for instance Menchicoff Island (see Fig. 2, Plate I.), 
which is sixty miles in length, and consists of three loops 
tied together. In far the greater number of cases an 
atoll consists of a simple elongated ring, with its outline 
moderately regular. 

The average width of the annular wreath may be taken 
as about a quarter of a mile. Capt. Beechey 1 says that in 
the atolls of the Low Archipelago it exceeded in no instance 
half a mile. The description given of the structure and 
proportional dimensions of the reef and islets of Keeling 
atoll, appears to apply perfectly to nearly all the atolls in 
the Pacific and Indian Oceans. The islets are first formed 
some way back either on the projecting points of the 
reef, especially if its form be angular, or on the sides 
of the main entrances into the lagoon — that is in both 
cases, on points where the breakers can act during gales 
of wind in somewhat different directions, so that the matter 
thrown up from one side may accumulate against that 
before thrown up from another. In Lutke's chart of the 
Caroline atolls, we see many instances of the former case ; 
and the occurrence of islets, as if placed for beacons, on 
the points where there is a gateway or breach through the 

1 Beechey' s Voyage to the i acific and Beering's Straits t chap. vni. 



38 CORAL-REEFS, 

reef, has been noticed by several authors. There are some 
atoll-formed reefs, rising to the surface of the sea and 
partly dry at low water, on which from some cause islets 
have never been formed ; and there are others on which 
they have been formed, but have subsequently been worn 
away. In atolls of small dimensions the islets frequently 
become united into a single horse-shoe or ring-formed 
strip ; but Diego Garcia, although an atoll of considerable 
size, being thirteen miles and a half in length, has its lagoon 
entirely surrounded, except at the northern end, by a belt 
of land, on an average a third of a mile in width. To show 
how small the total area of the annular reef and the land 
is in islands of this class, I may quote a remark from the 
voyage of Lutke, namely, that if the forty-three rings, or 
atolls, in the Caroline Archipelago, were put one within 
another, and over a steeple in the centre of St Petersburg, 
the whole world would not cover that city and its suburbs. 

The form of the bottom off Keeling atoll, which gradually 
slopes to about twenty fathoms at the distance of between one 
and two hundred yards from the edge of the reef, and then 
plunges at an angle of 45 into unfathomable depths, is 
exactly the same 1 with that of the sections of the atolls in 
the Low Archipelago given by Captain Beechey. The 
nature, however, of the bottom seems to differ, for this officer 2 
informs me that all the soundings, even the deepest, were 
on coral, but he does not know whether dead or alive. 

1 The form of the bottom round the Marshall atolls in the Northern 
Pacific is probably similar: Kotzebue {First Voyage, vol. ii. p. 16) says: 
" We had at a small distance from the reef, forty fathoms depth, which 
increased a little further so much that we could find no bottom." 

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



FZATE H 




Barthoto'rri&yv- -Eaxn? 



CORAL-REEFS. 39 

The slope round Christmas atoll (lat. i° 4' N., 157 45' W.), 
described by Cook, 1 is considerably less ; at about half a mile 
from the edge of the reef, the average depth was about four- 
teen fathoms on a fine sandy bottom, and at a mile, only 
between twenty and forty fathoms. It has no doubt been 
owing to this gentle slope, that the strip of land surrounding 
its lagoon has increased in one part to the extraordinary 
width of three miles ; it is formed of successive ridges of 
broken shells and corals, like those on the beach. I know 
of no other instance of such width in the reef of an atoll ; 
but Mr. F. D. Bennett informs me that the inclination of the 
bottom round Caroline atoll in the Pacific, is like that off 
Christmas Island, very gentle. Off the Maldiva and Chagos 
atolls, the inclination is much more abrupt; thus at 
Heawandoo Pholo, Lieutenant Powell 2 found fifty and sixty 
fathoms close to the edge of the reef, and at 300 yards dis- 
tance there was no bottom with a 300-yard line. Captain 
Moresby informs me, that at 100 fathoms from the mouth 
of the lagoon of Diego Garcia, he found no bottom with 
150 fathoms; this is the more remarkable, as the slope is 
generally less abrupt in front of channels through a reef, 
owing to the accumulation of sediment. At Egmont 
Island, also, at 150 fathoms from the reef, soundings were 
struck with 150 fathoms. Lastly, at Cardoo atoll, only 
sixty yards from the reef, no bottom was obtained, as I am 
informed by Captain Moresby, with a line of 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, 

1 Cook's Third Voyage^ vol. ii. chap. 10. 

2 This fact is taken from a MS. account of these groups lent me by 
Captain Moresby. See also Captain Moresby's paper on the Maldiva 
atolls in the Geographical Journal > vol. v. p. 401. 



40 CORAL-REEFS. -m 

that wherever soundings were obtained off these islands, 
the bottom was invariably sandy : nor was there any reason 
to suspect the existence of submarine cliffs, as there was at 
Keeling Island. 1 Here then occurs a difficulty ; can sand 
accumulate on a slope, which, in some cases, appears to 
exceed fifty-five degrees? It must be observed, that I 
speak of slopes where soundings were obtained, and not of 
such cases, as that of Cardoo, where the nature of the 
bottom is unknown, and where its inclination must be 
nearly vertical. M. Elie de Beaumont 2 has argued, and 
there is no higher authority on this subject, from the 
inclination at which snow slides down in avalanches, that a 
bed of sand or mud cannot be formed at a greater angle 
than thirty degrees. Considering the number of soundings 
on sand, obtained round the Maldiva and Chagos atolls, 
which appears to indicate a greater angle, and the extreme 
abruptness of the sand-banks in the West Indies, as will 
be mentioned in the Appendix, I must conclude that the 
adhesive property of wet sand counteracts its gravity, in a 
much greater ratio than has been allowed for by M. Elie de 
Beaumont. From the facility with which calcareous sand 
becomes agglutinated, it is not necessary to suppose that 
the bed of loose sand is thick. 

Captain Beechey has observed, that the submarine slope 

1 Off some of the islands in the Low Archipelago the bottom appears 
to descend by ledges. Off Elizabeth Island, which, however, consists 
of raised coral, Capt. Beechey (p. 45, quarto ed.) describes three 
ledges: the first had an easy slope from the beach to a distance of 
about fifty yards : the second extended two hundred yards with twenty- 
five fathoms on it, and then ended abruptly, like the first; and 
immediately beyond this there was no bottom with two hundred 
fathoms. 

2 Memoires pour servir a une description Geolog. de France, tome iv. 
p, 216. 



CORAL-REEFS. 41 

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 1 the buttress, as it may be called, 
which " has the most powerful enemy (the S.W. swell) to 
oppose, is carried out much further, and with less abrupt- 
ness than the other." In some cases, the less inclination of 
a certain part of the external slope, for instance of the 
northern extremities of the two Keeling atolls, is caused by 
a prevailing current which there accumulates a bed of sand. 
Where the water is perfectly tranquil, as within a lagoon, 
the reefs generally grow up perpendicularly, and sometimes 
even overhang their bases ; on the other hand, on the 
leeward side of Mauritius, where the water is generally 
tranquil, although not invariably so, the reef is very gently 
inclined. Hence it appears that the exterior angle varies 
much ; nevertheless in the close similarity in form between 
the sections of Keeling atoll and of the atolls in the Low 
Archipelago, in the general steepness of the reefs of the 
Maldiva and Chagos atolls, and in the perpendicularity of 
those rising out of water always tranquil, we may discern 
the effects of uniform laws ; but from the complex action of 
the surf and currents, on the growing powers of the coral 
and on the deposition of sediment, we can by no means 
follow out all the results. 

Where islets have been formed on the reef, that part 
which I have sometimes called the 'flat' and which is 
partly dry at low water, appears similar in every atoll. In 
the Marshall group in the North Pacific, it may be inferred 
from Chamisso's description, that the reef, where islets 
have not been formed on it, slopes gently from the external 
margin to the shores of the lagoon : Flinders states that 
the Australian barrier has a similar inclination inwards, 
1 Beechey's Voyage, quarto ed., p. 44. 



42 CORAL-REEFS. 

and I have no doubt it is of general occurrence, although, 
according to Ehrenberg, the reefs of the Red Sea offer 
an exception. Chamisso observes that " the red colour 
of the reef (at the Marshall atolls) under the breakers is 
caused by a Nullipora, which covers the stone wherever 
the waves beat ; and, under favourable circumstances, 
assumes a stalactical form," — a description perfectly applic- 
able to the margin of Keeling atoll. 1 Although Chamisso 
does not state that the masses of Nulliporse form points 
or a mound, higher than the flat, yet I believe that this 
is the case ; for Kotzebue, 2 in another part, speaks of the 
rocks on the edge of the reef " as visible for about two 
feet at low water," and these rocks we may feel quite 
certain are not formed of true coral. 3 Whether a smooth 
convex mound of Nulliporse, like that which appears as 



1 Kotzebue's First Voyage, vol. iii. p. 142. Near Porto Praya, 
in the Cape de Verde Islands, some basaltic rocks, lashed by no in- 
considerable surf, were completely enveloped with a layer of Nulliporse. 
The entire surface over many square inches was coloured of a peach- 
blossomed red ; the layer, however, was of no greater thickness 
than paper. Another kind, in the form of projecting knobs, grew in 
the same situation. These Nulliporse are closely related to those 
described on the coral-reefs, but I believe are of different species. 

2 Kotzebue's First Voyage, vol. ii. p. 16. Lieut. Nelson, in his 
excellent memoir in the Geological Transactions (vol. ii. p. 105), 
alludes to the rocky points mentioned by Kotzebue, and infers that 
they consist of Serpulas, which compose incrusting masses on the reefs 
of Bermudas, as they likewise do on a sandstone bar off the coast of 
Brazil (which I have described in London Phil. Journal, Oct. 1841). 
These masses of Serpulae hold the same position, relatively to the 
action of the sea, with the Nulliporse on the coral-reefs in Indian 
and Pacific Oceans. 

3 Captain 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. 



CORAL-REEFS. 43 

if artificially constructed to protect the margin of Keeling 
Island, is of frequent occurrence round atolls, I know not ; 
but we shall presently meet with it, under precisely the 
same form, on the outer edge of the ' barrier-reefs ' which 
encircle the Society Islands. 

There appears to be scarcely a feature in the structure 
of Keeling reef which is not of common, if not of universal 
occurrence, in other atolls. Thus Chamisso describes 1 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, 2 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 Capt. Beechey's Voyage^ it is not 
apparent that any conglomerate coral-rock was there 
observed. 

The lagoon in Keeling atoll is shallow ; in the atolls of 
the Low Archipelago the depth varies from 20 to 38 
fathoms, and in the Marshall group, according to 
Chamisso, from 30 to 35 ; in the Caroline atolls it is only 
a little less. Within the Maldiva atolls there are large 
spaces with 45 fathoms, and some soundings are laid 
down of 49 fathoms. The greater part of the bottom in 
most lagoons is formed of sediment; large spaces have 
exactly the same depth, or the depth varies so insensibly, 
that it is evident that no other means, excepting aqueous 
deposition, could have levelled the surface so equally. In 

1 Kotzetue's First Voyage, vol. iii. p. 144. 

2 See also Moresby on the Northern atolls of the Maldivas, Geo- 
graphical Journal, vol. v. p. 400. 



44 CORAL-REEFS. 

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 day; and Kotzebue 
speaks of clay having been found within one of the Marshall 
atolls. No doubt this clay is calcareous mud, similar 
to that at Keeling Island, and to that at Bermuda 
already referred to, as un distinguishable from disintegrated 
chalk, and which Lieut. Nelson says is called there pipe- 
clay. 1 

Where the waves act with unequal force on the two sides 
of an atoll, the islets appear to be first formed, and are 
generally of greater continuity on the more exposed shore. 
The islets, also, which are placed to leeward, are in most 
parts of the Pacific liable to be occasionally swept entirely 
away by gales, equalling hurricanes in violence, which blow 
in an opposite direction to the ordinary trade-wind. The 
absence of the islets on the leeward side of atolls, or when 
present their lesser dimensions compared with those to 
windward, is a comparatively unimportant fact; but in 
several instances the reef itself on the leeward side, retain- 
ing its usual defined outline, does not rise to the surface by 
several fathoms. This is the case with the southern side of 
Peros Banhos (Plate IV., Fig. 3) in the Chagos group, with 

1 I may here observe that on the coast of Brazil, where there is much 
coral, the soundings near the land are described by Admiral Roussin, 
in the Pilote du Brteil, as siliceous sand, mingled with much finely 
comminuted particles of shells and coral. Further in the offing, for a 
space of 1,300 miles along the coast, from the Abrolhos Islands to 
Maranham, the bottom in many places is composed of "tuf blanc, 
mele ou forme de madrepores broyes." This white substance, probably, 
is analogous to that which occurs within the above-mentioned lagoons; 
it is sometimes, according to Roussin, firm, and he compares it to 
mortar. 



CORAL-REEFS. 



45 



Mourileu atoll, 1 in the Caroline Archipelago, and with the 
barrier-reef (Plate II., Fig. 5) of the Gambier Islands. 
I allude to the latter reef, although belonging to another 
class, because Captain Beechey was first led by it to observe 
the peculiarity in the question. At Peros Banhos the sub- 
merged part is nine miles in length, and lies at an average 
depth of about five fathoms ; its surface is nearly level, and 
consists of hard stone, with a thin covering of loose sand. 
There is scarcely any living coral on it, even on the outer 
margin, as I have been particularly assured by Captain 
Moresby ; it is, in fact, a wall of dead coral-rock, having 
the same width and transverse section with the reef in its 
ordinary state, of which it is a continuous portion. The 
living and perfect parts terminate abruptly, and abut on the 
submerged portions, in the same manner as on the sides of 
an ordinary passage through the reef. The reef to leeward 
in other cases is nearly or quite obliterated, and one side of 
the lagoon is left open ; for instance, at Oulleay (Caroline 
Archipelago), where a crescent-formed reef is fronted by an 
irregular bank, on which the other half of the annular reef 
probably once stood. At Namonouito, in the same Archi- 
pelago, both these modifications of the reef concur ; it 
consists of a great flat bank, with from 20 to 25 fathoms 
water on it; for a length of more than 40 miles on its 
southern side it is open and without any reef, whilst on the 
other sides it is bounded by a reef, in parts rising to the 
surface and perfectly characterised, in parts lying some 
fathoms submerged. In the Chagos group there are 
annular reefs, entirely submerged, which have the same 
structure as the submerged and defined portions just 

1 Frederick 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. 



46 CORAL-REEFS. 

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 6 and 8 fathoms beneath the surface, and at the 
same depth there are scattered knolls in the lagoon. Cap- 
tain Moresby believes the rim consists of dead rock, thinly 
covered with sand, and he is certain this is the case with 
the external rim of the Great Chagos Bank, which is also 
essentially a submerged atoll. In both these cases, as in 
the submerged portion of the reef at Peros Banhos, Captain 
Moresby feels sure that the quantity of living coral, even on 
the outer edge overhanging the deep-sea water, is quite 
insignificant. Lastly, in several parts of the Pacific and 
Indian Oceans there are banks, lying at greater depths 
than in the cases just mentioned, of the same form and size 
with the neighbouring atolls, but with their atoll-like struc- 
ture wholly obliterated. It appears from the survey of 
Freycinet, that there are banks of this kind in the Caroline 
Archipelago, and, as is reported, in the Low Archipelago. 
When we discuss the origin of the different classes of coral 
formations, we shall see that the submerged state of the 
whole of some atoll-formed reefs, and of portions of others, 
generally but not invariably on the leeward side, and the 
existence of more deeply submerged banks now possessing 
little or no signs of their original atoll-like structure, are 
probably the effects of a uniform cause, — namely, the death 
of the coral, during the subsidence of the area, in which the 
atolls or banks are situated. 

There is seldom, with the exception of the Maldiva atolls, 
more than two or three channels, and generally only one 
leading into the lagoon, of sufficient depth for a ship to 
enter. In small atolls, there is usually not even one. 



CORAL-REEFS. 47 

Where there is deep water, for instance above twenty fathoms, 
in the middle of the lagoon, the channels through the reef 
are seldom as deep as the centre, — it may be said that the 
rim only of the saucer-shaped hollow forming the lagoon is 
notched. Mr. Lyell 1 has observed that the growth of the 
coral would tend to obstruct all the channels through a reef, 
except those kept open by discharging the water, which 
during high tide and the greater part of each ebb is thrown 
over its circumference. Several facts indicate that a con- 
siderable quantity of sediment is likewise discharged through 
these channels ; and Captain Moresby informs me that he 
has observed, during the change of the monsoon, the sea 
discoloured to a distance off the entrances into the Maldiva 
and Chagos atolls. This, probably, would check the growth 
of the coral in them, far more effectually than a mere 
current of water. In the many small atolls without any 
channel, these causes have not prevented the entire ring 
attaining the surface. The channels, like the submerged 
and effaced parts of the reef, very generally though not 
invariably occur on the leeward side of the atoll, or on that 
side, according to Beechey, 2 which, from running in the 
same direction with the prevalent wind, is not fully exposed 
to it. Passages between the islets on the reef, through 
which boats can pass at high water, must not be confounded 
with ship-channels, by which the annular reef itself is 
breached. The passages between the islets occur, of course, 
on the windward as well as on the leeward side ; but they 
are more frequent and broader to leeward, owing to the 
lesser dimensions of the islets on that side. 

At Keeling atoll the shores of the lagoon shelve gradually, 
where the bottom is of sediment, and irregularly or abruptly 

1 Principles of Geology ', vol. iii. p. 289. 

2 Beechey's Voyage^ 4to ed., vol. i. p. 189. 



48 CORAL-REEFS. 

where there are coral-reefs ; but this is by no means the 
universal structure in other atolls. Chamisso, 1 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 of Vanikoro 
have a similar structure. Captain Beechey has described a 
modification of this structure (and he believes it is not un- 
common) in two atolls in the Low Archipelago, in which 
the shores of the lagoon descend by a few, broad, slightly 
inclined ledges or steps : thus at Matilda atoll, 2 the great 
exterior reef, the surface of which is gently inclined 
towards and beneath the surface of the lagoon, ends 
abruptly in a little cliff three fathoms deep ; at its foot, a 
ledge forty yards wide extends, shelving gently inwards 
like the surface-reef, and terminated by a second little cliff 
five fathoms deep ; beyond this, the bottom of the lagoon 
slopes to twenty fathoms, which is the average depth of its 
centre. These ledges seem to be formed of coral-rock; 
and Captain Beechey says that the lead often descended 
several fathoms through holes in them. In some atolls, all 
the coral-reefs or knolls in the lagoon come to the surface 
at low water ; in other cases of rarer occurrence, all lie at 
nearly the same depth beneath it, but most frequently they 
are quite irregular, — some with perpendicular, some with 
sloping sides, — some rising to the surface, and others lying 

1 Kotzebue's First Voyage, vol. iii. p. 142. 

2 Beechey's Voyage, 4to ed., vol. i. p. 160. At Whitsunday 
Island the bottom of the lagoon slopes gradually towards the centre, 
and then deepens suddenly, the edge of the bank being nearly perpen- 
dicular. This bank is formed of coral and dead shells. 



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CORAL-REEFS. 49 

at all intermediate depths from the bottom upwards. I 
cannot, therefore, suppose that the union of such reefs could 
produce even one uniformly sloping ledge, and much less 
two or three, one beneath the other, and each terminated 
by an abrupt wall. At Matilda Island, which offers the 
best example of the step-like structure, Captain Beechey 
observes that the coral-knolls within the lagoon are quite 
irregular in their height. We shall hereafter see that the 
theory which accounts for the ordinary form of atolls, 
apparently includes this occasional peculiarity in their 
structure. 

In the midst of a group of atolls, there sometimes occur 
small, flat, very low islands of coral formation, which 
probably once included a lagoon, since filled up with 
sediment and coral-reefs. Captain Beechey entertains no 
doubt that this has been the case with the two small 
islands, which alone of thirty-one surveyed by him in the 
Low Archipelago, did not contain lagoons. Romanzoff 
Island (in lat. 15 S.) is described by Chamisso 1 as formed 
by a dam of madreporitic rock inclosing a flat space, thinly 
covered with trees, into which the sea on the leeward side 
occasionally breaks. North Keeling atoll appears to be in 
a rather less forward stage of conversion into land; it 
consists of a horse-shoe shaped strip of land surrounding a 
muddy flat, one mile in its longest axis, which is covered by 
the sea only at high water. When describing South Keeling 
atoll, I endeavoured to show how slow the final process of 
filling up a lagoon must be ; nevertheless, as all causes do 
tend to produce this effect, it is very remarkable that not 
one instance, as I believe, is known of a moderately sized 
lagoon being filled up even to the low water-line at spring- 

1 Kotzebue's First Voyage , vol. iii. p. 221. 

S69 



50 CORAL-REEFS. 

tides, much less of such a one being converted into land. 
It is, likewise, in some degree remarkable, how few atolls, 
except small ones, are surrounded by a single linear strip of 
land, formed by the union of separate islets. We cannot 
suppose that the many atolls in the Pacific and Indian 
Oceans all have had a late origin, and yet should they 
remain at their present level, subjected only to the action of 
the sea and to the growing powers of the coral, during as 
many centuries as must have elapsed since any of the 
earlier tertiary epochs, it cannot, I think, be doubted that 
their lagoons and the islets on their reef would present a 
totally different appearance from what they now do. This 
consideration leads to the suspicion that some renovating 
agency (namely subsidence) comes into play at intervals, 
and perpetuates their original structure. 

Section Third. 

Maldiva archipelago. — Ring-formed reefs marginal and central. — Great 
depth in the lagoons of the S. 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. — Destrttction 
of islets. — Connection in the position and submarine foundation of 
distinct atolls. — The apparent dissever?nent of large atolls. — The 
Great Chagos Bank. — Its submerged condition and extraordinary 
structure. 

Although occasional references have been made to the 
Maldiva atolls, and to the banks in the Chagos group, some 
points of their structure deserve further consideration. My 
description is derived from an examination of the admirable 
charts lately published from the survey of Captain Moresby 
and Lieut. Powell, and more especially from information 
which Captain Moresby has communicated to me in the 
kindest manner. 



CORAL-REEFS. gi 

The Maldiva Archipelago is 470 miles in length, with an 
average breadth of about 50 miles. The form and dimen- 
sions of the atolls, and their singular position in a double 
line, may be seen, but not well, in the greatly reduced chart 
(Fig. 2) in Plate III. The dimensions of the longest atoll 
in the group (called by the double name of Milla-dou-Madou 
and Tilla-dou-Matte) have already been given ; it is 88 miles 
in a medial and slightly curved line, and is less than 20 
miles in its broadest part. Suadiva, also, is a noble atoll, 
being 44 miles across in one direction, and 34 in another, 
and the great included expanse of water has a depth of 
between 250 and 300 feet The smaller atolls in this group 
differ in no respect from ordinary ones ; but the larger ones 
are remarkable from being breached by numerous deep- 
water channels leading into the lagoon ; for instance, there 
are 42 channels, through which a ship could enter the 
lagoon of Suadiva. In the three southern large atolls, the 
separate portions of reef between these channels have the 
ordinary structure, and are linear ; but in the other atolls, 
especially the more northern ones, these portions are ring- 
formed, like miniature atolls. Other ring-formed reefs rise 
out of the lagoons, in the place of those irregular ones which 
ordinarily occur there. In the reduction of the chart of 
Mahlos Mahdoo (Plate III., 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 imperfectly shown : in 
the large published charts of Tilla-dou-Matte, the appearance 
of these rings, from standing further apart from each other, 
is very remarkable. The rings on the margin are generally 
elongated; many of them are three, and some even five 
miles, in diameter; those within the lagoon are usually 
smaller, few being more than two miles across, and the 
greater number rather less than one. The depth of the 



52 CORAL-REEFS. 

little lagoon within these small annular reefs is generally 
from five to seven fathoms, but occasionally more ; and in 
Ari atoll many of the central ones are twelve, and some 
even more than twelve fathoms deep. These rings rise 
abruptly from the platform or bank, on which they are 
placed ; their outer margin is invariably bordered by living 
coral 1 within which there is a flat surface of coral rock; on 
this flat, sand and fragments have in many cases accumu- 
lated and been converted into islets, clothed with vegetation. 
I can, in fact, point out no essential difference between 
these little ring-formed reefs (which, however, are larger, 
and contain deeper lagoons than many true atolls that stand 
in the open sea), and the most perfectly characterised atolls, 
excepting that the ring-formed reefs are based on a shallow 
foundation, instead of on the floor of the open sea, and that 
instead of being scattered irregularly, they are grouped 
closely together on one large platform, with the marginal 
rings arranged in a rudely formed circle. 

The perfect series which can be traced from portions of 
simple linear reef, to others including long linear lagoons, 
and from these again to oval or almost circular rings, 
renders it probable that the latter are merely modifications 
of the linear or normal state. It is conformable with this 
view, that the ring-formed reefs on the margin, even where 
most perfect and standing furthest apart, generally have 
their longest axes directed in the line which the reef would 
have held, if the atoll had been bounded by an ordinary 
wall. We may also infer that the central ring-formed reefs 
are modifications of those irregular ones, which are found 
in the lagoons of all common atolls. It appears from the 
charts on a large scale, that the ring-like structure is 

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



CORAL-REEFS. 53 

contingent on the marginal channels or breaches being 
wide ; and, consequently, on the whole interior of the atoll 
being freely exposed to the waters of the open sea. When 
the channels are narrow or few in number, although the 
lagoon be of great size and depth (as in Suadiva), there are 
no ring-formed reefs; where the channels are somewhat 
broader, the marginal portions of reef, and especially those 
close to the larger channels, are ring-formed, but the central 
ones are not so; where they are broadest, almost every 
reef throughout the atoll is more or less perfectly ring- 
formed. Although their presence is thus contingent on 
the openness of the marginal channels, the theory of their 
formation, as we shall hereafter see, is included in that of 
the parent atolls, of which they form the separate portions. 

The lagoons of all the atolls in the southern part of the 
Archipelago are from ten to twenty fathoms deeper than 
those in the northern part. This is well exemplified in the 
case of Addoo, the southernmost atoll in the group, for 
although only 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. In the central 
and deepest part of the lagoons, the bottom consists, as 
I am informed by Capt. Moresby, of stiff clay (probably a 
calcareous mud) ; nearer the border it consists of sand, and 
in the channels through the reef, of hard sand-banks, 
sandstone, conglomerate rubble, and a little live coral. 
Close outside the reef and the line joining its detached 
portions (where intersected by many channels), the bottom 
is sandy, and it slopes abruptly into unfathomable depths. 
In most lagoons the depth is considerably greater in the 
centre than in the channels ; but in Tilla-dou-Matte, where 
the marginal ring-formed reefs stand far apart, the same 



54 CORAL-REEFS. 

depth is carried across the entire atoll, from the deep-water 
line on one side to that on the other. I cannot refrain from 
once again remarking on the singularity of these atolls, — a 
great sandy and generally concave disc rises abruptly from 
the unfathomable ocean, with its central expanse studded 
and its border symmetrically fringed with oval basins of 
coral-rock, just lipping the surface of the sea, sometimes 
clothed with vegetation, and each containing a little lake of 
clear water ! 

In the southern Maldiva atolls, of which there are nine 
large ones, all the small reefs within the lagoons come to 
the surface, and are dry at low water spring-tides ; hence in 
navigating them, there is no danger from submarine banks. 
This circumstance is very remarkable, as within some atolls, 
for instance those of the neighbouring Chagos group, not a 
single reef comes to the surface, and in most other cases a 
few only do, and the rest lie at all intermediate depths from 
the bottom upwards. When treating of the growth of coral 
I shall again refer to this subject. 

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

In groups of atolls, exposed to a trade-wind, the ship 



CORAL-REEFS. 55 

channels into the lagoons are almost invariably situated on 
the leeward or less exposed side of the reef, and the reef 
itself is sometimes either wanting there, or is submerged. 
A strictly analogous, but different fact, may be observed at 
the Maldiva atolls — namely, that where two atolls stand in 
front of each other, the breaches in the reef are the most 
numerous on their near, and therefore less exposed, sides. 
Thus on the near sides of Ari and the two Nillandoo atolls, 
which face S. Male, Pbaleedoo, and Moloque atolls, there 
are seventy-three deep-water channels, and only twenty-five 
on their outer sides ; on the near side of the three latter 
named atolls there are fifty-six openings, and only thirty- 
seven on their outsides. It is scarcely possible to attribute 
this difference to any other cause than the somewhat 
different action of the sea on the two sides, which would 
ensue from the protection afforded by the two rows of atolls 
to each olher. I may here remark that in most cases, the 
conditions favourable to the greater accumulation of frag- 
ments on the reef and to its more perfect continuity on one 
side of the atoll than on the other, have concurred, but 
this has not been the case with the Maldivas ; for we have 
seen that the islets are placed on the eastern or south- 
eastern sides, whilst the breaches in the reef occur 
indifferently on any side, where protected by an opposite 
atoll. The reef being more continuous on the outer and 
more exposed sides of those atolls which stand near each 
other, accords with the fact, that the reef of the southern 
atolls is more continuous than that of the northern ones ; 
for the former, as I am informed by Capt. Moresby, are 
more constantly exposed than the northern atolls to a heavy 
surf. 

The date of the first formation of some of the islets in 
this Archipelago is known to the inhabitants \ on the other 



56 CORAL-REEFS. 

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. Capt. 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 madrepore." In the northern part, also, of the 
Maldiva Archipelago and in the Chagos group, it is known 
that some of the islets are disappearing. The natives 
attribute these effects to variations in the currents of the 
sea. For my own part I cannot avoid suspecting that there 
must be some further cause, which gives rise to such a cycle 
of change in the action of the currents of the great and 
open ocean. 

Several of the atolls in this Archipelago are so related to 
each other in form and position, that at the first glance one 
is led to suspect that they have originated in the dissever- 
ment of a single one. Male consists of three perfectly 
characterised atolls, of which the shape and relative position 
are such, that a line drawn closely round all three, gives a 
symmetrical figure; to see this clearly, a larger chart is 
required than that of the Archipelago in Plate III.; the 
channel separating the two northern Male atolls is only 
little more than a mile wide, and no bottom was found in it 
with ioo fathoms. Powell's Island is situated at the 
distance of two miles and a half off the northern end of 



PLA1£ IT 



GKEAT CHAGOS BASK 

The sttadalpartr arc from 4iolPi mJ itndirMiilrr 



"^-.. JrS& 






"~ <$ 






;~ 



C? 



0. ' -' 



I 



«> S 



v 



Level of tke Sea 




East * West Section across the Gr. Cha-os Bank 76 milcr in lm<rtii 

Fiy.2. 




PEJROS BANHOS ATOLL 



BartJiolofftew. Edln? 



CORAL-REEFS. 57 

Mahlos Mahdoo (see Fig. 4, Plate III.), at the exact point 
where the two sides of the latter, if prolonged, would meet ; 
no bottom, however, was found in the channel with 200' 
fathoms ; in the wider channel between Horsburgh atoll 
and the southern end of Mahlos Mahdoo, no bottom was 
found with 250 fathoms. In these and similar cases, the 
relation consists only in the form and position of the atolls. 
But in the channel between the two Nillandoo atolls, 
although three miles and a quarter wide, soundings were 
struck at the depth of 200 fathoms ; the channel between 
Ross and Ari atolls is four miles wide, and only 150 
fathoms deep. Here then we have, besides the relation 
of form, a submarine connection. The fact of soundings 
having been obtained between two separate and perfectly 
characterised atolls is in itself interesting, as it has 
never, I believe, been effected in any of the many 
other groups of atolls in the Pacific and Indian seas. In 
continuing to trace the connection of adjoining atolls, if a 
hasty glance be taken at the chart (Fig. 4, Plate III.) 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 bifur- 
cating channel, of which the northern arm is about one mile 
and three-quarters in width, with an average depth of 125 
fathoms, and the southern one three-quarters of a mile wide, 
and rather less deep. These channels resemble in the slope 
of their sides and general form, those which separate atolls 
in every respect distinct; and the northern arm is wider 
than that dividing two of the Male atolls. The ring-formed 
reefs on the sides of this bifurcating channel are elongated, 
so that the northern and southern portions of Mahlos 
Mahdoo may claim, as far as their external outline is 
concerned, to be considered as distinct and perfect atolls. 



58 CORAL-REEFS. 

But the intermediate portion, lying in the fork of the 
channel, is bordered by reefs less perfect than those which 
surround any other atoll in the group of equally small 
dimensions. Mahlos Mahdoo, therefore, is in every respect 
in so intermediate a condition, that it may be considered 
either as a single atoll nearly dissevered into three portions, 
or as three atolls almost perfect and intimately connected. 
This is an instance of a very early stage of the apparent 
disseverment of an atoll, but a still earlier one in many 
respects is exhibited at Tilla-dou-Matte. In one part of 
this atoll, the ring-formed reefs stand so far apart from each 
other, that the inhabitants have given different names to the 
northern and southern halfs ; nearly all the rings, moreover, 
are so perfect and stand so separate, and the space from 
which they rise is so level and unlike a true lagoon, that we 
can easily imagine the conversion of this one great atoll, 
not into two or three portions, but into a whole group of 
miniature atolls. A perfect series such as we have here 
traced, impresses the mind with an idea of actual change ; 
and it will hereafter be seen, that the theory of subsidence, 
with the upward growth of the coral, modified by accidents 
of probable occurrence, will account for the occasional 
disseverment of large atolls. 

The Great Chagos Bank alone remains to be described. 
In the Chagos group there are some ordinary atolls, some 
annular reefs rising to the surface but without any islets on 
them, and some atoll-formed banks, either quite submerged, 
or nearly so. Of the latter, the Great Chagos Bank is much 
the largest, and differs in its structure from the others : a 
plan of it is given in Plate IV., Fig. i, 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 necessarily 



CORAL-REEFS. 59 

exaggerated. Its longest axis is ninety nautical miles, and 
another line drawn at right angles to the first, across the 
broadest part, is seventy. The central part consists of a 
level muddy flat, between forty and fifty fathoms deep, 
which is surrounded on all sides, with the exception of 
some breaches, by the steep edges of a set of banks, rudely 
arranged in a circle. These banks consist of sand, with a 
very little live coral ; they vary in breadth from five to 
twelve miles, and on an average lie about sixteen fathoms 
beneath the surface ; they are bordered by the steep edges 
of a third narrow and upper bank, which forms the rim to 
the whole. This rim is about a mile in width, and with the 
exception of two or three spots where islets have been 
formed, is submerged between five and ten fathoms. It 
consists of smooth hard rock, covered with a thin layer of 
sand, but with scarcely any live coral ; it is steep on both 
sides, and outwards slopes abruptly into unfathomable 
depths. At the distance of less than half a mile from one 
parr, no bottom was found with 190 fathoms; and off 
another point, at a somewhat greater distance, there was 
none with 210 fathoms. Small steep-sided banks or knolls, 
covered with luxuriantly growing coral, rise from the interior 
expanse to the same level with the external rim, which, as 
we have seen, is formed only of dead rock. It is impossible 
to look at the plan (Fig. 1, Plate IV.), although reduced to 
so small a scale, without at once perceiving that the Great 
Chagos Bank is, in the words of Capt. Moresby, 1 " nothing 
more than a half- drowned atoll." But of what great 
dimensions, and of how extraordinary an internal structure ? 

1 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 Capt. 
Moresby, the above account of the Great Chagos Bank is taken. 



6o CORAL-REEFS. 

We shall hereafter have to consider both the cause of its 
submerged condition, a state common to other banks in the 
group, and the origin of the singular submarine terraces, 
which bound the central expanse : these, I think, it can 
be shown, have resulted from a cause analogous to that 
which has produced the bifurcating channel across Mahlos 
Mahdoo. 



CHAPTER II. 

BARRIER-REEFS. 

Closely resemble in general form and structure atoll-reefs. — Width and 
depth of the lagoon-channels. — Breaches through the reef in front of 
valleys, and generally on the leeward side. — Checks to the filling up 
of the lagoon- channels. — Size and constitution of the encircled 
islands. — Nzwiber of islands within the same reef — Barrier-reefs 
of New Caledonia and Australia. — Position of the reef relative to 
the slope of the adjoining land. — Probable great thickness of 
barrier-reefs. 

The term 'barrier' has been generally applied to that 
vast reef which fronts the N.E. shore of Australia, and by 
most voyagers likewise to that on the western coast of 
New Caledonia. At one time I thought it convenient 
thus to restrict the term, but as these reefs are similar 
in structure, and in position relatively to the land, to those, 
which, like a wall with a deep moat within, encircle many 
smaller islands, I have classed them together. The reef, 
also, on the west coast of New Caledonia, circling round 
the extremities of the island, is an intermediate form 
between a small encircling reef and the Australian barrier, 
which stretches for a thousand miles in nearly a straight 
line. 

The geographer Balbi has in effect described those 
barrier-reefs, which encircle moderately sized islands, by 
calling them atolls with high land rising from within their 
central expanse. The general resemblance between the 



62 CORAL-REEFS. 

reefs of the barrier and atoll classes may be seen in the 
small, but accurately reduced charts on the Plates, 1 and this 
resemblance can be further shown to extend to every part 
of the structure. Beginning with the outside of the reef; 
many scattered soundings off Gambier, Oualan, and some 
other encircled islands, show that close to the breakers 
there exists a narrow shelving margin, beyond which the 
ocean becomes suddenly unfathomable ; but off the west 
coast of New Caledonia, Capt. Kent 2 found no bottom 
with 150 fathoms, at two ships' length from the reef; so 
that the slope here must be nearly as precipitous as off the 
Maldiva atolls. 

I can give litde information regarding the kinds of corals 
which live on the outer margin. When I visited the reef 
at Tahiti, although it was low water, the surf was too violent 
for me to see the living masses ; but, according to what I 
heard from some intelligent native chiefs, they resemble 
in their rounded and branchless forms, those on the margin 
of Keeling atoll. The extreme verge of the reef, which 
was visible between the breaking waves at low water, con- 
sisted of a rounded, convex, artificial-like breakwater, entirely 
coated with Nulliporse, and absolutely similar to that which 
I have described at Keeling atoll. From what I heard 
when at Tahiti, and from the writings of the Revs. W. Ellis 
and J. Williams, I conclude that this peculiar structure 
is common to most of the encircled islands of the 
Society Archipelago. The reef within this mound or 
breakwater, has an extremely irregular surface, even 
more so than between the islets on the reef of Keeling 

1 The authorities from which these charts have been reduced, 
together with some remarks on them, are given in a separately appended 
page, descriptive of the Plates. 

2 Dalrymple, Hydrog. Ale/n., vol. iii. 



CORAL-REEFS. 63 

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 
(Plate I., Fig. 1, and Plate II., Fig. 5), it is quite as regular, and 
of the same average width, as in true atolls. Most barrier- 
reefs on the inner side slope irregularly into the lagoon- 
channel (as the space of deep water separating the reef from 
the included land may be called), but at Vanikoro the reef 
slopes only for a short distance, and then terminates 
abruptly in a submarine wall, forty feet high, — a structure 
absolutely similar to that described by Chamisso in the 
Marshall atolls. 

In the Society Archipelago, Ellis 1 states, that the reefs 
generally lie at the distance of from one to one and a half 
miles, and, occasionally, even at more than three miles, 
from the shore. The central mountains are generally 
bordered by a fringe of flat, and often marshy, alluvial 
land, from one to four miles in width. This fringe consists 
of coral-sand and detritus thrown up from the lagoon- 
channel, and of soil washed down from the hills; it is an 
encroachment on the channel, analogous to that low and 
inner part of the islets in many atolls which is formed 
by the accumulation of matter from the lagoon. At 
Hogoleu (Fig. 3, Plate II.), in the Caroline Archipelago, 2 
the reef on the south side is no less than twenty miles; 
on the east side, five; and on the north side, fourteen 
miles from the encircled high islands. 

The lagoon channels may be compared in every respect 

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

2 See Hydrographical Mem. and the Atlas of the Voyage of the 
Astrolabe by Captain Dumont D'Urville, p. 428. 



6 4 CORAL-REEFS, 

with true lagoons. In some cases they are open, with a 
level bottom of fine sand ; in others they are choked up 
with reefs of delicately branched corals, which have the 
same general character as those within the Keeling atoll. 
These internal reefs either stand separately, or more com- 
monly skirt the shores of the included high islands. The 
depth of the lagoon-channel round the Society Islands 
varies from two or three to thirty fathoms; in Cook's 1 chart 
of Ulietea, however, there is one sounding laid down of 
48 fathoms ; at Vanikoro there are several of 54 and one of 
56-I 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 II., Fig. 1) form a single linear strip. The 
islets first appear either on the angles of the reef, or on the 
sides of the breaches through it, and are generally most 
numerous on the windward side. The reef to leeward 
retaining its usual width, sometimes lies submerged several 
fathoms beneath the surface ; I have already mentioned 
Gambier Island as an instance of this structure. Sub- 
merged reefs, having a less defined outline, dead, and 
covered with sand, have been observed (see Appendix) off 
some parts of Huaheine and Tahiti. The reef is more 
frequently breached to leeward than to windward; thus I 
find in Krusenstern's Memoir on the Pacific that there 
are passages through the encircling reef on the leeward 
side of each of the seven Society Islands, which possess 
ship-harbours ; but that there are openings to windward 
through the reef of only three of them. The breaches in 
the reef are seldom as deep as the interior lagoon-like 

1 See the chart in vol. i. of Hawkesworlh's 410 ed. of Cook's First 
Voyage. 



CORAL-REEFS. 65 

channel ; they generally occur in front of the main valleys, 
a circumstance which can be accounted for, as will be 
seen in the fourth chapter, without much difficulty. The 
breaches being situated in front of the valleys, which 
descend indifferently on all sides, explains their more 
frequent occurrence through the windward side of barrier- 
reefs than through the windward side of atolls, — for in 
atolls there is no included land to influence the position of 
the breaches. 

It is remarkable, that the lagoon -channels round 
mountainous islands have not in every instance been long 
ago filled up with coral and sediment ; but it is more easily 
accounted for than appears at first sight. In cases like that 
of Hogoleu and the Gambier Islands, where a few small 
peaks rise out of a great lagoon, the conditions scarcely 
differ from those of an atoll, and I have already shown, at 
some length, that the filling up of a true lagoon must be an 
extremely slow process. Where the channel is narrow, the 
agency, which on unprotected coasts is most productive of 
sediment, namely the force of the breakers, is here entirely 
excluded, and the reef being breached in the front of the 
main valleys, much of the finer mud from the rivers must 
be transported into the open sea. As a current is formed 
by the water thrown over the edge of atoll-formed reefs, 
which carries sediment with it through the deep-water 
breaches, the same thing probably takes place in barrier- 
reefs, and this would greatly aid in preventing the lagoon- 
channel from being filled up. The low alluvial border, 
however, at the foot of the encircled mountains, shows that 
the work of filling up is in progress ; and at Maurua (Plate 
III., Fig. 1), 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 

870 



66 CORAL-REEFS. 

in imagination the encircled land, we shall find that, besides 
the many points already noticed of resemblance, or rather 
of identity in structure with atolls, there is a close general 
agreement in form, average dimensions, and grouping. 
Encircling barrier-reefs, like atolls, are generally elongated, 
with an irregularly rounded, though sometimes angular out- 
line. There are atolls of all sizes, from less than two miles 
in diameter to sixty miles (excluding Tilla-dou-Matte, as it 
consists of a number of almost independent atoll-formed 
reefs) ; and there are encircling barrier-reefs from three 
miles and a half to forty-six miles in diameter, — Turtle 
Island being an instance of the former, and Hogoleu of the 
latter. At Tahiti the encircled island is thirty-six miles in 
its longest axis, whilst at Maurua it is only a little more than 
two miles. It will be shown, in the last chapter in this 
volume, that there is the strictest resemblance in the group- 
ing of atolls and of common islands, and consequently there 
must be the same resemblance in the grouping of atolls and 
of encircling barrier-reefs. 

The islands lying within reefs of this class are of very 
various heights. Tahiti 1 is 7,000 feet ; Maurua about 800; 
Aitutaki 360, and Manouai only 50. The geological nature 
of the included land varies : in most cases it is of ancient 
volcanic origin, owing apparently to the fact that islands of 
this nature are most frequent within all great seas ; some, 
however, are of madreporitic limestone, and others of 
primary formation, of which latter kind New Caledonia 

1 The height of Tahiti is given from Captain Beechey ; Maurua from 
Mr. F. D. Bennett {GeograJ>h. Jour., vol. viii. p. 220); Aitutaki from 
measurements made on board the Beagle ; and Manouai or Harvey 
Island, from an estimate by the Rev. J. Williams. The two latter 
islands, however, are not in some respects well characterised examples 
of the encircled class. 



CORAL-REEFS. 67 

offers the best example. The central land consists either of 
one island, or of several : thus, in the Society group, Eimeo 
stands by itself; while Taha and Raiatea (Fig. 4, Plate II.), 
both moderately large islands of nearly equal size, are 
included in one reef. Within the reef of the Gambier group 
there are four large and some smaller islands (Fig. 5, 
Plate II.); within that of Hogoleu (Fig. 3, Plate II.) nearly 
a dozen small islands are scattered over the expanse of one 
vast lagoon. 

After the details now given, it may be asserted that there 
is not one point of essential difference between encircling 
barrier-reefs and atolls : the latter enclose a simple sheet of 
water, the former encircle an expanse with one or more 
islands rising from it. I was much struck with this fact, 
when viewing, from the heights of Tahiti, the distant 
island of Eimeo standing within smooth water, and encircled 
by a ring of snow-white breakers. Remove the central 
land, and an annular reef like that of an atoll in an early 
stage of its formation is left ; remove it from Bolabola, and 
there remains a circle of linear coral-islets, crowned with tall 
cocoa-nut trees, like one of the many atolls scattered over 
the Pacific and Indian Oceans. 

The barrier-reefs of Australia and of New Caledonia 
deserve a separate notice from their great dimensions. The 
reef on the west coast of New Caledonia (Fig. 3, Plate III.) 
is 400 miles in length ; and for a length of many leagues it 
seldom approaches within eight miles of the shore; and 
near the southern end of the island, the space between the 
reef and the land is sixteen miles in width. The Australian 
barrier extends, with a few interruptions, for nearly a 
thousand miles ; its average distance from the land is 
between twenty and thirty miles, and in some parts from 
fifty to seventy. The great arm of the sea thus included is 



68 CORAL-REEFS. 

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 1 has 
described the surface of this reef as consisting of a hard 
white agglomerate of different kinds of coral, with rough 
projecting points. The outer edge is the highest part ; it 
is traversed by narrow gullies, and at rare intervals is 
breached by ship-channels. The sea close outside is 
profoundly deep; but, in front of the main breaches, 
soundings can sometimes be obtained. Some low islets 
have been formed on the reef. 

There is one important point in the structure of barrier- 
reefs which must here be considered. The accompanying 
diagrams represent north and south vertical sections, taken 
through the highest points of Vanikoro, Gambier, and 
Maurua Islands, and through their encircling reefs. The 
scale both in the horizontal and vertical direction is the 
same, namely, a quarter of an inch to a nautical mile. The 
height and width of these islands is known ; and I have 
attempted to represent the form of the land from the 
shading of the hills in the large published charts. It has 
long been remarked, even from the time of Dampier, that 
considerable degree of relation subsists between the in- 
clination of that part of the land which is beneath water 
and that above it; hence the dotted line in the three 
sections, probably, does not widely differ in inclination from 
the actual submarine prolongation of the land. If we now 
look at the outer edge of the reef (AA), and bear in mind 
that the plummet on the right hand represents a depth of 
1,200 feet, we must conclude that the vertical thickness of 
these barrier coral-reefs is very great. 

1 Flinders' Voyage to Terra Australis, vol. ii. p. 88. 



CORAL-REEFS. 



69 




***fcta^ 



South. 



Korth. 



1. Yanikoro, from the Atlas of the Voyage of the Astrolabe, by D. D'Urville. 

2. Gambier Island, from Beecbey. 

3. Maurua, from the Atlas of the Voyage of the Coquille, by Duperrey. 

The horizontal line is the level of the sea, from which on the right hand a 
plummet descends, representing a depth of 200 fathoms, or 1,200 feet. The 
vertical shading shows the section of the land, and the horizontal shading that 
of the encircling barrier-reef : from the smallness of the scale, the lagoon- 
channel could not be represented. 

AA. — Outer edge of the coral-reefs, where the sea breaks. 

BB.— The shore of the encircled islands. 

I must observe that if the sections had been taken in 
any other direction across these islands, or across other 
encircled islands, 1 the result would have been the same. 
In the succeeding chapter it will be shown that reef-build- 
ing polypifers cannot flourish at great depths, — for instance, 
it is highly improbable that they can exist at a quarter of 
the depth represented by the plummet on the right hand of 
the woodcut. Here there is a great apparent difficulty — 
how were the basal parts of these barrier-reefs formed ? It 
will, perhaps, occur to some, that the actual reefs formed of 



1 In the fifth chapter an East and West section across the Island of 
Bolabola and its barrier-reefs is given, for the sake 



of illustrating 



another point. The unbroken line in it (woodcut No. 5) is the section 
referred to ; it is taken from the Atlas of the Voyage of the Coquille, by 
Duperrey. The depth of the lagoon-channel is exaggerated. 



70 CORAL-REEFS. 

coral are not of great thickness, but that before their first 
growth, the coasts of these encircled islands were deeply 
eaten into, and a broad but shallow submarine ledge thus 
left, on the edge of which the coral grew ; but if this had 
been the case, the shore would have been invariably 
bounded by lofty cliffs, and not have sloped down to the 
lagoon-channel, as it does in many instances. On this 
view, 1 moreover, the cause of the reef springing up at such 
a great distance from the land, leaving a deep and 
broad moat within, remains altogether unexplained. A 
supposition of the same nature, and appearing at first 
more probable, is, that the reefs sprung up from banks 
of sediment, which had accumulated round the shore 
previously to the growth of the coral; but the extension 
of a bank to the same distance round an unbroken 
coast, and in front of those deep arms of the sea (as in 
Raiatea, see Plate II., Fig. 4) which penetrate nearly to the 
heart of some encircled islands, is exceedingly improbable. 
And why, again, should the reef spring up, in some cases 
steep on both sides like a wall, at a distance of two, three 
or more miles from the shore, leaving a channel often 
between 200 and 300 feet deep, and rising from a depth 
which we have reason to believe is destructive to the growth 
of coral ? An admission of this nature cannot possibly be 
made. The existence, also, of the deep channel, utterly 
precludes the idea of the reef having grown outwards, on 
a foundation slowly formed on its outside, by the accumula- 
tion of sediment and coral detritus. Nor, again, can it 
be asserted, that the reef-building corals will not grow, 
excepting at a great distance from the land; for, as we 

1 The Rev. D. Tyerman and Mr. Bennett (Jonrn. of Voyage and 
Travels, vol. i. p. 215) have briefly suggested this explanation of the 
origin of the encircling reefs of the Society Islands. 



CORAL-REEFS. 71 

shall soon see, there is a whole class of reefs, which take 
their name from growing closely attached (especially where 
the sea is deep) to the beach. At New Caledonia (see 
Plate III., Fig. 3) the reefs which run in front of the west 
coast are prolonged in the same line 150 miles beyond the 
northern extremity of the island, and this shows that some 
explanation, quite different from any of those just suggested, 
is required. The continuation of the reefs on each side of 
the submarine prolongation of New Caledonia is an exceed- 
ingly interesting fact, if this part formerly existed as the 
northern extremity of the island, and before the attachment 
of the coral had been worn down by the action of the sea, 
or if it originally existed at its present height, with or 
without beds of sediment on each flank, how can we 
possibly account for the reefs, not growing on the crest of 
this submarine portion, but fronting its sides, in the same 
line with the reefs which front the shores of the lofty island ? 
We shall hereafter see, that there is one, and I believe only 
one, solution of this difficulty. 

One other supposition to account for the position of 
encircling barrier-reefs remains, but it is almost too pre- 
posterous to be mentioned; — namely, that they rest on 
enormous submarine craters, surrounding the included 
islands. When the size, height, and form of the islands in 
the Society group are considered, together with the fact 
that all are thus encircled, such a notion will be rejected by 
almost every one. New Caledonia, moreover, besides its 
size, is composed of primitive formations, as are some of 
the Comoro Islands \ x and Aitutaki consists of calcareous 
rock. We must, therefore, reject these several explanations, 
and conclude that the vertical thickness of barrier-reefs, 

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



72 CORAL-REEFS. 

from their outer edges to the foundation on which they 
rest (from AA in the section to the dotted lines), is really 
great ; but in this, there is no difficulty, for it is not 
necessary to suppose that the coral has sprung up from an 
immense depth, as will be evident when the theory of the 
upward growth of coral-reefs, during the slow subsidence of 
their foundation, is discussed. 



CHAPTER III. 

FRINGING OR SHORE-REEFS, 

Reefs of Mauritius. — Shallow channel within the reef. — Its slow filling 
up. — Currents of water formed within it. — Upraised reefs. — Narrozu 

fringing-reefs in deep seas. — Reefs on the coast of E. Africa and of 
Brazil. — Fringing-reefs in very shallozv 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 part of 
a continent, might at first be thought to differ little, except 
in generally being of less breadth, from barrier-reefs. As 
far as the superficies of the actual reef is concerned this is 
the case ; but the absence of an interior deep-water channel, 
and the close relation in their horizontal extension with the 
probable slope beneath the sea of the adjoining land, 
present essential points of difference. 

The reefs which fringe the island of Mauritius offer a 
good example of this class. They extend round its whole 
circumference, with the exception of two or three parts, 1 
where the coast is almost precipitous, and where, if as is 

• 1 This fact is stated on the authority of the Ofhcier du Roi, in 
his extremely interesting Voyage a Vlsle de France, undertaken in 
1768. According to Captain Carmichael (Hooker's Bot. Misc., vol. 
ii. p. 316), on one part of the coast there is a space for sixteen miles 
without a reef. * 



74 CORAL-REEFS. 

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 
S.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 lies at 
the distance of about half a mile from the shore; but in 
some parts it is distant from one to two, and even three 
miles. But even in this last case, as the coast-land is 
gently inclined from the foot of the mountains to the sea- 
beach, and as the soundings outside the reef indicate an 
equally gentle slope beneath the water, there is no reason 
for supposing that the basis of the reef, formed by the pro- 
longation of the strata of the island, lies at a greater depth 
than that at which the polypifers could begin constructing 
the reef. Some allowance, however, must be made for the 
outward extension of the corals on a foundation of sand 
and detritus, formed from their own wear, which would give 
to the reef a somewhat greater vertical thickness than would 
otherwise be possible. 

The outer edge of the reef on the western or leeward 
side of the island is tolerably well defined, and is a little 
higher than any other part. It chiefly consists of large 
strongly branched corals, of the genus Madrepora, which also 
form a sloping bed some way out to sea : the kinds of coral 
growing in this part will be described in the ensuing 
chapter. Between the outer margin and the beach, there 
is a flat space with a sandy bottom and a few tufts of living 
coral ; in some parts it is so shallow, that people, by avoid- 
ing the deeper holes and gullies, can wade across it at low 
water; in other parts it is deeper, seldom however exceeding 



CORAL-REEFS. 75 

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 10 or 12 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 con- 
ditions unfavourable to the growth of coral are considered. 
Low coral-islets, like those on barrier-reefs and atolls, are 
seldom formed on reefs of this class, owing apparently in 
some cases to their narrowness, and in others to the gentle 
slope of the reef outside not yielding many fragments to 
the breakers. On the windward side, however, of the 
Mauritius, two or three small islets have been formed. 

It appears, as will be shown in the ensuing chapter, 
that the action of the surf is favourable to the vigorous 
growth of the stronger corals, and that sand or sediment, if 
agitated by the waves, is injurious to them. Hence it is 
probable that a reef on a shelving shore, like that of 
Mauritius, would at first grow up, not attached to the actual 
beach, but at some little distance from it ; and the corals 
on the outer margin would be the most vigorous. A 
shallow channel would thus be formed within the reef, and 



76 CORAL-REEFS. 

as the breakers are prevented acting on the shores of the 
island, and as they do not ordinarily tear up many frag- 
ments from the outside, and as every streamlet has its bed 
prolonged in a straight line through the reef, this channel 
could be filled up only very slowly with sediment. But a 
beach of sand and of fragments of the smaller kinds of 
coral seems, in the case of Mauritius, to be slowly 
encroaching on the shallow channel. On many shelving 
and sandy coasts, the breakers tend to form a bar of sand a 
little way from the beach, with a slight increase of depth 
within it; for instance, Capt. Grey 1 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, more or less perfect, occur on other coasts. 
In these cases I suspect that the shallow channel (which no 
doubt during storms is occasionally obliterated) is scooped 
out by the flowing away of the water thrown beyond the 
line, on which the waves break with the greatest force. At 
Pernambuco a bar of hard sandstone, 2 which has the same 
external form and height as a coral-reef, extends nearly 
parallel to the coast ; within this bar currents, apparently 
caused by the water thrown over it during the greater part 
of each tide, run strongly, and are wearing away its inner 
wall. From these facts it can hardly be doubted, that 
within most fringing-reefs, especially within those lying 
some distance from the land, a return stream must carry 
away the water thrown over the outer edge; and the 
current thus produced, would tend to prevent the channel 
being filled up with sediment, and might even deepen it 

1 Capt. Grey's Journal of Two Expeditions, vol. i. p. 369. 

2 I have described this singular structure in the Lond. and Edin. 
Phil. Mag., October 1S41. 



CORAL-REEFS. 77 

under certain circumstances. To this latter belief I am 
led, by rinding that channels are almost universally present 
within the fringing-reefs of those islands which have under- 
gone recent elevatory movements; and this could hardly 
have been the case, if the conversion of the very shallow 
channel into land had not been counteracted to a certain 
extent. 

A fringing-reef, if elevated in a perfect condition above 
the level of the sea, ought to present the singular appear- 
ance of a broad dry moat within a low mound. The 
author 1 of an interesting pedestrian tour round the Mauritius 
seems to have met with a structure of this kind : he says, 
"J'observai que la, oil la mer &ale, independamment des 
rescifs du large, il y a k terre une espece d' effoncement ou 
chemin couvert naturel. On y pourrait mettre du canon," 
etc. In another place he adds, " Avant de passer le Cap, 
on remarque un gros banc de corail eleVe* de plus de quinze 
pieds : c'est une espece de rescif, que la mer a abandonne* : 
il regne au pied une longue flaque d'eau, dont on pourrait 
faire un bassin pour de petits vaisseaux." But the margin 
of the reef, although the highest and most perfect part, 
from being most exposed to the surf, would generally 
during a slow rise of the land be either partially or entirely 
worn down to that level, at which corals could renew their 
growth on its upper edge. On some parts of the coast-land 
of Mauritius there are little hillocks of coral-rock, which are 
either the last remnants of a continuous reef, or of low islets 
formed on it. I observed 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 

1 Voyage a tlsle de France ; par un Officier du Roi> part i. pp. 192, 
200. 



7 8 CORAL-REEFS. 

smooth surface, strewed with worn fragments of coral. They 
consisted in their lower part of hard calcareous sandstone, 
and in their upper of great blocks of several species 
of Astraea and Madrepora, loosely aggregated; they were 
divided into irregular beds, dipping seaward, in one hillock 
at an angle of 8°, and in the other at i8°. I suspect that 
the superficial parts of the reefs, which have been upraised 
together with the islands they fringe, have generally been 
much more modified by the wearing action of the sea, than 
those of Mauritius. 

Many islands 1 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 limited 
extension seems evidently to depend on the high inclination 
of the submarine slope ; — a relation, which, as we have 
seen, does not exist in reefs of the barrier class. The 
fringing-reefs on steep coasts are frequently not more than 
from 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 greater distance from the land. The 
fragments torn up during gales from the outer margin 
are thrown over the reef on the shores of the island. I may 
give as instances, Wateeo, where the reef is described by 
Cook as being a hundred yards wide; and Mauti and 

1 I may give Cuba, as another instance ; Mr. Taylor (Loudo?is 
Mag. of Nat. Htst., 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 encloses a space of shallow water, with a sandy bottom 
and tufts of coral. Outside the edge of the reef, which is formed of 
great branching corals, the depth is six and seven fathoms. This coast 
has been upheaved at no very distant geological period. 



CORAL-REEFS. 79 

Elizabeth 1 Islands, where it is only fifty yards in width : 
the sea round these islands is very deep. 

Fringing-reefs, like barrier-reefs, both surround islands, 
and front the shores of continents. In the charts of the 
eastern coast of Africa, by Capt. Owen, many extensive 
fringing-reefs are laid down ; — thus, for a space of nearly 
forty miles, from lat. i° 15' to i° 45' S., a reef fringes 
the shore at an average distance of rather more than one 
mile, and therefore at a greater distance than is usual in 
reefs of this class ; but as the coast-land is not lofty, and as 
the bottom shoals vary gradually (the depth being only from 
eight to fourteen fathoms at a mile and a half outside the 
reef), its extension thus far from the land offers no difficulty. 
The external margin of this reef is described as formed of 
projecting points, within which there is a space, from six to 
twelve feet deep, with patches of living coral on it. At 
Mukdeesha (lat. 2 1' N.) "the port is formed," it is said, 2 
" by a long reef extending eastward, four or five miles, 
within which there is a narrow channel, with ten to twelve 
feet of water at low spring-tides ; " it lies at the distance of 
a quarter of a mile from the shore. Again, in the plan of 
Mombas (lat. 4 S.), a reef extends for thirty-six miles, at 
the distance of from half a mile to one mile and a quarter 
from the shore ; within it, there is a channel navigable " for 
canoes and small craft," between six and fifteen feet deep : 
outside the reef the depth is about 30 fathoms at the distance 
of nearly half a 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 

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

2 Owen's Africa, vol. i. p. 357, from which work the foregoing facts 
are likewise taken. 



80 CORAL-REEFS. 

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 1 so closely every 
turn of the shore, that I can hardly doubt it is of coral ; it 
runs at the distance of three-quarters of a mile from the 
land, and within it the depth is from ten to fifteen feet. I 
was assured by an intelligent pilot that at Ports Frances and 
Maceio, the outer part of the reef consists of living coral, 
and the inner of a white stone, full of large irregular cavities, 
communicating with the sea. The bottom of the sea off 
the coast of Brazil shoals gradually to between 30 and 40 
fathoms, at the distance of between nine and ten leagues 
from the land. 

From the description now given, we must conclude that 
the dimensions and structure of fringing -reefs depend 
entirely on the greater or less inclination of the submarine 
slope, conjoined with the fact, that reef-building polypifers 
can exist only at limited depths. It follows from this, 
that where the sea is very shallow, as in the Persian Gulf 
and in parts of the East Indian Archipelago, the reefs lose 
their fringing character, and appear as separate and irregu- 
larly scattered patches, often of considerable area. From 
the more vigorous growth of the coral on the outside, and 
from the conditions being less favourable in several respects 
within, such reefs are generally higher and more perfect in 
their marginal than in their central parts : hence these reefs 
sometimes assume (and this circumstance ought not to be 
overlooked) the appearance of atolls ; but they differ from 
atolls in their central expanse being much less deep, in 
their form being less defined, and in being based on a 
shallow foundation. But when in a deep sea reefs fringe 

1 See Baron Roussin's Pilote du Bristly and accompanying hydro- 
graphical memoir. 



CORAL-REEFS. 81 

banks of sediment, which have accumulated beneath the 
surface, round either islands or submerged rocks, they are 
distinguished with difficulty on the one hand from encircling 
barrier-reefs, and on the other from atolls. In the West 
Indies there are reefs, which I should probably have 
arranged under both these classes, had not the existence 
of large and level banks, lying a little beneath the surface, 
ready to serve as the basis for the attachment of coral, been 
occasionally brought into view by the entire or partial 
absence of reefs on them, — and had not the formation of 
such banks, through the accumulation of sediment now in 
progress, been sufficiently evident. Fringing-reefs some- 
times coat, and thus protect the foundations of islands, 
which have been worn down by the surf to the level of the 
sea. According to Ehrenberg, this has been extensively 
the case with the islands in the Red Sea, which formerly 
ranged parallel to the shores of the mainland, with deep 
water within them : hence the reefs now coating their bases 
are situated relatively to the land like barrier-reefs, although 
not belonging to that class ; — but there are, as I believe, in 
the Red Sea some true barrier-reefs. The reefs of this sea 
and of the West Indies will be described in the Appendix. 
In some cases, fringing-reefs appear to be considerably 
modified in outline by the course of the prevailing currents. 
Dr. J. Allan informs me that on the east coast of Madagascar 
almost every headland and low point of sand has a coral- 
reef extending from it in a S.W. and N.E. line, parallel to 
the currents on that shore. I should think the influence of 
the currents chiefly consisted in causing an extension, in a 
certain direction, of a proper foundation for the attachment 
of the coral. Round many intertropical islands, for instance 
the Abrolhos on the coast of Brazil surveyed by Capt. 
Fitzroy, and, as I am informed by Mr. Cuming, round the 

871 



82 CORAL-REEFS. 

Philippines, the bottom of the sea is entirely coated by 
irregular masses of coral, which although often of large size, 
do not reach the surface and form proper reefs. This 
must be owing, either to insufficient growth, or to the 
absence of those kinds of corals which can withstand the 
breaking of the waves. 

The three classes, atoll-formed, barrier, and fringing- 
reefs, together with the modifications just described of 
the latter, include all the most remarkable coral formations 
anywhere existing. At the commencement of the last 
chapter in the volume, where I detail the principles on 
which the map (Plate V.) is coloured, the exceptional 
cases will be enumerated. 



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 First. 

On the distribution of coral-reefs, and on the conditions favourable to 

their increase. 

With regard to the limits of latitude, over which coral- 
reefs extend, I have nothing new to add. The Bermuda 
Islands, in 3 2° 15' N., is the point furthest removed from 
the equator, in which they appear to exist; and it has 
been suggested that their extension so far northward in 
this instance is owing to the warmth of the Gulf Stream. 
In the Pacific, the Loo Choo Islands, in lat. 27 N., have 
reefs on their shores, and there is an atoll in 2 8° 30', 
situated N.W. of the Sandwich Archipelago. In the Red 
Sea there are coral-reefs in lat. 30 . In the southern hemi- 
sphere coral-reefs do not extend so far from the equatorial 
sea. In the Southern Pacific there are only a few reefs 



84 CORAL-REEFS. 

beyond the line of the tropics, 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, not much 
foundation for this view ; for nowhere are coral-reefs more 
extensive than on the shores of New Caledonia, and of 
north-eastern Australia, which consist of primary formations; 
and in the largest groups of atolls, namely the Maldiva, 
Chagos, Marshall, Gilbert, and Low Archipelagoes, there 
is no volcanic or other kind of rock, excepting that formed 
of coral. 

The entire absence of coral-reefs in certain large areas 
within the tropical seas, is a remarkable fact. Thus no 
coral-reefs were observed, during the surveying voyages of 
the Beagle and her Tender on the west coast of South 
America south of the equator, or round the Galapagos 
Islands. It appears, also, that there are none 1 north of the 
equator ; Air. Lloyd, who surveyed the Isthmus of Panama, 
remarked to me, that although he had seen corals living in 
the Bay of Panama, yet he had never observed any reefs 
formed by them. I at first attributed this absence of reefs 
on the coasts of Peru and of the Galapagos Islands, 2 to the 

1 I have been informed that this is the case, by Lieut. Ryder, R.N., 
and others who have had ample opportunities for observation. 

2 The mean temperature of the surface sea from observations made 
by the direction of Capt. 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 south-west 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 
observations made on board the Beagle, was (although further from the 



CORAL-REEFS. 85 

coldness of the currents from the south, but the Gulf of 
Panama is one of the hottest pelagic districts in the world. 1 
In the central parts of the Pacific there are islands entirely 
free from reefs ; in some few of these cases I have thought 
that this was owing to recent volcanic action ; but the 
existence of reefs round the greater part of Hawaii, one of 
the Sandwich Islands, shows that recent volcanic action 
does not necessarily prevent their growth. 

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

I have been assured by several people, that there are no 
coral-reefs on the west coast of Africa, 2 or round the islands 
in the Gulf of Guinea. This perhaps may be attributed, 
in part, to the sediment brought down by the many rivers 
debouching on that coast, and to the extensive mud-banks, 
which line great part of it. But the islands of St. Helena, 
Ascension, the Cape Verdes, St. Paul's, and Fernando 
Noronha, are, also, entirely without reefs, although they 
lie far out at sea, are composed of the same ancient 
volcanic rocks, and have the same general form, with those 

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. 

1 Humboldt's Personal Narrative, vol. vii. p. 434. 

2 It might be concluded, from a paper by Capt. Owen {Geography 
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 resemble those in the West Indies. 



86 CORAL-REEFS. 

islands in the Pacific, the shores of which are surrounded 
by gigantic walls of coral-rock. With the exception of 
Bermuda, there is not a single coral-reef in the central 
expanse of the Atlantic Ocean. It will, perhaps, be 
suggested that the quantity of carbonate of lime in different 
parts of the sea, may regulate the presence of reefs. But 
this cannot be the case, for at Ascension, the waves charged 
to excess precipitate a thick layer of calcareous matter on 
the tidal rocks ; and at St. Jago, in the Cape Verdes, 
carbonate of lime not only is abundant on the shores, but 
it forms the chief part of some upraised post-tertiary strata. 
The apparently capricious distribution, therefore, of coral- 
reefs, cannot be explained by any of these obvious causes ; 
but as the study of the terrestrial and better known half of 
the world must convince every one that no station capable 
of supporting life is lost, — nay more, that there is a struggle 
for each station, between the different orders of nature, — 
we may conclude that in those parts of the intertropical sea, 
in which there are no coral-reefs, there are other organic 
bodies supplying the place of the reef-building polypifers. 
It has been shown in the chapter on Keeling atoll that 
there are some species of large fish, and the whole tribe of 
Holothuriae which prey on the tenderer parts of the corals. 
On the other hand, the polypifers in their turn must prey 
on some other organic beings ; the decrease of which from 
any cause would cause a proportionate destruction of the 
living coral. The relations, therefore, which determine the 
formation of reefs on any shore, by the vigorous growth of 
the efficient kinds of coral, must be very complex, and with 
our imperfect knowledge quite inexplicable. From these 
considerations, we may infer that changes in the condition 
of the sea, not obvious to our senses, might destroy all the 
coral-reefs in one area, and cause them to appear in 



CORAL-REEFS. 87 

another : thus, the Pacific or Indian Ocean might become 
as barren of coral-reefs as the Atlantic now is, without 
our being able to assign any adequate cause for such a 
change. 

It has been a question with some naturalists, which part 
of a reef is most favourable to the growth of coral. The 
great mounds of living Pontes and of Millepora round 
Keeling atoll occur exclusively on the extreme verge of 
the reef, which is washed by a constant succession of 
breakers ; and living coral nowhere else forms solid masses. 
At the Marshall islands the larger kinds of coral (chiefly 
species of Astrsea, a genus closely allied to Porites) "which 
form rocks measuring several fathoms in thickness," prefer, 
according to Chamisso, 1 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 2 remarks, that in the Red Sea the strongest 
corals live on the outer reefs, and appear to love the surf; 
he adds, that the more branched kinds abound a little way 
within, but that even these in still more protected places 
become smaller. Many other facts having a similar tend- 
ency might be adduced. 3 It. has, however, been doubted 

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

2 Ehrenberg, titer die Natur und Bildung der Corailen B'dnke im 
rothen Meere> p. 49. 

3 In the West Indies, as I am informed by Capt. Bird Allen, R.M., 
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. 



88 CORAL-REEFS. 

by MM. Quoy and Gaimard, whether any kind of coral can 
even withgtand, much less flourish in, the breakers of an 
open sea: 1 they affirm that the saxigenous lithophytes 
flourish only where the water is tranquil, and the heat 
intense. This statement has passed from one geological 
work to another ; nevertheless, the protection of the whole 
reef undoubtedly is due to those kinds of coral, which cannot 
exist in the situations thought by these naturalists to be 
most favourable to them. For should the outer and living 
margin perish, of any one of the many low coral-islands, 
round which a line of great breakers is incessantly foaming, 
the whole, it is scarcely possible to doubt, would be washed 
away and destroyed, in less than half a century. But the 
vital energies of the corals conquer the mechanical power 
of the waves ; and the large fragments of reef torn up by 
every storm, are replaced by the slow but steady growth of 
the innumerable polypifers, which form the living zone on 
its outer edge. 

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

1 Annates des Sciences Naturelles^ tome vi. pp. 276, 278. — " La oil 
les ondes sont agitees, les Lytophytes ne peuvent travailler, parce 
qu'elles detruiraient leurs fragiles edifices," etc. 






CORAL-REEFS. 89 

the heat intense. The total number of species of coral in 
the circumtropical seas must be very great : in the Red 
Sea alone, 120 kinds, according to Ehrenberg, 1 have been 
observed. 

The same author has observed that the recoil of the sea 
from a steep shore is injurious to the growth of coral, 
although waves breaking over a bank are not so. Ehrenberg 
also states, that where there is much sediment, placed so as 
to be liable to be moved by the waves, there is little or no 
coral ; and a collection of living specimens placed by him 
on a sandy shore died in the course of a few days. 2 An 
experiment, however, will presently be related in which 
some large masses of living coral increased rapidly in size, 
after having been secured by stakes on a sandbank. That 
loose sediment should be injurious to the living polypifers, 
appears, at first sight, probable ; and accordingly, in sound- 
ing off Keeling atoll, and (as will hereafter be shown) off 
Mauritius, the arming of the lead invariably came up clean, 
where the coral was growing vigorously. This same circum- 
stance has probably given rise to a strange belief, which, 
according to Capt. Owen, 3 is general amongst the inhabi- 
tants of the Maldiva atolls, namely that corals have roots, 
and therefore that if merely broken down to the surface, 
they grow up again ; but, if rooted out, they are permanently 
destroyed. By this means the inhabitants keep their 
harbours clear; and thus the French Governor of St. 
Mary's in Madagascar, " cleared out and made a beautiful 
little port at that place." For it is probable that sand 
would accumulate in the hollows formed by tearing out the 

1 Ehrenberg, Uber die Natur, etc. , etc. , p. 46. 

2 Ibid., p. 49. 

3 Capt. Owen on the Geography of the Maldiva Islands, Geograph. 
Jotirnal) vol. ii. p. '&*>. 



go CORAL-REEFS. 

corals, but not on the broken and projecting stumps, and 
therefore, in the former case, the fresh growth of the coral 
might be thus prevented. 

In the last chapter I remarked that fringing-reefs are 
almost universally breached, where streams enter the sea. 1 
Most authors have attributed this fact to the injurious 
effects of the fresh water, even where it enters the sea only 
in small quantity, and during a part of the year. No 
doubt brackish water would prevent or retard the growth of 
coral; but I believe that the mud and' sand which is 
deposited, even by rivulets when flooded, is a much more 
efficient check. The reef on each side of the channel 
leading into Port Louis at Mauritius, ends abruptly in a 
wall, at the foot of which I sounded and found a bed of 
thick mud. This steepness of the sides appears to be a 
general character in such breaches : Cook, 2 speaking of one 
at Raiatea, says, " Like all the rest, it is very steep on both 
sides." Now, if it were the fresh water mingling with the 
salt which prevented the growth of coral, the reef certainly 
would not terminate abruptly, but as the polypifers nearest 
the impure stream would grow less vigorously than those 
farther off, so would the reef gradually thin away. On the 
other hand, the sediment brought down from the land 
would only prevent the growth of the coral in the line of its 
deposition, but would not check it on the side, so that the 

1 Lieut. Wellstead and others have remarked that this is the case in 
the Red Sea; Dr. Riippell {Reise. in Abyss., Band. i. p. 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. 

2 Cook's First Voyage, vol. ii. p. 271. — (Hawkesworth's Edit.) 



CORAL-REEFS. 91 

reefs might increase till they overhung the bed of the 
channel. The breaches are much fewer in number, and 
front only the larger valleys in reefs of the encircling barrier 
class. They probably are kept open in the same manner 
as those into the lagoon of an atoll, namely, by the force of 
the currents and the drifting outwards of fine sediment. 
Their position in front of valleys, although often separated 
from the land by deep water lagoon-channels, which it might 
be thought would entirely remove the injurious effects both 
of the fresh water and the sediment, will receive a simple 
explanation when we discuss the origin of barrier-reefs. 

In" the vegetable kingdom every different station has its 
peculiar group of plants, and similar relations appear to 
prevail with corals. We have already described the great 
difference between the corals within the lagoon of an atoll 
and those on its outer margin. The corals, also, on the 
margin of Keeling Island occurred in zones ; thus the 
Porites and Millepora complanata grow to a large size only 
where they are washed by a heavy sea, and are killed 
by a short exposure to the air; whereas, three species of 
Nullipora also live amidst the breakers, but are able to 
survive uncovered for a part of each tide; at greater 
depths, a strong Madrepora and Millepora alcicornis are the 
commonest kinds ; the former appearing to be confined to 
this part : beneath the zone of massive corals, minute 
encrusting corallines and other organic bodies live. If we 
compare the external margin of the reef at Keeling atoll 
with that on the leeward side of Mauritius, which are very 
differently circumstanced, we shall find a corresponding 
difference in the appearance of the corals. At the latter 
place, the genus Madrepora is preponderant over every 
other kind, and beneath the zone of massive corals there 
are large beds of Seriatopora. There is also a marked 



92 CORAL-REEFS. 

difference, according to Captain Moresby, 1 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 circum- 
stance which has been pointed out to me by Capt. 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 ; w T hereas all those, 
with equally few exceptions, within Solomon and Egmont 
atolls in the same group, and likewise within the large 
southern Maldiva atolls, reach the surface. I make these 
statements, after having examined the charts of each atoll. 
In the lagoon of Peros Banhos, which is nearly twenty 
miles across, there is only one single reef which rises to the 
surface ; in Diego Garcia there are seven, but several of 
these lie close to the margin of the lagoon, and need 
scarcely have been reckoned : in the Great Chagos Bank 
there is not one. On the other hand, in the lagoons of 
some of the great southern Maldiva atolls, although thickly 
studded with reefs, every one without exception rises to the 
surface ; and on an average there are less than two sub- 
merged reefs in each atoll ; in the northern atolls, however, 
the submerged lagoon-reefs are not quite so rare. The 
submerged reefs in the Chagos atolls generally have from 
one to seven fathoms water on them, but some have from 
seven to ten. Most of them are small, with very steep sides ; 2 

1 Capt. Moresby on the Northern Maldiva atolls, Geograph. Journ.> 
vol. v. p. 401. 

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



CORAL-REEFS. 93 

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, Capt. 
Moresby informs me, with living and healthy coral, two and 
three feet high, consisting of several species. Why then 
have not these lagoon-reefs reached the surface, like the 
innumerable ones in the atolls above-named? If we 
attempt to assign any difference in their external conditions, 
as the cause of this diversity, we are at once baffled : the 
lagoon of Diego Garcia is not deep, and is almost wholly 
surrounded by its reef; Peros Banhos is very deep, much 
larger, with many wide passages communicating with the 
open sea. On the other hand, of those atolls, in which all, 
or nearly all the lagoon-reefs have reached the surface, some 
are small, others large, some shallow, others deep, some 
well-enclosed, and others open. 

Capt. 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 con- 
verted into land — an unparalleled case, I believe, in an atoll 
of such large size, — and as the strip of land is for consider- 
able 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, 



94 CORAL-REEFS, 

that no sensible change has taken place during eighty 
years in the coral-knolls, and considering that every 
single reef has reached the surface in other atolls, which 
do not present the smallest appearance of being older 
than Diego Garcia and Peros Banhos, and which are 
placed under the same external conditions with them, one 
is led to conclude that these submerged reefs, although 
covered with luxuriant coral, have no tendency to grow 
upwards, and that they would remain at their present levels 
for an almost indefinite period. 

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

The Great Chagos Bank is situated in the centre of the 
Chagos group, and the Pitt and Speaker Banks at its two 
extreme points. These banks resemble atolls, except in 



CORAL-REEFS. 95 

their external rim being about eight fathoms submerged, 
and in being formed of dead rock, with very little living 
coral on it : a portion nine miles long of the annular reef of 
Peros Banhos atoll is in the same condition. These facts, 
as will hereafter be shown, render it very probable that the 
whole group at some former period subsided seven or eight 
fathoms ; and that the coral perished on the outer margin of 
those atolls which are now submerged, but that it continued 
alive, and grew up to the surface on those which are now 
perfect. If these atolls did subside, and if from the sud- 
denness of the movement or from any other cause, those 
corals which are better adapted to live at a certain depth 
than at the surface, once got possession of the knolls, 
supplanting the former occupants, they would exert little or 
no tendency to grow upwards. To illustrate this, I may 
observe, that if the corals of the upper zone on the outer 
edge of Keeling atoll were to perish, it is improbable that 
those of the lower zone would grow to the surface, and thus 
become exposed to conditions for which they do not appear 
to be adapted. The conjecture, that the corals on the 
submerged knolls within the Chagos atolls have analogous 
habits with those of the lower zone outside Keeling atoll, 
receives some support from a remark by Captain Moresby, 
namely, that they have a different appearance from those on 
the reefs in the Maldiva atolls, which, as we have seen, all 
rise to the surface : he compares the kind of difference to 
that of the vegetation under different climates. I have 
entered at considerable length into this case, although 
unable to throw much light on it, in order to show that an 
equal tendency to upward growth ought not to be attributed 
to all coral-reefs, — to those situated at different depths, — to 
those forming the ring of an atoll or those on the knolls 
within a lagoon, — to those in one area and those in another. 



96 CORAL-REEFS. 

The inference, therefore, that one reef could not grow up 
to the surface within a given time, because another, not 
known to be covered with the same species of corals, and 
not known to be placed under conditions exactly the same, 
has not within the same time reached the surface, is 
unsound. 

Section Second. 

On the rate of growth of coral-reefs. 

The remark made at the close of the last section, 
naturally leads to this division of our subject, which 
has not, I think, hitherto been considered under a right 
point of view. Ehrenberg 1 has stated, that in the Red 
Sea, the corals only coat other rocks in a layer from 
one to two feet in thickness, or at most to a fathom and a 
half; and he disbelieves that, in any case, they form, by 
their own proper growth, great masses, stratum over 
stratum. A nearly similar observation has been made by 
MM. Quoy and Gaimard, 2 with respect to the thickness of 
some upraised beds of coral, which they examined at Timor 
and some other places. Ehrenberg 3 saw certain large 
massive corals in the Red Sea, which he imagines to be of 
such vast antiquity, that they might have been beheld by 
Pharaoh ; and according to Mr. Lyell 4 there are certain 
corals at Bermuda, which are known by tradition, to have 
been living for centuries. To show how slowly coral-reefs 
grow upwards, Captain Beechey 5 has adduced the case of 

1 Ehrenberg, as before cited, pp. 39, 46, and 50. 

2 Annates des Sciences Nat., torn. vi. p. 28. 

3 Ehrenberg, ut sup., p. 42. 

4 Lyell's Principles of Geology, book iii. chap, xviii. 

5 Beechey's Voyage to the Pacific, chap. viii. 



CORAL-REEFS. 97 

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 1 to 
have increased in dimensions during the last half-century, 
and from the comparison of old charts with recent surveys, 
probably not during the last two hundred years. These, 
and other similar facts, have so strongly impressed many 
with the belief of the extreme slowness of the growth of 
corals, that they have even doubted the possibility of 
islands in the great oceans having been formed by their 
agency. Others, again, who have not been overwhelmed 
by this difficulty, have admitted that it would require thou- 
sands, and tens of thousands of years, to form a mass, 
even of inconsiderable thickness ; but the subject has not, 
I believe, been viewed in the proper light. 

That masses of considerable thickness have been formed 
by the growth of coral, may be inferred with certainty from 
the following facts : — In the deep lagoons of Peros Banhos 
and of the Great Chagos Bank, there are, as already 
described, small steep-sided knolls covered with living 
coral. There are similar knolls in the southern Maldiva 
atolls, some of which, as Captain Moresby assures me, are 
less than a hundred yards in diameter, and rise to the 
surface from a depth of between 250 and 300 feet. Con- 
sidering their number, form, and position, it would be 
preposterous to suppose that they are based on pinnacles of 
any rock, not of coral formation ; or that sediment could 
have been heaped up into such small and steep isolated 
cones. As no kind of living coral grows above the height 
of a few feet, we are compelled to suppose that these knolls 
have been formed by the successive growth and death of 

1 Ehrenberg, ut sup. , p. 43. 

872 



98 CORAL-REEFS. 

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, 1 that at 
Bermuda a process of this kind takes place beneath water, 
without the aid of evaporation. In reefs, also, of the 
barrier class, we may feel sure, as I have shown, that 
masses of great thickness have been formed by the growth 
of the coral ; in the case of Vanikoro, judging only from the 
depth of the moat between the land and the reef, the wall 
of coral-rock must be at least 300 feet in vertical thickness. 

It is unfortunate that the upraised coral-islands in the 
Pacific have not been examined by a geologist. The cliffs 
of Elizabeth Island, in the Low Archipelago, are eighty feet 
high, and appear, from Captain Beechey's description, to 
consist of a homogeneous coral-rock. From the isolated 
position of this island, we may safely infer that it is an 
upraised atoll, and therefore that it has been formed by 
masses of coral, grown together : Savage Island seems, from 
the description of the younger Forster, 2 to have a similar 
structure, and its shores are about forty feet high : some of 
the Cook Islands also appear 3 to be similarly composed. 
Capt. Belcher, R.N., in a letter which Capt. Beaufort 
showed me at the Admiralty, speaking of Bow atoll, says, 
" I have succeeded in boring forty-five feet through coral- 
sand, when the auger became jammed by the falling in of 
the surrounding creamy matter." On one of the Maldiva 

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

2 Forster's Voyage round the World with Cook, vol. ii. pp. 163, 167. 
5 Williams's Narrative of Missionary Enterprise, p. 30. 



CORAL-REEFS. 99 

atolls, Capt. Moresby bored to a depth of twenty-six feet, 
when his auger also broke : he has had the kindness to give 
me the matter brought up ; it is perfectly white, and like 
finely triturated coral-rock. 

In my description of Keeling atoll, I have given some 
facts, which show that the reef probably has grown out- 
wards j and I have found, just within the outer margin, 
the great mounds of Porites and of Millepora, with their 
summits lately killed, and their sides subsequently thickened 
by the growth of the coral : a layer, also, of Nullipora had 
already coated the dead surface. As the external slope of 
the reef is the same round the whole of this atoll, and round 
many other atolls, the angle of inclination must result from 
an adaption between the growing powers of the coral, and the 
force of the breakers, and their action on the loose sediment. 
The reef, therefore, could not increase outwards, without a 
nearly equal addition to every part of the slope, so that the 
original inclination might be preserved, and this would 
require a large amount of sediment, all derived from the 
wear of corals and shells, to be added to the lower part. 
Moreover, at Keeling atoll, and probably in many other 
cases, the different kinds of corals would have to encroach 
on each other ; thus the Nulliporse cannot increase outwards 
without encroaching on the Porites and Millepora com- 
planata, as is now taking place ; nor these latter without 
encroaching on the strongly branched Madrepora, the Mille- 
pora alcicornis, and some Astrseas ; nor these again without 
a foundation being formed for them within the requisite 
depth, by the accumulation of sediment. How slow, then, 
must be the ordinary lateral or outward growth of such reefs. 
But off Christmas atoll, where the sea is much more shallow 
than is usual, we have good reason to believe that, within a 
period not very remote, the reef has increased considerably 



ioo CORAL-REEFS. 

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, 1 "that the island has been produced by accessions 
from the sea, and is in a state of increase." The land is 
fronted by a coral-reef, and from the manner in which islets 
are known to be formed, we may feel confident that the reef 
was not three miles wide, when the first, or most backward 
ridge, was thrown up; and, therefore, we must conclude 
that the reef has grown outwards during the accumulation 
of the successive ridges. Here then, a wall of coral-rock 
of very considerable breadth has been formed by the out- 
ward 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 Capt. Beechey, that Matilda atoll, in the 
Low Archipelago, has been converted in the space of thirty- 
four years, from being, as described by the crew of a 
wrecked whaling vessel, a "reef of rocks" into a lagoon- 
island, fourteen miles in length, with " one of its sides 
covered nearly the whole way with high trees." 2 The islets, 
also, on Keeling atoll, it has been shown, have increased 
in length, and since the construction of an old chart, several 
of them have become united into one long islet; but in this 
case, and in that of Matilda atoll, we have no proof, and 
can only infer as probable, that the reef, that is the founda- 
tion of the islets, has increased as well as the islets 
themselves. 

After these considerations, I attach little importance, as 
indicating the ordinary and still less the possible rate of 
outward growth of coral-reefs, to the fact that certain reefs 

1 Cook's Third Voyage, book iii. chap. x. 

2 Beechey's Voyage to the Pacific, chap. vii. and viii. 



CORAL-REEFS. 101 

in the Red Sea have not increased during a long interval 
of time \ or to other such cases, as that of Ouluthy atoll in 
the Caroline group, where every islet, described a hundred 
years before by Cantova, was found in the same state by 
Lutke', 1 — without it could be shown that, in these cases, the 
conditions were favourable to the vigorous and unopposed 
growth of the corals living in the different zones of depth, 
and that a proper basis for the extension of the reef was 
present. The former conditions must depend on many 
contingencies, and in the deep oceans where coral forma- 
tions most abound, a basis within the requisite depth can 
rarely be present. 

Nor do I attach any importance to the fact of certain 
submerged reefs, as those off Tahiti, or those within Diego 
Garcia, not now being nearer the surface than they were 
many years ago, as an indication of the rate under favourable 
circumstances of the upward growth of reefs ; after it has 
been shown, that all the reefs have grown to the surface in 
some of the Chagos atolls, but that in neighbouring atolls 
which appear to be of equal antiquity and to be exposed to 
the same external conditions, every reef remains submerged ; 
for we are almost driven to attribute this to a difference, not 
in the rate of growth, but in the habits of the corals in the 
two cases. 

In an old-standing reef, the corals, which are so different 
in kind on different parts of it, are probably all adapted to 
the stations they occupy, and hold their places, like other 
organic beings, by a struggle one with another, and with 
external nature; hence we may infer that their growth 

1 F. Lutke 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 Banc de Falipi. It is not stated whether this has been 
caused by the growth of coral, or by the accumulation of sand. 



102 CORAL-REEFS. 

would generally be slow, except under peculiarly favourable 
circumstances. Almost the only natural condition, allowing 
a quick upward growth of the whole surface of a reef, 
would be a slow subsidence of the area in which it stood ; — 
if, for instance, Keeling atoll were to subside two or three 
feet, can we doubt that the projecting margin of live coral, 
about half an inch in thickness, which surrounds the dead 
upper surfaces of the mounds of Porites, would in this case 
form a concentric layer over them, and the reef thus 
increase upwards, instead of, as at present, outwards? 
The Nulliporse 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 Nulli- 
poras ? After a subsidence of this kind, the sea would gain 
on the islets, and the great fields of dead but upright corals 
in the lagoon, would be covered by a sheet of clear water ; 
and might we not then expect that these reefs would rise to 
the surface, as they anciently did when the lagoon was less 
confined by islets, and as they ditf. 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, instead of this, it 
had quietly subsided, surely every part of the island which 
offered a solid foundation, would in a like manner have 
become coated with living coral. 

Through steps such as these, any thickness of rock, 
composed of a singular intermixture of various kinds of 
corals, shells, and calcareous sediment, might be formed; 
but without subsidence, the thickness would necessarily 
be determined by the depth at which the reef-building 



CORAL-REEFS. 103 

polypifers can exist. If it be asked, at what rate in years I 
suppose a reef of coral favourably circumstanced could 
grow up from a given depth ; I should answer, that we 
have no precise evidence on this point, and comparatively 
little concern with it. We see, in innumerable points over 
wide areas, that the rate has been sufficient, either to bring 
up the reefs from various depths to the surface, or, as is 
more probable, to keep them at the surface, during pro- 
gressive subsidences ; and this is a much more important 
standard of comparison than any cycle of years. 

It may, however, be inferred from the following facts, that 
the rate in years under favourable circumstances would 
be very far from slow. Dr. Allan, of Forres, has, in his 
MS. Thesis deposited in the library of the Edinburgh 
University (extracts from which I owe to the kindness 
of Dr. Malcolmson), the following account of some experi- 
ments, 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 species 
of coral were taken off the reef and planted apart on a 
sand-bank three feet deep at low water. Each portion 
weighed ten pounds, and was kept in its place by stakes. 
Similar quantities were placed in a clump and secured as the 
rest. This was done in December 1830. In July follow- 
ing, each detached mass was nearly level with the sea at 
low water, quite immovable, and several feet long, stretch- 
ing as the parent reef, with the coast current from north to 
south. The masses accumulated in a clump were found 
equally increased, but some of the species in such unequal 
ratios, as to be growing over each other." The loss of 
Dr. Allan's magnificent collection by shipwreck, unfortu- 
nately prevents its being known to what genera these corals 



104 CORAL-REEFS. 

belonged ; but from the numbers experimented on, it is 
certain that all the more conspicuous kinds must have been 
included. Dr. Allan informs me, in a letter, that he 
believes it was a Madrepora, which grew most vigorously. 
One may be permitted to suspect that the level of the 
sea might possibly have been somewhat different at the two 
stated periods; nevertheless, it is quite evident that the 
growth of the ten-pound masses, during the six or seven 
months, at the end of which they were found immovably 
fixed 1 and several feet in length, must have been very 
great. The fact of the different kinds of coral, when placed 
in one clump, having increased in extremely unequal ratios, 
is very interesting, as it shows the manner in which a reef, 
supporting many species of coral, would probably be affected 
by a change in the external conditions favouring one kind 
more than another. The growth of the masses of coral 
in N. and S. lines parallel to the prevailing currents, 
whether due to the drifting of sediment or to the simple 
movement of the water, is, also, a very interesting circum- 
stance. 

A fact, communicated to me by Lieut. Wellstead, I.N., 
in some degree corroborates the result of Dr. Allan's experi- 
ments : it is, that in the Persian Gulf a ship had her copper 
bottom encrusted in the course of twenty months with a 
layer of coral, two feet in thickness, which it required great 
force to remove, when the vessel was docked : it was not 
ascertained to what order this coral belonged. The case of 
the schooner-channel choked up with coral in an interval of 

1 It is stated by Mr. de la Beche {Geological Manual, p. 143), on the 
authority of Mr. Lloyd, who surveyed the Isthmus of Panama, that 
some specimens of Polypifers, placed by him in a sheltered pool of 
water, were found in the course of a few days firmly fixed by the 
secretion of a stony matter, to the bottom. 



CORAL-REEFS. 105 

less than ten years, in the lagoon of Keeling atoll, should 
be here borne in mind. We may also infer, from the 
trouble which the inhabitants of the Maldiva atolls 
take to root out, as they express it, the coral-knolls from 
their harbours, that their growth can hardly be very 
slow. 1 

From the facts given in this section, it may be con- 
cluded, first, that considerable thicknesses of rock have 
certainly been formed within the present geological sera by 
the growth of coral and the accumulation of its detritus ; 
and, secondly, that the increase of individual corals and 

1 Mr. Stutchbury {West of England Journal, No. I, p. 50) has 
described a specimen of Agaricia, "weighing 2 lbs. 9 oz., which 
surrounds a species of oyster, whose age could not be more than two 
years, and yet is completely enveloped by this dense coral." I pre- 
sume that the oyster was living when the specimen was procured ; 
otherwise the fact tells nothing. Mr. Stutchbury also mentions an 
anchor, which had become entirely encrusted with coral in fifty years ; 
other cases, however, are recorded of anchors which have long 
remained amidst coral-reefs without having become coated. The 
anchor of the Beagle, in 1832, after having been down exactly one 
month at Rio de Janeiro, was so thickly coated by two species of 
Tubularia, that large spaces of the iron were entirely concealed ; the 
tufts of this horny zoophyte were between two and three inches in 
length. It has been attempted to compute, but I believe erroneously, 
the rate of growth of a reef, from the fact mentioned by Capt. Beechey, 
of the Chama gigas being embedded in coral-rock. But it should be 
remembered, that some species of this genus invariably live, both 
whilst young and old, in cavities, which the animal has the power of 
enlarging with its growth. I saw many of these shells thus embedded 
in the outer ' flat ' of Keeling atoll, which is composed of dead rock ; 
and therefore the cavities in this case had no relation whatever with 
the growth of coral. M. Lesson, also, speaking of this shell (Partie 
Zoolog., Voyage de la Coquille), has remarked, "que constam- 
ment ses valves etaient engages completement dans la masse des 
Madrepores." 



106 CORAL-REEFS. 

of reefs, both outwards or horizontally and upwards or 
vertically, under the peculiar conditions favourable to such 
increase, is not slow, when referred either to the standard 
of the average oscillations of level in the earth's crust, or 
to the more precise but less important one of a cycle of 
years. 

Section Third. 

On the Depths at ivhich Reef-building Polypifers live. 

I have already described in detail, which might have 
appeared trivial, the nature of the bottom of the sea 
immediately surrounding Keeling atoll; and I will now 
describe with almost equal care the soundings off the 
fringing-reefs of Mauritius. I have preferred this arrange- 
ment, for the sake of grouping together facts of a similar 
nature. I sounded with the wide bell-shaped lead which 
Capt. Fitzroy used at Keeling Island, but my examination 
of the bottom was confined to a few miles of coast (between 
Port Louis and Tomb Bay) on the leeward side of the island. 
The edge of the reef is formed of great shapeless masses of 
branching Madrepores, which chiefly consist of two species, 
— apparently M. corymbosa and pocillifera^ — mingled with a 
few other kinds of coral. These masses are separated from 
each other by the most irregular gullies and cavities, into 
which the lead sinks many feet Outside this irregular 
border of Madrepores, the water deepens gradually to 
twenty fathoms, which depth generally is found at the 
distance of from half to three-quarters of a mile from the 
reef. A little further out the depth is thirty fathoms, and 
thence the bank slopes rapidly into the depths of the ocean. 
This inclination is very gentle compared with that outside 



CORAL-REEFS. 107 

Keeling and other atolls, but compared with most coasts it 
is steep. The water was so clear outside the reef, that I 
could distinguish every object forming the rugged bottom. 
In this part, and to a depth of eight fathoms, I sounded 
repeatedly, and at each cast pounded the bottom with the 
broad lead, nevertheless the arming invariably came up 
perfectly clean, but deeply indented. From eight to fifteen 
fathoms a little calcareous sand was occasionally brought 
up, but more frequently the arming was simply indented. 
In all this space the two Madrepores above mentioned, and 
two species of Astraea, with rather large 1 stars, seemed the 
commonest kinds ; and it must be noticed that twice at the 
depth of fifteen fathoms, the arming was marked with a 
clean impression of an Astrsea. Besides these lithophytes, 
some fragments of the Millefiora alcicornis, which occurs in 
the same relative position at Keeling Island, were brought 
up ; and in the deeper parts there were large beds of a 
Seriatopora, different from S. subulata, but closely allied to 
it. On the beach within the reef, the rolled fragments 
consisted chiefly of the corals just mentioned, and of a 

1 Since the preceding pages were printed off, I have received from 
Mr. Lyell a very interesting pamphlet, entitled Remarks upon Coral 
Formations, etc., by J. Couthouy, Boston, United States, 1842. 
There is a statement (p. 6), on the authority of the Rev. J. Williams, 
corroborating the remarks made by Ehrenberg and Lyell (p. 118 of 
this volume), on the antiquity of certain individual corals in the Red 
Sea and at Bermuda ; namely, that at Upolu, one of the Navigator 
Islands, " particular clumps of coral are known to the fishermen by 
name, derived from either some particular configuration or tradition 
attached to them, and handed down from time immemorial." With 
respect to the thickness of masses of coral-rock, it clearly appears, 
from the descriptions given by Mr. Couthouy (pp. 34, 58), that Man- 
gaia and Aurora Islands are upraised atolls, composed of coral rock: 
the level summit of the former is about 300 feet, and that of Aurora 
Island is 200 feet above the sea-level. 



io8 CORAL-REEFS. 

massive Porites, like that at Keeling atoll, of a Meandrina, 
Pocillopora verrucosa, and of numerous fragments of Nulli- 
pora. From fifteen to twenty fathoms the bottom was, 
with few exceptions, either formed of sand, or thickly 
covered with Seriatopora : this delicate- coral seems to form 
at these depths extensive beds unmingled with any other 
kind. At 20 fathoms, one sounding brought up a frag- 
ment of Madrepora apparently M. pocUlifera, and I believe 
it is the same species (for I neglected to bring speci- 
mens from both stations) which mainly forms the upper 
margin of the reef; if so, it grows in depths varying from 

to 20 fathoms. Between twenty and thirty-three fathoms 

1 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 cir- 
cumstance of the arming having invariably come up quite 
clean, when sounding within a certain number of fathoms 
off the reefs of Mauritius and Keeling atoll (eight fathoms 
in the former case, and twelve in the latter), and of its 
having always come up (with one exception) smoothed and 
covered with sand, when the depth exceeded 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 circumstance of a gradual change, in 
the two cases, from a field of clean coral to a smooth sandy 



CORAL-REEFS. 109 

bottom, is far more important in indicating the depth at 
which the larger kinds of coral nourish than almost any 
number of separate observations on the depth, at which 
certain species have been dredged up. For we can under- 
stand the gradation, only as a prolonged struggle against 
unfavourable conditions. If a person were to find the 
soil clothed with turf on the banks of a stream of water, 
but on going to some distance on one side of it, he 
observed the blades of grass growing thinner and thinner, 
with intervening patches of sand, until he entered a 
desert of sand, he would safely conclude, especially if 
changes of the same kind were noticed in other places, that 
the presence of the water was absolutely necessary to the 
formation of a thick bed of turf : so may we conclude, with 
the same feeling of certainty, that thick beds of coral 
are formed only at small depths beneath the surface of the 
sea. 

I have endeavoured to collect every fact, which might 
either invalidate or corroborate this conclusion. Capt. 
Moresby, whose opportunities for observation during his 
survey of the Maldiva and Chagos Archipelagoes have been 
unrivalled, informs me, that the upper part or zone of the 
steep-sided reefs, on the inner and outer coasts of the atolls 
in both groups, invariably consists of coral, and the lower 
parts of sand. At seven or eight fathoms depth, the 
bottom is formed, as could be seen through the clear 
water, of great living masses of coral, which at about ten 
fathoms generally stand some way apart from each other, 
with patches of white sand between them, and at a little 
greater depth these patches become united into a smooth 
steep slope, without any coral. Capt. Moresby, also, 
informs me in support of his statement, that he found only 
decayed coral on the Padua Bank (northern part of the 



i to CORAL-REEFS. 

• 
Laccadive group) which has an average depth between 
25 and 35 fathoms, but that on some other banks in 
the same group with only ten or twelve fathoms water on 
them (for instance, the Tillacapeni bank), the coral was 
living. 

With regard to the coral-reefs in the Red Sea, Ehrenberg 
has the following passage : — " The living corals do not 
descend there into great depths. On the edges of islets 
and near reefs, where the depth was small, very many 
lived; but we found no more even at six fathoms. The 
pearl-fishers at Yemen and Massaua asserted that there was 
no coral near the pearl-banks at nine fathoms deep, but 
only sand. We were not able to institute any more special 
researches." 1 I am, however, assured both by Captain 
Moresby and Lieut. Wellstead, 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 ; and 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 
facts, and considering the manner in which the beds of 
clean coral off Mauritius, Keeling Island, the Maldiva and 

1 Ehrenberg, Uber die Natur, etc., p. 50. 



CORAL-REEFS. in 

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 1 believe that the growth of 
coral is confined within very limited depths ; and they state 
that they never found any fragment of an Astrsea (the genus 
they consider most efficient in forming 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 twice 
this depth) off the reefs of Mauritius, the arming was marked 
with the distinct impression of a living Astraea. Millepora 
alcicornis lives in from o to 12 fathoms, and the genera 
Madrepora and Seriatopora from o to 20 fathoms. Capt. 
Moresby has given me a specimen of Sideropora scabra 
(Pontes of Lamarck) brought up alive from 17 fathoms. 
Mr. Couthouy 2 states that he has dredged up on the 
Bahama banks considerable masses of Meandrina from 16 
fathoms, and he has seen this coral growing in 20 fathoms. 
A Caryophyllia, half an inch in diameter, was dredged up 
alive from 80 fathoms off Juan Fernandez (lat. 33 S.) by 
Capt. P. P. King : 3 this is the most remarkable fact with which 
I am acquainted, showing the depth at which a genus of 
corals often found on reefs, can exist. 4 We ought, however, 

1 Annates des Sci. Nat., torn. vi. 

2 Remarks on Coral Formations, p. 12. 

3 I am indebted to Mr. Stokes for having kindly communicated this 
fact to me, together with much other valuable information. 

4 I will record in the form of a note all the facts that I have been 
able to collect on the depths, both within and without the tropics, at 



112 



CORAL-REEFS, 



to feel less surprise at this fact, as Caryophyllia alone of the 
lamelliform genera, ranges far beyond the tropics ; it is 

which those corals and corallines can live, which there is no reason to 
suppose ever materially aid in the construction of a reef. 



Name of Zoophyte. 



Sertularia 

Cellaria 
,, A minute scarlet encrust- 
ing species, found living 
,, An allied, small stony 
sub -generic form 
A coral allied to Vincularia, 
with eight rows of cells . 
Tubulipora, near to T. patima . 

Do. do. 

Cellepora, several species, and 

allied sub-generic form 
Ditto . 
Ditto . 
Eschara 
Ditto . 
Retepora 



Depth in 
Fathoms. 



Ditto 

Millepora, a strong coral with' 
cylindrical branches, of a 
pink colour, about two 
inches high, resembling in 
the form of its orifices M. 
aspera of Lamarck . 



Coralium 



Antipathes . 



Gorgonia (or an allied form) . 



40 
Ditto 

190 

48 

40 

Ditto 

94 

40 
40 and 5; 
48 
30 
48 
40 

100 



94 and 30 

120 

16 

160 



Country and S. 
Latitude. 



Cape Horn 60° 
Ditto 

Keeling At. 12° 

S. Cruz Riv. 50° 

Cape Horn 
Ditto 
East Chiloe 43° 

Cape Horn 

Chonos Arch. 45° 

S. Cruz 50° 

Tierra del Fue°x> 53° 

S. Cruz R. 50° 

Cape Horn 

C. Good Hope 34 ; 



E. Chiloe 43°, Tierra 
del Fuego 53° 



Barbary 33° X. 



Chonos 45° 



[ Where none 
is given, the 
observation is 
my own.] 



Authority. 



/ Quoy & Gai- 
j rnard, Ann. 
i Seien. Nat., 
( t. vi. p. 284. 



/"Peyssonel in 
I pa per read to 
| Roval Soc, 
(May 1752. 



/Cp. Beechey 
informed me 



\ coast of Brazil 18° 



1 of this fact 

[in a letter. 



Ellis {A T at. Hist, of Coralline, p. 96) states that Ombellularia was 
procured in lat. 79 N. sticking to a line from the depth of 236 fathoms ; 
hence this coral either must have been floating loose, or was entangled 
in stray line at the bottom. Off Keeling atoll a compound Ascidia 
(Sigillina) was brought up from 39 fathoms, and a piece of sponge, 
apparently living, from 70, and a fragment of Nullipora also apparently 



CORAL-REEFS. i 1 3 

found in Zetland 1 in Lat. 6o° N, in deep water, and I 
procured a small species from Tierra del Fuego in Lat. 53 
S. Capt. Beechey informs me, that branches of pink and 
yellow coral were frequently brought up from between 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 far from being accurately 
known ; yet when we bear in mind the manner in which 
the clumps of coral gradually became infrequent at about 
the same depth, and wholly disappeared at a greater depth 
than 20 fathoms, on the slope round Keeling atoll, on the 
leeward side of the Mauritius, and at rather less depth, 
both without and within the atolls of the Maldiva and 
Chagos Archipelagoes ; and when we know that the reefs 
round these islands do not differ from other coral forma- 
tions in their form and structure, we may, I think, conclude 
that in ordinary cases, reef-building polypifers do not 

living from 92 fathoms. At a greater depth than 90 fathoms off this 
coral island, the bottom was thickly strewed with joints of Halimeda 
and small fragments of other Nullipone, but all dead. Captain B. Allen, 
R.N.j informs me that in the survey of the West Indies it was noticed 
that between the depth of 10 and 200 fathoms, the sounding lead very 
generally came up coated with the dead joints of a Halimeda, of which 
he showed me specimens. Off Pernambuco, in Brazil, in about twelve 
fathoms, the bottom was covered with fragments dead and alive of 
a dull red Nullipora, and I infer from Roussin's chart, that a bottom of 
this kind extends over a wide area. On the beach, within the coral-reefs 
of Mauritius, vast quantities of fragments of Nulliporse were piled up. 
From these facts it appears, that these simply organised bodies are 
amongst the most abundant productions of the sea. 
1 Fleming's British Animals, genus Caryophyllia. 

873 



ti4 CORAL-REEFS. 

flourish at greater depths than between 20 and 30 
fathoms. 

It has been argued 1 that reefs may possibly rise from 
very great depths through the means of small corals, first 
making a platform for the growth of the stronger kinds. 
This, however, is an arbitrary supposition : it is not always 
remembered, that in such cases there is an antagonist power 
in action, namely, the decay of organic bodies, when not 
protected by a covering of sediment, or by their own rapid 
growth. We have, moreover, no right to calculate on 
unlimited time for the accumulation of small organic bodies 
into great masses. Every fact in geology proclaims that 
neither the land, nor the bed of the sea retain for indefinite 
periods the same level. As well might it be imagined that 
the British Seas would in time become choked up with beds 
of oysters, or that the numerous small corallines off the 
inhospitable shores of Tierra del Fuego would in time form 
a solid and extensive coral-reef. 

1 Journal of 'the Royal Geographical Society, 1831, p. 21S. 



CHAPTER V. 

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

The atolls of the larger archipelagoes are not formed on submerged 
craters, or on banks of sediment. — Immense areas interspersed with 
atolls. — Their subsidence. — The effects of storms and earthquakes 
on atolls. — Recent changes in their state. — The origin of barrier- 
reefs and of atolls. — Their relative for ?ns. — The step-formed ledges 
and walls round the shores of some lagoons. — The ring-formed 
reefs of the Maldiva atolls. — The submerged condition of parts or 
of the whole of some annular reefs. — The disseverment of large 
atolls. — The union of atolls by linear reefs. — The Great Chagos 
Bank. — Objections from the area and amount of subsidence required 
by the theory, considered. — The probable composition of the loxver 
parts of atolls. 

The naturalists who have visited the Pacific, seem to have 
had their attention riveted by the lagoon-islands, or atolls, 
— those singular rings of coral-land which rise abruptly out 
of the unfathomable ocean — and have passed over, almost 
unnoticed, the scarcely less wonderful encircling barrier- 
reefs. The theory most generally received on the formation 
of atolls, is that they are based on submarine craters ; but 
where can we find a crater of the shape of Bow atoll, which 
is five times as long as it is broad (Plate III., Fig. 5) ; or like 
that of Menchicoff Island (Plate I., Fig. 2), with its three 
loops, together sixty miles in length; or like Rimsky 
Korsacoff, narrow, crooked, and fifty-four miles long ; or 



n6 CORAL-REEFS. 

like the northern Maldiva atolls, made up of numerous 
ring-formed reefs, placed on the margin of a disc, — one of 
which discs is eighty-eight miles in length, and only from 
ten to twenty in breadth. It is, also, not a little improbable 
that there should have existed as many craters of immense 
size crowded together beneath the sea, as there are now in 
some parts atolls. But this theory lies under a greater 
difficulty, as will be evident, when we consider on what 
foundations the atolls of the larger archipelagoes rest : 
nevertheless, if the rim of a crater afforded a basis at the 
proper depth, I am far from denying that a reef like a 
perfectly characterised atoll might not be formed ; some 
such, perhaps, now exist ; but I cannot believe in the 
possibility of the greater number having thus originated. 

An earlier and better theory was proposed by Chamisso; 1 
he supposes that as the more massive kinds of corals prefer 
the surf, the outer portions, in a reef rising from a sub- 
marine basis, would first reach the surface and consequently 
form a ring. But on this view it must be assumed, that in 
every case the basis consists of a flat bank ; for if it were 
conically formed, like a mountainous mass, we can see no 
reason why the coral should spring up from the flanks, 
instead of from the central and highest parts : considering 
the number of the atolls in the Pacific and Indian Oceans, 
this assumption is very improbable. As the lagoons of 
atolls are sometimes even more than forty fathoms deep, it 
must, also, be assumed on this view, that at a depth at 
which the waves do not break, the coral grows more 
vigorously on the edges of a bank than on its central part ; 
and this is an assumption without any evidence in support 
of it. I remarked, in the third chapter, that a reef, growing 

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



CORAL-REEFS. 117 

on a detached bank, would tend to assume an atoll-like 
structure ; if, therefore, corals were to grow up from a bank, 
with a level surface some fathoms submerged, having steep 
sides and being situated in a deep sea, a reef not to be 
distinguished from an atoll, might be formed : I believe 
some such exist in the West Indies. But a difficulty of the 
same kind with that affecting the crater theory, renders, as 
we -shall presently see, this view inapplicable to the greater 
number of atolls. 

No theory worthy of notice has been advanced to account 
for those barrier-reefs, which encircle islands of moderate 
dimensions. The great reef which fronts the coast of 
Australia has been supposed, but without any special facts, 
to rest on the edge of a submarine precipice, extending 
parallel to the shore. The origin of the third class or of 
fringing-reefs presents, I believe, scarcely any difficulty, and 
is simply consequent on the polypifers not growing up from 
great depths, and their not flourishing close to gently 
shelving beaches where the water is often turbid. 

What cause, then, has given to atolls and barrier-reefs 
their characteristic forms? Let us see whether an im- 
portant deduction will not follow from the consideration of 
these two circumstances, — first, the reef-building corals 
flourishing only at limited depths, — and secondly, the 
vastness of the areas interspersed with coral-reefs and coral- 
islets, none of which rise to a greater height above the level 
of the sea, than that attained by matter thrown up by the 
waves and winds. I do not make this latter statement 
vaguely ; I have carefully sought for descriptions of every 
island in the intertropical seas ; and my task has been in 
some degree abridged by a map of the Pacific, corrected in 
1834 by MM. D'Urville and Lottin, in which the low 
islands are distinguished from the high ones (even from 



n8 CORAL-REEFS. 

those much less than a hundred feet in height) by being 
written without a capital letter ; I have detected a few 
errors in this map, respecting the height of some of the 
islands, which will be noticed in the Appendix, where I 
treat of coral formations in geographical order. To the 
Appendix, also, I must refer for a more particular account 
of the data on which the statements on the next page 
are grounded. I have ascertained, and chiefly from the 
writings of Cook, Kotzebue, Bellinghausen, Duperrey, 
Beechey, and Lutke, regarding the Pacific; and from 
Moresby 1 with respect to the Indian Ocean, that in the 
following cases the term "low island" strictly means 
land of the height commonly attained by matter thrown 
up by the winds and the waves of an open sea. 
If we draw a line (the plan I have always adopted) 
joining the external atolls of that part of the Low 
Archipelago in which the islands are numerous, the 
figure will be a pointed ellipse (reaching from Hood to 
Lazaref Island), of which the longer axis is 840 geographical 
miles, and the shorter 420 miles ; in this space 2 none of the 

1 See also Capt. Owen's and Lieut. Wood's papers in the Geo- 
graphical 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 Capt. 
Moresby. 

2 I find from Mr. Couthouy's pamphlet (p. 58) that Aurora Island is 
about 200 feet in height; it consists of coral-rock, and seems to have 
been formed by the elevation of an atoll. It lies north-east of Tahiti, 
close without the line bounding the space coloured dark blue in the 
map appended to this volume. Honden Island, which is situated in 
the extreme north-west part of the Low Archipelago, according to 
measurements made on board the Beagle, whilst sailing by, is 114 
feet from the summit of the trees to the water's edge. This island 
appeared to resemble the other atolls of the group. 



CORAL-REEFS. n 9 

innumerable islets united into great rings rise above the 
stated level. The Gilbert group is very narrow, and 300 
miles in length. In a prolonged line from this group, at 
the distance of 240 miles, is the Marshall Archipelago, 
the figure of which is an irregular square, one end being 
broader than the other; its length is 520 miles, with an 
average width of 240 : these two groups together are 1,040 
miles in length, and all their islets are low. Between the 
southern end of the Gilbert and the northern end of Low 
Archipelago, the ocean is thinly strewed with islands, all 
of which, as far as I have been able to ascertain, are low ; 
so that from nearly the southern end of the Low Archi- 
pelago, to the northern end of the Marshall Archipelago, 
there is a narrow band of ocean, more than 4,000 miles 
in length, containing a great number of islands, all of 
which are low. In the western part of the Caroline 
Archipelago, there is a space of 480 miles in length, and 
about 100 broad, thinly interspersed with low islands. 
Lastly, in the Indian Ocean, the archipelago of the 
Maldivas is 470 miles in length, and 60 in breadth; that 
cf the Laccadives is 150 by 100 miles : as there is a low 
island between these two groups, they may be considered 
as one group of 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 breadth. 
So striking is the uniformity in direction of these three 
archipelagoes, all the islands of which are low, that Captain 
Moresby, in one of his papers, speaks of them as parts 
of one great chain, nearly 1,500 miles long. I am, then, 
fully justified in repeating, that enormous spaces, both in 
the Pacific and Indian Oceans, are intersper ed with islands, 



120 CORAL-REEFS. 

of which not one rises above that height, to which the 
waves and winds in an open sea can heap up matter. 

On what foundations, then, have these reefs and islets 
of coral been constructed ? A foundation must originally 
have been present beneath each atoll at that limited depth, 
which is indispensable for the first growth of the reef- 
building polypifers. A conjecture will perhaps be hazarded, 
that the requisite bases might have been afforded by the 
accumulation of great banks of sediment, which owing to 
the action of superficial currents (aided possibly by the 
undulatory movement of the sea) did not quite reach the 
surface, — as actually appears to have been the case in some 
parts of the West Indian Sea. But in the form and dis- 
position of the groups of atolls, there is nothing to 
countenance this notion ; and the assumption without any 
proof, that a number of immense piles of sediment have 
been heaped on the floor of the great Pacific and Indian 
Oceans, in their central parts far remote from land, and 
where the dark blue colour of the limpid water bespeaks 
its purity, cannot for one moment be admitted. 

The many widely-scattered atolls must, therefore, rest on 
rocky bases. But we cannot believe that the broad summit 
of a mountain lies buried at the depth of a few fathoms 
beneath every atoll, and nevertheless throughout the 
immense areas above-named, with not one point of rock 
projecting above the level of the sea; for we may judge 
with some accuracy of mountains beneath the sea, by those 
on the land ; and where can we find a single chain several 
hundred miles in length and of considerable breadth, much 
less several such chains, with their many broad summits 
attaining the same height, within from 120 to 180 feet? 
If the data be thought insufficient, on which I have 
grounded my belief, respecting the depth at which the 



CORAL-REEFS. 121 

reef-building polypifers can exist, and it be assumed that 
they can flourish at a depth of even 100 fathoms, yet the 
weight of the above argument is but little diminished, for 
it is almost equally improbable, that as many submarine 
mountains, as there are low islands in the several great and 
widely separated areas above specified, should all rise 
within 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 of one or two hundred feet. So highly 
improbable is this supposition, that we are compelled to 
believe, that the bases of the many atolls did never at any 
one period all lie submerged within the depth of a few 
fathoms beneath the surface, but that they were brought 
into the requisite position or level, some at one period and 
some at another, through movements in the earth's crust. 
But this could not have been effected by elevation, for the 
belief that points so numerous and so widely separated 
were successively uplifted to a certain level, but that not 
one point was raised above that level, is quite as improbable 
as the former supposition, and indeed differs little from it. 
It will probably occur to those who have read Ehrenberg's 
account of the Reefs of the Red Sea, that many points in 
these great areas may have been elevated, but that as soon 
as raised, the protuberant parts were cut off by the destroy- 
ing action of the waves : a moment's reflection, however, 
on the bason-like form of the atolls, will show that this is 
impossible ; for the upheaval and subsequent abrasion of an 
island would leave a flat disc, which might become coated 
with coral, but not a deeply concave surface ; moreover, we 
should expect to see, in some parts at least, the rock of the 
foundation brought to the surface. If, then, the founda- 
tions of the many atolls were not uplifted into the requisite 
position, they must of necessity have subsided into it ; and 



122 CORAL-REEFS. 

this at once solves every difficulty, 1 for we may safely infer, 
from the facts given in the last chapter, that during a 
gradual subsidence the corals would be favourably circum- 
stanced for building 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 required for their growth a solid foundation 
within a few fathoms of the surface. 

It would be out of place here to do more than allude to 
the many facts, showing that the supposition of a gradual 
subsidence over large areas is by no means improbable. 

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- 
posed to have formed within the areas specified a vast number of craters, 
all rising within a few fathoms of the surface, and not one above it. 
The supposition that the craters were at different times upraised above 
the surface, and were there abraded by the surf and subsequently 
coated by corals, is subject to nearly the same objections with those 
given at the bottom of the last page; but I consider it superfluous to 
detail all the arguments opposed to such a notion. Chamisso's theory, 
from assuming the existence of so many banks, all lying at the proper 
depth beneath the water, is also vitally defective. The same observa- 
tion 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 likewise applies to 
the notion formerly entertained (Forster's Observ.^ p. 151), that lagoon- 
islands owe their peculiar form to the instinctive tendencies of the 
polypifers. According to this latter view, the corals on the outer 
margin of the reef instinctively expose themselves to the surf in order 
to afford protection to corals living in the lagoon, which belong to 
other genera, and to other families ! 



CORAL-REEFS. 123 

We have the clearest proof that a movement of this kind is 
possible, in the upright trees buried under the strata many 
thousand feet in thickness ; we have also every reason for 
believing that there are now large areas gradually sinking, 
in the same manner as others are rising. And when we 
consider how many parts of the surface of the globe have 
been elevated within recent geological periods, we must 
admit that there have been subsidences on a corresponding 
scale, for otherwise the whole globe would have swollen. 
It is very remarkable that Mr. Lyell, 1 even in the first 
edition of his Principles of Geology^ inferred that the 
amount of subsidence in the Pacific must have exceeded 
that of elevation, from the area of land being very small 
relatively to the agents there tending to form it, namely, 
the growth of coral and volcanic action. But it will be 
asked, are there any direct proofs of a subsiding movement 
in those areas, in which subsidence will explain a pheno- 
menon otherwise inexplicable ? This, however, can hardly 
be expected, for it must ever be most difficult, excepting in 
countries long civilised, to detect a movement, the tendency 
of which is to conceal the part affected. In barbarous and 
semi-civilised nations how long might not a slow movement, 
even of elevation such as that now affecting Scandinavia, 
have escaped attention ! 

Mr. Williams 2 insists strongly that the traditions of the 
natives, which he has taken much pains in collecting, do 
not indicate the appearance of any new islands : but on the 
theory of a gradual subsidence, all that would be apparent 
would be, the water sometimes encroaching slowly on the 
land, and the land again recovering by the accumulation 
of detritus its former extent, and perhaps sometimes the 

1 Principles of Geology, sixth edition, vol. iii. p. 386. 

2 Williams's Narrative of Missionary Enterprise, p. 31. 



124 CORAL-REEFS. 

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 distinguish- 
able. In Kotzebue's Voyage there are accounts of islands, 
both in the Caroline and Marshall Archipelagoes, which 
have been partly washed away during hurricanes ; and 
Kadu, the native who was on board one of the Russian 
vessels, said "he saw the sea at Radack rise to the feet of 
the cocoa-nut trees; but it was conjured in time." 1 A 
storm lately entirely swept away two of the Caroline islands, 
and converted them into shoals ; it partly, also, destroyed 
two other islands. 2 According to a tradition which was 
communicated to Capt. Fitzroy, it is believed in the Low 
Archipelago, that the arrival of the first ship caused a great 
inundation, which destroyed many lives. Mr. Stutchbury 
relates, that in 1825, the western side of Chain Atoll, in 
the same group, was completely devastated by a hurricane, 
and not less than 300 lives lost : " in this instance it was 
evident, even to the natives, that the hurricane alone was 
not sufficient to account for the violent agitation of the 
ocean/'* 3 That considerable changes have taken place 
recently in some of the atolls in the Low Archipelago, 
appears certain from the case already given of Matilda 
Island : with respect to Whitsunday and Gloucester Islands 
in this same group, we must either attribute great inaccuracy 
to their discoverer, the famous circumnavigator Wallis, or 
believe that they have undergone a considerable change in 
the period of fifty-nine years, between his voyage and that 

1 Kotzebue's First Voyage, vol. iii. p. 168. 

2 M. Desmoulins in Comptes Rendus, 1840, p. %^J. 
8 West of England Journ.il. No. I, p. 35. 



CORAL-REEFS. 125 

of Capt. 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 Capt. Beechey's words, 1 "has been 
accurately described by its discoverer, but its present form 
and extent differ materially." Blenheim reef, in the Chagos 
group, consists of a water-washed annular reef, thirteen 
miles in circumference, surrounding a lagoon ten fathoms 
deep : on its surface there were a few worn patches of 
conglomerate coral-rock, of about the size of hovels ; and 
these Capt. Moresby considered as being, without doubt, 
the last remnants of islets ; so that here an atoll has been 
converted into an atoll-formed reef. The inhabitants of the 
Maldiva Archipelago, as long ago as 1605, declared, "that 
the high tides and violent currents were diminishing the 
number of the islands :" 2 and I have already shown, on the 
authority of Capt. Moresby, that the work of destruction is 
still in progress ; but that on the other hand the first 
formation of some islets is known to the present inhabitants. 
In such cases, it would be exceedingly difficult to detect 
a gradual subsidence of the foundation, on which these 
mutable structures rest. 

Some of the archipelagoes of low coral-islands are subject 
to earthquakes : Capt. Moresby informs me that they are 
frequent, though not very strong, in the Chagos group, 
which occupies a very central position in the Indian Ocean, 
and is far from any land not of coral formation. One of 
the islands in this group was formerly covered by a bed of 
mould, which, after an earthquake, disappeared, and was 

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. 



126 CORAL-REEFS. 

believed by the residents to have been washed by the rain 
through the broken masses of underlying rock ; the island 
was thus rendered unproductive. Chamisso 1 states, that 
earthquakes are felt in the Marshall atolls, which are far 
from any high land, and likewise in the islands of the 
Caroline Archipelago. On one of the latter, namely 
Oulleay atoll, Admiral Lutke, as he had the kindness to 
inform me, observed several straight fissures about a foot in 
width, running for some hundred yards obliquely across the 
whole width of the reef. Fissures indicate a stretching of 
the earth's crust, and, therefore, probably changes in its 
level ; but these coral-islands, which have been shaken and 
fissured, certainly have not been elevated, and, therefore, 
probably they have subsided. In the chapter on Keeling 
atoll, I attempted to show by direct evidence, that the 
island underwent a movement of subsidence, during the 
earthquakes lately felt there. 

The facts stand thus; — there are many large tracts of 
ocean, without any high land, interspersed with reefs and 
islets, formed by the growth of those kinds of corals, which 
cannot live at great depths ; and the existence of these 
reefs and low islets, in such numbers and at such distant 
points, is quite inexplicable, excepting on the theory, that 
the bases on which the reefs first became attached, slowly 
and successively sank beneath the level of the sea, whilst 
the corals continued to grow upwards. No positive facts 
are opposed to this view, and some general considerations 
render it probable. There is evidence of change in form, 
whether or not from subsidence, or some of these coral- 
islands ; and there is evidence of subterranean disturbances 
beneath them. Will then the theory, to which we have 

1 See Chamisso, in Kotzebue's First Voyage, vol. iii. pp. 182 and 



CORAL-REEFS, 



127 



thus been led, solve the curious problem, — what has given 
to each class of reef its peculiar form ? 

Let us in imagination place within one of the subsiding 
areas, an island surrounded by a " fringing-reef," — that 
kind, which alone offers no difficulty in the explanation 
of its origin. Let the unbroken lines and the oblique 
shading in the woodcut (No. 4) represent a vertical section 
through such an island; and the horizontal shading will 




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

BB — Shores of the island. 

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

CC — The lagoon-channel between the reef and the shores of the now encircled 
land. 

B'B' — The shores of the encircled island. 

N.B. — In this, and the following woodcut, the subsidence of the land could 
only be represented by an apparent rise in the level of the sea. 

represent the section of the reef. Now, as the island sinks 
down, either a few feet at a time or quite insensibly, we 
may safely infer from what we know of the conditions 
favourable to the growth of coral, that the living masses 
bathed by the surf on the margin of the reef, will soon 
regain the surface. The water, however, will encroach, 
little by little, on the shore, the island becoming lower 
and smaller, and the space between the edge of the reef 
and the beach proportionately broader. A section of the 
reef and island in this state, after a subsidence of several 
hundred feet, is given by the dotted lines : coral-islets are 



128 CORAL-REEFS. 

supposed to have been formed on the new reef, and a ship 
is anchored in the lagoon-channel. This section is in 
every respect that of an encircling barrier-reef; it is, in 
fact, a section taken 1 E. and W. through the highest point 
of the encircled island of Bolabola ; of which a plan is 
given in Plate II., Fig. i. The same section is more clearly 
shown in the following woodcut (No. 5) by the unbroken 
lines. The width of the reef, and its slope, both on the 
outer and inner side, will have been determined by the 
growing powers of the coral, under the conditions (for 
instance, the force of the breakers and of the currents) 
to which it has been exposed ; and the lagoon-channel 
will be deeper or shallower, in proportion to the growth of 
the delicately branched corals within the reef, and to the 
accumulation of sediment, relatively, also, to the rate of 
subsidence and the length of the intervening stationary 
periods. 

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

As the space between the reef and the subsiding shore 
continued to increase in breadth and depth, and as the 

1 The section has been made from the chart given in the Atlas of 
the Voyage of the Coquille. 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. 



CORAL-REEFS. 



129 



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

Referring to the following diagram, in which the newly 
formed barrier-reef is represented by unbroken lines, instead 
of by dots as in the former woodcut, let the work of 
subsidence go on, and the doubly pointed hill will form two 



, A' ' i^a*. C - A" 

^^ -;:r,.. B j HB j H ...„.,,...— »<■ ST 




[No. 5.] 

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

CC — The lagoon channel. 

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

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

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



874 



130 CORAL-REEFS. 

small islands (or more, according to the number of the hills) 
included within one annular reef. Let the island continue 
subsiding, and the coral-reef will continue growing up on its 
own foundation, whilst the water gains inch by inch on the 
land, until the last and highest pinnacle is covered, and 
there remains a perfect atoll. A vertical section of this atoll 
is shown in the woodcut by the dotted lines; — a ship is 
anchored in its lagoon, but islets are not supposed yet to have 
been formed on the reef. The depth of the lagoon and the 
width and slope of the reef, will depend on the circum- 
stances just referred to under barrier-reefs. Any further 
subsidence will produce no change in the atoll, except 
perhaps a diminution in its size, from the reef not growing 
vertically upwards ; but should the currents of the sea act 
violently upon it, and should the corals perish on part or on 
the whole of its margin, changes would result during sub- 
sidence which will be presently noticed. I may here 
observe, that a bank either of rock or of hardened 
sediment, level with the surface of the sea, and fringed 
with living coral, would (if not so small as to allow the 
central space to be quickly filled up with detritus) by 
subsidence be converted immediately into an atoll, without 
passing, as in the case of a reef fringing the shore of an 
island, through the intermediate form of a barrier-reef. If 
such a bank lay a few fathoms submerged, the simple 
growth of the coral (as remarked in the third chapter) 
without the aid of subsidence, would produce a structure 
scarcely to be distinguished from a true atoll ; for in all cases 
the corals on the outer margin of a reef, from having space 
and being freely exposed to the open sea, will grow 
vigorously and tend to form a continuous ring whilst the 
growth of the less massive kinds on the central expanse, 
will be checked by the sediment formed there, and by that 



CORAL-REEFS. 131 

washed inwards by the breakers ; and as the space becomes 
shallower, their growth will, also, be checked by the 
impurities of the water, and probably by the small amount 
of food brought by the enfeebled currents, in proportion 
to the surface of living reefs studded with innumerable 
craving mouths : the subsidence of a reef based on a bank 
of this kind, would give depth to its central expanse or 
lagoon, steepness to its flanks, and through the free growth 
of the coral, symmetry to its outline : — I may here repeat 
that the larger groups of atolls in the Pacific and Indian 
Oceans cannot be supposed to be founded on banks of this 
nature. 

If, instead of the island in the diagram, the shore of a 
continent fringed by a reef had subsided, a great barrier- 
reef, like that on the N.E. coast of Australia, would have 
necessarily resulted; and it would have been separated 
from the main land by a deep-water channel, broad in pro- 
portion to the amount of subsidence, and to the less or 
greater inclination of the neighbouring coast-line. The 
effect of the continued subsidence of a great barrier-reef 
of this kind, and its probable conversion into a chain of 
separate atolls, will be noticed, when we discuss the 
apparent progressive disseverment of the larger Maldiva 
atolls. 

We now are able to perceive that the close similarity in 
form, dimensions, structure, and relative position (which 
latter point will hereafter be more fully noticed) between 
fringing and encircling barrier-reefs, and between these 
latter and atolls, is the necessary result of the transforma- 
tion, during subsidence of the one class into the other. 
On this view, the three classes of reefs ought to graduate 
into each other. Reefs having an intermediate character 
between those of the fringing and barrier classes do exist ; 



132 CORAL-REEFS. 

for instance, on the S.W. coast of Madagascar, a reef 
extends for several miles, within which there is a broad 
channel from seven to eight fathoms deep, but the sea does 
not deepen abruptly outside the reef. Such cases, however, 
are open to some doubts, for an old fringing-reef, which had 
extended itself a little on a basis of its own formation, 
would hardly be distinguishable from a barrier-reef, pro- 
duced by a small amount of subsidence, and with its 
lagoon-channel nearly filled up with sediment during a long 
stationary period. Between barrier-reefs, encircling either 
one lofty island or several small low ones, and atolls includ- 
ing a mere expanse of water, a striking series can be shown: 
in proof of this, I need only refer to the plates in this 
volume, which speak more plainly to the eye, than any 
description could to the ear. The authorities from which 
the charts have been engraved, together with some remarks 
on them, are given on a separate page descriptive of the 
plates. At New Caledonia (Plate III., Fig. 3) the barrier- 
reefs extend for 150 miles on each side of the submarine 
prolongation of the island ; and at their northern extremity 
they appear broken up and converted into a vast atoll- 
formed reef, supporting a few low coral-islets : we may 
imagine that we here see the effects of subsidence actually 
in progress, — the water always encroaching on the northern 
end of the island, towards which the mountains slope down, 
and the reefs steadily building up their massive fabrics in 
the lines of their ancient growth. 

We have as yet only considered the origin of barrier-reefs 
and atolls in their simplest form; but there remain some 
peculiarities in structure and some special cases, described 
in the two first chapters, to be accounted for by our theory. 
These consist — in the inclined ledge terminated by a wall, 
and sometimes succeeded by a second ledge with a wall, 



CORAL-REEFS. 133 

round the shores of certain lagoons and lagoon-channels; 
a structure which cannot, as I endeavoured to show, be 
explained by the simple growing powers of the corals, — in 
the ring or bason-like forms of the central reefs, as well as 
of the separate marginal portions of the northern Maldiva 
atolls, — in the submerged condition of the whole, or of parts 
of certain barrier and atoll-formed reefs; where only a 
part is submerged, this being generally to leeward, — in the 
apparent progressive disseverment of some of the Maldiva 
atolls, — in the existence of irregularly formed atolls, some 
being tied together by linear reefs, and others with spurs 
projecting from them, — and, lastly, in the structure and 
origin of the Great Chagos Bank. 

Step-formed ledges round certain lagoons. — If we suppose 
an atoll to subside at an extremely slow rate, it is difficult 
to follow out the complex results. The living corals would 
grow up on the outer margin ; and likewise probably in the 
gullies and deeper parts of the bare surface of the annular 
reef; the water would encroach on the islets, but the 
accumulation of fresh detritus might possibly prevent 
their entire submergence. After a subsidence of this very 
slow nature, the surface of the annular reef sloping gently 
into the lagoon, would probably become united with the 
irregular reefs and banks of sand, which line the shores 
of most lagoons, Should, however, the atoll be carried 
down by a more rapid movement, the whole surface 
of the annular reef, where there was a foundation of 
solid matter, would be favourably circumstanced for the 
fresh growth of coral ; but as the corals grew upwards on 
its exterior margin, and the waves broke heavily on this 
part, the increase of the massive polypifers on the inner 
side would be checked from the want of water. Conse- 
quently, the exterior parts would first reach the surface, 



i34 CORAL-REEFS. 

and the new annular reef thus formed on the old one, 
would have its summit inclined inwards, and be terminated 
by a subaqueous wall, formed by the upward growth of the 
coral (before being much checked), from the inner edge of 
the solid parts of the old reef. The inner portion of the 
new reef, from not having grown to the surface, would be 
covered by the waters of the lagoon. Should a subsidence 
of the same kind be repeated, the corals would again grow 
up in a wall, from all the solid parts of the resunken reef, 
and, therefore, not from within the sandy shores of the 
lagoon ; and the inner part of the new annular reef would, 
from being as before checked in its upward growth, be of 
less height than the exterior parts, and therefore would not 
reach the surface of the lagoon. In this case the shores of 
the lagoon would be surrounded by two inclined ledges, 
one beneath the other, and both abruptly terminated by 
subaqueous cliffs. 1 

The ring or bason-formed reefs of the northe7'n Maldiva 
atolls. — I may first observe, that the reefs within the 
lagoons of atolls and within lagoon-channels, would, if 
favourably circumstanced, grow upwards during subsidence 
in the same manner as the annular rim ; and, therefore, we 
might expect that such lagoon-reefs, when not surrounded 

1 According to Mr. Couthouy (p. 26), the external reef round many 
atolls descends by a succession of ledges or terraces. He attempts, I 
doubt whether successfully, to explain this structure somewhat in the 
same manner as I have attempted, with respect to the internal ledges 
round the lagoons of some atolls. More facts are wanted regarding 
the nature both of the interior and exterior step-like ledges: are all the 
ledges, or only the upper ones, covered with living coral ? If they are 
all covered, are the kinds different on the ledges according to the 
depth? Do the interior and exterior ledges occur together in the same 
atolls; if so, what is their total width, and is the intervening surface- 
reef narrow, etc. ? 



CORAL-REEFS. 135 

and buried by an accumulation of sediment more rapid 
than the rate of subsidence, would rise abruptly from a 
greater depth than that at which the efficient polypifers can 
flourish : we see this well exemplified in the small abruptly- 
sided reefs, with which the deep lagoons of the Chagos and 
Southern Maldiva atolls are studded. With respect to the 
ring or bason-formed reefs of the Northern Maldiva atolls, 
it is evident, from the perfectly continuous series which 
exists, that the marginal rings, although wider than the 
exterior or bounding reef of ordinary atolls, are only modi- 
fied portions of such a reef ; it is also evident that the cen- 
tral rings, although wider than the knolls or reefs which 
commonly occur in lagoons, occupy their place. The ring- 
like structure has been shown to be contingent on the 
breaches into the lagoon being broad and numerous, so 
that all the reefs which are bathed by the waters of the 
lagoon are placed under nearly the same conditions with 
the outer coast of an atoll standing in the open sea. 
Hence the exterior and living margins of these reefs must 
have been favourably circumstanced for growing outwards, 
and increasing beyond the usual breadth ; and they must 
likewise have been favourably circumstanced for growing 
vigorously upwards, during the subsiding movements, to 
which by our theory the whole archipelago has been sub- 
jected; and subsidence with this upward growth of the 
margins would convert the central space of each little reef 
into a small lagoon. This, however, could only take place 
with those reefs, which had increased to a breadth sufficient 
to prevent their central spaces from being almost imme- 
diately filled up with the sand and detritus driven inwards 
from all sides : hence it is that few reefs, which are less 
than half a mile in diameter, even in the atolls where the 
bason-like structure is most strikingly exhibited, include 



136 CORAL-REEFS. 

lagoons. This remark, I may add, applies to all coral- 
reefs wherever found. The bason-formed reefs of the 
Maldiva Archipelago may, in fact, be briefly described, 
as small atolls formed during subsidence over the separate 
portions of large and broken atolls, in the same manner 
as these latter were formed over the barrier-reefs, which 
encircled the islands of a large archipelago now wholly 
submerged. 

Submerged and dead reefs. — In the second section of the 
first chapter, I have shown that there are in the neighbour- 
hood of atolls, some deeply submerged banks, with level 
surfaces ; that there are others, less deeply but yet wholly 
submerged, having all the characters of perfect atolls, but 
consisting merely of dead coral-rock ; that there are barrier- 
reefs and atolls with merely a portion of their reef, generally 
on the leeward side, submerged; and that such portions 
either retain their perfect outline, or they appear to be 
quite effaced, their former place being marked only by a 
bank, conforming in outline with that part of the reef which 
remains perfect. These several cases are, I believe, inti- 
mately related together, and can be explained by the 
same means. There, perhaps, exist some submerged reefs, 
covered with living coral and growing upwards, but to these 
I do not here refer. 

As we see that in those parts of the ocean, where coral- 
reefs are most abundant, one island is fringed and another 
neighbouring one is not fringed ; as we see in the same 
archipelago, that all the reefs are more perfect in one part 
of it than in another, — for instance, in the southern half 
compared with the northern half of the Maldiva Archi- 
pelago, and likewise on the outer coasts compared with 
the inner coasts of the atolls in this same group, which 
are placed in a double row ; as we know that the existence 



CORAL-REEFS. 137 

of the innumerable polypifers forming a reef, depends on 
their sustenance, and that they are preyed on by other 
organic beings ; and, lastly, as we know that some inorganic 
causes are highly injurious to the growth of coral, it cannot 
be expected that during the round of change to which 
earth, air, and water are exposed, the reef-building polypifers 
should keep alive for perpetuity in any one place; and 
still less can this be expected, during the progressive sub- 
sidences, perhaps at some periods more rapid than at 
others, to which by our theory these reefs and islands 
have been subjected and are liable. It is, then, not im- 
probable that the corals should sometimes perish either 
on the whole or on part of a reef; if on part, the dead 
portion, after a small amount of subsidence, would still 
retain its proper outline and position beneath the water. 
After a more prolonged subsidence, it would probably form, 
owing to the accumulation of sediment, only the margin of 
a flat bank, marking the limits of the former lagoon. Such 
dead portions of reef would generally lie on the leeward 
side, 1 for the impure water and fine sediment would more 
easily flow out from the lagoon over this side of the reef, 

1 Mr. Lyell, in the first edition of his Principles of Geology, offered a 
somewhat different explanation of this structure. He supposes that 
there has been subsidence ; but he was not aware that the submerged 
portions of reef were in most cases, if not in all, dead ; and he attributes 
the difference in height in the two sides of most atolls, chiefly to the 
greater accumulation of detritus to windward than to leeward. But as 
matter is accumulated only on the backward part of the reef, the front 
part would remain of the same height on both sides. I may here 
observe that in most cases (for instance, at Peros Banhos, the Gambier 
group and the Great Chagos Bank), and I suspect in all cases, the dead 
and submerged portions do not blend or slope into the living and 
perfect parts, but are separated from them by an abrupt line. In some 
instances small patches of living reef rise to the surface from the middle 
of the submerged and dead parts. 



138 CORAL-REEFS. 

where the force of the breakers is less than to windward ; 
and therefore the corals would be less vigorous on this side, 
and be less able to resist any destroying agent It is like- 
wise owing to this same cause, that reefs are more frequently 
breached to leeward by narrow channels, serving as by ship- 
channels, than to windward. If the corals perished entirely, 
or on the greater part of the circumference of an atoll, 
an atoll-shaped bank of dead rock, more or less entirely 
submerged, would be produced; and further subsidence, 
together with the accumulation of sediment, would often 
obliterate its atoll-like structure, and leave only a bank with 
a level surface. 

In the Chagos group of atolls, within an area of 160 
miles by 60, there are two atoll-formed banks of dead rock 
(besides another very imperfect one), entirely submerged; 
a third, with merely two or three very small pieces of living 
reef rising to the surface ; and a fourth, namely, Peros 
Banhos (Plate IV., Fig. 3), with a portion nine miles in 
length dead and submerged. As by our theory this area 
has subsided, and as there is nothing improbable in the 
death, either from changes in the state of the surrounding 
sea or from the subsidence being great or sudden, of the 
corals on the whole, or on portions of some of the atolls, 
the case of the Chagos group presents no difficulty. So 
far indeed are any of the above-mentioned cases of sub- 
merged reefs from being inexplicable, that their occurrence 
might have been anticipated on our theory; and as fresh 
atolls are supposed to be in progressive formation by the 
subsidence of encircling barrier-reefs, a weighty objection, 
namely that the number of atolls must be increasing 
infinitely, might even have been raised, if proofs of the 
occasional destruction and loss of atolls could not have 
been adduced. 



CORAL-REEFS. 139 

The disseverment of the larger Maldiva atolls. — The 
apparent progressive disseverment in the Maldiva Archi- 
pelago of large atolls into smaller ones, is, in many respects, 
an important consideration, and requires an explanation. 
The graduated series which marks, as I believe, this process, 
can be observed only in the northern half of the group, 
where the atolls have exceedingly imperfect margins, con- 
sisting of detached bason-formed reefs. The currents of the 
sea flow across these atolls, as I am informed by Capt. 
Moresby, with considerable force, and drift the sediment 
from side to side during the monsoons, transporting much 
of it seaward; yet the currents sweep with greater force 
round their flanks. It is historically known that these atolls 
have long existed in their present state ; and we can believe, 
that even during a very slow subsidence they might thus 
remain, the central expanse being kept at nearly its original 
depth by the accumulation of sediment. But in the action 
of such nicely balanced forces during a progressive sub- 
sidence (like that, to which by our theory this archipelago 
has been subjected), it would be strange if the currents of 
the sea should never make a direct passage across some one 
of the atolls, through the many wide breaches in their 
margins. If this were once effected, a deep-water channel 
would soon be formed by the removal of the finer sediment, 
and the check to its further accumulation ; and the sides of 
the channel would be worn into a slope like that on the 
outer coasts, which are exposed to the same force of the 
currents. In fact, a channel precisely like that bifurcating 
one which divides Mahlos Mahdoo (Plate III., Fig. 4), 
would almost necessarily be formed. The scattered reefs 
situated near the borders of the new ocean-channel, from 
being favourably placed for the growth of coral, would, by 
their extension, tend to produce fresh margins to the 



140 CORAL-REEFS. 

dissevered portions ; such a tendency is very evident (as may 
be seen in the large published chart) in the elongated reefs 
on the borders of the two channels intersecting Mahlos 
Mahdoo. Such channels would become deeper with con- 
tinued subsidence, and probably from the reefs not growing 
up perpendicularly, somewhat broader. In this case, and 
more especially if the channels had been formed originally 
of considerable breadth, the dissevered portions would be- 
come perfect and distinct atolls, like Ari and Ross atolls 
(Plate III., Fig. 2), or like the two Nillandoo atolls, which 
must be considered as distinct, although related in form and 
position, and separated from each other by channels, which 
though deep have been sounded. Further subsidence 
would render such channels unfathomable, and the dis- 
severed portions would then resemble Phaleedoo and 
Moluque atolls, or Mahlos Mahdoo and Horsburgh atolls 
(Plate III., Fig. 4), which are related to each other in no 
respect except in proximity and position. Hence, on the 
theory of subsidence, the disseverment of large atolls, which 
have imperfect margins (for otherwise their disseverment 
would be scarcely possible), and which are exposed to 
strong currents, is far from being an improbable event ; and 
the several stages, from close relation to entire isolation in 
the atolls of the Maldiva Archipelago, are readily explicable. 
We might go even further, and assert as not improbable, 
that the first formation of the Maldiva Archipelago was due 
to a barrier-reef, of nearly the same dimensions with that 
of New Caledonia (Plate III., Fig. 3), for if, in imagination, 
we complete the subsidence of that great island, we might 
anticipate from the present broken condition of the northern 
portion of the reef, and from the almost entire absence of 
reefs on the eastern coast, that the barrier-reef after repeated 
subsidences, would become during its upward growth 



CORAL-REEFS. 141 

separated into distinct portions ; and these portions would 
tend to assume an atoll-like structure, from the coral growing 
with vigour round their entire circumferences, when freely 
exposed to an open sea. As we have some large islands 
partly submerged with barrier-reefs marking their former 
limits, such as New Caledonia, so our theory makes it 
probable that there should be other large islands wholly 
submerged; and these, we may now infer, would be sur- 
mounted, not by one enormous atoll, but by several large 
elongated ones, like the atolls in the Maldiva group ; 
and these again, during long periods of subsidence, would 
sometimes become dissevered into smaller atolls. I may 
add, that both in the Marshall and Caroline Archipel- 
agoes, there are atolls standing close together, which have 
an evident relationship in form : we may suppose, in such 
cases, either that two or more encircled islands originally 
stood close together, and afforded bases for two or more 
atolls, or that one atoll has been dissevered. From the 
position, as well as form, of three atolls in the Caroline 
Archipelago (the Namourrek and Elato group), which 
are placed in an irregular circle, I am strongly tempted 
to believe that they have originated by the process of 
disseverment. 1 

Irregularly-formed atolls. — In the Marshall group, Mus- 
quillo atoll consists of two loops united in one point ; and 

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 forms a semicircle. 
Consult also, the atlas of Lutke's voyage ; and for the Marshall group 
that of Kotzebue ; for the Gilbert group consult the atlas of Duperrey's 
voyage. Most of the points here referred to may, however, be seen 
in Krusenstern's general Atlas of the Pacific. 



i 4 2 CORAL-REEFS. 

MenchicofT atoll is formed of three loops, two of which (as 
may be seen in Fig. 2, Plate I.) are connected by a mere 
ribbon-shaped reef, and the three together are 60 miles in 
length. In the Gilbert group some of the atolls have 
narrow strips of reef, like spurs, projecting from them. 
There occur also in parts of the open sea, a few linear and 
straight reefs, standing by themselves ; and likewise some 
few reefs in the form of crescents, with their extremities 
more or less curled inwards. Now, the upward growth of a 
barrier-reef which fronted only one side of an island, or one 
side of an elongated island with its extremities (of which 
cases exist), would produce after the complete subsidence 
of the land, mere strips or crescent or hook-formed reefs : 
if the island thus partially fronted became divided during 
subsidence into two or more islands, these islands would be 
united together by linear reefs ; and from the further 
growth of the coral along their shores together with sub- 
sidence, reefs of various forms might ultimately be produced, 
either atolls united together by linear reefs, or atolls with 
spurs projecting from them. Some, however, of the more 
simple forms above specified, might, as we have seen, be 
equally well produced by the coral perishing during sub- 
sidence on part of the circumference of an atoll, whilst on 
the other parts it continued to grow up till it reached the 
surface. 

The Great Chagos Bank. — I have already shown that 
the submerged condition of the Great Chagos Bank (Plate 
IV., Fig. 1, with its section Fig. 2), and of some other 
banks in the Chagos group, may in all probability be 
attributed to the coral having perished before or during the 
movements of subsidence, to which this whole area by our 
theory has been subjected. The external rim or upper 
ledge (shaded in the chart), consist of dead coral-rock thinly 



CORAL-REEFS. 143 

covered with sand ; it lies at an average depth of between 
five and eight fathoms, and perfectly resembles in form the 
annular reef of an atoll. The banks of the second level, 
the boundaries of which are marked by dotted lines in the 
chart, lie from about fifteen to twenty fathoms beneath the 
surface ; they are several miles broad, and terminate in a 
very steep slope round the central expanse. This central 
expanse I have already described, as consisting of a level 
muddy flat between thirty and forty fathoms deep. The 
banks of the second level, might at first sight be thought 
analogous to the internal step-like ledge of coral-rock which 
borders the lagoons of some atolls, but their much greater 
width, and their being formed of sand, are points of 
essential difference. On the eastern side of the atoll some 
of the banks are linear and parallel, resembling islets in a 
great river, and pointed directly towards a great breach on 
the opposite side of the atoll ; these are best seen in the 
large published chart I inferred from this circumstance, 
that strong currents sometimes set directly across this vast 
bank ; and I have since heard from Capt. Moresby that this 
is the case. I observed, also, that the channels or breaches 
through the rim, were all of the same depth as the central 
lagoon-like space into which they lead ; whereas the 
channels into the other atolls of the Chagos group, and as I 
believe into most other large atolls, are not nearly as deep 
as their lagoons : — for instance at Peros Banhos, the 
channels are only of the same depth, namely between 10 
and 20 fathoms, as the bottom of the lagoon for a space 
about a mile and a half in width round its shores, whilst the 
central expanse of the lagoon is from 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 



144 CORAL-REEFS. 

sediment could be formed from it ; and consequently the 
channels leading into the lagoon from not being filled 
up with drifted sand and coral detritus, would continue 
increasing in depth, as the whole sank down. In this case, 
we might expect that the currents of the open sea, instead 
of any longer sweeping round the submarine flanks, would 
flow directly through the breaches across the lagoon, 
removing in their course the finer sediment, and preventing 
its further accumulation. We should then have the sub- 
merged reef forming an external and upper rim of rock^ and 
beneath this portion of the sandy bottom of the old lagoon, 
intersected by deep-water channels or breaches, and thus 
formed into separate marginal banks ; and these would be 
cut off by steep slopes, overhanging the central space, worn 
down by the passage of the oceanic currents. 

By these means, I have scarcely any doubt that the 
Great Chagos Bank has originated, — a structure which at 
first appeared to me far more anomalous than any I had 
met with. The process of formation is nearly the same 
with that, by which Mahlos Mahdoo had been trisected ; 
but in the Chagos Bank the channels of the oceanic 
currents entering at several different quarters, have united 
in a central space. 

This great atoll-formed bank appears to be in an early 
stage of disseverment ; should the work of subsidence go 
on, from the submerged and dead condition of the whole 
reef, and the imperfection of the S.E. quarter, a mere wreck 
would probably be left. The Pitt's Bank, situated not far 
southward, appears to be precisely in this state ; it consists 
of a moderately level, oblong bank of sand, lying from 10 
to 20 fathoms beneath the surface, with two sides protected 
by a narrow ledge of rock which is submerged between 5 
and 8 fathoms. A little further south, at about the same 



CORAL-REEFS. 145 

distance as the southern rim of the Great Chagos Bank is 
from the northern rim, there are two other small banks with 
from 10 to 20 fathoms on them; and not far eastward 
soundings were struck on a sandy bottom, with between 
no and 145 fathoms. The northern portion with its 
ledge-like margin, closely resembles any one segment of 
the Great Chagos Bank, between two of the deep-water 
channels, and the scattered banks, southward appear to be 
the last wrecks of less perfect portions. 

I have examined with care the charts of the Indian and 
Pacific Oceans, and have now brought before the reader all 
the examples, which I have met with, of reefs differing from 
the type of the class to which they belong ; and I think it 
has been satisfactorily shown, that they are all included in 
our theory, modified by occasional accidents which might 
have been anticipated as probable. In this course we have 
seen, that in the lapse of ages encircling barrier-reefs are 
occasionally converted into atolls, — the name of atoll being 
properly applicable, at the moment when the last pinnacle 
of encircled land sinks beneath the surface of the sea. We 
have, also, seen that large atolls during the progressive 
subsidence of the areas in which they stand, sometimes 
become dissevered into smaller ones ; at other times, the 
reef-building polypifers having entirely perished, atolls are 
converted into atoll-formed banks of dead rock ; and these 
again through further subsidence and the accumulation 
of sediment modified by the force of the oceanic currents, 
pass into level banks with scarcely any distinguishing 
character. Thus may the history of an atoll be followed 
from its first origin, through the occasional accidents of its 
existence, to its destruction and final obliteration. 

Objections to the theory of the formation of Atolls and 
Barrier-reefs. — The vast amount of subsidence, both 

87s 



146 CORAL-REEFS. 

horizontally or in area, and vertically or in depth, necessary 
to have submerged every mountain, even the highest, 
throughout the immense spaces of ocean interspersed with 
atolls, will probably strike most people as a formidable 
objection to my theory. But as continents, as large as the 
spaces supposed to have subsided, have been raised above 
the level of the sea, — as whole regions are now rising, 
for instance, in Scandinavia and South America, — and as 
no reason can be assigned, why subsidences should not have 
occurred in some parts of the earth's crust on as great a 
scale both in extent and amount as those of elevation, 
objections of this nature strike me as of little force. The 
remarkable point is that movements to such an extent should 
have taken place within a period, during which the poly- 
pifers have continued adding matter on and above the same 
reefs. Another and less obvious objection to the theory will 
perhaps be advanced from the circumstance, of the lagoons 
within atolls and within barrier-reefs never having become 
in any one instance during prolonged subsidences of a 
greater depth than 60 fathoms, and seldom more than 40 
fathoms ; but we already admit, if the theory be worth con- 
sidering, that the rate of subsidence has not exceeded that 
of the upward growth of the coral on the exterior margin ; 
we are, therefore, only further required to admit, that the 
subsidence has not exceeded in rate the filling up of the 
interior spaces by the growth of the corals living there, and 
by the accumulation of sediment. As this filling up must 
take place very slowly within barrier-reefs lying far from the 
land, and within atolls which are of large dimensions and 
which have open lagoons with very few reefs, we are led to 
conclude that the subsidence thus counterbalanced, must 
have been slow in an extraordinary degree ; a conclusion 
which accords with our only means, namely, with what is 



CORAL-REEFS. 147 

known of the rate and manner of recent elevatory move- 
ments, of judging by analogy what is the probable rate of 
subsidence. 

In this chapter it has, I think, been shown, that the theory 
of subsidence, which we were compelled to receive from the 
necessity of giving to the corals, in certain large areas, 
foundations at the requisite depth, explains both the normal 
structure and the less regular forms of those two great 
classes of reefs, which have justly excited the astonishment 
of all persons who have sailed through the Pacific and 
Indian Oceans. But further to test the truth of the theory, 
a crowd of questions will occur to the reader : Do the 
different kinds of reefs, which have been produced by the 
same kind of movement, generally lie within the same areas ? 
What is their relation of form and position, — for instance, 
do adjoining groups of atolls, and the separate atolls in these 
groups, bear the same relation to each other which islands 
do in common archipelagoes ? Have we reason to believe, 
that where there are fringing-reefs, there has not lately been 
subsidence ; or, for it is almost our only way of ascertaining 
this point, are there frequently proofs of recent elevation ? 
Can we by this means account for the presence of certain 
classes of reefs in some large areas, and their entire absence 
in others? Do the areas which have subsided, as indicated 
by the presence of atolls and barrier-reefs, and the areas 
which have remained stationary or have been upraised, as 
shown by fringing-reefs, bear any determinate relation to 
each other ; and are the dimensions of these areas such as 
harmonise with the greatness of the subterranean changes, 
which, it must be supposed, have lately taken place 
beneath them? Is there any connection between the 
movements thus indicated, and recent volcanic action ? All 
these questions ought to receive answers in accordance with 



148 CORAL-REEFS. 

the theory ; and if this can be satisfactorily shown, not 
only is the theory confirmed, but as deductions, the answers 
are in themselves important Under this latter point of 
view, these questions will be chiefly considered in the 
following chapter. 1 

1 I may take this opportunity of briefly considering the appearances, 
which would probably be presented by a vertical and deep section 
across a coral formation (referring chiefly to an atoll), formed by the 
upward growth of coral during successive subsidences. This is a 
subject worthy of attention, as a means of comparison with ancient 
coral-strata. The circumferential parts would consist of massive 
species, in a vertical position, with their interstices filled up with 
detritus; but this would be the part most subject to subsequent denuda- 
tion and removal. It is useless to speculate how large a portion of the 
exterior annular reef would consist of upright coral, and how much of 
fragmentary rock, for this would depend on many contingencies, — 
such as on the rate of subsidence, occasionally allowing a fresh growth 
of coral to cover the whole surface, and on the breakers having force 
sufficient to throw fragments over this same space. The conglomerate 
which composes the base of the islets, would (if not removed by 
denudation together with the exterior reef on which it rests) be con- 
spicuous from the size of the fragments, — the different degrees in which 
they have been rounded, — the presence of fragments of conglomerate 
torn up, rounded, and recemented, — and from the oblique stratification. 
The corals which lived in the lagoon-reefs at each successive level, 
would be preserved upright, and they would consist of many kinds, 
generally much branched. In this part, however, a very large propor- 
tion of the rock (and in some cases nearly all of it) would be formed of 
sedimentary matter, either in an excessively fine, or in a moderately 
coarse state, and with the particles almost blended together. The 
conglomerate which was formed of rounded pieces of the branched 
corals, on the shores of the lagoon, would differ from that formed on 
the islets and derived from the outer coast; yet both might have 
accumulated very near each other. I have seen a conglomerate lime- 
stone from Devonshire like a conglomerate now forming on the shores 
of the Maldiva atolls. The stratification taken as a whole, would be 
horizontal; but the conglomerate beds resting on the exterior reef, and 
the beds of sandstone on the shores of the lagoon (and no doubt on the 



CORAL-REEFS. 149 

external flanks) would probably be divided (as at Keeling atoll and at 
Mauritius) by numerous layers dipping at considerable angles in 
different directions. The calcareous sandstone and coral-rock would 
almost necessarily contain innumerable shells, echini, and the bones of 
fish, turtle, and perhaps of birds; possibly, also, the bones of small 
saurians, as these animals find their way to the islands far remote from 
any continent. The large shells of some species of Tridacna would be 
found vertically imbedded n the solid rock, in the position in which 
they lived. We might expect also to find a mixture of the remains of 
pelagic and littoral animals in the strata formed in the lagoon, for 
pumice and the seeds of plants are floated from distant countries into 
the lagoons of many atolls : on the outer coast of Keeling atoll, near 
the mouth of the lagoon, the case of a pelagic Pteropodous animal was 
brought up on the arming of the sounding lead. All the loose blocks 
of coral on Keeling atoll were burrowed by vermiform animals ; and as 
every cavity, no doubt, ultimately becomes filled with spathose lime- 
stone, slabs of the rock taken from a considerable depth, would, if 
polished, probably exhibit the excavations of such burrowing animals. 
The conglomerate and fine-grained beds of coral-rock would be 
hard, sonorous, white and composed of nearly pure calcareous 
matter; in some few parts, judging from the specimens at Keeling 
atoll, they would probably contain a small quantity of iron. Floating 
pumice and scoriae, and occasionally stones transported in the root of 
trees (see my Journal of Researches, 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 : Capt. Moresby informs me that 
during the change of monsoons the sea is discoloured to a considerable 
distance off the Maldiva and Chagos atolls. The sediment of fringing 
and barrier coral-reefs must be mingled with the mud, which is 
brought down from the land, and is transported seaward through the 
breaches, which occur in front of almost every valley. If the atolls of 
the larger archipelagoes were upraised, the bed of the ocean being 
converted into land, they would form flat-topped mountains, varying in 
diameter from a few miles (the smallest atolls being worn away) to 
sixty miles ; and from being horizontally stratified and of similar com- 
position, they would, as Mr. Lyell has remarked, falsely appear as if 
they had originally been united into one vast continuous mass. Such 
great strata of coral-rock would rarely be associated with erupted 
volcanic matter, for this could only take place, as may be inferred 



150 CORAL-REEFS. 

from what follows in the next chapter, when the area, in which they 
were situated, commenced to rise, or at least ceased to subside. 
During the enormous period necessary to effect an elevation of the 
kind just alluded to, the surface would necessarily be denuded to a 
great thickness ; hence it is highly improbable that any fringing-reef, 
or even any barrier-reef, at least of those encircling small islands, 
would be preserved. From this same cause, the strata which were 
formed within the lagoons of atolls and lagoon-channels of barrier- 
reefs, and which must consist in a large part of sedimentary matter, 
would more often be preserved to future ages, than the exterior solid 
reef, composed of massive corals in an upright position ; although it is 
on this exterior part that the present existence and further growth of 
atolls and barrier-reefs entirely depend. 



CHAPTER VI. 

ON THE DISTRIBUTION OF CORAL-REEFS WITH 
REFERENCE TO THE THEORY OF THEIR FOR- 
MATION. 

Description of the coloured map. — Proximity of atolls and barrier-reefs. 
— Relation in form and position of atolls zvith ordinary islands. — 
Direct evidence of subsidence difficult to be detected. — Proofs of 
recent elevation where fringing-reefs occur. — Oscillations of level. — 
Absence of active volcanoes in the areas of subsidence. — Immensity 
of the areas which have been elevated and have subsided. — Their 
relation to the present distribution of the land. — Areas of sub- 
sidence elongated, their intersection and alternation with those 
of elevation. — Amount ', and slow rate of the subsidence. — Re- 
capitulation. 

It will be convenient to give here a short account of the 
appended map (Plate V.) : a fuller one, with the data for 
colouring each spot, is reserved for the Appendix; and 
every place there referred to may be found in the Index. 
A larger chart would have been desirable; but, small as 
the adjoined one is, it is the result of many months' labour. 
I have consulted, as far as I was able, every original voyage 
and map; and the colours were first laid down on charts 
on a larger scale. The same blue colour, with merely a 
difference in the depth of tint, is used for atolls or lagoon- 
islands, and barrier-reefs, for we have seen, that as far as 



152 CORAL-REEFS. 

the actual coral-formation is concerned, they have no dis- 
tinguishing character. Fringing-reefs have been coloured 
red, for between them on the one hand, and barrier-reefs 
and atolls on the other, there is an important distinction 
with respect to the depth beneath the surface, at which 
we are compelled to believe their foundations lie. The 
two distinct colours, therefore, mark two great types of 
structure. 

The dark blue colour represents atolls and submerged 
annular reefs, with deep water in their centres. I have 
coloured as atolls, a few low and small coral-islands, 
without lagoons ; but this has been done only when it 
clearly appeared that they originally contained lagoons, 
since filled up with sediment : when there were not good 
grounds for this belief, they have been left uncoloured. 

The pale blue colour represents barrier-reefs. The most 
obvious character of reefs of this class is the broad 
and deep-water moat within the reef: but this, like the 
lagoons of small atolls, is liable to become filled up with 
detritus and with reefs of delicately branched corals : when, 
therefore, a reef round the entire circumference of an island 
extends very far into a profoundly deep sea, so that it can 
hardly be confounded with a fringing-reef which must rest 
on a foundation of rock within a small depth, it has been 
coloured pale blue, although it does not include a deep- 
water moat : but this has only been done rarely, and each 
case is distinctly mentioned in the Appendix. 

The red colour represents reefs fringing the land quite 
closely where the sea is deep, and where the bottom is 
gently inclined extending to a moderate distance from it, 
but not having a deep-water moat or lagoon-like space 
parallel to the shore. It must be remembered that fringing- 
reefs are frequently breached in front of rivers and valleys by 



CORAL-REEFS. 153 

deepish channels, where mud has been deposited. 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 
conspicuous on the appended map, which is reduced to so 
small a scale. 

The vermilion spots and streaks represent volcanoes 
now in action, or historically known to have been so. They 
are chiefly laid down from Von Buch's work on the Canary 
Islands ; and my reasons for making a few alterations are 
given in the note below. 1 

1 I have also made considerable use of the geological part of 
Berghaus' Physical Atlas. Beginning at the eastern side of the 
Pacific, I have added to the number of the volcanoes in the southern 
part of the Cordillera, and have coloured Juan Fernandez according to 
observations collected during the voyage of the Beagle {Geol. Trans., 
vol. v. p. 601). I have added a volcano to Albemarle Island, one of 
the Galapagos Archipelago (the author's Journal of Researches : , p. 457). 
In the Sandwich group there are no active volcanoes, except at Hawaii; 
but the Rev. W. Ellis informs me, there are streams of lava apparently 
modern on Maui, having a very recent appearance, which can be traced 
to the craters whence they flowed. The same gentleman informs me, 
that there is no reason to believe that any active volcano exists in the 
Society Archipelago ; nor are there any known in the Samoa or Navi- 
gator group, although some of the streams of lava and craters there 
appear recent. In the Friendly group, the Rev. J. Williams says 
{Narrative of Missionary Enterprise, p. 29) that Toofoa and Proby 
Islands are active volcanoes. 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 Captain Edwards (Von Buch, p. 386) found the lava of 
recent eruption at Amargura still smoking. Berghaus marks four active 
volcanoes actually within the Friendly group ; but I do not know on 
what authority: I-may mention that Maurelle describes Latte as having 
a burnt-up appearance: I have marked only Toofoa and Amargura. 
South of the New Hebrides lies Matthews Rock, which is drawn and 
described as an active crater in the Voyage of the Astrolabe. Between 
it, and the volcano on the eastern side of New Zealand, lies Brimstone 



154 CORAL-REEFS. 

The uncoloured coasts consist, first and chiefly, of those 
where there are no coral-reefs, or such small portions as 
to be quite insignificant. Secondly, of those coasts where 
there are reefs, but where the sea is very shallow, for in this 
case the reefs generally lie far from the land, and become 
very irregular, in their forms : where they have not become 
irregular, they have been coloured. Thirdly, if I had the 
means of ascertaining the fact, I should not colour a reef 
merely coating the edges of a submarine crater, or of a level 
submerged bank; for such superficial formations differ 
essentially, even when not in external appearance, from reefs 
whose foundations as well as superficies have been wholly 
formed by the growth of coral. Fourthly, in the Red 

Island, which from the high temperature of the water in the crater, may 
be ranked as active (Berghaus, Vorbemerk, n 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 a 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 volcanoes between these 
islands and Japan. Gemelli Careri (Churchill's Collect., vol. iv. p. 458) 
says there are two active volcanoes in lat. 23 30', and in lat. 24 : but 
I have not coloured them. From the statements in Beechey's Voyage 
(p. 518, 4to ed.), I have coloured one in the northern part of the Bonin 
group. M. S. Julien has clearly made out from Chinese manuscripts 
not very ancient {Comptes Rendus, 1840, p. 832), that there are two 
active volcanoes on the eastern side of Formosa. In Torres Straits, on 
Cap Island (9° 48' S., 142 39' E.) a volcano was seen burning with 
great violence in 1793 by Capt. Bampton (see Introduction to Flinders' 
Voyage, p. 41). Mr. M'Clelland {Report of Committee for investigating 
Coal in India, p. 39) has shown that the volcanic band passing through 
Barren Island must be extended northwards. It appears by an old 
chart, that Cheduba was once an active volcano (see also Silli- 
tnaris North American Journal, vol. xxxviii. p. 385). In Berghaus' 



CORAL-REEFS. 155 

Sea, and within some parts of the East Indian Archipelago 
(if the imperfect charts of the latter can be trusted), there 
are many scattered reefs, of small size, represented in the 
chart by mere dots, which rise out of deep water : these 
cannot be arranged under either of the three classes : in 
the Red Sea, however, some of these little reefs, from their 
position, seem once to have formed parts of a continuous 
barrier. There exist, also, scattered in the open ocean, 
some linear and irregularly formed strips of coral-reef, 
which, as shown in the last chapter, are probably allied in 
their origin to atolls ; but as they do not belong to that 
class, they have not been coloured ; they are very few in 
number and of insignificant dimensions. Lastly, some 
reefs are left uncoloured from the want of information 
respecting them, and some because they are of an inter- 
mediate structure between the barrier and fringing classes. 
The value of the map is lessened, in proportion to the 
number of reefs which I have been obliged to leave 
uncoloured, although, in a theoretical point of view, few 
of them present any great difficulty : but their number 
is not very great, as will be found by comparing the map 
with the statements in the Appendix. I have experienced 

Phys. Atlas, 1840, No. 7 of Geological Part, a volcano on the 
coast of Pondicherry is said to have burst forth in 1757. Ordin 
aire {Hist. Nat. des Vo leans, p. 218) says that there is one at 
the mouth of the Persian Gulf, but I have not coloured it, as 
he gives no particulars. A volcano in Amsterdam, or St, Paul's, 
in the southern part of the Indian Ocean, has been seen {Naut. Mag., 
1838, p. 842) in action. Dr. J. Allan, of Forres, informs me in a 
letter, that when he was at Joanna, he saw at night flames apparently 
volcanic, issuing from the chief Comoro Island, and that the Arabs 
assured him that they were volcanic, adding that the volcano burned 
more during the wet season. I have marked this as a volcano, though 
with some hesitation, on account of the possibility of the flame arising 
from gaseous sources. 



156 CORAL-REEFS. 

more difficulty in colouring fringing-reefs than in colouring 
barrier-reefs, as the former, from their much less dimensions, 
have less attracted the attention of navigators. As I have 
had to seek my information from all kinds of sources, and 
often from indirect ones, I do not venture to hope that 
the map is free from many errors. Nevertheless, I trust 
it will give an approximately correct view of the general 
distribution of the coral-reefs over the whole world (with 
the exception of some fringing-reefs on the coast of Brazil, 
not included within the limits of the map), and of their 
arrangement into the three great classes, which, though 
necessarily very imperfect from the nature of the objects 
classified, have been adopted by most voyagers. I may 
further remark, that the dark blue colour represents land 
entirely composed of coral-rock ; the pale blue, land with a 
wide and thick border of coral-rock ; and the red, a mere 
narrow fringe of coral-rock. 

Looking now at the map under the theoretical point of 
view indicated in the last chapter, the two blue tints signify 
that the foundations of the reefs thus coloured have subsided 
to a considerable amount, at a slower rate than that of the 
upward growth of the corals, and that probably in many cases 
they are still subsiding. The red signifies that the shores 
which support fringing-reefs have not subsided (at least to 
any considerable amount, for the effects of a subsidence on 
a small scale would in no case be distinguishable) ; but that 
they have remained nearly stationary since the period when 
they first became fringed by reefs; or that they are now 
rising or have been upraised, with new lines of reefs suc- 
cessively formed on them : these latter alternatives are 
obviously implied, as newly formed lines of shore, after 
elevations of the land, would be in the same state with 
respect to the growth of fringing-reefs, as stationary coasts. 



CORAL-REEFS. 157 

If during the prolonged subsidence of a shore, coral-reefs 
grew for the first time on it, or if an old barrier-reef were 
destroyed and submerged, and new reefs became attached 
to the land, these would necessarily at first belong to the 
fringing class, and, therefore, be coloured red, although the 
coast was sinking; but I have no reason to believe, that 
from this source of error, any coast has been coloured 
wrongly with respect to movement indicated. Well charac- 
terised atolls and encircling barrier-reefs, where several 
occur in a group, or a single barrier-reef if of large 
dimensions, leave scarcely any doubt on the mind respect- 
ing the movement by which they have been produced; 
and even a small amount of subsequent elevation is soon 
betrayed. The evidence from a single atoll or a single 
encircling barrier-reef, must be received with some caution, 
for the former may possibly be based upon a submerged 
crater or bank, and the latter on a submerged margin of 
sediment, or of worn-down rock. From these remarks we 
may with greater certainty infer that the spaces, especially 
the larger ones, tinted blue in the map, have subsided, 
than that the red spaces have remained stationary, or have 
been upraised. 

On the grouping of the different classes of reefs. — Having 
made these preliminary remarks, I will consider first how 
far the grouping of the different kinds of coral-islands and 
reefs is corroborative of the truth of the theory. A glance 
at the map shows that the reefs, coloured blue and red, 
produced under widely different conditions, are not indis- 
criminately mixed together. Atolls and barrier-reefs, on 
the other hand, as may be seen by the two blue tints, 
generally lie near each other ; and this would be the natural 
result of both having been produced during the subsidence 
of the areas in which they stand. Thus, the largest group 



158 CORAL-REEFS. 

of encircled islands is that of the Society Archipelago ; and 
these islands are surrounded by atolls, and only separated 
by a narrow space from the large group of Low atolls. 
In the midst of the Caroline atolls, there are three fine 
encircled islands. The northern point of the barrier-reef of 
New Caledonia seems itself, as before remarked, to form a 
complete large atoll. The great Australian barrier is de- 
scribed as including both atolls and small encircled islands. 
Capt. King 1 mentions many atoll-formed and encircling 
coral-reefs, some of which lie within the barrier, and others 
may be said (for instance between lat. 16 and 13 ) to form 
part of it. Flinders 2 has described an atoll-formed reef in 
lat. io°, seven miles long and from one to three broad, 
resembling a boot in shape, with apparently very deep water 
within. Eight miles westward of this, and forming part of 
the barrier, lie the Murray Islands, which are high and are 
encircled. In the Corallian Sea, between the two great 
barriers of Australia and New Caledonia, there are many 
low islets and coral-reefs, some of which are annular, or 
horse-shoe shaped. Observing the smallness of the scale 
of the map, the parallels of latitude being 900 miles apart, 
we see that none of the large groups of reefs and islands 
supposed to have been produced by long-continued sub- 
sidence, lie near extensive lines of coast coloured red, 
which are supposed to have remained stationary since the 
growth of their reefs, or to "have been upraised and new 
lines of reefs formed on them. Where the red and blue 
circles do occur near each other, I am able, in several 
instances, to show that there have been oscillations of level, 
subsidence having preceded the elevation of the red spots ; 

1 Sailing Directions, appended to vol. ii. of his Surveying Voyage to 
Australia. 

2 Voyage to Terra Australis, vol. ii. p. 336. 



CORAL-REEFS. 159 

and elevation having preceded the subsidence of the blue 
spots : and in this case the juxtaposition of reefs belonging 
to the two great types of structure is little surprising. We 
may, therefore, conclude that the proximity in the same 
areas of the two classes of reefs, which owe their origin to 
the subsidence of the earth's crust, and their separation 
from those formed during its stationary or uprising con- 
dition, holds good to the full extent, which might have 
been anticipated by our theory. 

As groups of atolls have originated in the upward growth, 
at each fresh sinking of the land, of those reefs which 
primarily fringed the shores of one great island, or of 
several smaller ones ; so we might expect that these rings 
of coral-rock, like so many rude outline charts, will still 
retain some traces of the general form, or at least general 
range, of the land, round which they were first modelled. 
That this is the case with the atolls in the Southern Pacific 
as far as their range is concerned, seems highly probable, 
when we observe that the three principal groups are directed 
in N.W. and S.E. lines, and that nearly all the land in the 
S. Pacific ranges in this same direction ; namely, N. Western 
Australia, New Caledonia, the northern half of New Zea- 
land, the New Hebrides, Saloman, Navigator, Society, 
Marquesas, and Austral archipelagoes : in the Northern 
Pacific, the Caroline atolls abut against the N.W. line of 
the Marshall atolls, much in the same manner as the 
E. and W. line of islands from Ceram to New Britain do 
on New Ireland : in the Indian Ocean the Laccadive and 
Maldiva atolls extend nearly parallel to the western and 
mountainous coast of India. In most respects, there is a 
perfect resemblance with ordinary islands in the grouping 
of atolls and in their form : thus the outline of all the 
larger groups is elongated ; and the greater number of the 



160 CORAL-REEFS. 

individual atolls are elongated in the same direction with 
the group, in which they stand. The Chagos group is less 
elongated than is usual with other groups, and the individual 
atolls in it are likewise but little elongated ; this is strikingly 
seen by comparing them with the neighbouring Maldiva 
atolls. In the Marshall and Maldiva archipelagoes, the 
atolls are ranged in two parallel lines, like the mountains in 
a great double mountain-chain. Some of the atolls, in the 
larger archipelagoes, stand so near to each other, and have 
such an evident relationship in form, that they compose 
little sub-groups : in the Caroline Archipelago, one such 
sub-group consists of Pouynipete, a lofty island encircled by 
a barrier-reef, and separated by a channel only four miles and 
a half wide from Andeema atoll, with a second atoll a little 
further off. In all these respects an examination of a series 
of charts will show how perfectly groups of atolls resemble 
groups of common islands. 

On the direct evidence of the blue spaces in the map having 
subsided during the upward growth of the reefs so coloured, 
and of the red spaces having remained stationary, or having 
been upraised. — With respect to subsidence, I have shown 
in the last chapter, that we cannot expect to obtain in 
countries inhabited only by semi-civilised races, demon- 
strative proofs of a movement, which invariably tends to 
conceal its own evidence. But on the coral-islands 
supposed to have been produced by subsidence, we have 
proofs of changes in their external appearance — of a round 
of decay and renovation — of the last vestiges of land on 
some — of its first commencement on others : we hear of 
storms desolating them to the astonishment of their 
inhabitants : we know by the great fissures with which 
some of them are traversed, and by the earthquakes felt 
under others, that subterranean disturbances of some kind 



CORAL-REEFS. 161 

are in progress. These facts, if not directly connected with 
subsidence, as I believe they are, at least show how difficult 
it would be to discover proofs of such movement by 
ordinary means. At Keeling atoll, however, I have 
described some appearances, which seem directly to show 
that subsidence did take place there during the late earth- 
quakes. Vanikoro, according to the Chevalier Dillon, 1 is 
often violently shaken by earthquakes, and there, the 
unusual depth of the channel between the shore and the 
reef, — the almost entire absence of islets on the reef, — its 
wall-like structure on the inner side, and the small quantity 
of low alluvial land at the foot of the mountains, all seem 
to show that this island has not remained long at its present 
level, with the lagoon-channel subjected to the accumula- 
tion of sediment, and the reef to the wear and tear of the 

1 See Capt. Dillon's Voyage in search of La Peyrouse. M. Cordier, 
in his Report on the Voyage of the Astrolabe (p. cxi. vol. i. ), speaking 
of Vanikoro, says the shores are surrounded by reefs of madrepore, 
" qu' 'on assure etre de formation tout-a-fait moderne." I have in vain 
endeavoured to learn some further particulars about this remarkable 
passage. I may here add, that according to our theory, the island 
of Pouynipete (Plate I., Fig. 3), in the Caroline Archipelago, being 
encircled by a barrier-reef, must have subsided. In the New S. Wales 
Lit. Advert., Feb. 1835 (which I have seen through the favour of Dr. 
Lloghtsky), there is an account of this island (subsequently confirmed 
by Mr. Campbell), in which it is said, " At the N.E. end, at a place 
called Tamen, there are ruins of a town, now only accessible by boats, 
the waves reaching to the steps of the houses." Judging from this 
passage, one would be tempted to conclude that the island must have 
subsided, since these houses were built. I may, also, here append a 
statement in Malte Brun (vol. ix. p. 775, given without any authority), 
that the sea gains in an extraordinary manner on the coast of Cochin 
China, which lies in front and near the subsiding coral-reefs in the 
China Sea: as the coast is granitic, and not alluvial, it is scarcely 
possible that the encroachment of the sea can be owing to the washing 
away of the land ; and if so, it must be due to subsidence. 

876 



1 62 CORAL-REEFS. 

breakers. At the Society Archipelago, on the other hand, 
where a slight tremor is only rarely felt, the shoaliness of the 
lagoon-channels round some of the islands, the number of 
islets formed on the reefs of others, and the broad belt of 
low land at the foot of the mountains, indicate that, 
although there must have been great subsidence to have 
produced the barrier-reefs, there has since elapsed a long 
stationary period. 1 

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 those coral-reefs, which within most barrier-reefs 
merely fringe the land. From this circumstance, he arrives at the 
same conclusion as I have done, that the Society Islands since their 
subsidence, have remained stationary during a long period ; but he 
further believes that they have recently commenced rising, as well as 
the whole area of the Low Archipelago. He does not give any 
detailed proofs regarding the elevation of the Society Islands, but I 
shall refer to this subject in another part of this chapter. Before 
making some further comments, I may observe how satisfactory it is to 
me, to find Mr. Couthouy affirming, that " having personally examined 
a large number of coral-islands, and also residing eight months among 
the volcanic class, having shore and partially encircling reefs, I may 
be permitted to state that my own observations have impressed a 
conviction of the correctness of the theory of Mr. Darwin." 

This gentleman believes, that subsequently to the subsidence by 
which the atolls in the Low Archipelago were produced, the whole 
area has been elevated to the amount of a few feet ; this would indeed 
be a remarkable fact ; but as far as I am able to judge, the grounds of 
his conclusion are not sufficiently strong. He states that he found in 
almost every atoll which he visited, the shores of the lagoon raised 
from eighteen to thirty inches above the sea-level, and containing 
imbedded Tridacnae and corals standing as they grew ; some of the 
corals were dead in their upper parts, but below a certain line they 
continued to flourish. In the lagoons, also, he frequently met with 
clusters cf Madrepore, w?th their extremities standing from one inch to 
a foot above the surface of the water. Now, these appearances are 
exactly what I should have expected, without any subsequent elevation 



CORAL-REEFS. 163 

Turning now to the red colour ; as on our map, the areas 
which have sunk slowly downwards to great depths are 
many and large, we might naturally have been led to con- 
jecture, that with such great changes of level in progress, the 
coasts which have been fringed probably for ages (for we have 
no reason to believe that coral-reefs are of short duration), 

having taken place ; and I think Mr. Couthouy has not borne in mind 
the indisputable fact, that corals, when constantly bathed by the surf, 
can exist at a higher level than in quite tranquil water, as in a lagoon. 
As long, therefore, as the waves continued at low water to break 
entirely over parts of the annular reef of an atoll, submerged to a small 
depth, the corals and shells attached on these parts might continue 
living at a level above the smooth surface of the lagoon, into which the 
waves rolled ; but as soon as the outer edge of the reef grew up to its 
utmost possible height, or if the reef were very broad nearly to that 
height, the force of the breakers would be checked, and the corals and 
shells on the inner parts near the lagoon would occasionally be left 
dry, and thus be partially or wholly destroyed. Even in atolls, which 
have not lately subsided, if the outer margin of the reef continued to 
increase in breadth seaward (each fresh zone of corals rising to 
the same vertical height as at Keeling atoll), the line where the 
waves broke most heavily would advance outwards, and therefore 
the corals, which when living near the margin, were washed by 
the breaking waves during the whole of each tide, would cease being 
so, and would therefore be left on the backward part of the reef stand- 
ing exposed and dead. The case of the madrepores in the lagoons 
with the tops of their branches exposed, seems to be an analogous fact, 
to the great fields of dead but upright corals in the lagoon of Keeling 
atoll; — a condition of things which I have endeavoured to show, has 
resulted from thp lagoon having become more and more enclosed and 
choked up with reefs, so that during high winds, the rising of the tide 
(as observed by the inhabitants) is checked, and the corals, which had 
formerly grown to the greatest possible height, are occasionally exposed, 
and thus are killed : and this is a condition of things, towards which 
almost every atoll in the intervals of its subsidence must be tending. 
Or if we look to the state of an atoll directly after a subsidence of some 
fathoms, the waves would roll heavily over the entire circumference of 
the reef, and the surface of the lagoon would, like the ocean, never be 



1 64 CORAL-REEFS. 

would not have remained all this time stationary, but would 
frequently have undergone movements of elevation. This 
supposition, we shall immediately see, holds good to a 
remarkable extent; and although a stationary condition 
of the land can hardly ever be open to proof, from the 

quite at rest, and therefore the corals in the lagoon, from being con- 
stantly laved by the rippling water, might extend their branches to 
a little greater height than they could, when the lagoon became enclosed 
and protected. Christmas atoll (2 N. lat.), which has a very shallow 
lagoon, and differs in several respects from most atolls, possibly may 
have been elevated recently ; but its highest part appears (Couthouy, 
p. 46) to be only ten feet above the sea-level. The facts of a second 
class, adduced by Mr. Couthouy, in support of the alleged recent 
elevation of the Low Archipelago, are not all (especially those refer- 
ring to a shelf of rock) quite intelligible to me; he believes that certain 
enormous fragments of rock on the reef, must have been moved into 
their present position, when the reef was at a lower level; but here 
again the force of the breakers on any inner point of the reef being 
diminished by its outward growth without any change in its level, has 
not, I think, been borne in mind: We should, also, not overlook the 
occasional agency of waves caused by earthquakes and hurricanes. Mr. 
Couthouy further argues, that since these great fragments were deposited 
and fixed on the reef, they have been elevated; he infers this from the 
greatest amount of erosion not being near their bases, where they are 
unceasingly washed by the reflux of the tides, but at some height on 
their sides, near the line of high-water mark, as shown in an accompany- 
ing diagram. My former remark again applies here, with this further 
observation, that as the waves have to roll over a wide space of reef 
before they reach the fragments, their force must be greatly increased 
with the increasing depth of water as the tide rises, and therefore I 
should have expected that the chief line of present erosion would have 
coincided with the line of high-water mark; and if the reef had grown 
outwards, that there would have been lines of erosion at greater 
heights. The conclusion, to which I am finally led by the interesting 
observations of Mr. Couthouy is, that the atolls in the Low Archipelago 
have, like the Society Islands, remained at a stationary level for a long 
period : and this probably is the ordinary course of events, subsidence 
supervening after long intervals of rest. 



CORAL-REEFS. 165 

evidence being only negative, we are, in some degree, 
enabled to ascertain the correctness of the parts coloured 
red on the map, by the direct testimony of upraised organic 
remains of a modern date. Before going into the details 
on this head (printed in small type), I may mention, that 
when reading a memoir on coral formations by MM. Quoy 
and Gaimard 1 I was astonished to find, for I knew that 
they had crossed both the Pacific and Indian Oceans, that 
their descriptions were applicable only to reefs of the fringing 
class; but my astonishment ended satisfactorily, when I 
discovered that, by a strange chance, all the islands which 
these eminent naturalists had visited, though several in 
number, — namely, the Mauritius, Timor, New Guinea, the 
Mariana, and Sandwich Archipelagoes, could be shown by 
their own statements to have been elevated within a recent 
geological era. 

In the eastern half of the Pacific, the Sandwich Islands are 
all fringed, and almost every naturalist who has visited them, 
has remarked on the abundance of elevated corals and shells, 
apparently identical with living species. The Rev. W. Ellis 
informs me, that he has noticed round several parts of Hawaii, 
beds of coral-detritus, about twenty feet above the level of 
the sea, and where the coast is low they extend far inland. 
Upraised coral-rock forms a considerable part of the borders of 
Oahu ; and at Elizabeth Island 2 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 3 has lately described with interesting details, 
several upraised beaches, ancient reefs with their surfaces 

1 Annates des Sciences Nat., torn. vi. p. 279, etc. 

2 Zoology of Capt. Beecheys Voyage, p. 176. See also MM. Quoy 
and Gaimard in Annates des Scien. Nat., torn. vi. 

3 Remarks on Coral Formation, p. 51. 



1 66 CORAL-REEFS. 

perfectly preserved, and beds of recent shells and corals, at the 
islands of .Maui, Morokai, Oahu, and Tauai (or Kauai) in this 
group. Mr. Pierce, an intelligent resident at Oahu, is con- 
vinced, from changes which have taken place within his 
memory, during the last sixteen years, "that the elevation is at 
present going forward at a very perceptible rate." The natives 
at Kauai state that the land is there gaining rapidly on the sea, 
and Mr. Couthouy has no doubt, from the nature of the strata, 
that this has been effected by an elevation of the land. 

In the southern part of the Low Archipelago, Elizabeth 
Island is described by Capt. Beechey, 1 as being quite flat, and 
about eighty feet in height ; it is entirely composed of dead 
corals, forming a honeycombed, but compact rock. In cases 
like this, of an island having exactly the appearance, which the 
elevation of any one of the smaller surrounding atolls with a 
shallow lagoon would present, one is led to conclude (with little 
better reason, however, than the improbability of such small 
and low fabrics lasting, for an immense period, exposed to the 
many destroying agents of nature), that the elevation has taken 
place at an epoch not geologically remote. When merely the 
surface of an island of ordinary formation is strewed with marine 
bodies, and that continuously, or nearly so, from the beach 
to a certain height, and not above that height, it is exceedingly 
improbable that such organic remains, although they may not 
have been specially examined, should belong to any ancient 
period. It is necessary to bear these remarks in mind, in 
considering the evidence of the elevatory movements in the 
Pacific and Indian Oceans, as it does not often rest on specific 
determinations, and therefore should be received with caution. 
Six of the Cook and Austral Islands (S.W. of the Society 
group) are fringed ; of these, five were described to me by the 
Rev. J. Williams, as formed of coral-rock, associated with some 
basalt in Mangaia), and the sixth as lofty and basaltic. 
Mangaia is nearly three hundred feet high, with a level summit ; 
and according to Mr. S. Wilson 2 it is an upraised reef ; " and 

1 Beechey's Voyage in the Facific y p. 46, 4to ed. 

2 Couthouy's Remarks, p. 34. 



CORAL-REEFS. 167 

there are in the central hollow, formerly the bed of the lagoon, 
many scattered patches of coral-rock, some of them raised to a 
height of forty feet." These knolls of coral-rock were evidently 
. once separate reefs in the lagoon of an atoll. Mr. Martens, at 
Sydney, informed me that this island is surrounded by a 
terrace-like plain at about the height of a hundred feet, which 
probably marks a pause in its elevation. From these facts we 
may infer, perhaps, that the Cook and Austral Islands have 
been upheaved at a period probably not very remote. 

Savage Island (S.E. of the Friendly group) is about forty 
feet in height. Forster x describes the plants as already grow- 
ing out of the dead, but still upright and spreading trees of 
coral ; and the younger Forster 2 believes that an ancient 
lagoon is now represented by a central plain ; here we cannot 
doubt that the elevatory forces have recently acted. The 
same conclusion may be extended, though with somewhat less 
certainty, to the islands of the Friendly Groups which have 
been well described in the second and third voyages of Cook. 
The surface of Tongatabou is low and level, but with some 
parts a hundred feet high ; the whole consists of coral-rock, 
" which yet shows the cavities and irregularities worn into it 
by the action of the tides." 3 On Eoua the same appearances 
were noticed at an elevation of between 200 and 300 feet. 
Vavao, also, at the opposite or northern end of the group, 
consists, according to the Rev. J. Williams, of coral-rock. 
Tongatabou, with its northern extensive reefs, resembles either 
an upraised atoll with one half originally imperfect, or one 
unequally elevated ; and Anamouka, an atoll equally elevated. 
This latter island contains 4 in its centre a salt-water lake, about 
a mile and a half in diameter, without any communication with 
the sea, and around it the land rises gradually like a bank ; the 
highest part is only between twenty and thirty feet ; but on 
this part, as well as on the rest of the land (which, as Cook 

1 Observations made during Voyage round the World, p. 147. 

2 Voyage^ vol. ii. p. 163. 

3 Cook's Third Voyage (4to edition), vol. i. p. 314. 

4 J bid. t vol. i. p. 235. 



1 68 CORAL-REEFS. 

observes, rises above the height of true lagoon-islands), coral- 
rock, like that on the beach, was found. In the Navigator 
Archipelago, Mr. Couthouy 1 found on Manua many and very 
large fragments of coral at the height of eighty feet, " on a 
steep hill-side, rising half a mile inland from a low sandy plain 
abounding in marine remains." The fragments were em- 
bedded 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. Earth- 
quakes are very frequent in this archipelago. 

Still proceeding westward we come to the New Hebrides; on 
these islands, Mr. G, Bennett (author of Wanderings in New 
South Wales) informs me he found much coral at a great 
altitude, which he considered of recent origin. Respecting 
Santa Cruz and the Salomon Archipelago, I have no infor- 
mation ; but at New Ireland, which forms the northern point of 
the latter chain, both Labillardiere and Lesson have described 
large beds of an apparently very modern madreporitic rock, 
with the form of the corals little altered. The latter author 3 
states that this formation composes a newer line of coast, 
modelled round an ancient one. There only remains to be 
described in the Pacific, that curved line of fringed islands, 
of which the Marianas form the main part. Of these Guam, 
Rota, Tiniam, Saypan, and some islets farther north, are 
described by Quoy and Gaimard, 3 and Chamisso, 4 as chiefly 
composed of madreporitic limestone, which attains a consider- 
able 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. Fats, which lies in the prolonged line 

1 Remarks on Coral- Formations \ p. 50. 

2 Voyage de la Coquille, Part. Zoolog. 

3 Freycinet's Voyage atitour du Motide. See also the Hydrographical 
Metnoir, p. 215. 

4 Kotzebue's First Voyage, 



CORAL-REEFS. 169 

of the Marianas, is the only island in this part of the sea which 
is fringed; it is ninety feet high, and consists entirely of 
madreporitic rock. 1 

In the East Indian Archipelago, many authors have recorded 
proofs of recent elevation. M. Lesson 2 states, that near Port 
Dory, on the north coast of New Guinea, the shores are flanked, 
to the height of 150 feet, by madreporitic strata of modern date. 
He mentions similar formations at Waigiou, Amboina, Bourou, 
Ceram, Sonda, and Timor : at this latter place, MM. Quoy and 
Gaimard 3 have likewise described the primitive rocks, as coated 
to a considerable height with coral. Some small islets east- 
ward of Timor are said in Kolffs Voyage* to resemble small 
coral islets upraised some feet above the sea. Dr. Malcolmson 
informs me that Dr. Hardie found in Java an extensive for- 
mation, containing an abundance of shells, of which the greater 
part appear to be of existing species. Dr. Jack 5 has described 
some upraised shells and corals, apparently recent, on Pulo 
Nias off Sumatra; and Marsden relates in his history of this 
great island, that the names of many promontories show that 
they were originally islands. On part of the west coast of 
Borneo and at the Sooloo Islands, the form of the land, the nature 
of the soil, and the water-washed rocks, present appearances 6 

1 Lutke^s Voyage, vol. ii. p. 304. 

2 Partie Zoolog. , Voyage de la Coquille. 

3 Ann. des Scien. Nat., torn. vi. p. 281. 

4 Translated by Windsor Earl, chaps, vi., vii. 

5 Geolog. Transact., 2nd series, vol. i. p. 403. On the Peninsula of 
Malacca, in front of Pinang, 5 30' N., Dr. Ward collected some shells, 
which Dr. Malcolmson informs me, although not compared with 
existing species, had a recent appearance. Dr. Ward describes in this 
neighbourhood (Trans. Asiat. Soc, vol. xviii., part ii., p. 166) a single 
water-worn rock, with a conglomerate of sea-shells at its base, situated 
six miles inland, which, according to the traditions of the natives, was 
once surrounded by the sea. Capt, Low has also described {ibid., part 
i., p. 131) mounds of shells lying two miles inland on this line of coast. 

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



170 CORAL-REEFS. 

(although it is doubtful whether such vague evidence is worthy 
of mention) of having recently been covered by the sea; and 
the inhabitants of the Sooloo Islands believe that this has 
been the case. Mr. Cuming, who has lately investigated, with 
so much success, the natural history of the Philippines, found 
near Cabagan, in Luzon, about fifty feet above the level of the 
R. Cagayan, and seventy miles from its mouth, a large bed of 
fossil shells : these, he informs me, are of the same species with 
those now existing on the shores of the neighbouring islands. 
From the accounts given us by Capt. Basil Hall and Capt. 
Beechey 1 of the lines of inland reefs, and walls of coral-rock 
worn into caves, above the present reach of the waves, at the 
Loo C/wo Islands, there can be little doubt that they have been 
upraised at no very remote period. 

Dr. Davey 2 describes the northern province of Ceylon as 
being very low, and consisting of a limestone with shells and 
corals of very recent origin ; he adds, that it does not admit of 
a doubt that the sea has retired from this district even within 
the memory of man. There is also some reason for believing 
that the western shores of India, north of Ceylon, have been 
upraised within the recent period. 8 Mauritius has certainly 
been upraised within the recent period, as I have stated in the 

1 Capt. B. Hall, Voyage to Loo Choo, Append., pp. xxi. and xxv. 
Capt. Beechey's Voyage, p. 496. 

2 Travels in Ceylon, p. 13. This madreporitic formation is men- 
tioned by M. Cordier in his report to the Institute (May 4th, 1839), on 
the voyage of the Chevrette, as one of immense extent, and belonging to 
the latest tertiary period. 

3 Dr. Benza, in his Journey through the N. Circars (the Madras Lit. 
and Sc lent. Journ., vol. v.), has described a formation with recent fresh- 
water and marine shells, occurring at the distance of three or four miles 
from the present shore. Dr. Benza, in conversation with me, attributed 
their position to a rise of the land. Dr. Malcolmson, however (and 
there cannot be a higher authority on the geology of India), informs 
me that he suspects that these beds may have been formed by the mere 
action of the waves and currents accumulating sediment. From analogy 
I should much incline to Dr. Benza's opinion. 



CORAL-REEFS. 171 

chapter on fringing-reefs. The northern extremity of Mada- 
gascar is described by Capt. Owen 1 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 expression "madreporitic rock"; but at the same time 
it is, I think, scarcely possible to look at the chart of the linear 
islets, which rise to a greater height than can be accounted for 
by the growth of coral, in front of the coast, from the equator 
to 2 S., without feeling convinced that a line of fringing-reefs 
has been elevated at a period so recent, that no great changes 
have since taken place on the surface of this part of the globe, 
Some, also, of the higher islands of madreporitic rock on this 
coast, for instance Pemba, have very singular forms, which 
seem to show the combined effect of the growth of coral round 
submerged banks, and their subsequent upheaval. Dr. Allan 
informs me that he never observed any elevated organic remains 
on the Seychelles, which come under our fringed class. 

The nature of the formations round the shores of the Red 
Sea, as described by several authors, shows that the whole of 
this large area has been elevated within a very recent tertiary 
epoch. A part of this space in the appended map is coloured 
blue, indicating the presence of barrier-reefs : on which circum- 
stance I shall presently make some remarks. Ruppell 2 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 and 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, however, can hardly be quite 
accurate ; although possibly there may be a decrease in the 
elevation of the shores in the middle parts of the Red Sea, for 

1 Owen's Africa, vol. ii. p. 37, for Madagascar; and for S. Africa, 
vol. i. pp. 412 and 426. Lieutenant 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. 

2 Ruppell, Reise in Abyssinien, Band. i. s. 141. 



172 CORAL-REEFS. 

Dr. Malcolm son (as he informs me) collected from the cliffs of 
Camaran Island (Lat. 15 30' S.) shells and corals, apparently- 
recent, at a height between 30 and 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 says that the modern beds attain 
a height of only 30 to 40 feet, Mr. Burton 1 found a deposit 
replete with existing species of shells, at the height of 200 feet. 
In an admirable series of drawings by Capt. Moresby, I could 
see how continuously the cliff-bounded low plains of this forma- 
tion extended with a nearly equable height, both on the eastern 
and western shores. The southern coast of Arabia seems to 
have been subjected to the same elevatory movement, for Dr. 
Malcolmson found at Sahar low cliffs containing shells and 
corals, apparently of recent species. 

The Persian Gulf abounds with coral-reefs; but as it is 
difficult to distinguish them from sand-banks in this shallow 
sea, I have coloured only some near the mouth; towards the 
head of the gulf Mr. Ains worth 2 says that the land is worn into 
terraces, and that the beds contain organic remains of existing 
forms. The West Indian Archipelago of 'fringed' islands, 
alone remains to be mentioned ; evidence of an elevation within 
a late tertiary epoch of nearly the whole of this great area, may 
be found in the works of almost all the naturalists who have 
visited it. I will give some of the principal references in a 
note. 

1 Lyell's Principles of Geology, 5th edition, vol. iv. p. 25. 

2 Ainsworth's Assyria and Babylon, p. 217. 

3 On Florida and the north shores of the Gulf of Mexico, Rogers' 
Report to Brit. Assoc. , vol. iii. p. 14.— On the shores of Mexico, Hum- 
boldt, Polit. Essay on New Spain, vol. i. 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. t vol. xxx. p. 74. — St. 
Domingo, Courrojolles, four, de Phys.> torn. liv. p. 106. — Bahamas, 
United Service fournal, No. lxxi. pp. 218 and 224. Jamaica, De la 



CORAL-REEFS. 173 

It is very remarkable on reviewing these details, to 
observe in how many instances fringing-reefs round the 
shores, have coincided with the existence on the land of 
upraised organic remains, which seem, from evidence more 
or less satisfactory, to belong to a late tertiary period. It 
may, however, be objected, that similar proofs of elevation, 
perhaps, occur on the coasts coloured blue in our map : 
but this certainly is not the case with the few following and 
doubtful exceptions. 

The entire area of the Red Sea appears to have been 
upraised within a modern period ; nevertheless I have been 
compelled (though on unsatisfactory evidence, as given in 
the Appendix) to class the reefs in the middle part, as 
barrier-reefs ; should, however, the statements prove accu- 
rate of the less height of the tertiary bed in this middle 
part, compared with the northern and southern districts, we 
might well suspect that it had subsided subsequently to the 
general elevation by which the whole area has been upraised. 
Several authors 1 have stated that they have observed shells 

Beche, Geol. Man., p. 142. — Cuba, Taylor, in Lond. and E din. Mag., 
vol. xi. p. 17. Dr. Daubeny also, at a meeting of the Geolog. Soc, 
orally described some very modern beds lying on the N.W. parts of 
Cuba. I might have added many other less important references. 

1 Ellis, in his Polynesian Researches, was the first to call attention to 
these remains (vol. i. p. 38), 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. 213; also Mr. 
Couthouy's Remarks, p. 51 ; but this principal fact, namely, that there 
is a mass of upraised coral on the narrow peninsula of Tiarubu, is from 
hearsay evidence; also Mr. Stutchbury, West of England 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 inquired from 
several native chiefs, whether they knew of any change of this kind, 
and they were unanimous in giving me an answer in the negative, 



174 CORAL-REEFS. 

and corals high up on the mountains of the Society Islands, 
— a group encircled by barrier-reefs, and, therefore, sup- 
posed to have subsided : at Tahiti Mr. Stutchbury found 
on the apex of one of the highest mountains, between 5,000 
and 7,000 feet above the level of the sea, "a distinct and 
regular stratum of semi-fossil coral." At Tahiti, however, 
other naturalists, as well as myself, have searched in vain at 
a low level near the coast, for upraised shells or masses of 
coral-reef, where if present they could hardly have been 
overlooked. From this fact, I concluded that probably the 
organic remains strewed high up on the surface of the land, 
had originally been embedded in the volcanic strata, and 
had subsequently been washed out by the rain. I have 
since heard from the Rev. W. Ellis, that the remains which 
he met with, were (as he believes) interstratified with an 
argillaceous tuff; this likewise was the case with the shells 
observed by the Rev. D. Tyerman at Huaheine. These 
remains have not been specifically examined; they may, 
therefore, and especially the stratum observed by Mr. 
Stutchbury at an immense height, be contemporaneous with 
the first formation of the Society Islands, and be of any 
degree of antiquity ; or they may have been deposited at 
some subsequent, but probably not very recent, period of 
elevation ; for if the period had been recent, the entire sur- 
face of the coast land of these islands, where the reefs are 
so extensive, would have been coated with upraised coral, 
which certainly is not the case. Two of the Harvey, or 
Cook Islands, namely, Aitutaki and Manouai, are encircled 
by reefs, which extend so far from the land, that I have 
coloured them blue, although with much hesitation, as the 
space within the reef is shallow, and the outline of the land 
is not abrupt. These two islands consist of coral-rock ; but 
I have no evidence of their recent elevation, besides, the 



CORAL-REEFS. 175 

improbability of Mangaia, a fringed island in the same 
group (but distant 170 miles), having retained its nearly 
perfect atoll-like structure, during any immense lapse of 
time after its upheaval. The Red Sea, therefore, is the 
only area in which we have clear proofs of the recent 
elevation of a district, which, by our theory (although the 
barrier-reefs are there not well characterised), has lately 
subsided. But we have no reason to be surprised at 
oscillation, of level of this kind having occasionally taken 
place. There can be scarcely any doubt that Savage, 
Aurora, 1 and Mangaia Islands, and several of the islands in 
the Friendly group, existed originally as atolls, and these 
have undoubtedly since been upraised to some height 
above the level of the sea ; so that by our theory, there has 
here, also, been an oscillation of level, — elevation having 
succeeded subsidence, instead of, as in the middle part of 
the Red Sea and at the Harvey Islands, subsidence having 
probably succeeded recent elevation. 

It is an interesting fact, that Fais, which, from its com- 
position, form, height, and situation at the western end 
of the Caroline Archipelago, one is strongly induced to 
believe existed before its upheaval as an atoll, lies exactly 
in the prolongation of the curved line of the Mariana group, 

1 Aurora Island is described by Mr. Couthouy {Remarks, p. 58) ; it 
lies 120 miles N.E. of Tahiti ; it is not coloured in the appended map, 
because it does not appear to be fringed by living reefs. Mr. Couthouy 
describes its summit as <f presenting a broad table-land which declines 
a few feet towards the centre, where we may suppose the lagoon to 
have been placed." It is about 200 feet in height, and consists of reef- 
rock and conglomerate, with existing species of coral embedded in it. 
The island has been elevated at two successive periods ; the cliffs 
being marked half-way up with a horizontal water-worn line of deep 
excavations. Aurora Island seems closely to resemble in structure 
Elizabeth Island, at the southern end of the Low Archipelago. 



176 CORAL-REEFS. 

which we know to be a line of recent elevation. I 
may add, that Elizabeth Island, in the southern part 
of the Low Archipelago, which seems to have had the 
same kind of origin as the Fais, lies near Pitcairn 
Island, the only one in this part of the ocean which 
is high, and at the same time not surrounded by an 
encircling barrier-reef. 

On the absence of active volcanoes in the areas of subsidence, 
and on their frequent presence in the areas of elevation, — 
Before making some concluding remarks on the relations of 
the spaces coloured blue and red, it will be convenient to 
consider the position on our map of the volcanoes histori- 
cally known to have been in action. It is impossible not 
to be struck, first with the absence of volcanoes in the 
great areas of subsidence tinted pale and dark blue, — 
namely, in the central parts of the Indian Ocean, in the 
China Sea, in the sea between the barriers of Australia and 
New Caledonia, in the Caroline, Marshall, Gilbert, and 
Low Archipelagoes ; and, secondly, with the coincidence of 
the principal volcanic chains with the parts coloured red, 
which indicates the presence of fringing-reefs ; and, as we 
have just seen, the presence in most cases of upraised 
organic remains of a modern date. I may here remark that 
the reefs were all coloured before the volcanoes were added 
to the map, or indeed before I knew of the existence of 
several of them. 

The volcano in Torres Strait, at the northern point of 
Australia, is that which lies nearest to a large subsiding 
area, although situated 125 miles within the outer margin of 
the actual barrier-reef. The Great Comoro Island, which 
probably contains a volcano, is only twenty miles distant 
from the barrier-reef of Mohila; Ambil volcano, in the 
Philippines, is distant only a little more than sixty miles 



CORAL-REEFS. 177 

from the atoll-formed Appoo reef : and there are two other 
volcanoes in the map within ninety miles of circles coloured 
blue. These few cases, which thus offer partial exceptions 
to the rule, of volcanoes being placed remote from the 
areas of subsidence, lie either near single and isolated atolls, 
or near small groups of encircled islands ; and these by our 
theory can have, in few instances, subsided to the same 
amount in depth or area, as groups of atolls. There is not 
one active volcano within several hundred miles of an 
archipelago, or even a small group of atolls. It is, there- 
fore, a striking fact that in the Friendly Archipelago, which 
owes its origin to the elevation of a group of atolls, two 
volcanoes, and, perhaps, others, are known to be in 
action : on the other hand, on several of the encircled 
islands in the Pacific, supposed by our theory to have 
subsided, there are old craters and streams of lava, which 
show the effects of past and ancient eruptions. In these 
cases, it would appear as if the volcanoes had come 
into action, and had become extinguished on the same 
spots, according as the elevating or subsiding movements 
prevailed. 

There are some other coasts on the map, where volcanoes 
in a state of action concur with proofs of recent elevation, 
besides those coloured red from being fringed by coral- 
reefs. Thus I hope to show in a future volume, that nearly 
the whole line of the west coast of South America, which 
forms the greatest volcanic chain in the world, from near 
the equator for a space of between 2,000 and 3,000 miles 
southward, has undergone an upward movement during a 
late geological period. The islands on the north-western 
shores of the Pacific, which form the second greatest 
volcanic chain, are very imperfectly known ; but Luzon, in 
the Philippines, and the Loo Choo Islands, have been 

877 



178 CORAL-REEFS. 

recently elevated; and at Kamtschatka 1 there are extensive 
tertiary beds of modern date. Evidence of the same nature, 
but not very satisfactory, may be detected in Northern New 
Zealand where there are two volcanoes. The co-existence 
in other parts of the world of active volcanoes, with upraised 
beds of a modern tertiary origin, will occur to every 
geologist. 2 Nevertheless, until it could be shown that 
volcanoes were inactive, or did not exist in subsiding areas, 
the conclusion that their distribution depended on the 
nature of the subterranean movements in progress, would 
have been hazardous. But now, viewing the appended 
map, it may, I think, be considered as almost established, 
that volcanoes are often (not necessarily always) present in 
those areas where the subterranean motive power has lately 
forced, or is now forcing outwards the crust of the earth, 
but that they are invariably absent in those, where the 
surface has lately subsided or is still subsiding. 3 

On the relations of the areas of Subsidence and Elevation. 
— The immense surfaces on the map, which, both by our 
theory and by the plain evidence of upraised marine 
remains, have undergone a change of level either down- 
wards or upwards during a late period, is a most remarkable 
fact. The existence of continents shows that the areas 

1 At Sedanka, in Lat. 58° N. (Von Buch's Descrip. des Isles 
Canaries, p. 455). In a forthcoming part, I shall give the evidence 
referred to with respect to the elevation of New Zealand. 

2 During the subterranean disturbances which took place in Chile, 
in 1S35, I have shown (Geolog. Trans., 2nd Ser., vol. v. p. 606) that 
at the same moment that a large district was upraised, volcanic matter 
burst forth at widely separated points, through both new and old vents. 

3 We may infer from this rule, that in any old deposit, which con- 
tains interstratified beds of erupted matter, there was at the period, and 
in the area of its formation, a tendency to an upward movement in the 
earth's surface, and certainly no movement of subsidence. 



CORAL-REEFS. 179 

have been immense which at some period have been 
upraised : in South America we may feel sure, and on the 
north-western shores of the Indian Ocean we may suspect, 
that this rising is either now actually in progress, or has 
taken place quite recently. By our theory, we may con- 
clude that the areas are likewise immense which have lately 
subsided, or, judging from the earthquakes occasionally felt 
and from other appearances, are now subsiding. The 
smallness of the scale of our map should not be over- 
looked : each of the squares on it contains (not allowing 
for the curvature of the earth) 810,000 square miles. Look 
at the space of ocean from near the southern end of the 
Low Archipelago to the northern end of the Marshall 
Archipelago, — a length of 4,500 miles, in which, as far as is 
known, every island, except Aurora, which lies just without 
the Low Archipelago, is atoll-formed. The eastern and 
western boundaries of our map are continents, and they are 
rising areas : the central spaces of the great Indian and 
Pacific Oceans, are mostly subsiding ; between them, north 
of Australia, lies the most broken land on the globe, and 
there the rising parts are surrounded and penetrated by 
areas of subsidence, 1 so that the prevailing movements now 
in progress, seem to accord with the actual states of surface 
of the great divisions of the world. 

The blue spaces on the map are nearly all elongated ; but 
it does not necessarily follow from this (a caution, for which 
I am indebted to Mr. Lyell), that the areas of subsidence 
were likewise elongated; for the subsidence of a long, 
narrow space of the bed of the ocean, including in it a 

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 from my imperfect information, as given in the 
Appendix. 



180 CORAL-REEFS. 

transverse chain of mountains, surmounted by atolls, would 
only be marked on the map by a transverse blue band. 
But where a chain of atolls and barrier-reefs lies in an 
elongated area, between spaces coloured red, which there- 
fore have remained stationary or have been upraised, this 
must have resulted either from the area of subsidence 
having originally been elongated (owing to some tendency 
in the earth's crust thus to subside), or from the subsiding 
area having originally been of an irregular figure, or as 
broad as long, and having since been narrowed by the 
elevation of neighbouring districts. Thus the areas, which 
subsided during the formation of the great north and south 
lines of atolls in the Indian Ocean, — of the east and west 
line of the Caroline atolls, — and of the north-west and 
south-east line of the barrier-reefs of New Caledonia and 
Louisiade, must have originally been elongated, or if not 
so, they must have since been made elongated by elevations, 
which we know to belong to a recent period. 

I infer from Mr. Hopkins' researches, 1 that for the forma- 
tion of a long chain of mountains, with few lateral spurs, 
an area elongated in the same direction with the chain, 
must have been subjected to an elevatory movement. 
Mountain-chains, however, when already formed, although 
running in very different directions, it seems 2 may be 

1 "Researches in Physical Geology," Transact. Cambridge Phil. 
Soc, vol, vi. part i. 

2 For instance in S. America from lat. 34°, for many degrees south- 
ward there are upraised beds containing recent species of shells, on 
both the Atlantic and Pacific side of the continent, and from the 
gradual ascent of the land, although with very unequal slopes, on 
both sides towards the Cordillera, I think it can hardly be doubted 
that the entire width has been upraised in mass within the recent 
period. In this case the two W.N.W. and E.S.E. mountain-lines, 
namely the Sierra Ventana and the S. Tapalguen, and the great north 



CORAL-REEFS, 181 

raised together by a widely-acting force : so, perhaps, 
mountain-chains may subside together. Hence, we cannot 
tell, whether the Caroline and Marshall Archipelagoes, two 
groups of atolls running in different directions and meeting 
each other, have been formed by the subsidence of two 
areas, or of one large area, including two distinct lines of 
mountains. We have, however, in the southern prolonga- 
tion of the Mariana Islands, probable evidence of a line of 
recent elevation having intersected one of recent subsi- 
dence. A view of the map will show that, generally, there is 
a tendency to alternation in the parallel areas undergoing 
opposite kinds of movement ; as if the sinking of one area 
balanced the rising of another. 

The existence in many parts of the world of high table- 
land, proves that large surfaces have been upraised in mass 
to considerable heights above the level of the ocean ; 
although the highest points in almost every country con- 
sist of upturned strata, or erupted matter : and from the 
immense spaces scattered with atolls, which indicate that 
land originally existed there, although not one pinnacle 
now remains above the level of the sea, we may conclude 
that wide areas have subsided to an amount, sufficient to 
bury not only any formerly existing table-land, but even the 
heights formed by fractured strata, and erupted matter. 
The effects produced on the land by the later elevatory 
movements, namely, successively rising cliffs, lines of 
erosion, and beds of littoral shells and pebbles, all requiring 
time for their production, prove that these movements have 
been very slow; we can, however, infer this with safety, 

and south line of the Cordillera have been together raised. In the 
West Indies the N. and S. line of the Eastern Antilles, and the E. 
and W. line of Jamaica, appear both to have been upraised within the 
latest geological period. 



1 82 CORAL-REEFS. 

only with respect to the few last hundred feet of rise. But 
with reference to the whole vast amount of subsidence, 
necessary to have produced the many atolls widely scattered 
over immense spaces, it has already been shown (and it is, 
perhaps, the most interesting conclusion in this volume), 
that the movements must either have been uniform and 
exceedingly slow, or have been effected by small steps, 
separated from each other by long intervals of time, during 
which the reef-constructing polypifers were able to bring 
up their solid frameworks to the surface. We have little 
means of judging whether many considerable oscillations of 
level have generally occurred during the elevation of large 
tracts ; but we know, from clear geological evidence, that 
this has frequently taken place ; and we have seen on our 
map, that some of the same islands have both subsided and 
been upraised. I conclude, however, that most of the 
large blue spaces have subsided without many and great 
elevatory oscillations, because only a few upraised atolls 
have been observed : the supposition that such elevations 
have taken place, but that the upraised parts have been 
worn down by the surf, and thus have escaped observation, 
is overruled by the very considerable depth of the lagoons 
of all the larger atolls ; for this could not have been the 
case, if they had suffered repeated elevations and abrasion. 
From the comparative observations made in these latter 
pages, we may finally conclude, that the subterranean 
changes which have caused some large areas to rise, and 
others to subside, have acted in a very similar manner. 

Recapitulation. — In the three first chapters, the principal 
kinds of coral-reefs were described in detail, and they 
were found to differ little, as far as relates to the actual 
surface of the reef. An atoll differs from an encircling 
barrier-reef onlv in the absence of land within its central 



CORAL-REEFS. 183 

expanse; and a barrier-reef differs from a fringing-reef, in 
being placed at a much greater distance from the land 
with reference to the probable inclination of its submarine 
foundation, and in the presence of a deep-water lagoon- 
like space or moat within the reef. In the fourth chapter 
the growing powers of the reef-constructing polypifers were 
discussed; and it was shown, that they cannot flourish 
beneath a very limited depth. In accordance with this 
limit, there is no difficulty respecting the foundations on 
which fringing-reefs are based ; whereas, with barrier- 
reefs and atolls, there is a great apparent difficulty on 
this head ; — in barrier-reefs from the improbability of 
the rock of the coast or of banks of sediment extend- 
ing, in erery instance, so far seaward within the required 
depth ; — and in atolls, from the immensity of the spaces 
over which they are interspersed, and the apparent 
necessity for believing that they are all supported on 
mountain-summits, which, although rising very near to the 
surface-level of the sea, in no one instance emerge above 
it. To escape this latter most improbable admission, which 
implies the existence of submarine chains of mountains 
of almost the same height, extending over areas of many 
thousand square miles, there is but one alternative ; namely, 
the prolonged subsidence of the foundations, on which 
the atolls were primarily based, together with the upward 
growth of the reef-constructing corals. On this view every 
difficulty vanishes : fringing-reefs are thus converted into 
barrier-reefs ; and barrier-reefs, when encircling islands, are 
thus converted into atolls, the instant the last pinnacle of 
land sinks beneath the surface of the ocean. 

Thus the ordinary forms and certain peculiarities in the 
structure of atolls and barrier-reefs can be explained; — 
namely, the wall-like structure on their inner sides, the 



1 84 CORAL-REEFS. 

bason or ring-like shape both of the marginal and central 
reefs in the Maldiva atolls — the union of some atolls as 
if by a ribbon — the apparent disseverment of others — and 
the occurrence, in atolls as well as in barrier-reefs, of 
portions of reef, and of the whole of some reefs, in a dead 
and submerged state, but retaining the outline of living 
reefs. Thus can be explained the existence of breaches 
through barrier-reefs in front of valleys, though separated 
from them by a wide space of deep water; thus, also, the 
ordinary outline of groups of atolls and the relative forms 
of the separate atolls one to another ; thus can be explained 
the proxifnky of the two kinds of reefs formed during 
subsidence, and their separation from the spaces where 
fringing-reefs abound. On searching for other evidence 
of the movements supposed by our theory, we find marks 
of change in atolls and in barrier-reefs, and of subterranean 
disturbances under them ; but from the nature of things, 
it is scarcely possible to detect any direct proofs of sub- 
sidence, although some appearances are strongly in favour 
of it. On the fringed coasts, however, the presence of 
sipraised marine bodies of a recent epoch, plainly show, 
that these coasts, instead of having remained stationary, 
which is all that can be directly inferred from our theory, 
have generally been elevated. 

Finally, when the two great types of structure, namely 
barrier-reefs and atolls on the one hand, and fringing-reefs 
on the other, were laid down in colours on our map, a 
magnificent and harmonious picture of the movements, 
which the crust of the earth has within a late period under- 
gone, is presented to us. We there see vast areas rising, 
with volcanic matter every now and then bursting forth 
through the vents or fissures with which they are traversed. 
We see other wide spaces slowly sinking without any 



CORAL-REEFS. 185 

volcanic outburst, and we may feel sure, that this sinking 
must have been immense in amount as well as in area, 
thus to have buried over the broad face of the ocean every 
one of those mountains, above which atolls now stand like 
monuments, marking the place of their former existence. 
Reflecting how powerful an agent with respect to denu- 
dation, and consequently to the nature and thickness of 
the deposits in accumulation, the sea must ever be, when 
acting for prolonged periods on the land, during either its 
slow emergence or subsidence ; reflecting, also, on the final 
effects of these movements in the interchange of land and 
ocean-water on the climate of the earth, and on the dis- 
tribution of organic beings, I may be permitted to hope, 
that the conclusions derived from the study of coral- 
formations, originally attempted merely to explain their 
peculiar forms, may be thought worthy of the attention of, 
geologists. 



APPENDIX, 



CONTAINING A DETAILED DESCRIPTION OF THE 
REEFS AND ISLANDS IN PLATE V. 



In the beginning of the last chapter I stated the principles 
on which the map is coloured. There only remains to be 
said, that it is an exact copy of one by M. C. Gressier, 
published by the D£pot general de la Marine, in 1835. 
The names have been altered into English, and the 
longitude has been reduced to that of Greenwich. The 
colours were first laid down on accurate charts, on a large 
scale. The data, on which the volcanoes historically known 
to have been in action, have been marked with 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 consecutively, proceeding 
westward across the Pacific and Indian Oceans, but ending 
with the West Indies. 

The Western Shores of America appear to be entirely 
without coral-reefs ; south of the equator the survey of the 
Beagle^ and north of it, the published charts show that this 
is the case, Even in the Bay of Panama^ where corals 
flourish, there are no true coral-reefs, as I have been 
informed by Mr. Lloyd. There are no coral-reefs in the 
Galapagos Archipelago, as I know from personal inspec- 
tion ; and I believe there are none on the Cocos, Reviila- 
gigedo, and other neighbouring islands. Clipfierton rock, 



iSS APPENDIX. 

io° N., 109 W., has lately been surveyed by Capt. Belcher; 
in form it is like the crater of a volcano. From a drawing 
appended to the MS. plan in the Admiralty, it evidently is 
not an atoll. The eastern parts of the Pacific present an 
enormous area, without any islands, except Easter, and 
Sa/a, and Gomez Islands, which do not appear to be 
surrounded by reefs. 

The Low Archipelago. — This group consists of about 
eighty 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. 
Capt. 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. 
Bellinghausen (see an account of his 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. 
Kotzebue has given plans of several; Cook and Bligh 
mention others ; a few were seen during the voyage of the 
Beagle; and notices of other atolls are scattered through 
several publications. The Actaoti group in this archipelago 
has lately been discovered (Geograflh. jfourn., vol. ii. p. 
454) ; it consists of three small and low islets, one of which 
has a lagoon. Another lagoon-island has been discovered 
(Niaut. Mag., 1839, p. 770), in 22° 4' S., and 136 20' W. 
Towards the S.E. part of the group, there are some islands 
of different formation : Elizabeth Island is described by 
Beechey (p. 46, 4to ed.) as fringed by reefs, at the distance 



APPENDIX. 189 

of between two and three hundred yards ; coloured red. 
Pitcairn Island, in the immediate neighbourhood, according 
to the same authority, has no reefs of any kind, although 
numerous pieces of coral are thrown up on the beach ; the 
sea close to its shore is very deep (see Zool. of Beechefs 
Voyage, p. 164); it is left uncoloured. Gambier Islands 
(see Plate II., Fig. 5) are encircled by a barrier-reef; the 
greatest depth within is 38 fathoms ; coloured pale blue. 
Aurora Island, which lies N.E. of Tahiti close to the large 
space coloured dark blue in the map, has been already 
described in a note (p. 118), on the authority of Mr. 
Couthouy; it is an upraised atoll, but as it does not 
appear to be fringed by living reefs, it is left uncoloured. 

The Society Arch, is separated by a narrow space from 
the Low Arch. ; and in their parallel direction they manifest 
some relation to each other. I have already described the 
general character of the reefs of these fine encircled islands. 
In the Atlas of the Coquille's Voyage there is a good 
general chart of the group, and separate plans of some of 
the islands. Tahiti, the largest island in the group, is 
almost surrounded, as seen in Cook's chart, by a reef from 
half a mile to a mile and a half from the shore, with from 
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 (JVaut. Mag., 
1836, p. 264) on the N.E. coast of the island, where none 
are laid down by Cook. At Eimeo the reef " which like a 
ring surrounds it, is in some places one or two miles distant 
from the shore, in others united to the beach" (Ellis, 
Polynesian Researches, vol. i. p. 18, i2mo 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 



190 APPENDIX. 

the barrier- reef and the shore, has been almost filled 
up, — " a nearly continuous fringing-reef surrounding the 
island, and varying from a few yards to rather more 
than a mile in width, the lagoons merely forming canals 
between this and the sea-reef," that is the barrier-reef. 
Tapa?nanoa is surrounded by a reef at a considerable 
distance from the shore; from the island being small, 
it is breached, as I am informed by the Rev. W. 
Ellis, only by a narrow and crooked boat channel. This is 
the lowest island in the group, its height probably not 
exceeding 500 feet. A little way north of Tahiti, the low 
coral-islets of Teiuroa are situated ; from the description of 
them given me by the Rev. J. Williams (the author of the 
Narrative of Missionary Enterprise), I should have thought 
they had formed a small atoll, and likewise from the 
description given by the Rev. D. Tyerman and G. Bennett 
(Journ. of Voy. and Travels, vol. i. p. 183), who say that ten 
low coral-islets " are comprehended within one general reef, 
and separated from each other by interjacent lagoons ; " but 
as Mr. Stutchbury (West of England Journal, vol. i. p. 54) 
describes it as consisting of a mere narrow ridge, I have left 
it uncoloured. Maitea, eastward of the group, is classed by 
Forster as a high encircled island ; but from the account 
given by the Rev. D. Tyerman and G. Bennett (vol. i. p. 
57) it appears to be an exceedingly abrupt cone, rising from 
the sea without any reef; I have left it uncoloured. It 
would be superfluous to describe the northern islands in 
this group, as they may be well seen in the chart accompany- 
ing 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 4^2 fathoms ; but the great width of the 
reef, stretching three miles and a half southward of the land 



APPENDIX. 191 

(which is represented in the drawing in the atlas of the 
Ccquille's voyage as descending abruptly to the water), 
shows, on the principle explained in the beginning of the last 
chapter, that it belongs to the barrier class. I may here 
mention, from information communicated to me by the Rev. 
W. Ellis, that on the N.E. side of Huaheine there is a bank 
of sand, about a quarter of a mile wide, extending parallel 
to the shore, and separated from it by an extensive and 
deep lagoon : this bank of sand rests on coral-rock, and un- 
doubtedly was originally a living reef. North of Bolabola 
lies the atoll of Toubai (Motou-iti of the Coquille's Atlas), 
which is coloured dark blue ; the other islands, surrounded 
by barrier-reefs, are pale blue : three of them are represented 
in Figs. 1 and 4 in Plate II., and Fig. 5 in Plate III. There 
are three low coral-groups lying a little E. of the Society 
Arch., and almost forming part of it, namely, Bellinghausen, 
which is said by Kotzebue {^Second Voyage, vol. ii. p. 255) 
to be a lagoon island ; Mophea, which, from Cook's descrip- 
tion (Second Voyage, book iii. chap, i.), no doubt is an atoll ; 
and the Sally Islands, which are said by Wallis (Voyage, 
chap, ix.) to form a group of low islets and shoals, and, 
therefore, probably, they compose an atoll : the two former 
have been coloured blue, but not the latter. 

Mendana or Marquesas Group. — These islands are 
entirely without reefs, as may be seen in Krusenstern's 
Atlas, making a remarkable contrast with the adjacent group 
of the Society Islands. Mr. F. D. Bennett has given some 
account of this group, in the seventh volume of the 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, Domini- 
eana, Christiana, and Roapoa; their beaches are strewed with 
rounded masses of coral, and although no regular reefs 



192 APPENDIX. 

exist, yet the shore is in many places lined by coral-rock, sc 
that a boat grounds on this formation. Hence these islands 
ought probably to come within the class of fringed islands 
and be coloured red ; but as I am determined to err on the 
cautious side, I have left them uncoloured. 

Cook or Harvey and Austral Isl. — Pahnersion Island 
is minutely described as an atoll by Capt. Cook during his 
voyage in 1774; coloured blue. Aitutaki was partially 
surveyed by the Beagle (see map accompanying Voyages of 
Adveiiture a?id Beagle) ; the land is hilly, sloping gently to 
the beach ; the highest point is 360 feet ; on the southern 
side the reef projects five miles from the land : off this 
point the Beagle found no bottom with 270 fathoms: the 
reef is surmounted by many low coral-islets. Although 
within the reef the water is exceedingly shallow, not being 
more than a few feet deep, as I am informed by the Rev. J. 
Williams, nevertheless, from the great extension of this reef 
into a profoundly deep ocean, this island probably belongs, 
on the principle lately adverted to, to the barrier class, and 
I have coloured it pale blue ; although with much hesita- 
tion. — Manonai or Harvey Isld. The highest point is 
about 50 feet : the Rev. J. Williams informs me that the 
reef here, although it lies far from the shore, is less distant 
than at Aitutaki, but the water within the reef is rather 
deeper: I have also coloured this pale blue, with many 
doubts. — Round Mitiaro Isld., as I am informed by Mr. 
Williams, the reef is attached to the shore ; coloured red. — 
Mauki or Maouti ; the reef round this isld. (under the 
name of Parry Isld., 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 corro- 
borated by Mr. Williams, who calls the reef attached; 
coloured red. — Atiu^ or Wateeo; a moderately elevated, 



APPENDIX. 193 

hilly island, like the others of this group. The reef is 
described in Cook's Voyage, as attached to the shore, and 
about 100 yards wide; coloured red. — Fenoua-iti ; Cook 
describes this isld. as very low, not more than six or 
seven feet high (vol. i., bk. ii. chap, iii., 1777); in the 
chart published in the Coquiltts Atlas, a reef is engraved 
close to the shore : this isld. is not mentioned in the list 
given by Mr. Williams (p. 16) in the Narrative of Missionary 
Enterprise ; nature doubtful. As it is so near Atiu, it has 
been unavoidably coloured red. — Rarotonga ; Mr. Williams 
informs me that it is a lofty basaltic isld. with an attached 
reef; coloured red. — There are three islands, Rourouti, 
Roxburgh, and Hull, of which I have not been able to 
obtain any account, and have left them uncoloured. Hull 
Isld., in the French chart, is written with small letters as 
being low. — Mangaia ; height about three hundred feet; 
"the surrounding reef joins the shore" (Williams's Nar- 
rative, p. 18); coloured red. — Rimetara ; Mr. Williams 
informs me that the reef is rather close to the shore ; but, 
from information given me by Mr. Ellis, the reef does not 
appear to be quite so closely attached to it as in the 
foregoing cases : the island is about three hundred feet 
high (Naut. Mag., 1839, 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 sur- 
rounded, like the neighbouring islds. by a reef; he must 
have meant a distant reef. — Toubouai ; in Cook's chart 
{2nd Voyage, vol. ii. p. 2) the reef is laid down in part one 
mile, and in part two miles from the shore. Mr. Ellis 
{Polynes. Res., vol. iii. p. 381) says the low land round the 
base of the isld. is very extensive; and this gentleman 
informs me that the water within the reef appears deep ; 

878 



194 APPENDIX. 

coloured blue. — Raivaivai, or Vivitao ; Mr. Williams in- 
forms me that the reef is here distant : Mr. Ellis, however, 
says that this is certainly not the case on one side of the 
isld. ; 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. I have not coloured 
it. — Papa, or Oparree ; from the accounts given of it by 
Ellis and Vancouver, there does not appear to be any 
reef. — I. de Bass is an adjoining isld., of which I cannot 
find any account. — Kemin Isld. ; Krusenstern seems hardly 
to know its position, and gives no further particulars. 



Islands between the Low and Gilbert Archipelagoes. 

Caroline Isld. (io° S., 150 W.) is described by Mr. F. 
D. Bennett {Geograph. Journ., vol. vii. p. 225) as contain- 
ing a fine lagoon; coloured blue. — Flint Isld. (n° S., 
15 1 ° W.)j Krusenstern believes that it is the same with 
Peregrino, which is described by Quiros (Burney's Chron, 
Hist., vol. ii. p. 283) as "a cluster of small islands con- 
nected by a reef, and forming a lagoon in the middle;" 
coloured blue. — Wostock is an isld. a little more than half 
a mile in diameter, and apparently quite flat and low, and 
was discovered by Bellinghausen ; it is situated a little west 
of Caroline Isld., but it is not placed on the French charts ; 
I have not coloured it, although I entertain little doubt 
from the chart of Bellinghausen, that it originally contained 
a small lagoon. — Penrhyn Isld. (9 S., 158 W.); a plan 
of it in the atlas of the first voyage of Kotzebue, shows 
that it is an atoll ; blue. — Starbuck Isld. (5 S., 156° W.) 
is described in Lord Byron's Voyage in the Blonde (p. 206) 



APPENDIX. 195 

as formed of a flat coral-rock, with no trees ; the height not 
given; not coloured. — Maiden Isld. (4 S., 154 W.); in 
the same voyage (p. 205) this isld. 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.— farm's, or Bunker Isld. (o° 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 isld. between the two 
latter; the position, and perhaps even the existence of it 
is doubtful ; not coloured. — Pescado and Humphrey Islands ; 
I can find out nothing about these islands, except that 
the latter appears to be small and low; not coloured. — 
Rear son, or Grand Duke Alexander's (io° S., 161 W.) ; 
an atoll, of which a plan is given by Bellinghausen ; 
blue. — Souvoroff Islands (13 S., 163 W.); Admiral 
Krusenstern, in the most obliging manner, obtained for 
me an account of these islands from Admiral LazarerT 
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 Arch.) formed a single atoll ; 
but I have not coloured them. — Danger Isld. (io° S., 166 
W.) ; described as low by Com. Byron, and more lately 
surveyed by Bellinghausen ; it is a small atoll with three 
islets on it; blue. — Clarence Isld. (9 S., 172 W.); dis- 
covered 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 



196 APPENDIX. 

of other low islands, and as it is said, that the natives make 
reservoirs of water in old cocoa-nut trees (which shows the 
nature of the land), I have no doubt it is an atoll, and have 
coloured it blue. York Isld. (8° S., 17 2° W.) is described 
by Commodore Byron (chap. x. of his Voyage) as an atoll ; 
blue. — Sydney Isld. (4 S., 172 W.) is about three miles in 
diameter, with its interior occupied by a lagoon (Capt. 
Tromelin, Annal. Marit, 1829, p. 297); blue. — Phoznix 
Isld. (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 in- 
ferred that this isld. originally contained a lagoon, but I 
have not coloured it. — New Nantucket (o° 15' N., 174 W.). 
From the French chart it must be a low isld. ; I can find 
nothing more about it or about Mary Isld. ; both un- 
coloured. — Gardtier Isld. (5 S., 174 W.) from its position 
is certainly the same as Kemin Isld. described (Krusenstern, 
p. 435, Appen. to Mem., publ. 1827) as having a lagoon in 
its centre ; blue. 

Islands south of the Sandwich Archipelago. 

Christmas Isld. (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 islets on the reef 
is unusually great, and the sea near it does not deepen so 
suddenly as is generally the case. It has more lately been 
visited by Mr. F. D. Bennett (Geograph. Journ., vol. vii. 
p. 226); and he assures me that it is low and of coral- 
formation : I particularly mention this, because it is en- 
graved with a capital letter, signifying a high isld., in 
D'Urville and Lottin's chart. Mr. Couthouy, also, has 
given some account of it {Remarks, p. 46) from the 



APPENDIX. 197 

Hawaiian Spectator ; he believes it has lately undergone a 
small elevation, but his evidence does not appear to me 
satisfactory; the deepest part of the lagoon is said to be 
only ten feet; nevertheless, I have coloured it blue. — 
Fanning Isld. (4 N., 15 8° W.), according to Capt. 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 Isld. (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 Isld., and hence I presume it is not 
an atoll; not coloured. — Palmyra Isld. (6° N., 162 W.) is 
an atoll divided into two parts (Krusenstern's Mem. Supply 
p. 50, also Fanning's Voyage, p. 233); blue. — Smytfts or 
Johnston's Islds. (17 N., 170 W.). Capt. 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. Capt. Smyth does not recollect whether 
these islets, together with the reef, surrounded a lagoon; 
uncoloured. 

Sandwich Arch. — Hawaii; in the chart in Freycinet's 
Atlas, small portions of the coast are fringed by reefs ; and 
in the accompanying Hydrog. Memoir, reefs are mentioned 
in several places, and the coral is said to injure the cables. 
On one side of the islet of Kohaihai there is a bank of 
sand and coral with five feet water on it, running parallel 
to the shore, and leaving a channel of about fifteen feet 
deep within. I have coloured this isld. red, but it is very 
much less perfectly fringed than others of the group. — 
Maui; in Freycinet's chart of the anchorage of Raheina, 
two or three miles of coast are seen to be fringed ; and in 
the Hydrog. Memoir, "banks of coral along shore" are 



198 APPENDIX. 

spoken of, Mr. F. D. Bennett informs me that the reefs, 
on an average, extend about a quarter of a mile from the 
beach \ the land is not very steep, and outside the reefs the 
sea does not become deep very suddenly ; coloured red. — 
Morotoi, I presume, is fringed : Freycinet speaks of the 
breakers extending along the shore at a little distance from 
it. From the chart, I believe it is fringed ; coloured red. — 
Oahn; Freycinet, in his Hydrog. Memoir, mentions some 
of the reefs. Mr. F. D. Bennett informs me that the shore 
is skirted for forty or fifty miles in length. There is even 
a harbour for ships formed by the reefs, but it is at the 
mouth of a valley; red. — Atooi, in La Peyrouse's charts, 
is represented as fringed by a reef, in the same manner as 
Oahu and Morotoi ; and this, as I have been informed by 
Mr. Ellis, on part at least of the shore, is of coral-forma- 
tion : the reef does not leave a deep channel within ; red.— 
O?ieehow ; Mr. Ellis believes that this island is also fringed 
by a coral-reef: considering its close proximity to the other 
islands, I have ventured to colour it red. I have in vain 
consulted the works of Cook, Vancouver, La Peyrouse, 
and Lisiansky, for any satisfactory account of the small 
islands and reefs, which lie scattered in a N.W. line pro- 
longed from the Sandwich group, and hence have left them 
uncoloured, with one exception ; for I am indebted to Mr. 
F. D. Bennett for informing me of an atoll-formed reef, in 
lat. 28 22', long. 178 30' W., on which the Gledstanes was 
wrecked in 1837. It is apparently of large size, and 
extends in a N.W. and S.E. line: very few islets have been 
formed on it The lagoon seems to be shallow; at least, 
the deepest part which was surveyed was only three 
fathoms. Mr. Couthouy {Remarks, p. 38) describes this 
isld. under the name of Ocean Isld. Considerable doubts 
should be entertained regarding the nature of a reef of this 



APPENDIX. 199 

kind, with a very shallow lagoon, and standing far from any 
other atoll, on account of the possibility of a crater or flat 
bank of rock lying at the proper depth beneath the surface 
of the water, thus affording a foundation for a ring-formed 
coral-reef. I have, however, thought myself compelled, 
from its large size and symmetrical outline, to colour it blue. 
Samoa or .Navigator Group. — Kotzebue, in his second 
voyage, contrasts the structure of these islands with many 
others in the Pacific, in not being furnished with harbours for 
ships, formed by distant coral-reefs. The Rev. J. Williams, 
however, informs me, that coral-reefs do occur in irregular 
patches on the shores of these islands ; but that they do 
not form a continuous band, as round Mangaia, and other 
such perfect cases of fringed islands. From the charts 
accompanying La Peyrouse's voyage, it appears that the 
north shore of Savaii Maouna, Orosenga i 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 to all the islands which he visited. 
Mr. Williams in his Narrative, speaks of a reef going round 
a small island attached to Oyo/ava, and returning again to 
it : all these islands have been coloured red. — A chart of 
Pose 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 (Pemarks, 
p. 43), it consists of a reef, only a league in circuit, sur- 
mounted by a very few low islets ; the lagoon is very 
shallow, and is strewed with numerous large boulders of 
volcanic rock. This island, therefore, probably consists 
of a bank of rock, a few feet submerged, with the outer 



20© APPENDIX. 

margin of its upper surface fringed with reefs; hence it 
cannot be properly classed with atolls, in which the founda- 
tions are always supposed to lie at a depth, greater than 
that at which the reef-constructing polypifers can live ; not 
coloured. 

Beveridge Reef, 20 S., 167 W., is described in the 
Nairt. Mag. (May 1833, p. 442) as ten miles long in a 
N. and S. line, and eight wide ; " in the inside of the reef 
there appears deep water ; " there is a passage near the 
S.W. corner: this therefore seems to be a submerged atoll, 
and is coloured blue. 

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

Friendly Arch. — Pylstaart Isld. : judging from the 
chart in Freycinet's At/as, I should have supposed that it 
had been regularly fringed 3 but as nothing is said in the 
Hydrog. Memoir (or in the voyage of Tasman, the dis- 
coverer) about coral-reefs, I have left it uncoloured. — 
Tongatabou: In the atlas of the voyage of the Astrolabe, 
the whole south side of the island is represented as narrowly 
fringed by the same reef which forms an extensive platform 
on the northern side. The origin of this latter reef, which 
might have been mistaken for a barrier-reef, has already been 
attempted to be explained, when giving the proofs of the 
recent elevation of this island. — In Cook's charts the little 
outlying island also of Eoaigee, is represented as fringed; 
coloured red. — Eoua. I cannot make out from Capt. 
Cook's charts and descriptions, that this island has any 
reef, although the bottom of the neighbouring sea seems 



APPENDIX. 201 

to be corally, and the island itself is formed of coral-rock. 
Forster, however, distinctly {Observations ; p. 14) classes 
it with 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. 
The several islands lying N.W. of Tongatabou, namely, 
Anamouka, Ko?nango, Kotou, Lefouga, Foa, etc., are seen 
in Capt. Cook's chart to be fringed by reefs, and several 
of them are connected together. From the various state- 
ments in the first volume of Cook's third voyage, and 
especially in the fourth and sixth chapters, it appears that 
these reefs are of coral-formation, and certainly do not 
belong to the barrier class ; coloured red. — Toufoa and 
Kao, forming the western part of the group, according to 
Forster have no reefs ; the former is an active volcano. — 
Vavao. There is a chart of this singularly formed island, 
by Espinoza : according to Mr. Williams it consists of 
coral-rock : the Chevalier Dillon informs me that it is not 
fringed ; not coloured. Nor are the islands of Latte and 
Amargura, for I have not seen plans on a large scale of 
them, and do not know whether they are fringed. 

Niouha, 16 S., 174 W., or Keppel Island of Wallis, or 
Cocos Isld. From a view and chart of this island given 
in Wallis's Voyage (4to edit.) it is evidently encircled by a 
reef; coloured blue : it is however remarkable that Boscawen 
Island, immediately adjoining, has no reef of any kind; 
uncoloured. 

Wallis Island, 13 S., 17 6° 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; blue. 



202 APPENDIX. 

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

Fidji or Viti Group. — The best chart of the numerous 
islands of this group, will be found in the Atlas of the 
Astrolabe's Voyage. From this, and from the description 
given in the Hydrog. Memoir, accompanying it, it appears 
that many of these islands are bold and mountainous, rising 
to the height of between 3,000 and 4,000 feet. Most of the 
islands are surrounded by reefs, lying far from the land, 
and outside of which the ocean appears very deep. The 
Astrolabe sounded with 90 fathoms in several places about 
a mile from the reefs, and found no bottom. Although the 
depth within the reef is not laid down, it is evident from 
several expressions, that Capt. D'Urville believes that ships 
could anchor within, if passages existed through the outer 
barriers. The Chev. Dillon informs me that this is the 
case : hence I have coloured this group blue. In the S.E. 
part lies Batoa, or Turtle Island of Cook {2nd Voyage, 
vol. ii. p. 23, 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 unfathomable; coloured pale blue. At the 
distance of a few miles, Capt. Cook {ibid. p. 24) found a 
circular coral-reef, four or five leagues in circuit, with deep 
water within ; " in short, the bank wants only a few little 
islets to make it exactly like one of the half-drowned isles 
so often mentioned," — namely, atolls. South of Batoa, lies 
the high island of Ono, which appears in Bellinghausen's 
atlas to be encircled ; as do some other small islands to the 
south ; coloured pale blue : near Ono, there is an annular 
reef, quite similar to the one just described in the words of 
Capt. Cook; coloured dark blue. 



APPENDIX. 203 

Rotoumah) 13 S., 179 E. — From the chart in Duperrey's 
atlas, I thought this isl. was encircled, and had coloured it 
blue, but the Chev. Dillon assures me that the reef is only 
a shore or fringing one ; red. 

Independence Isl., io° S., 179 E., is described by Mr. G. 
Bennett {United Service Jonrn., 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 y Peyster, and Ellice Islds. are 
figured in Arrowsmith's chart of the Pacific (corrected to 
1832) as atolls, and are said to be very low; blue. — 
Nederlandisch Isld. I am greatly indebted to the kindness 
of Admiral Krusenstern, for sending me the original 
documents concerning this island. From the plans given 
by Capts. 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. Capt. Eeg compares the 
lagoon with that of other coral-islands; and he distinctly 
says, the land is "very low." I have therefore coloured 
it blue. Admiral Krusenstern {Memoir on the Pacific^ 
Append., 1835) states that its shores are 80 feet high; this 
probably arose from the height of the cocoa-nut trees, with 
which it is covered, being mistaken for land. — Gran Cocal 
is said in Krusenstern's Memoir to be low, and to be 
surrounded by a reef; it is small, and therefore probably 
once contained a lagoon; uncoloured. — St. Augustin. 
From a chart and view of it, given in the Atlas of the 
Coquiltes Voyage^ it appears to be a small atoll, with its 
lagoon partly filled up ; coloured blue. 



204 APPENDIX. 

Gilbert Group. — The chart of this group, given in the 
Atlas of the Coquille's Voyage, at once shows that it is 
composed of ten well-characterised atolls. In D'Urville 
and Lottin's chart, Syde?iham is written with a capital letter, 
signifying that it is high ; but this certainly is not the 
case, for it is a perfectly characterised atoll, and a sketch, 
showing how low it is, is given in the Coquille's Atlas. 
Some narrow strip-like reefs project from the southern side 
of Drwmnond 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 (Geografih. Journ., vol. vii. p. 229) 
a low extensive island in nearly the same latitude, about 
three degrees westward of the longitude assigned to 
Rotches, but very probably it is the same island. Mr. 
Bennett informs me that the man at the masthead reported 
an appearance of lagoon-water in the centre ; and, therefore, 
considering its position, I have coloured it blue. — Pitt 
Isld., at the extreme northern point of the group, is left 
uncoloured, as its exact position and nature is not known. 
— Byron Isld., 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 Islds. all lie considerably 
to the west of the Gilbert group : I have been unable to find 
any distinct account of them. Ocean Island is written with 
small letters in the French chart, but in Krusenstern's 
Memoir it is said to be high. 

Marshall Group. — We are well acquainted with this 
group from the excellent charts of the separate islands, made 
during the two voyages of Kotzebue : a reduced one of the 
whole group may be easily seen in Krusenstern's Atlas, and 



APPENDIX. 205 

in Kotzebue's Seco?id Voyage. The group consists (with the 
exception of two little islands which probably have had 
their lagoon filled up) of a double row of 23 large and 
well-characterised atolls, from the examination of which 
Chamisso has given us his well-known account of coral- 
formations. I include Gaspar-Pico, or Cornwallis Isld. in 
this group, which is described by Chamisso (Kotzebue's 
First Voyage, vol. iii. p. 179) "as a low sickle-formed group, 
with mould only on the windward side." Gaspard Island 
is considered by some geographers as a distinct island lying 
N.E. of the group, but it is not entered in the chart by 
Krusenstern; left uncoloured. In the S.W. part of this 
group lies Baring Island, of which little is known (see 
Krusenstern's Appendix, 1835, P- J 49)- * have left it un- 
coloured; but Boston Isld. I have coloured blue, as it is 
described {ibid.) as consisting of 14 small islands, which, no 
doubt, enclose a lagoon, as represented in a chart in the 
Coqiiille^s Atlas. — Two islands, Aur Kawen and Gaspar 
Rico, are written in the French chart with capital letters ; 
but this is an error, for from the account given by Chamisso 
in Kotzebue's First Voyage, they are certainly low. The 
nature, position, and even existence, of the shoals and small 
islands north of the Marshall group, are doubtful. 

New Hebrides. — Any chart, on even a small scale, of 
these islands, will show that their shores are almost without 
reefs, presenting a remarkable contrast with those of New 
Caledonia on the one hand, and the Fidji group on the 
other. Nevertheless, I have been assured by Mr. G. 
Bennett, that coral grows vigorously on their shores; as 
indeed, will be further shown in some of the following 
notices. As, therefore, these islands are not encircled, and 
as coral grows vigorously on their shores, we might almost 
conclude, without further evidence, that they were fringed, 



206 APPENDIX. 

and hence I have applied the red colour with rather greater 
freedom than in other instances. — Matthew's Pock, an active 
volcano, some way south of the group, (of which a plan is 
given in the Atlas of the Astrolabe's Voyage), does not 
appear to have reefs of any kind about it. — Annatom, the 
southernmost of the Hebrides ; from a rough woodcut 
given in the United Service Journal (1831, part iii. p. 
190), accompanying 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 madrepores; and the younger Forster, 
in his account (vol. ii. p. 269) speaking of the harbour 
says, the whole S.E. side consists of coral-reefs, which are 
overflowed at high-water; part of the southern shore in 
Cook's chart is represented as fringed ; coloured red. — 
Immer is described {United Service Journ., 1831, part iii. 
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. — Errontango Isld. ; Cook 
(2nd Voyage, vol. ii. p. 45, 4to edit.) speaks of rocks every- 
where lining the coast, and the natives offered to haul his 
boat over the breakers to the sandy beach : Mr. Bennett, in 
a letter to the editor of the Singapore Chron., alludes to 
the reefs on its shores. It may, I think, be safely inferred 
from these passages that the shore is fringed in parts by 
coral-reefs; coloured red. — Sandwich Isld., the east coast 
is said (Cook's 2?id Voyage, vol. ii. 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 



APPENDIX. 207 

reef is about thirty 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 repre- 
sented as fringed ; coloured red. — Aurora and Pentecost 
Islds., according to Bougainville, apparently have no reefs; 
nor has the large isld. of S. Espiritu, nor Bligh Isld. or 
Banks' Isld., which latter lie to the N.E. of the Hebrides. 
But in none of these cases have I met with any detailed 
account of their shores, or seen plans on a large scale ; and 
it will be evident, that a fringing-reef of only thirty or even 
a few hundred yards in width, is of so little importance to 
navigation, that it will seldom be noticed, excepting by 
chance; and hence I do not doubt that several of these 
islands, now left uncoloured, ought to be red. 

Santa-Cruz Group. — Vanikoro (Fig. 1, PL 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 isld., 
surrounded by cliffs {Astrolabe Hydrog., and Krusenstern, 
Mem., vol. ii. p. 432) ; these are uncoloured. Toupoua 
{Otooboa of Dillon) is stated by Capt. Tromelin {Annates 
Marit., 1829, p. 289) to be almost entirely included in a 
reef, lying at the distance of two miles from the shore. 
There is a space of three miles without any reef, which, 
although indented with bays, offers no anchorage from the 
extreme depth of the water close to the shore : Capt. 
Dillon also speaks of the reefs fronting this island; 
coloured blue. — Santa-Cruz. I have carefully examined 
the works of Carteret, D'Entrecasteaux, Wilson, and 
Tromelin, and I cannot discover any mention of reefs on 



208 APPENDIX. 

its shores; left uncoloured. — Tinakoro is a constantly 
active volcano without reefs. — Mendana Isles (mentioned 
by Dillon under the name of Mammee, etc.) ; said by 
Krusenstern to be low, and intertwined with reefs. I do 
not believe they include a lagoon ; I have left them un- 
coloured. — Duff's Islds. compose a small group directed in 
a N.W. and S.E. band; they are described by Wilson 
(p. 296, Miss. Voy.y 4to edit.), as formed by bold-peaked 
land, with the islands surrounded by coral-reefs, extending 
about half a mile from the shore ; at a distance of a mile 
from the reefs he found only seven fathoms. As I have no 
reason for supposing there is deep water within these reefs, 
I have coloured them red. Kennedy Isld., N.E. of Duff's; 
I have been unable to find any account of it. 

New Caledonia. — The great barrier-reefs on the shores 
of this island have already been described (Fig. 3, Plate 
III.). They have been visited by Labillardiere, Cook, and 
the northern point by D'Urville ; this latter part so closely 
resembles an atoll that I have coloured it dark blue. The 
Loyalty group is situated eastward of this island; from 
the chart and description given in the voyage of the 
Astrolabe^ they do not appear to have any reefs; north 
of this group, there are some extensive low reefs (called 
Astrolabe and Beaupre) which do not seem to be atoll- 
formed ; these are left uncoloured. 

Australian Barrier-Reef. — The limits of this great 
reef, which has already been described, have been coloured 
from the charts of Flinders and King. In the northern 
parts, an atoll-formed reef, lying outside the barrier, has 
been described by Bligh, and is coloured dark blue. In 
the space between Australia and New Caledonia, called 
by Flinders the Corallian Sea, there are numerous reefs. 
Of these, some are represented in Krusenstern's atlas as 



APPENDIX. 209 

having an atoll-like structure; namely, Bampton shoal, 
Frederic^ Vine or Horse-shoe, and Alert reefs; these have 
been coloured dark blue. 

Louisiade ; the dangerous reefs which front and surround 
the western, southern, and northern coasts of this so-called 
peninsula and archipelago, seem evidently to belong to the 
barrier class. The land is lofty, with a low fringe on the 
coast; the reefs are distant, and the sea outside them 
profoundly deep. Nearly all that is known of this group 
is derived from the labours of D'Entrecasteaux and Bou- 
gainville : the latter has represented one continuous reef 
ninety miles long, parallel to the shore, and in places as 
much as ten miles from it; coloured pale blue. A little 
distance northward we have the Laughlan Islds., the reefs 
round which are engraved in the atlas of the voyage of the 
Astrolabe^ in the same manner as in the encircled islands 
of the Caroline Arch., the reef is, in parts, a mile and a 
half from the shore, to which it does not appear to be 
attached ; coloured blue. At some little distance from the 
extremity of the Louisiade lies the Wells reef, described 
in G. Hamilton's Voyage in If. M.S. Pandora (p. 100): it 
is said, "We found we had got embayed in a double reef, 
which will soon be an island." As this statement is only 
intelligible on the supposition of the reef being crescent or 
horse-shoe formed, like so many other submerged annular 
reefs, I have ventured to colour it blue. 

Salomon Archipelago : the chart in Krusenstern's 
atlas shows that these islands are not encircled, and as 
coral appears from the works of Surville, Bougainville, and 
Labillardiere, to grow on their shores, this circumstance, 
as in the case of the New Hebrides, is a presumption 
that they are fringed. I cannot find out anything from 
D'Entrecasteaux's Voyage^ regarding the southern islds. of 

879 



210 APPENDIX. 

the group, so have left them uncoloured. — Malayta Isld. in 
a rough MS. chart in the Admiralty has its northern shore 
fringed. — Ysabel Isld., the N.E. part of this island, in the 
same chart, is also fringed : Mendana, speaking (Burney, 
vol. i. p. 280) of an islet adjoining the northern coast, says 
it is surrounded by reefs ; the shores, also, of Port Praslin 
appear regularly fringed. — Choiseul Isld. ; in Bougainville's 
Chart of Choiseul Pay, parts of the shores are fringed by coral- 
reefs. — Bougainville Isld. ; according to D'Entrecasteaux the 
western shore abounds with coral-reefs, and the smaller 
islands are said to be attached to the larger ones by reefs ; 
all the before-mentioned islands have been coloured red. 
Bouka Islds. ; Capt. Duperrey has kindly informed me in a 
letter that he passed close round the northern side of this 
island (of which a plan is given in his atlas of the Coquiltes 
voyage), and that it was " garnie d'une bande de recifs a 
fleur d'eau adherentes au rivage ; " and he infers, from the 
abundance of coral on the islands north and south of 
Bouka, that the reef probably is of coral ; coloured red. 

Off the north coast of the Salomon Arch, there are 
several small groups which are little known ; they appear 
to be low, and of coral-formation ; and some of them 
probably have an atoll-like structure ; the Chev. Dillon, 
however, informs me that this is not the case with the B. de 
Candelaria. — Outong Java, according to the Spanish navi- 
gator, Maurelle, is thus characterised ; but this is the only 
one which I have ventured to colour blue. 

New Ireland. — The shores of the S.W. point of this 
island and some adjoining islets, are fringed by reefs, as 
may be seen in the atlases of the voyages of the Coquille 
and Astrolabe. M. Lesson observes that the reefs are open 
in front of each streamlet. The Duke of York's Isld. is 
also fringed ; but with regard to the other parts of New 



APPENDIX. 211 

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 speci- 
fied portions. 

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

Admiralty Group. — From the accounts by Bougainville, 
Maurelle, D'Entrecasteaux, and the scattered notices col- 
lected by Horsburgh, it appears, that some of the many 
islands composing it are high, with a bold outline; and 
others are very low, small and interlaced with reefs. All 
the high islands appear to be fronted by distant reefs rising 
abruptly from the sea, and within some of which there is 
reason to believe that the water is deep. I have therefore 
little doubt they are of the barrier class. — In the southern 
part of the group we have Elizabeth isld., which is sur- 
rounded 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 diameter, having deep water inside 
with an opening at the N.N.W. part, and on the outside 
steep too." I have from these data, coloured the group pale 
blue, and circular-reef dark blue. — The Anachorites, Eche- 
quier, and Hermites, consist of innumerable low islands of 
coral-formation, which probably have atoll-like forms ; but 
not being able to ascertain this, I have not coloured 



212 APPENDIX. 

them, nor Durour is/d, which is described by Carteret 
as low. 

The Caroline Arch, is now well known, chiefly from 
the hydrographical labours of Lutke ; it contains about forty 
groups of atolls, and three encircled islands, two of which 
are engraved in Fig. 3, Plate L, and Fig. 3, Plate II. Com- 
mencing with the eastern part; the encircling reef round 
Ualen appears to be only about half a mile from the shore ; 
but as the land is low and covered with mangroves ( Voyage 
autour du Monde, par F. Lutke, voL i. p. 339), the real 
margin has not probably been ascertained. The extreme 
depth in one of the harbours within the reef is thirty-three 
fathoms (see charts in atlas of Coquille's voyage), and out- 
side at half a mile distant from the reef, no bottom was 
obtained with 250 fathoms. The reef is surmounted by 
many islets, and the lagoon-like channel within is mostly 
shallow, and appears to have been much encroached on by 
the low land surrounding the central mountains ; these 
facts show that time has allowed much detritus to accu- 
mulate ; coloured pale blue. — Pouynipete, or Seniavine. In 
the greater part of the circumference of this island, the reef 
is about one mile and three quarters distant ; on the north 
side it is five miles off the included high islets. The reef 
is broken in several places ; and just within it, the depth 
in one place is 30 fathoms, and in another, 28, beyond 
which, to all appearance, there was " un porte vaste et sur " 
(Lutkd, vol. ii. p. 4); coloured pale blue. — Hogoleu 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 Astrolabe, and the large accompanying 
chart taken chiefly from that given by Duperrey) are high, 
and the rest are all small, low, and formed on the reef. 



APPENDIX. 213 

The depth of the great interior lake has not been ascer- 
tained ; but Captain D'Urville appears to have entertained 
no doubt about the possibility of taking in a frigate. The 
reef lies no less than fourteen miles distant from the 
northern coasts of the interior high islds., seven from their 
western sides, and twenty from the southern ; the sea is 
deep outside. This island is a likeness on a grand scale 
to the Gambier group in the Low Archipelago. Of the 
groups of low 1 islands forming the chief part of the 
Caroline Archipelago, all those of larger size, have the true 
atoll-structure (as may be seen in the atlas by Captain 
Lutke), and some even of the very small ones, as 
Macaskill and Duperrey, of which plans are given in the 
atlas of the Coquiitts voyage. There are, however, some 
low small islands of coral-formation, namely, Ollap, Tama- 
tam, Bigali, Satahoual, which do not contain lagoons; 
but it is probable that lagoons originally existed, but have 
since filled up : Lutke (vol. ii. p. 304) seems to have 
thought that all the low islands, with only one exception, 
contained lagoons. From the sketches, and from the 
manner in which the margins of these islands are engraved 
in the atlas of the voyage of the Coquille, it might have 
been thought that they were not low ; but by a comparison 
with the remarks of Lutke* (vol. ii. p. 107, regarding Bigali) 
and of Freycinet (ffydrog. Memoir EUranie Voyage, p. 188, 
regarding Tamatam, Ollap, etc.), it will be seen that the 
artist must have represented the land incorrectly. The 
most southern isld. in the group, namely Piguiram, is not 
coloured, because I have found no account of it. Nougouor, 
or Monte Verdison, which was not visited by Lutkd, is 

1 In D'Urville and Lottin's chart, Peserare is written with capital 
letters ; but this evidently is an error, for it is one of the low islets on 
the reef of Namonouyto (see Lutk£'s charts), — a regular atoll. 



214 APPENDIX. 

described and figured by Mr. Bennett {United Service 
Journal) Jan. 1832) as an atoll. All the above-mentioned 
islands have been coloured blue. 

Western Part of the Caroline Archipelago. — 
Fats Island is ninety feet high, and is surrounded, as I 
have been informed by Admiral Lutke, by a narrow reef 
of living coral, of which the broadest part, as represented 
in the charts, is only 150 yards; coloured red. — Philip 
Isld., I believe, is low; but Hunter, in his Historical 
Journal^ gives no clear account of it; uncoloured. — Elivi ; 
from the manner in which the islets on the reefs are 
engraved, in the atlas of the Astrolabe's voyage, I should 
have thought they were above the ordinary height, but 
Admiral Lutke assures me this is not the case : they form 
a regular atoll; coloured blue. — Gouap (Eap of Chamisso) 
is a high island with a reef (see chart in Voy. of Astrolabe), 
more than a mile distant in most parts from the shore, 
and two miles in one part. Capt. D'Urville thinks that 
there would be anchorage {Hydrog. Descript. Astrolabe 
Voyage, p. 436) for ships within the reef, if a passage could 
be found; coloured pale blue. — Goulou, from the chart 
in the Astrolabe's atlas, appears to be an atoll. D'Urville 
(Hydrog. Descript., p. 437) speaks of the low islets on 
the reef; coloured dark blue. 

Pelew Islds. — Krusenstern speaks of some of the 
islands being mountainous ; the reefs are distant from the 
shore, and there are spaces within them, and not opposite 
valleys, with from ten to fifteen fathoms. According to 
a MS. chart of the group by Lieut. Elmer in the Admiralty, 
there is a large space within the reef with deepish water ; 
although the high land does not hold a central position 
with respect to the reefs, as is generally the case, I have 
little doubt that the reefs of the Pelew Islands ought to 



APPENDIX. 215 

be ranked with the barrier class, and I have coloured them 
pale blue. In Lieut. Elmer's chart there is a horse-shoe- 
formed shoal, laid down thirteen miles N.W. of Pelew, 
with fifteen fathoms within the reef, and some dry banks 
on it; coloured dark blue. — Spanish^ Martires, Sanserot, 
Pulo Anna y and Mariere Islands are not coloured, because 
I know nothing about them, excepting that according to 
Krusenstern, the second, third, and fourth mentioned, are 
low, placed on coral-reefs, and therefore, perhaps, contain 
lagoons ; but Pulo Mariere is a little higher. 

Mariana Archipelago, or Ladrones. — Guahan. 
Almost the whole of this island is fringed by reefs, which 
extend in most parts about a third of a mile from the land. 
Even where the reefs are most extensive, the water within 
them is shallow. In several parts there is a navigable 
channel for boats and canoes within the reefs. In 
Freycinet's Hydrog. Mem. there is an account of these reefs, 
and in the atlas, a map on a large scale ; coloured red. — 
Rota. " L'ile est presque entierement entourde des re'cifs" 
(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 likewise the western 
shores appear to be fringed ; but there is a great, irregular 
horn-like reef projecting far from this side ; coloured red. 
— Farallon de Medinilla appears so regularly and closely 
fringed in Freycinet's charts, that I have ventured to 
colour it red, although nothing is said about reefs in the 
Hydrographical Memoir. The several islands which form 
the northern part of the group are volcanic (with the 
exception perhaps of Torres, which resembles in form the 
madreporitic island of Medinilla), and appear to be without 



216 APPENDIX. 

reefs. — Jllangs, however, is described (by Freycinet, p. 219, 
Hydrog.) from some Spanish charts, as formed of small 
islands placed " aii milieu des nombreux recifs ; " and as 
these reefs in the general chart of the group do not project 
so much as a mile; and as there is no appearance from a 
double line, of the existence of deep water within, I have 
ventured, although with much hesitation, to colour them 
red. Respecting Folger and Marshall Islds. which lie 
some way east of the Marianas, I can find out nothing, 
excepting that they are probably low. Krusenstern says 
this of Marshall Isld. ; and Folger Isld. is written with 
small letters in D'Urville's chart ; uncoloured. 

Bonin or Arzobispo Group. — Peel Isld. has been 
examined by Capt. 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." Capt. Beechey, in another part of 
his letter to me, alludes to the reefs fringing the island in 
all directions ; but at the same time it must be observed 
that the surf washes the volcanic rocks of the coast in the 
greater part of its circumference. I do not know whether 
the other islands of the Archipelago are fringed; I have 
coloured Peel Isld. red. — Gra?nfius Isld. to the eastward, 
does not appear (Meare's Voyage, p. 95) to have any reefs, 
nor does Rosario Isld. (from Lutke's chart), which lies to 
the westward. Respecting the few other islds. in this part 
of the sea, namely the. - Sulphur Islds. , with an active 
volcano, and those lying between Bonin and Japan (which 
are situated near the extreme limit in latitude, at which reefs 
are formed), I have not been able to find any clear account. 

West End of New Guinea. — Port Dory. From the 
charts in the Voyage of the Coquille, it would appear that 



APPENDIX. 217 

the coast in this part is fringed by coral-reefs ; M. Lesson, 
however, remarks that the coral is sickly ; coloured red. — 
Waigiou. A considerable portion of the northern shores 
of these islands is seen in the charts (on a large scale) in 
Freycinet's Atlas to be fringed by coral-reefs. Forrest 
(p. 21, Voyage to New Guinea) alludes to the coral-reefs 
lining the heads of Piapis Bay ; and Horsburgh (vol. ii. p. 
599, 4th 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 to 
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 53' N., are described as circular, 
with stony points showing all round, with deeper water in 
the middle; coloured blue. — Aiou; the plan of this group, 
given in the atlas of the voyage of the Astrolabe, shows 
that it is an atoll ; and, from a chart in Forrest's Voyage, it 
appears that there is twelve fathoms within the circular 
reef; coloured blue. — The S.W. coast of New Guinea 
appears to be low, muddy, and devoid of reefs. The Arru, 
Timor-laut, and Tenimber groups have lately been examined 
by Capt. Kolff, the MS. translation of which, by Mr. W. 
Earl, I have been permitted to read, through the kindness 
of Capt. 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 



218 APPENDIX. 

to come within the barrier class, and be coloured blue ; but 
I have been forced to leave them uncoloured. — The last- 
mentioned groups are connected with the east end of 
Ceram by a chain of small islands, of which the small 
groups of Ceram-laut, Goram, and Keffi?ig 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 Araficra Sea, p. 9). 

Ceram. — In various charts which I have examined, 
several parts of the coast are represented as fringed by 
reefs. — Manipa Island, between Ceram and Bourou, in an 
old MS. chart in the Admiralty, is fringed by a very 
irregular reef, partly dry at low water, which I do not doubt 
is of coral-formation ; both islands coloured red. — Bourou; 
parts of this island appear fringed by coral-reefs, namely, 
the eastern coast, as seen in Freycinet's chart; and Cajeli 
Bay, which is said by Horsburgh (vol. ii. p. 630) to be 
lined by coral-reefs, that stretch out a little way, and have 
only a few feet water on them. In several charts, portions of 
the islands forming the Amboina Group are fringed by reefs; 
for instance, Koessa, Harenca, and Ucaster, in Freycinet's 
charts. The above-mentioned islands have been coloured 
red, although the evidence is not very satisfactory. — North 
of Bourou the parallel line of the Xulla Isles extends : I have 
not been able to find out anything about them, excepting 
that Horsburgh (vol. ii. p. 543) says that the northern shore 
is surrounded by a reef at the distance of two or three miles ; 
uncoloured. — My sol Group ; the Kanary Islands are said 
by Forrest (Voyage, p. 130) to be divided from each other 
by deep straits, and are lined with coral-rocks ; coloured 
red. — Guebe, lying between Waigiou and Gilolo, is engraved 



APPENDIX, 219 

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 
Patientid), 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 cotes (of Batchian) comme 
dans les plupart des iles de cet archipel, il y a de rocs 
de madrepores d'une beaute et d'une variete 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. ii. p. 506) says the 
northern coast is lined by reefs, projecting one or two miles, 
and having no soundings close to them ; I have left it 
uncoloured, although, as in some former cases, it ought 
probably to be pale blue. — Celebes. The western and 
northern coasts appear in the charts to be bold and with- 
out reefs. Near the extreme northern point, however, an 
islet in the Straits of Limbe, and parts of the adjoining 
shore, appear to be fringed : the east side of the bay 
of ManadO) has deep water, and is fringed by sand and 
coral (Astrol. Voyage, Hydrog. Part, pp. 453-4); this 
extreme point, therefore, I have coloured red. — Of the 
islands leading from this point to Magindanao, I have 
not been able to find any account, except of Seranganz] 
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 



220 APPENDIX. 

islands is taken from Capt. D. Kolffs 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." — Mo a has reefs on the S.W. part. — Lakor 
has a reef lining its shore ; these islands are coloured red. — 
Still more eastward, Luan has, differently from the last- 
mentioned islands, an extensive reef; it is steep outside, 
and within there is a depth of twelve feet ; from these facts, 
it is impossible to decide to which class this island belongs. 
— Kissa, off the point of Timor, has its "shore fronted by 
a reef, steep too on the outer side, over which small proahs 
can go at the time of high water;" coloured red. — Timor; 
most of the points, and some considerable spaces of the 
northern shore, are seen in Freycinet's chart to be fringed 
by coral-reefs; and mention is made of them in the 
accompanying Hydrog. Memoir ; coloured red. — Savu, S.E. 
of Timor, appears in Flinders' chart to be fringed; but 
I have not coloured it, as I do not know that the reefs are 
of coraL — Sandalwood Isld. 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 of Australia. — It appears, in Capt. King's 
Sailing Directions (Narrative of Survey, vol. ii. pp. 325-369), 
that there are many extensive coral-reefs skirting, often at 
considerable distances, the N.W. shores, and encompassing 
the small adjoining islets. Deep water, in no instance, is 
represented in the charts between these reefs and the land ; 
and, therefore, they probably belong to the fringing class. 
But as they extend far into the sea, which is generally 
shallow, even in places where the land seems to be some- 
what precipitous ; I have not coloured them. Houtman's 



APPENDIX. 221 

Abrolhos (lat. 2 8° S. on west coast) have lately been 
surveyed by Capt Wickham (as described in Naut. Mag., 
1 84 1, 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 island has been formed on their inland 
sides, by the accumulation of fragments of coral ; the sea- 
ward face consisting of nearly bare ledges of rock. Some 
of the specimens, brought home by Capt. Wickham, con- 
tained fragments of marine shells, but others did not ; and 
these closely resembled a formation at King George's 
Sound, principally due to the action of the wind on 
calcareous dust, which I shall describe in a forthcoming 
part. From the extreme irregularity of these reefs with 
their lagoons, and from their position on a bank, the usual 
depth of which is only 30 fathoms, I have not ventured 
to class them with atolls, and hence have left them un- 
coloured. — Rowley Shoals. These lie some way from the 
N.W. coast of Australia : according to Capt. King (Narra- 
tive of Survey, vol. i. p. 60), they are of coral-formation. 
They rise abruptly from the sea, and Capt. King had no 
bottom with 170 fathoms close to them. Three of them 
are crescent-shaped; they are mentioned by Mr. Lyell, on 
the authority of Capt. King, with reference to the direction 
of their open sides. " A third oval reef of the same group 
is entirely submerged " (Principles of Geo log., book iii. 
chap, xviii.); coloured blue. — Scoffs Reefs, lying north of 
Rowley Shoals, are briefly described by Capt. Wickham 
(Naut. Mag., 1841, p. 440) : they appear to be of great 
size, of a circular form, and " with smooth water within, 
forming probably a lagoon of great extent," There is a break 



222 APPENDIX. 

on the western side, where there probably is an entrance : 
the water is very deep off these reefs ; coloured blue. 

Proceeding westward along the great volcanic chain of 
the East Indian Archipelago, Solor Strait is represented in 
a chart published by Dalrymple from a Dutch MS., as 
fringed ; as are parts of Plores, of Adenara, and of Solor. 
Horsburgh speaks of coral growing on these shores ; and 
therefore I have no doubt that the reefs are of coral, and 
accordingly have coloured them red. We hear from 
Horsburgh (vol. ii. p. 602) that a coral-flat bounds the 
shores of Sapy Bay. From the same authority it appears 
(p. 610) that reefs fringe the island of Timor-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 a distance of a hundred fathoms, with 
channels through it for boats ; these places, therefore, have 
been coloured red. — Bally Isld. In a Dutch MS. chart on 
a large scale of Java, which was brought from that island 
by Dr. Horsfield, who had the kindness to show it me at 
the India House, its western, northern, and southern 
shores appear very regularly fringed by a reef (see also 
Horsburgh, vol. ii. p. 593) ; and as coral is found abund- 
antly there, I have not the least doubt that the reef is of 
coral, and therefore have coloured it red. 

Java. — My information regarding the reefs of this great 
island is derived from the chart just mentioned. The 
greater part of Madura is represented in it as regularly 
fringed, and likewise portions of the coast of Java imme- 
diately south of it. Dr. Horsfield informs me that coral 
is very abundant near Sourabaya. The islets and parts of 
the N. coast of Java, west of Point Buang, or Japara, are 
fringed by reefs, said to be of coral. Lubeck, or Bavian 
Islands, lying at some distance from the shore of Java, 



APPENDIX. 223 

are regularly fringed by coral-reefs. Carimon Java appears 
equally so, though it is not directly said that the reefs are 
of coral j there is a depth between 30 and 40 fathoms 
round these islands. Parts of the shores of Sunda Sir., 
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. 
portions are likewise fringed by coral-reefs ; all the above- 
mentioned places coloured red. 

Macassar Sir. ; the east coast of Borneo appears, in most 
parts, free from reefs, and where they occur, as on the east 
coast of Pamaroong, the sea is very shallow ; hence no part 
is coloured. In Macassar Str., itself, 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 fifty 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 surveys, in a manner which might 
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 uncoloured : at the same 
time I may remark, that these two shoals make a nearer 
approach to the atoll-like structure than any other within 
the E. Indian Arch. Southward of these shoals there are 
other low islands and irregular coral-reefs ; and in the space 
of sea, north of the great volcanic chain, from Timor to 
Java, we have also other islands, such as the Postillions, 
Kalatoa, Tokan-Bessees, etc., which are chiefly low, and are 



224 APPENDIX. 

surrounded by very irregular and distant reefs. From the 
imperfect charts I have seen, I have not been able to 
decide whether they belong to the atoll or barrier-classes, 
or whether they merely fringe submarine banks, and gently 
sloping land. In the Bay of Bonin, between the two 
southern arms of Celebes, there are numerous coral-reefs ; 
but none of them seem to have an atoll-like structure. I 
have, therefore, not coloured any of the islands in this part 
of the sea ; I think it, however, exceedingly probable that 
some of them ought to be blue. I may add that there is a 
harbour on the S.E. coast of Bonton which, according to an 
old chart, is formed by a reef, parallel to the shore, with 
deep water within ■ and in the voyage of the Coquille, some 
neighbouring islands are represented with reefs a good way 
distant, but I do not know whether with deep water within. 
I have not thought the evidence sufficient to permit me to 
colour them. 

Sumatra. — Commencing with the west coast and outlying 
islands, Engano Isld. is represented in the published chart 
as surrounded by a narrow reef, and Napier, in his sailing 
directions, speaks of the reef being of coral (also Horsburgh, 
vol. ii. p. 115) j coloured red. — Rat Isld. (3 51' S.) is sur- 
rounded by reefs of coral, partly dry at low water (Hors- 
burgh, 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 con- 
sists 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. 3 ). In 
an old chart it is said there are chasms in the reefs round 
the island, admitting boats to the watering-place, and that 
the southern islet consists of a mass of sand and coral. — 
Pulo Pisang ; Horsburgh (vol. ii. p. 86) says that the rocky 



APPENDIX. 225 

coral-bank, which stretches about forty yards from the shore, 
is steep to all round : in a chart, also, which I have seen, 
the island is represented as regularly fringed. — Pnlo Mintao 
is lined with reefs on its west side (Horsburgh, vol. ii. 
p. 107). — Pnlo Baniak ; the same authority (vol. ii. p. 105), 
speaking of a part, says it is faced with coral-rocks. — 
Minguin (3 36' N.). A coral-reef fronts this place, and 
projects into the sea nearly a quarter of a mile (Notices of 
the India?i Arch., published at Singapore, p. 105). — Pulo 
Brassa (5 46' N.). A reef 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 these nowhere have the structure of 
a barrier-reef, and Marsden (History of Sumatra) states, 
that where the coast is flat, the fringing-reefs extend furthest 
from it. The northern and southern points, and the greater 
part of the east coast, are low, and faced with mud banks, 
and therefore without coral. 

Nicobar Islands. — The chart represents the islands of 
this group as fringed by reefs. With regard to Great 
Nicobar, Capt. Moresby informs me, that it is 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 Islands. — From an examination of the MS. 
chart, on a large scale, of this island, by Capt. 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 

880 



226 APPENDIX. 

Researches (vol. iv. p. 402) led me to doubt the existence of 
reefs ; uncoloured. 

The coast of Malacca, Tenasserim, 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 coloured them. In the sea, how- 
ever, 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 Islds. 
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 Islds. (i° N.) are represented in the English charts as 
surrounded by a very regular fringe. — St. Barbes (o° 15' N.) 
is said by Horsburgh (vol. ii. 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 also fronted by a reef, 
extending not far from the land (Horsburgh, vol. ii. 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 Str., Lucepara, and Carimata; 
but as the sea is confined and shallow, and the reefs not 
very regular, I have left them uncoloured. 

The water shoals gradually towards the whole west coast 
of Borneo : I cannot make out that it has any reefs of coral. 
The islands, however, off the northern extremity, and near 
the S.W. end of Palawan, are fringed by very distant coral- 
reefs ; thus the reefs in the case of Balabac are no less than 
five miles from the land; but the sea, in the whole of this 
district, is so shallow, that the reefs might be expected to 
extend very far from the land. I have not, therefore, 



APPENDIX. 227 

thought myself authorised to colour them. The N.E. 
point of Borneo, where the water is very shoal, is con- 
nected with Magindanao by a chain of islands called the 
Sooloo Archipelago, about which I have been able to obtain 
very little information ; Pangooiaran, 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 sandbank, which forms the 
only dry part; these breakers are steep to outside; there 
appears to be an opening through them on one side, with 
four or five fathoms within: from this description, I 
strongly suspect that Cavilli ought to be considered an 
atoll ; but, as I have not seen any chart of it, on even a 
moderately large scale, I have not coloured it. The islets 
off the northern end of Palawan, are in the same case as 
those off the southern end, namely, they are fringed by 
reefs, some way distant from the shore, but the water is 
exceedingly shallow; uncoloured. The western shore of 
Palawan will be treated of under the head of China Sea. 
Philippine Archipelago. — A chart on a large scale of 



228 APPENDIX. 

Appoo Shoal, which lies near the S.E. coast of Mindoro, 
has been executed by Capt. D. Ross : it appears atoll- 
formed, but with rather an irregular outline; its diameter 
is about ten miles ; there are two well-defined passages 
leading into the interior lagoon, which appears open ; close 
outside the reef all round, there is no bottom with seventy 
fathoms; coloured blue. — Mindoro: the N.W. coast is 
represented in several charts, as fringed by a reef, and 
Luban Isld. is said, by Horsburgh (vol. ii. p. 436), to be 
"lined by a reef." — Luzon: Mr. Cuming, who has lately 
investigated with so much success the Natural History of 
the Philippines, informs me, that about three miles of the 
shore north of Point St. Jago, is fringed by a reef; as are 
(Horsburgh, 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 parts of this coast, namely, near Puebla, Iba, and 
Mansinglor. In the neighbourhood of Solon-solon Bay, 
the shore is lined (Horsburgh, ii. p. 439) by coral-reefs, 
stretching out a great way : there are also reefs about the 
islets off Solamague ; and as I am informed by Mr. Cuming, 
near St. Catalina, and a little north of it. The same 
gentleman informs me there are reefs on the S.E. point 
of this island in front of Samar, extending from Malalabon 
to Bulusan. These appear to be the principal fringing-reefs 
on the coasts of Luzon ; and they have all been coloured 
red. Mr. Cuming informs me that none of them have 
deep water within ; although it appears from Horsburgh 
that some few extend to a considerable distance from the 
shore. Within the Philippine Archipelago, the shores of 
the islands do not appear to be commonly fringed, with 
the exception of the S. shore of Masbate, and nearly the 



APPENDIX. 229 

whole of Bohol; which are both coloured red. On the S. 
shore of Magindanao^ Bunwoot Isld. is surrounded (accord- 
ing to Forrest, Voyage^ p. 253) by a coral-reef, which in the 
chart appears one of the fringing class. With respect to 
the eastern coasts of the whole Archipelago, I have not 
been able to obtain any account. 

Babuyan Islands. — Horsburgh says (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) ; about a mile off shore there is 
between 30 and 35 fathoms. The plan of Port San Pio 
Quinto shows that its shores are fringed with coral ; 
coloured red. — Bashee Islands : Horsburgh, speaking of 
the southern part of the group (vol. ii. 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 most regularly fringed; coloured red. 
The northern islands are left uncoloured, as I have been 
unable to find any account of them. — Formosa. The 
shores, especially the western one, seem chiefly composed 
of mud and sand, and I cannot make out that they are 
anywhere lined by reefs ; except in a harbour (Horsburgh, 
vol. ii. 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 Madjiko-Sima Groups. Patchuson has been described 
by Capt. Broughton {Voy. to the N. Pacific^ p. 191); he 
says, the boats, with some difficulty, found a passage 
through the coral-reefs, which extend along the coast, nearly 
half a mile off it. The boats were well sheltered within the 



2 3 o APPENDIX. 

reef; but it does not appear that the water is deep there. 
Outside the reef the depth is very irregular, varying from 
five to fifty fathoms ; the form of the land is not very 
abrupt; coloured red. — Taypin-san ; from the description 
given (p. 195) by the same author, it appears that a very 
irregular reef extends, to the distance of several miles, from 
the southern island ; but whether it encircles a space of 
deep water is not evident ; nor, indeed, whether these 
outlying reefs are connected with those more immediately 
adjoining the land; left uncoloured. I may here just add 
that the shore of Kumi (lying west of Patchow) has a 
narrow reef attached to it in the plan of it, in La Peyrouse's 
atlas ; but it does not appear in the account of the voyage 
that it is of coral ; uncoloured. — Loo Choo. The greater 
part of the coast of this moderately hilly island is skirted 
by reefs, which do not extend far from the shore, and 
which do not leave a channel of deep water within them, as 
may be seen in the charts accompanying Capt. B. Hall's 
voyage to Loo Choo (see also remarks in Appendix, pp. xxi. 
and xxv.). There are, however, some ports with deep 
water, formed by reefs in front of valleys, in the same 
manner as happens at Mauritius. Capt. Beechey, in a 
letter to me, compares these reefs with those encircling 
the Society Islands ; but there appears to me a marked 
difference between them, in the less distance at which the 
Loo Choo reefs lie from the land with relation to the 
probable submarine inclination, and in the absence of an 
interior deep water-moat or channel, parallel to the land. 
Hence, I have classed these reefs with fringing-reefs, and 
coloured them red. — Pescadores (west of Formosa). 
Dampier (vol. i. p. 416) has compared the appearance of 
the land to the southern parts of England. The islands 
are interlaced with coral-reefs; but as the water is very 



APPENDIX. 231 

shoal, and as spits of sand and gravel (^Horsburgh, vol. ii. 
p. 450) extend far out from them, it is impossible to draw 
any inferences regarding the nature of the reefs. 

China Sea. — Proceeding from north to south, we first 
meet the Pratas Shoal (lat. 20° N.) which, according to 
Horsburgh (vol. ii. p. 335), is composed of coral, is of a 
circular form, and has a low islet on it. The reef is on 
a level with the water's edge, and when the sea runs high, 
there are breakers mostly all round, " but the water within 
seems pretty deep in some places ; although steep-to in 
most parts outside, there appear to be several parts where 
a ship might find anchorage outside the breakers ; " coloured 
blue. — The Paracells have been accurately surveyed by 
Capt. D. Ross, and charts on a large scale published : but few 
low islets have been formed on these shoals, and this seems 
to be a general circumstance in the China Sea; the sea 
close outside the reefs is very deep ; several of them have 
a lagoon-like structure; or separate islets {Prattle, Robert, 
Drummond, etc.) are so arranged round a moderately 
shallow space, as to appear as if they had once formed 
one large atoll. — Bombay Shoal (one of the Paracells) has 
the form of an annular reef, and is "apparently deep with- 
in ; " it seems to have an entrance (Horsburgh, vol. ii. 
p. 332) on its west side ; it is very steep outside. — Discovery 
Shoal, also, is of an oval form, with a lagoon-like space 
within, and three openings leading into it, in which there 
is a depth from two to twenty fathoms. Outside, at the 
distance (Horsburgh, vol. ii. p. 333) of only twenty yards 
from the reef, soundings could not be obtained. The 
Paracells are coloured blue. — Macclesfield Bank: this h 
a coral-bank of great size, lying east of the Paracells ; some 
parts of the bank are level, with a sandy bottom, but, 
generally, the depth is very irregular. Tt is intersected by 



232 APPENDIX. 

deep cuts or channels. I am not able to perceive in the 
published charts (its limits, however, are not very accurately 
known) whether the central part is deeper, which I suspect 
is the case, as in the Great Chagos Bank, in the Indian 
Ocean ; not coloured. — Scarborough Shoal : this coral-shoal 
is engraved with a double row of crosses, forming a circle, 
as if there was deep water within the reef: close outside 
there was no bottom, with a hundred fathoms; coloured 
blue. — The sea off the west coast of Palawan and the 
northern part of Borneo is strewed with shoals : Swallow 
Shoal, according to Horsburgh (vol. ii. p. 431), "is formed, 
like most of the shoals hereabouts, of a belt of coral-rocks, 
with a basin of deeper water within." — PPalf-Moon Shoal 
has a similar structure ; Capt. D. Ross describes it, as a 
narrow belt of coral-rock, " with a basin of deep water in 
the centre," and deep sea close outside. — Bo?nbay Shoal 
appears (Horsburgh, vol. ii. p. 432) "to be a basin of 
smooth water surrounded by breakers." These three shoals 
I have coloured blue. — The Paraquas Shoals are of a 
circular form, with deep gaps running through them; not 
coloured. — A bank gradually shoaling to the depth of 30 
fathoms, extends to a distance of about 20 miles from the 
northern part of Borneo, and to 30 miles from the northern 
part of Pala-wan. Near the land this bank appears 
tolerably free from danger, but a little further out it is 
thickly studded with coral-shoals, which do not generally 
rise quite to the surface ; some of them are very steep-to, 
and others have a fringe of shoal-water round them. I 
should have thought that these shoals had level surfaces, 
had it not been for the statement made by Horsburgh 
" that most of the shoals hereabouts are formed of a belt 
of coral." But, perhaps that expression was more particu- 
larly applied to the shoals further in the offing. If these 



APPENDIX. 233 

reefs of coral have a- lagoon-like structure, they should 
have been coloured blue, and they would have formed an 
imperfect barrier in front of Palawan and the northern 
part of Borneo. But, as the water is not very deep, these 
reefs may have grown up from inequalities on the bank : 
I have not coloured them. — The coast of China, Tonquin, 
and Cochi?i- China, forming the western boundary of the China 
Sea, appear to be without reefs : with regard to the two last- 
mentioned coasts, I speak after examining the charts on a 
large scale in the atlas of the voyage of the Favourite. 

Indian Ocean. — South Keeling atoll has been specially 
described ; nine miles north of it lies North Keeling, a very 
small atoll, surveyed by the Beagle, the lagoon of which is 
dry at low water. — Christmas Island, lying to the east, is a 
high island, without, as I have been informed by a person 
who passed it, any reefs at all. — Ceylon : a space about 
eighty miles in length of the S.-western and southern shores 
of these islands has been described by Mr. Twynam (JVaut. 
Mag., 1836, pp. 365 and 518); parts of this space appear 
to be very regularly fringed by coral-reefs, which extend 
from a quarter to half a mile from the shore. These reefs 
are in places breached, and afford safe anchorage for the 
small trading craft. Outside, the sea gradually deepens; 
there is 40 fathoms about six miles off shore : this part I 
have coloured red. In the published charts of Ceylon there 
appear to be fringing-reefs in several parts of the south- 
eastern shores, which I have also coloured red. — At Venloos 
Bay the shore is likewise fringed. North of Trincomalee 
there are also reefs of the same kind. The sea off the 
northern part of Ceylon is exceedingly shallow ; and there- 
fore I have not coloured the reefs which fringe portions of 
its shores, and the adjoining islets, as well as the Indian 
promontory of Madura. 



234 APPENDIX. 

Chagos, Maldiva, and Laccadive Archipelagoes. — 
These three great groups which have already been often 
noticed, are now well known from the admirable surveys 
of Capt. Moresby and Lieut. Powell. The published charts, 
which are worthy of the most attentive examination, at 
once show that the Chagos and Maldiva groups are entirely 
formed of great atolls, or lagoon-formed reefs, surmounted 
by islets. In the Laccadive group, this structure is less 
evident ; the islets are low, not exceeding the usual height 
of coral formations (see Lieut. Wood's account, Geograph. 
Journ., vol. vi. p. 29), and most of the reefs are circular, as 
may be seen in the published charts ; and within several of 
them, as I am informed by Capt. Moresby, there is deepish 
water; these, therefore, have been coloured blue. Directly 
north, and almost forming part of this group, there is a long, 
narrow, slightly curved bank, rising out of the depths of the 
ocean, composed of sand, shells, and decayed coral, with 
from twenty-three to thirty fathoms on it. I have no doubt 
that it has had the same origin with the other Laccadive 
banks ; but as it does not deepen towards the centre I have 
not coloured it. I might have referred to other authorities 
regarding these three archipelagoes ; but after the publica- 
tion of the charts by Capt. Moresby, to whose personal 
kindness in giving me much information I am exceedingly 
indebted, it would have been superfluous. 

Sahia dc Maiha bank consists of a series of narrow banks, 
with from 8 to 16 fathoms on them ; they are arranged in a 
semicircular manner, round a space about forty fathoms 
deep, which slopes on the S.E. quarter to unfathomable 
depths ; they are steep-to on both sides, but more especially 
on the ocean-side. Hence this bank closely resembles in 
structure, and I may add from Capt. Moresby's information 
in composition, the Pitt's Bank in the Chagos group ; and 



APPENDIX. 235 

the Pitt's Bank, must, after what has been shown of the 
Great Chagos Bank, be considered as a sunken, half- 
destroyed atoll; hence coloured blue. — Cargados Carajos 
Bank. Its southern portion consists of a large, curved, 
coral-shoal, with some low islets on its eastern edge, and 
likewise some on the western side, between which there is a 
depth of about twelve fathoms. Northward, a great bank 
extends. I cannot (probably owing to the want of perfect 
charts) refer this reef and bank to any class ; — therefore not 
coloured. — lie de Sable is a little island, lying west of 
C. Carajos, only some toises in height {Voyage of the 
Favourite, vol. i. p. 130); it is surrounded by reefs; but 
its structure is unintelligible to me. There are some small 
banks north of it, of which I can find no clear account. — 
Mauritius. The reefs round this island have been de- 
scribed in the chapter on fringing-reefs ; coloured red. — 
Rodriguez. The coral-reefs here are exceedingly extensive ; 
in one part they project even five miles from the shore. 
As far as I can make out, there is no deep-water moat 
within them ; and the sea outside does not deepen very 
suddenly. The 0'itline, 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 Capt. Carmichael (Hooker's Bot. Misc.) states that a 
portion, fifteen miles in length, on the S.E. side, is imper- 
fectly fringed with coral-reefs : I have not thought this 
sufficient to colour the island. 

Seychelles. — The rocky islands of primary formation, 
composing this group, rise from a very extensive and 
tolerably level bank, having a depth between 20 and 40 



236 APPENDIX. 

fathoms. In Capt. Owen's chart, and in that in the atlas of 
the voyage of the Favourite, it appears that the east side 
of Make and the adjoining islands of St. Anne and Cerf, 
are regularly fringed by coral-reefs. A portion of the S.E. 
part of Curieuse Isld., the N., and part of the S.W. shore of 
Praslin Isld., and the whole west side of Digue Isld., 
appear fringed. From a MS. account of these islands by 
Capt. F. Moresby, in the Admiralty, it appears that Silhouette 
is also fringed ; he states that all these islands are formed 
of granite and quartz, that they rise abruptly from the sea, 
and that "coral-reefs have grown round them, and project 
for some distance." Dr. Allan, of Forres, who visited these 
islands, informs me that there is no deep water between 
the reefs and the shore. The above specified points have 
been coloured red. Amir antes Islands : The small islands 
of this neighbouring group, according to the MS. account 
of them by Capt. F. Moresby, are situated on an exten- 
sive bank ; they consist of the debris of corals and shells ; 
are only about twenty feet in height, and are environed 
by reefs, some attached to the shore, and some rather dis- 
tant from it. — I have taken great pains to procure plans and 
information regarding the several islands lying between 
S.E. and S.W. of the Amirantes, and the Seychelles; 
relying chiefly on Capt, F. Moresby and Dr. Allan, it 
appears that the greater number, namely — Platte^ Alphonse, 
Coetivi, Galega, Providence, St. Pierre, Astova, Assomption, 
and Glorioso, are low, formed of sand or coral-rock, and 
irregularly shaped; they are situated on very extensive 
banks, and are connected with great coral-reefs. Galega is 
said by Dr. Allan, to be rather higher than the other 
islands ; and St. Pierre is described by Capt. F. Moresby, 
as being cavernous throughout, and as not consisting of 
either limestone or granite. These islands, as well as the 



APPENDIX. 237 

Amirantes, certainly are not atoll-formed, and they differ 
as a group from every other group with which I am 
acquainted ; I have not coloured them ; but probably the 
reefs belong to the fringing class. Their formation is 
attributed, both by Dr. Allan and Capt. F. Moresby, to the 
action of the currents, here exceedingly violent, on banks, 
which no doubt have had an independent geological origin. 
They resemble in many respects some islands and banks 
in the West Indies, which owe their origin to a similar 
agency, in conjunction with an elevation of the entire area. 
In close vicinity to the several islands, there are three 
others of an apparently different nature : first, Juan de 
JVova, which appears from some plans and accounts to be 
an atoll ; but from others does not appear to be so ; not 
coloured. Secondly, Cosmoledo ; "this group consists of a 
ring of coral, ten leagues in circumference, and a quarter 
of a mile broad in some places, enclosing a magnificent 
lagoon, into which there did not appear a single opening " 
(Horsburgh, vol. i. p. 151); coloured blue. Thirdly, 
Aldabra; it consists of three islets, about 25 feet in height, 
with red cliffs (Horsburgh, vol. i. p. 176) surrounding a very 
shallow basin or lagoon. The sea is profoundly deep close 
to the shore. Viewing thi? island in a chart, it would be 
thought an atoll ; but the foregoing description shows that 
there is something different in its nature; Dr. Allan also 
states that it is cavernous, and that the coral-rock has a 
vitrified appearance. Is it an upheaved atoll, or the crater 
of a volcano ? — uncoloured. 

Comoro Group. — Mayotta, according to Horsburgh 
(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 



238 APPENDIX. 

38 fathoms is laid down within the reef. In the same 
chart, the space of open water within the reef in some 
parts is even more than three miles wide : the land is bold 
and peaked; this island, therefore, is encircled by a well- 
characterised barrier-reef, and is coloured pale blue. — 
Johanna; Horsburgh says (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 
(Nar., 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, in from 30 to 45 fathoms, between a reef 
and the shore (Horsburgh, voL i. p. 214); in Capt. Owen's 
chart of Madagascar, this island is represented as encircled ; 
coloured blue. — Great Comoro Isld. is, as I am informed 
by Dr. Allan, about 8,000 feet high, and apparently 
volcanic ; it is not regularly encircled ; but reefs of various 
shapes and dimensions jut out from every headland on the 
W., S., and S.E. coasts, inside of which reefs there are 
channels, often parallel with the shore, with deep water. 
On the N. -western coasts the reefs appear attached to the 
shores. The land near the coast is in some places bold, 
but generally speaking it is flat ; Horsburgh says (vol. i. 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 in 
the group. 



APPENDIX. 239 

Madagascar.— My information is chiefly derived from 
the published charts by Capt. Owen, and the accounts 
given by him and by Lieut. Boteler. Commencing at the 
S.W. extremity of the island ; towards the northern part of 
the Star Bank (in lat. 25 S.) the coast for ten miles is 
fringed by a reef; coloured red. The shore immediately 
S. of St. Augustine's Bay appears fringed ; but Tullear 
Harbour, directly N. of it, is formed by a narrow reef ten 
miles long, extending parallel to the shore, with from four 
to ten fathoms within it. If this reef had been more 
extensive, it must have been classed as a barrier-reef; but 
as the line of coast falls inwards here, a submarine bank 
perhaps extends parallel to the shore, which has offered a 
foundation for the growth of the coral ; I have left this part 
un coloured. 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 ten fathoms between them and the 
shore, and the depth of the sea one mile and a half seaward, 
is 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 
outlying sandbanks, 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 



240 APPENDIX, 

Bays (14 40') and the islands in front of Pa Jama 
Harbour are represented in the plans as regularly fringed, 
and have accordingly been coloured red. With respect 
to the East coast of Madagascar, Dr. Allan informs me 
in a letter, that the whole line of coast, from Tamatave, in 
1 8° 12', to C. Amber, at the extreme northern point of the 
island, is bordered by coral-reefs. The land is low, uneven, 
and gradually rising from the coast. From Capt. Owen's 
charts, also, the existence of these reefs, which evidently 
belong to the fringing class, on some parts, namely, N. of 
British Sound and near Ngoncy\ of the above line of coast 
might have been inferred. Lieut. Boteler (vol. i. p. 155) 
speaks of "the reef surrounding the island of St. Mary's at 
a small distance from the shore." In a previous chapter I 
have described, from the information of Dr. Allan, the 
manner in which the reefs extend in N.E. lines from the 
headlands on this coast, thus sometimes forming rather 
deep channels within them ; this seems caused by the 
action of the currents, and the reefs spring up from the 
submarine prolongations of the sandy headlands. The 
above specified portion of the coast is coloured red. The 
remaining S.E. portions do not appear on any published 
chart to possess reefs of any kind ; and the Rev. W. Ellis, 
whose means of information regarding this side of Mada- 
gascar have been extensive, informs me he believes there 
are none. 

East Coast of Africa. — Proceeding from the northern 
part, the coast appears, for a considerable space, without 
reefs. My information, I may here observe, is derived from 
the survey by Capt. Owen, together with his Narrative ; 
and that by Lieut. Boteler. At Mukdeesha (io° 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 the 



APPENDIX. 241 

distance of a quarter of a mile from the shore, and has 
within it from six to ten feet water : this then is a fringing 
reef, and is coloured red, From Juba, a little S. of the 
equator, to Lamoo (in 2 20' S.) "the coast and islands are 
formed of madrepore " (Owen's Narrative^ vol. i. p. 363). 
The chart of this part (entitled Dundas Islds.) presents an 
extraordinary appearance ; the coast of the mainland is 
quite straight, and it is fronted at the average distance of 
two miles by exceedingly narrow, straight islets, fringed 
with reefs. Within the chain of islets, there are extensive 
tidal flats and muddy bays, into which many rivers enter; 
the depths of these spaces varies from one to four fathoms 
— the latter depth not being common, and about twelve 
feet the average. Outside the chain of islets, the sea, at 
the distance of a mile, varies in depth from eight to fifteen 
fathoms. Lieut. Boteler (JVar., vol. i. p. 369) describes the 
muddy bay of Patta, which seems to resemble other parts 
of this coast, as fronted by small, narrow, level islets formed 
of decomposing coral, the margin of which is seldom of 
greater height than twelve feet, overhanging the rocky 
surface from which the islets rise. Knowing that the islets 
are formed of coral, it is, I think, scarcely possible to view 
the coast, and not at once conclude that we here see a 
fringing-reef, which has been upraised a few feet : the 
unusual depth of from two to four fathoms within some of 
these islets, is probably due to muddy rivers having pre- 
vented the growth of coral near the shore. There is, how- 
ever, one difficulty on this view, namely, that before the 
elevation took place, which converted the reef into a chain 
of islets, the water must apparently have been still deeper ; 
on the other hand it may be supposed that the formation of 
' a nearly perfect barrier in front, of so large an extent of 
coast, would cause the currents (especially in front of the 

881 



242 APPENDIX. 

rivers) to deepen their muddy beds. When describing in 
the chapter on fringing-reefs, those of Mauritius, I have 
given my reasons for believing that the shoal spaces 
within reefs of this kind, must, in many instances, have 
been deepened. However this may be, as several parts of 
this line of coast are undoubtedly fringed by living reefs, 
I have coloured it red. — Maleenda (3 20' S.). In the 
plan of the harbour, the south headland appears fringed ; 
and in Owen's chart on a larger scale, the reefs are seen to 
extend nearly thirty miles southward; coloured red. — - 
Mombas (4 5' S.). 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 mainland, N. and S. of the harbour, is 
most regularly fringed by a coral-reef at a distance from half 
a mile to one mile and a quarter from the land ; within the 
reef the depth is from nine to fifteen feet ; outside the reef 
the depth at rather less than half a mile is thirty fathoms. 
From the charts it appears that a space about thirty-six 
miles in length, is here fringed ; coloured red. — Pemba 
(5 S.) is an isld. of coral-formation, level, and about 200 
feet in height (Owen's Nar.^ vol. i. p. 425) ; it is 35 miles 
long, and is separated from the mainland by a deep sea. 
The outer coast is represented in the chart as regularly 
fringed; coloured red. The mainland in front of Pemba 
is likewise fringed ; but there also appear to be some out- 
lying reefs with deep water between them and the shore. 
I do not understand their structure, either from the charts 
or the description, therefore have not coloured them. — 
Zanzibar resembles Pemba in most respects ; its southern 
half on the western side and the neighbouring islets are 
fringed ; coloured red. On the mainland, a little S. of 
Zanzibar, there are some banks parallel to the coast, which 



APPENDIX, 243 

I should have thought had been formed of coral, had it not 
been said (Boteler's JVar., vol. ii. p. 39) that they were 
composed of sand; not coloured. — Lathanis Bank is a 
small island, fringed by coral-reefs ; but being only ten feet 
high, it has not been coloured. — Monfeea is an island of the 
same character as Pemba ; its outer shore is fringed, and 
its southern extremity is connected with Keelwa Point on 
the mainland by a chain of islands fringed by reefs ; 
coloured red. The four last-mentioned islands resemble 
in many respects some of the islands in the Red Sea, which 
will presently be described. — Keelwa. In a plan of the 
shore, a space of 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. The coast 
in the plans of the rivers Lindy and Monghow (9 59' and 
io° 7' S.) has the same structure ; coloured red. — Querimba 
Islands (from io° 40' to 13 S.). A chart on a large scale 
is given of these islands ; they are low, and of coral-forma- 
tion (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 their insides they are separated from the continent 
by a channel, or rather a succession of bays, with an 
average depth of ten fathoms. The small headlands 
on the continent also have coral-banks attached to 
them ; and the Querimba islands and banks are placed 
on the lines of prolongation of these headlands, and are 
separated from them by very shallow channels. It is 
evident that whatever cause, whether the drifting of sediment 
or subterranean movements, produced the headlands, like- 
wise produced, as might have been expected, submarine 
prolongations to them; and these towards their outer 
extremities have since afforded a favourable basis for the 



244 APPENDIX. 

growth of coral-reefs, and subsequently for the formation of 
islets. As these reefs clearly belong to the fringing class, 
the Querimba islands have been coloured red. — Monabila 
(13 32' S.). In the plan of this harbour, the headlands 
outside are fringed by reefs apparently of coral ; coloured 
red. — Mozambique (150 S.). The outer part of the island 
on which the city is built, and the neighbouring islands, are 
fringed by coral-reefs ; coloured red. From the description 
given in Owen's 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 small size and lowness, it is 
not possible, from the charts, to know whether they are 
truly fringed. Hence this portion of coast is left un- 
coloured, as are likewise those parts more northward, of 
which no mention has been made in the foregoing pages 
from the want of information. 

Persian Gulf. — From the charts lately published on a 
large scale by the East India Company, it appears that 
several parts, especially the southern shores of this gulf, are 
fringed by coral-reefs ; but as the water is very shallow, and 
as there are numerous sandbanks, which are difficult to 
distinguish on the chart from reefs, I have not coloured the 
upper part red. Towards the mouth, however, where the 
water is rather deeper, the islands of Ormuz and Larrack 
appear so regularly fringed, that I have coloured them red. 
There are certainly no atolls in the Persian Gulf. The 
shores of Immaum, and of the promontory forming the 
southern headland of the Persian Gulf, seem to be without 
reefs. The whole S.W. part (except one or two small 
patches) of Arabia Felix, and the shores of Socotra^ appear 
from the charts and memoir of Capt. Haines (Geograph. 
Journ.) 1839, P- I2 5) t0 be without any reefs. I believe 



APPENDIX. 245 

there are no extensive coral-reefs on any part of the coasts 
of Indict^ except on the low promontory of Madura (as 
already mentioned) in front of Ceylon. 

Red Sea. — My information is chiefly derived from the 
admirable charts published by the East India Company in 
1836, from personal communication with Capt. Moresby, 
one of the surveyors, and from the excellent memoir, 
Tiber die Natur der Corallen-Bdnken des Rothen Meeres, 
by Ehrenberg. The plains immediately bordering the Red 
Sea seem chiefly to consist of a sedimentary formation of 
the newer tertiary period. The shore is, with the exception 
of a few parts, fringed by coral-reefs. The water is generally 
profoundly deep close to the shore ; but this fact, which 
has attracted the attention of most voyagers, seems to 
have no necessary connection with the presence of reefs ; 
for Capt. Moresby particularly observed to me, that, in 
lat. 24 io' on the eastern side, there is a piece of 
coast, with very deep water close to it, without any reefs, 
but not differing in other respects from the usual nature of 
the coast-line. The most remarkable feature in the Red Sea 
is the chain of submerged banks, reefs, and islands, lying 
some way from the shore, chiefly on the eastern side; the 
space within being deep enough to admit a safe navigation 
in small vessels. The banks are generally of an oval form, 
and some miles in width; but some of them are very long in 
proportion to their width. Capt. 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 5 to 30 fathoms beneath it, with 
irregular soundings on them. They consist of sand and 
living coral; coral on most of them, according to Capt. 
Moresby, covering the greater part of their surface. They 



246 APPENDIX. 

extend parallel to the shore, and they are not unfrequently 
connected in their middle parts by short transverse banks 
with the mainland. The sea is generally profoundly deep 
quite close to them, as it is near most parts of the coast 
of the mainland; but this is not universally the case, for 
between lat. 15 and 17 the water deepens quite gradually 
from the banks, both on the eastern and western shores, 
towards the middle of the sea. Islands in many parts arise 
from these banks; they are low, flat-topped, and consist of 
the same horizontally stratified formation with that forming 
the plain-like margin of the mainland. Some of the smaller 
and lower islands consist of mere sand. Capt. Moresby 
informs me, that small masses of rock, the remnants of 
islands, are left on many banks where there is now no dry 
land. Ehrenberg also asserts that most of the islets, even 
the lowest, have a flat abraded basis, composed of the same 
tertiary formation: he believes that as soon as the surf 
wears down the protuberant parts of a bank, just beneath 
the level of the sea, the surface becomes protected from 
further abrasion by the growth of coral, and he thus 
accounts for the existence of so many banks standing on a 
level with the surface of this sea. It appears that most of 
the islands are certainly decreasing in size. 

The form of the banks and islands is most singular in 
the part just referred to, namely, from 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 very irregularly shaped, 
and it includes a bay seven miles long, by four across, in 
which no bottom was found with 252 feet: there is only one 
entrance into this bay, half a mile wide, and with an island 
in front of it The submerged banks on the eastern coast, 
within the same latitudes, round Farsan Isld., are, likewise 



APPENDIX, 247 

penetrated by many narrow creeks of deep water; one is 
twelve miles long, in the form of a hatchet, in which, close 
to its broad upper end, soundings were not struck with 360 
feet, and its entrance is only half a mile wide: in another 
creek of the same nature, but even with a more irregular 
outline, there was no bottom with 480 feet. The island of 
Farsan, itself, has as singular a form as any of its surround- 
ing banks. The bottom of the sea round the Dhalac and 
Farsan Islands consists chiefly of sand and agglutinated 
fragments, but, in the deep and narrow creeks, it consists of 
mud; the islands themselves consist of thin, horizontally 
stratified, modern tertiary beds, containing but little broken 
coral, 1 their shores are fringed by living coral-reefs. 

From the account given by Riippell 2 of the manner in 
which Dhalac has been rent by fissures, the opposite sides 
of which have been unequally elevated (in one instance to 
the amount of 50 feet), it seems probable that its irregular 
form, as well as probably that of Farsan, may have been 
partly caused by unequal elevations ; but, considering the 
general form of the banks, and of the deep-water creeks, 
together with the composition of the land, I think their 
configuration is more probably due in great part to strong 
currents having drifted sediment over an uneven bottom : 
it is almost certain that their form cannot be attributed to 
the growth of coral. Whatever may have been the precise 
origin of the Dhalac and Farsan Archipelagoes, the greater 
number of the banks on the eastern side of the Red Sea 
seem to have originated through nearly similar means. I 
judge of this from their similarity in configuration (in proof 
of which I may instance a bank on the east coast in lat. 22 ; 
and although it is true that the northern banks generally 

1 Riippell, Reise in Abyssinien, Band, i., s. 247. 2 Ibid., s. 245, 



248 APPENDIX. 

have a less complicated outline), and from their similarity 
in composition, as may be observed in their upraised 
portions. The depth within the banks northward of lat. 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 might easily 
have been caused by a difference in the action of the 
currents during their formation : moreover, the greater 
quantity of living coral, which, according to Capt. Moresby, 
exists on the northern banks, would tend to give them 
steeper margins. 

From this account, brief and imperfect as it is, we can 
see that the great chain of banks on the eastern coast, and 
on the western side in the southern portion, differ greatly 
from true barrier-reefs wholly formed by the growth of coral. 
It is indeed the direct conclusion of Ehrenberg ( Uber die, 
etc., pp. 45 and 51), that they are connected in their origin 
quite secondarily with the growth of coral ; and he remarks 
that the islands off the coast of Norway, if worn down level 
with the sea, and merely coated with living coral, would 
present a nearly similar appearance. I cannot, however, 
avoid suspecting, from information given me by Dr. 
Malcolmson and Capt. Moresby, that Ehrenberg has 
rather under-rated the influence of corals, in some places 
at least, on the formation of the tertiary deposits of the 
Red Sea. 

The West Coast of the Red Sea between lat. 19 and 22 . 
— There are, in this space, reefs which, if I had known 
nothing of those in other parts of the B.ed Sea, I should 
unhesitatingly have considered as barrier-reefs ; and, after 
deliberation, I have come to the same conclusion. One of 
these reefs, in 20 15', is twenty miles long, less than a mile 
in width (but expanding at the northern end into a disc), 



APPENDIX. 249 

slightly sinuous, and extending parallel to the mainland at 
the distance of five miles from it, with very deep water 
within; in one spot soundings were not obtained with 205 
fathoms, Some leagues further south, there is another 
linear reef, very narrow, ten miles long, with other small 
portions of reef, north and south, almost connected with it ; 
and within this line of reefs (as well as outside) the water 
is profoundly deep. There are also some small linear and 
sickle-formed reefs, lying a little way out at sea. All these 
reefs are covered, as I am informed by Capt. Moresby, by 
living corals. Here, then, we have all the characters of 
reefs of the barrier class; and in some outlying reefs we 
have an approach to the structure of atolls. The source of 
my doubts about the classification of these reefs, arises 
from having observed in the Dhalac and Farsan groups 
the narrowness and straightness of several spits of sand and 
rock : one of these spits in the Dhalac group is nearly 
fifteen miles long, only two broad, and it is bordered on 
each side with deep water ; so that, if worn down by the 
surf, and coated with living corals, it would form a reef 
nearly similar to those within the space under consideration. 
There is, also, in this space (lat. 21 ) a peninsula, bordered 
by cliffs, with its extremity worn down to the level of the 
sea. and its basis fringed with reefs : in the line of prolonga- 
tion of this peninsula, there lies the island of Macowa 
(formed, according to Capt. Moresby, of the usual tertiary 
deposit), and some smaller islands, large parts of which 
likewise appear to have been worn down, and are now 
coated with living corals. If the removal of the strata 
in these several cases had been more complete, the reefs 
thus formed would have nearly resembled those barrier- 
like ones now under discussion. Notwithstanding these 
facts. I cannot persuade myself that the many very small, 



250 APPENDIX, 

isolated, and sickle-formed reefs and others, long, nearly 
straight, and very narrow, with the water unfathomably 
deep close round them, could possibly have been formed 
by corals merely coating banks of sediment, or the abraded 
surfaces of irregularly-shaped islands. I feel compelled to 
believe that the foundations of these reefs have subsided, 
and that the corals, during their upward growth, have given 
to these reefs their present forms : I may remark that the 
subsidence of narrow and irregularly-shaped peninsulas and 
islands, such as those existing on the coasts of the Red 
Sea, would afford the requisite foundations for the reefs in 
question. 

The West Coast from lat. 2 2° to 24 . — This part of the 
coast (north of the space coloured blue on the map) is 
fronted by an irregularly shelving bank, from about 10 to 
30 fathoms deep; numerous little reefs, some of which 
have the most singular shapes, rise from this bank. It 
may be observed, respecting one of them, in lat. 23 10', 
that if the promontory in lat. 24° were worn down to the 
level of the sea, and coated with corals, a very similar and 
grotesquely formed reef would be produced. Many of the 
reefs on this part of the coast vn^y thus have originated ; 
but there are some sickle, and almost atoll-formed reefs 
lying in deep water off the promontory in lat. 24 , which 
lead me to suppose that all these reefs are more probably 
allied to the barrier or atoll classes. I have not, however, 
ventured to colour this portion of coast. — On the west 
coast from lat. 19° to 17 (south of space coloured blue on 
the map), there are many low islets of very small dimen- 
sions, not much elongated, and rising out of great depths 
at a distance from the coast ; these cannot be classed either 
with atolls, or barrier or fringing-reefs. I may here remark 
that the outlying reefs on the west coast, between lat. 19 



APPENDIX. 251 

and 24 , are the only ones in the Red Sea, which approach 
in structure to the true atolls of the Indian and Pacific 
Oceans, but they present only imperfect miniature likenesses 
of them. 

Eastern Coast — I have felt the greatest doubt about 
colouring any portion of this coast, north of the fringing- 
reefs round the Farsan Islands in 16 10'. There are many 
small outlying coral-reefs along the whole line of coast ; but 
as the greater number rise from banks not very deeply sub- 
merged (the formation of which has been shown to be only 
secondarily connected with the growth of coral), their origin 
may be due simply to the growth of knolls of corals, from an 
irregular foundation situated within a limited depth. But 
between 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 blue 
a portion of the west coast, have induced me to do the same 
in this part. There exist some small outlying reefs rising 
from deep water, north of lat. 20 (the northern limit 
coloured blue), on the east coast ; but as they are not very 
numerous and scarcely any of them linear, I have thought 
it right to leave them uncoloured. 

In the souther ?i parts of the Red Sea, considerable spaces 
of the mainland, and of some of the Dhalac islands, are 
skirted by reefs, which, as I am informed by Capt 
Moresby, are of living coral, and have all the characters of 
the fringing class. As in these latitudes, there are no out- 
lying linear or sickle-formed reefs, rising out of unfathom- 
able depths, I have coloured these parts of the coast 
red. On similar grounds, I have coloured red the 
northern parts of the western coast (north of lat. 24 30'), 
and likewise the shores of the chief part of the Gulf of 
Suez. In the Gulf of Acaba^ as I am informed by Capt. 



252 APPENDIX. 

Moresby, there are no coral-reefs, and the water is pro- 
foundly deep. 

West Indies. — My information regarding the reefs of 
this area, is derived from various sources, and from an 
examination of numerous charts ; especially of those lately 
executed during the survey under Capt. Owen, R.N. I lay 
under particular obligation to Capt. Bird Allen, R.N., one 
of the members of the late survey, for many personal com- 
munications on this subject. As in the case of the Red Sea, 
it is necessary to make some preliminary remarks on the 
submerged banks of the West Indies, which are in some 
degree connected with coral-reefs, and cause considerable 
doubts in their classification. That large accumulations of 
sediment are in progress on the West Indian shores, will be 
evident to any one who examines the charts of that sea, 
especially of the portion north of a line joining Yucutan 
and Florida. The area of deposition seems less intimately 
connected with the debouchement of the great rivers, than 
with the course of the sea-currents ; as is evident from the 
vast extension of the banks from the promontories of 
Yucutan and Mosquito. 

Besides the coast-banks, there are many of various dimen- 
sions which stand quite 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 only to the amount of a 
few fathoms, very gradually all round towards their edges, 
where they plunge abruptly into the unfathomable sea. 
This steep inclination of their sides, which is likewise 
characteristic of the coast-banks, is very remarkable : I may 
give as an instance, the Misteriosa Bank, on the edges of 
which the soundings change in 250 fathoms horizontal 



APPENDIX. 253 

distance, from 11 to 210 fathoms; off the northern point 
of the bank of Old Providence, in 200 fathoms horizontal 
distance, the change is from 19 to 152 fathoms; off the 
Great Bahama Bank, in 160 fathoms horizontal distance, 
the inclination is in many places from 10 fathoms to no 
bottom with 190 fathoms. On coasts in all parts of the 
world, where sediment is accumulating, something of this 
kind may be observed ; the banks shelve very gently far out 
to sea, and then terminate abruptly. The form and com- 
position of the banks standing in the middle parts of the 
W. Indian Sea, clearly show that their origin must be 
chiefly attributed to the accumulation of sediment ; and the 
only obvious explanation of their isolated position is the 
presence of a nucleus, round which the currents have 
collected fine drift matter. Any" one who will compare the 
character of the bank surrounding the hilly island of Old 
Providence, with those banks in its neighbourhood which 
stand isolated, will scarcely doubt that they surround sub- 
merged mountains. We are led to the same conclusion 
by examining the bank called Thunder Knoll, which is 
separated from the Great Mosquito Bank by a channel 
only seven miles wide, and 145 fathoms deep. There 
cannot be any doubt that the Mosquito Bank has been 
formed by the accumulation of sediment round the pro- 
montory of the same name ; and Thunder Knoll resembles 
the Mosquito Bank, in the state of its surface submerged 
twenty fathoms, in the inclinations of its sides, in com- 
position, and in every other respect. I may observe, 
although the remark is here irrelevant, that geologists 
should be cautious in concluding that all the outlyers of 
any formation have once been connected together, for we 
here see that deposits, doubtless of exactly the same nature, 
may be deposited with large valley-like spaces between them. 



254 APPENDIX. 

Linear strips of coral-reefs and small knolls project from 
many of the isolated, as well as coast-banks ; sometimes 
they occur quite irregularly placed, as on the Mosquito 
Bank, but more generally they form crescents on the 
windward side, situated some little distance within the outer 
edge of the banks : — thus on the Serranilla Bank they form 
an interrupted chain which ranges between two and three 
miles within the windward margin : generally they occur, as 
on Roncador, Courtown, and Anegada Banks, nearer the 
line of deep water. Their occurrence on the windward 
side is conformable to the general rule, of the efficient 
kinds of corals flourishing best where most exposed ; but 
their position some way within the line of deep water I 
cannot explain, without it be, that a depth somewhat less 
than that close to the outer margin of the banks, is most 
favourable to their growth. Where the corals have formed 
a nearly continuous rim, close to the windward edge of a 
bank some fathoms submerged, the reef closely resembles 
an atoll ; but if the bank surrounds an island (as in the 
case of Old Providence), the reef resembles an encircling 
barrier-reef. I should undoubtedly have classed some of 
these fringed banks as imperfect atolls, or barrier-reefs, 
if the sedimentary nature of their foundations had not 
been evident from the presence of other neighbouring 
banks, of similar forms and of similar composition, but 
without the crescent-like marginal reef: in the third 
chapter, I observed that probably some atoll-like reefs 
did exist, which had originated in the manner here 
supposed. 

Proofs of elevation within recent tertiary periods abound, 
as referred to in the sixth chapter, over nearly the whole 
area of the West Indies. Hence it is easy to understand 
the origin of the low land on the coasts, where sediment 



APPENDIX. 255 

is now accumulating ; for instance on the northern part 
of Yucutan, and on the N.E. part of Mosquito, where the 
land is low, and where extensive banks appear to be in 
progressive formation. Hence, also, the origin of the 
Great Bahama Banks, which are bordered on their western 
and southern edges by very narrow, long, singularly shaped 
islands, formed of sand, shells, and coral-rock, and some 
of them about a hundred feet in height, is easily explained 
by the elevation of banks fringed on their windward (western 
and southern) sides by coral-reefs. On this view, however, 
we must suppose either that the chief part of the surfaces 
of the great Bahama sandbanks were all originally deeply 
submerged, and were brought up to their present level by 
the same elevatory action, which formed the linear islands , 
or that during the elevation of the banks, the superficial 
currents and swell of the waves continued wearing them 
down and keeping them at a nearly uniform level : the 
level is not quite uniform ; for, in proceeding from the 
N.W. end of the Bahama group towards the S.E. end, 
the depth of the banks increases, and the area of land 
decreases, in a very gradual and remarkable manner. The 
latter view, namely, that these banks have been worn down 
by the currents and swell during their elevation, seems to 
me the most probable one. It is, also, I believe, applicable 
to many banks, situated in widely distant parts of the West 
Indian Sea, which are wholly submerged ; for, on any other 
view, we must suppose, that the elevatory forces have acted 
with astonishing uniformity. 

The shores of the Gulf of Mexico, for the space of many 
hundred miles, is formed by a chain of lagoons, from one 
to twenty miles in breadth {Columbian Navigator, p. 178, 
etc.) containing either fresh or salt water, and separated 
from the sea by linear strips of sand. Great spaces of 



256 APPENDIX. 

the shores of Southern Brazil, 1 and of the United States 
from Long Island (as observed by Professor Rogers) to 
Florida have the same character. Professor Rogers, in his 
Report to the British Association (vol. iii. p. 13), speculates 
on the origin of these low, sandy, linear islets ; he 
states that the layers of which they are composed are too 
homogeneous, and contain too large a proportion of 
shells, to permit the common supposition of their formation 
being simply due to matter thrown up, where it now lies, 
by the surf: he considers these islands as upheaved bars 
or shoals, which were deposited in lines where opposed 
currents met. It is evident that these islands and spits of 
sand parallel to the coast, and separated from it by shallow 
lagoons, have no necessary connection with coral-forma- 
tions. But in Southern Florida, from the accounts I have 
received from persons who have resided there, the upraised 
islands seem to be formed of strata, containing a good deal 
of coral, and they are extensively fringed by living reefs; 
the channels within these islands are in some places 
between two and three miles wide, and five or six fathoms 
deep, though generally 2 they are less in depth than width. 
After having seen how frequently banks of sediment in the 
West Indian Sea are fringed by reefs, we can readily con- 
ceive that bars of sediment might be greatly aided in their 
formation along a line of coast, by the growth of corals; 
and such bars would, in that case, have a deceptive 
resemblance with true barrier-reefs. 

1 In the London and Edinburgh Philosophical Journal \ 1S41, p. 257, 
I have described a singular bar of sandstone lying parallel to the coast 
off Pernambuco in Brazil, which probably is an analogous formation. 

2 In the ordinary sea-charts, no lagoons appear on the coast of 
Florida, north of 26 ; but Major Whiting [Sillimans Journal, vol, 
xxxv. p. 54) says that many are formed by sand thrown up along the 
whole line of coast from St. Augustine's to Jupiter Inlet. 



APPENDIX. 257 

Having now endeavoured to remove some sources of 
doubt in classifying the reefs of the West Indies, I will give 
my authorities for colouring such portions of the coast as I 
have thought myself warranted in doing. Capt. Bird Allen 
informs me, that most of the islands on the Bahama Banks 
are fringed, especially on their windward sides, with living 
reefs ; and hence I have coloured those, which are thus repre- 
sented in Capt. Owen's late chart, red. The same officer 
informs me, that the islands along the southern part of 
Florida are similarly fringed; coloured red. — Cuba : Pro- 
ceeding along the northern coast, at the distance of forty miles 
from the extreme S.E. point, the shores are fringed by reefs, 
which extend westward for a space of 160 miles, with only 
a few breaks. Parts of these reefs are represented in the 
plans of the harbours on this coast by Capt. 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 enclosed a space called the " baxo" from half to 
three-quarters of a mile in width, with a sandy bottom, and 
a little coral. In most parts people can wade, at low water, 
to the reef; but in some parts the depth is between two and 
three fathoms. Close outside the reef, the depth is between 
six and seven fathoms; these well-characterised fringing- 
reefs are coloured red. — Westward of long. 77 30', on the 
northern side of Cuba, a great bank commences, which 
extends along the coast for nearly four degrees of longitude. 
In the place of its commencement, in its structure, and in 
the " cays" or low islands on its edge, there is a marked 
correspondence (as observed by Humboldt, Pers. Nar., 
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 both these sets of 
banks ; namely, the accumulation of sediment, conjoined 

882 



258 APPENDIX. 

with an elevatory movement, and the growth of coral on 
their outward edges; those parts which appear fringed 
by living reefs are coloured red. Westward of these 
banks, there is a portion of coast apparently without reefs, 
except in the harbours, the shores of which seem in the 
published plans to be fringed.- — The Colorado Skoals (see 
Capt. Owen's charts), and the low land at the western end 
of Cuba, correspond as closely in relative position and 
structure to the banks at the extreme point of Florida, as 
the banks above described on the north side of Cuba do to 
the Bahamas. The depth within the islets and reefs on the 
outer edge of the Colorados, is generally between two and 
three fathoms, increasing to twelve fathoms in the southern 
part, where the bank becomes nearly open, without islets or 
coral-reefs; the portions which are fringed are coloured 
red. — The southern shore of Cuba is deeply concave, and 
the included space is filled up with mud and sandbanks, 
low islands and coral-reefs. Between the mountainous Isle 
of Pines and the southern shore of Cuba, the general depth 
is only between two and three fathoms; and in this part 
small islands, formed of fragmentary rock and broken 
madrepores (Humboldt, Pers. Nar., vol. vii. 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 sandbanks. The charts represent the 
southern part of the Isle of Pines as fringed by reefs, 
which the Columb. Navig. says extend some way from the 
coast, but have only from nine to twelve feet water on 
them; these are coloured red. — I have not been able to 
procure any detailed description of the large groups of 



APPENDIX. 259 

banks and "cays" further eastward on the southern side of 
Cuba ; within them there is a large expanse, with a muddy 
bottom, from eight to twelve fathoms deep; although some 
parts of this line of coast are represented in the general 
charts of the West Indies, as fringed, I have not thought it 
prudent to colour them. The remaining portion of the 
south coast of Cuba appears to be without coral-reefs. 

Yucutan. — The N.E. part of the promontory appears in 
Capt. 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 
Capt. 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 increases 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 characteristics of a barrier-reef; never- 
theless, from observing, first, that the channel within the 
reef is a continuation of a great irregular bay, which 
penetrates the mainland to the depth of 50 miles; and 
secondly, that considerable spaces of this barrier-like reef 
are described in the charts (for instance, in lat. 16 45' and 
16 12') as formed of pure sand; and thirdly, from knowing 
that sediment is accumulating in many parts of the West 
Indies in banks parallel to the shore ; I have not ventured 
to colour this reef as a barrier, without further evidence 
that it has really been formed by the growth of corals, and 
that it is not merely in parts a spit of sand, and in other 



260 APPENDIX. 

parts a worn-down promontory, partially coated and fringed 
by reefs : I lean, however, to the probability of its being 
a barrier-reef, produced by subsidence. To add to my 
doubts, immediately on the outside of this barrier-like reef, 
Turneffe A Lighthouse, and Glover reefs are situated, and 
these reefs have so completely the form of atolls, that if 
they had occurred in the Pacific, I should not have hesi- 
tated about colouring them blue. Turnejfe 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 be- 
tween 70 and 80 feet, and to Cozu??iel Island, the level 
surface of which is likewise between 70 and 80 feet in 
height, I consider it more probable that the three foregoing 
banks are the worn-down bases of upheaved shoals, fringed 
with corals, than that they are true atolls, wholly produced 
by the growth of coral during subsidence; left uncoloured. 

In front of the eastern Mosquito coast, there are between 
lat. 12 and 16 some extensive banks (already mentioned, 
p. 253), with high islands rising from their centres; and 
there are other banks wholly submerged, both of which 
kinds of banks are bordered, near their windward margins, 
by crescent-shaped coral-reefs. But it can hardly be 
doubted, as was observed in the preliminary remarks, that 
these banks owe their origin, like the great bank extending 
from the Mosquito promontory, almost entirely to the accu- 
mulation 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 Capt. B. Allen informs me that the reefs 
extend about a mile from the shore, and have only from 



APPENDIX. 261 

5 to 12 feet water within them; coloured red.— Jamaica : 
judging from the charts, about fifteen miles of the S.E. 
extremity, and about twice that length on the S.W. ex- 
tremity, and some portions on the S. side near Kingston and 
Port Royal, are regularly fringed, and therefore are coloured 
red. From the plans of some harbours on the N. side of 
Jamaica, parts of the coast appear to be fringed; but as 
these are not represented in the charts of the whole island, 
I have not coloured them. — St. Domingo : I have not been 
able to obtain sufficient information, either from plans of 
the harbours, or from general charts, to enable me to 
colour any part of the coast, except 60 miles from Port 
de Plata westward, which seems very regularly fringed; 
many other parts, however, of the coast are probably 
fringed, especially towards the eastern end of the island. 
— Puerto. Rico : considerable portions of the southern, 
western, and eastern coasts, and some parts of the 
northern coast, appear in the charts to be fringed ; 
coloured red. Some miles in length of the southern 
side of the Island of St. Thomas is fringed ; most of 
the Virgin Gorda Islands, as I am informed by Mr. 
Schomburgk, are fringed ; the shores of Anegada, as well 
as the bank on which it stands, are likewise fringed ; these 
islands have been coloured red. The greater part of the 
southern side of Santa Cruz appears in the Danish survey 
to be fringed (see also Prof. Hovey's account of this island, 
in Silliman's Journal, vol. xxxv. p. 74) ; the reefs extend 
along the shore for a considerable space, and project rather 
more than a mile ; the depth within the reef is three 
fathoms ; coloured red. — The Antilles, as remarked by Von 
Buch (DescriJ>. lies Canaries, p. 494), may be divided into 
two linear groups, the western row being volcanic, and the 
eastern of modern calcareous origin ; my information is very 



262 APPENDIX. 

defective on the whole group. Of the eastern islands, Bar- 
buda and the western coasts of Antigua and Mariagalante 
appear to be fringed : this is also the case with Barbadoes, 
as I have been informed by a resident ; these islands are 
coloured red. On the shores of the Western Antilles, of 
volcanic origin, very few coral-reefs appear to exist. The 
island of Martinique, of which there are beautifully executed 
French charts, on a very large scale, alone presents any 
appearance worthy of special notice. The south-western, 
southern, and eastern coasts, together forming about half 
the circumference of the island, are skirted by very 
irregular banks, projecting generally rather less than a mile 
from the shore, and lying from two to five fathoms sub- 
merged. In front of almost every valley, they are breached 
by narrow, crooked, steep-sided passages. The French 
engineers ascertained by boring, that these submerged banks 
consisted of madreporitic rocks, which were covered in 
many parts by thin layers of mud or sand. From this fact, 
and especially from the structure of the narrow breaches, I 
think there can be little doubt that these banks once 
formed living reefs, which fringed the shores of the island, 
and like other reefs probably reached the surface. From 
some of these submerged banks reefs of living coral rise 
abruptly, either in small detached patches, or in lines 
parallel to, but some way within the outer edges of the 
banks on which they are based. Besides the above banks 
which skirt the shores of the island, there is on the eastern 
side a range of linear banks, similarly constituted, 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 five to fllteen 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 



APPENDIX. 263 

they do not front more than a sixth part of the circumference 
of the island), they would necessarily from their position 
have been coloured as barrier-reefs ; as the case stands they 
are left uncoloured. I suspect that after a small amount of 
subsidence, the corals were killed by sand and mud being 
deposited on them, and the reefs being thus prevented from 
growing upwards, the banks of madreporitic rock were left 
in their present submerged condition. 

The Bermuda Islands have been carefully described 
by Lieut. Nelson, in an excellent Memoir in the Geol. 
Transactions (vol. v. part i. p. 103). In the form of the 
bank or reef, on one side of which the islands stand, there 
is a close general resemblance to an atoll; but in the 
following respects there is a considerable difference, — first, 
in the margin of the reef not forming (as I have been 
informed by Mr. Chaffers, R.N.) a flat, solid surface, laid 
bare at low water, and regularly bounding the internal 
space of shallow water or lagoon ; secondly, in the border 
of gradually shoaling water, nearly a mile and a half in 
width, which surrounds the entire outside of the reef (as is 
laid down in Capt. Hurd's chart) ; and thirdly, in the size, 
height, and extraordinary form of the islands, which present 
little resemblance to the long, narrow, simple islets, seldom 
exceeding half a mile in breadth, which surmount the 
annular reefs of almost all the atolls in the Indian and 
Pacific Oceans. Moreover, there are evident proofs 
(Nelson, ibid., 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 thirty feet in height; whereas, Mr. Nelson 
estimates the highest point of the Bermuda Islands to be 
260 feet; if, however, Mr. Nelson's view, that the whole 
of the land consists of sand drifted by the winds, and 



264 APPENDIX. 

agglutinated together, were proved correct, this difference 
would be immaterial; but, from his own account (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 as many violent catastrophes; but 
further investigation in such cases has generally succeeded 
in explaining phenomena of this kind by ordinary and 
simpler means. Finally, I may remark, that these islands 
have a considerable resemblance in shape to Barbuda in 
the West Indies, and to Pemba on the eastern coast of 
Africa, which latter island is about 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 uncoloured, on account 
of their general resemblance in external form to a lagoon- 
island or atoll. 



INDEX. 



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. 



PAGE 

Abrolhos, Brazil, coated by 

corals . . . .81 
Abrolhos {Australia) . .2.2.1 
Absence of coral-reefs from 

certain coasts . . .84 
Acaba, gulf of . . .251 
Admiralty group . . .211 
Africa, east coast, fringing- 

reef of . . • 79 

Madreporitic rock of .171 

Africa, east coast . . . 240 
Age of individual corals 95, 109 

Aiou 217 

Aitutaki . . . .192 
Aldabra .... 237 
Alert reef .... 209 
Alexander, Grand Duke,island 195 
Allan, Dr., on Holuthurise 

feeding on corals . . 30 
— — on quick growth of corals 

at Madagascar . 103, 104 
- on reefs affected by cur- 
rents . 
Alloufatou . 
Alphonse . . 
Amargura . 
Amboina 

America, west coast 
Amirantes . 
Anachorites . 
Anambas 

Anamouka, description of 
Anamouka . 
Andaman is Ids. . • 
Antilles , . 



i*7, 



Appoo reef 



XI 
202 
236 
20I 
2l8 
188 
236 
211 
226 
167 
. 201 
. 225 
26l, 262 
. 228 



179 

179 



182 
217 
2l6 
112 

236 
236 

204 



PAGE 

Arabia Felix . . .. 244 

Areas, great extent of, inter- 
spersed with low islands 118, 119 

of subsidence and of 

elevation . .178, 

■ of subsidence appear to 

be elongated 

of subsidence alternat- 
ing with areas of ele- 
vations . . 181, 

Arru group .... 

Arzobispo .... 

Ascidia, depth at which found 

Assomption .... 

Astova ..... 

Atlantic islands . 

Atolls, breaches in their reefs 

47, 136, 137 

dimensions of 36, 37, 38 

dimensions of groups of 

118, 119 

— — not based on craters or 

on banks of sediment, 

or of rock . 115, 116, 

120, 121, 122, 181, 182 

of irregular forms 36, ^7, 

_ 38, 141, 142 

steepness of their flanks 38, 39 

width of their reef and 

islets . . 37, 38 

their lowness . 117,118 

lagoons . 41, 42, 43 

general range . 157, 158 

with part of their reef 

submerged, and theory 

of . 44,45, 136, 137 



266 



INDEX. 



PAGE 

Augustine, St. , . 203 
Auroraisland, an upraised atoll 

" 107, iiS, 175 
Aurora . , . .189 
Austral islands, recently ele- 
vated . , 166, 174, 175 
Austral islands , . .192 
Australia, N. IV. coast . . 220 
Australian barrier-reef . 67, 158 
Australian barrier . . 208 

Babuyan group . . . 229 

Bahama banks . . 255, 257 
Bat abac .... 226 

Bally 222 

Baring . . . .205 

Earrier-reef of Australia 67, 158 

of New Caledonia . 71 

Barrier-reefs, breaches through 129 

not based on worn down 

margin of rock . . 70 

on banks of sediment . 7° 

on submarine craters . 7 1 

steepness of their flanks 62, 63 

their probable vertical 

thickness . . 7°s l2 % 

theory of their formation 

127, 128, 129, 130 
Bamp'on shoal . . . 209 
Banks islds. .... 207 
Banks in the West Indies . 252 
Bashee islds. . . . 229 

Bass is Id. . . . .194 
Batoa ..... 202 
Beaupri reef , . . 20S 

Beechey, Captain, obligations 
of the author to . . 38 

on submerged reefs . 40 

account of Matilda island IOO 

Belcher, Captain, on boring 

through coral-reef . . 98 

Bellinghausen . . . 191 

Bermuda islds. , . .263 

Bevendge reef . . . 200 

Bligh 207 

Bolabola, view of . . 17 

Bombay shoal , . .232 

Benin Bay .... 224 



PAGE 

Bonin group . . . 216 

Borings through coral-reefs 98, 99 
Borneo, W. coast, recently ele- 
vated . . . 169, 170 
Borneo, E. coast . . . 223 
S. W. and W. coast 226, 227 



N. coast 
•western bank 



227 
232 
201 
205 
210 



Boscawen 

Boston .... 
Bouka .... 
Bourbon . . . . 235 
Bourou .... 218 

Bout on .... 224 

Brazil, fringing-reefs on coast 

of 80 

Breaches through barrier-reefs 129 

Brook 195 

Bunker . . . . 195 

Bunoa ..... 226 
Byron , , , 204 



Cagayanes ...» 227 
Candelaria . , . .210 
Cargados Carajos , . . 235 

Caroline arch. . . .212 

Caroline isld. ■ . -194 

Carteret shoal . , .217 

Caryophyllia, depth at which 

it lives . . . .108 
Cavilli .... 227 

Cayma?i isld. . . . 260 

Celebes ..... 219 
Ceram ..... 218 

Ceylon, recently elevated . 170 
Ceylon .... 233 

Chagos Great Bank, descrip- 
tion and theory of 58, 59, 142, 143 

Chagos group . , . 145 

Chagos group . . . 234 

Chama-shells embedded in 
coral-rock .... 105 

Chamisso, on corals preferring 
the surf . . . . 87 

Changes in the state of Keel- 
ing atoll . . . . 31 

of atolls . . . 124 



INDEX. 



267 



Channels leading into the 
lagoons of atolls 45, 46, 



PAGE 



the Maldiva atolls 51, 54 



through barrier-reefs 

Chase .... 
China sea . . 
Christmas atoll 
Christmas atoll 



99: 



129 
204 
231 
164 

118 



Christmas isld. (Indian Ocean) 233 
Clarence . . . . 195 
Clipperton rock . . .187 
Cocos, or Keeling atoll . . 19 
Cocos (or Keeling) . . 233 

Cocos isld. (Pacific) . . 187 

Cochin China, encroachments 

of the sea on the coast . 161 
Cochin China . . . 233 

Coettvi . .... 236 
Comoro group . . .237 

Composition of coral forma- 
tions .... 148, 149 
Conglomerate coral-rock on 
Keeling atoll . . .28 

on other atolls . . 43 

coral-rock . . . 148 

Cook islds., recently elevated, 

165, 174 
Cook islds. . . . .192 
Coral-blocks bored by vermi- 
form animals . . 30, 149 
Coral-reefs, their distribution 
and absence from certain 
areas . . . '83 
destroyed by loose sedi- 
ment . . . .89 
Coral-rock at Keeling atoll 27, 28 

77 
149 

3i 

108 
22 
20 



Mauritius 

organic remains of 

Corals dead but upright in 
Keeling lagoon . 

depths at which they 

live .... 107, 

off Keeling atoll . 

killed by a short exposure 

living in the lagoon of 

Keeling atoll . 28, 

quick growth of, in Keel- 
ing lagoon .... 



29 



PAGE 

Corals merely coating the bot- 
tom of the sea . . 81, 82 
standing exposed in the 

Low arch. . . .162 
Corallian sea . . . 158 
Corallian sea . . . 208 
Cornwallis .... 205 
Cosmo ledo . . . . 237 
Couthouy, Mr., alleged proofs 
of recent elevation of the 
Low arch. . . .162 
on coral -rock at Mangaia 

and Aurora islds. . 107 
on external ledges round 

coral-islands . . 134 
remarks confirmatory of 

the author's theory . 162 

Crescent-formed reefs . .142 

Cuba ..... 257 

Cuming, Mr., on the recent 

elevation of the Philippines 170 

Dangerous, or Low arch. . 188 
Danger islds. . . -195 
Depths at which reef-building 

corals live . . . 106 

at Mauritius, the Red 

Sea, and in the Maldiva 

arch no 

at which other corals and 

corallines can live . 1 12 
Dhalac group . . . 246 
Diego Garcia, slow growth of 

reef . . . . -93 
Dimensions of the larger 

groups of atolls . . 118, 119 
Disseverment of the Maldiva 

atolls, and theory of . 58, 139 
Distribution of coral-reefs . 83 
Domingo, St. . . .261 

Dory, Port, recently elevated . 169 
Dory, Port . . . .216 
Duff islds. .... 208 
Durour .... 212 

Eap 214 

Earthquakes at Keeling atoll . 34 



263 



INDEX. 



PAGE 

Earthquakes in groups of 
atolls . . . 125, 126 

in Navigator arch. . 168 I 

East Indian arch., recently 

elevated .... 169 | 
Easter ..... iSS 
Echequier . . . . 211 ! 

Ehrenberg, on the banks of 

the Red Sea . . 81, 245 i 
on depths at which corals 

live in the Red Sea . no 
on corals preferring the 

surf . . . .^7 
on the antiquity of certain 

corals. . . .96 
Eimeo . . . . .189 
Elevated reef of Mauritius . 77 
Elevations, recent proofs of . 164 

immense areas of . 178, 179 

Eli vi . . . . .214 

Elizabeth isld. . . 98 

recently elevated . 165, 176 

Elizabeth isld. . . .188 
El lice group . . . .203 
Encircled islds. , their height. 66 

geol. composition 6S, 70, 71 

Eoua, description of . . 167 
Eoua ..... 200 
Erupted matter probably not 
associated with thick masses 
of coral-rock . . . 149 



168, 175 
. 214 
. 197 
. 215 

246, 247 
. 207 



Fais, recently elevated 

Fiats 

Fanning 

Farallon de Medinilla 

Farsan group 

Fataka . 

Fiji arch. .... 202 

Fish, feeding on corals . ; 30 

Fish killed in Keeling lagoon 

by heavy rain . . 36 

Fissures across coral-islds. . 126 
Fitzroy, Capt. , on a submerged 

shed at Keeling atoll . . 34 
— — on an inundation in the 

Low arch. . . . 124 



PAGE 

194 
222 
256 
216 
229 



122 
209 
217 



Flint 

Flores ..... 
Florida .... 

Folger ..... 
Formosa .... 
Forster, theory of coral-forma- 
tions .... 
Frederick reef 

Freewill .... 
Friendly group recently ele- 
vated . . . 167, 177 
Friendly arch. . . .200 
Fringing-reefs, absent where 

coast precipitous . 73, 74 
breached in front of 

streams 
described by MM. Quoy 

and Gaimard 
not closely attached to 

shelving coasts . . 

of east coast of Africa . 

of Cuba . . 

of Mauritius . 

on worn down banks of 

rock .... 

on banks of sediment 

their appearance when 

elevated 
their growth influenced 

by currents 
by shallowness of sea 



90 
165 

75 
79 
7S 
73 

80 
81 

77 

81 
80 



Galapagos arch. 

Galega . 

Gambier islds., section of 

Gambler islds. 

Gardner 

Gaspar-Rico . 

Geological composition 

coral-formations 
Gilbert arch. 
Gilolo . • 
Gloriosa 
Gloucester isld. 
Glover reef . 
Gomez . 
Gouap . 



. 188 
• 236 
. 69 
. 189 
. 196 
. 205 
of 
[48, 149 
. 204 
. 219 
. 236 
. 124 
. 260 
. 188 
. 214 



INDEX. 



269 



PAGE 

Goulou . . . .214 

Grampus . . . .216 

Gra.fi Cocal .... 203 
Great Chagos Bank, descrip- 
tion and theory of . 58, 142 
Grey, Capt. , on sandbars . 76 
Grouping of the different 

classes of reefs . . .157 
Guedes . . . .217 

Hall, Capt. B., on Loo Choo 170 
Harvey islds., recently ele- 
vated . . . -174 
Height of encircled islds. . 66 
Her mites . . . .211 
Ha?-vey or Cook islds. . .192 
Hogoleu .... 212 
Holuthurise feeding on coral . 30 
Honden isld., height of .118 
Honduras , reef off . . 259 
Horn ..... 202 
Houtmari s Abrolhos . .2.2.1 
Huaheine; alleged proofs of 

its recent elevation . . 174 
Htiaheine . . . .191 
Humphrey . . . . 195 
Hunter .... 202 

Hurricanes, effects of, on 
coral-islds. . . . 124 



PAGE 

Juan de Nova {Madagascar) . 239 



Imrnaum . 


244 


Independence 


203 


India, west coast, recently 




elevated .... 


170 


India ..... 


245 


Irregular reefs in shallow seas 


80 


Islets of coral-rock, their for- 




mation .... 


26 


their destruction in 




the Maldiva atolls 


56 


Jamaica ► . . . 


261 


Jarvis ..... 


195 


Java, recently elevated . 


169 


Java ..... 


222 


Johnston isld. , , 


197 


Juan de Nova 


237 



Kalatoa .... 

Kamtschatka, proofs of its 

recent elevation . 
Karkalang .... 
Keeling atoll, section of reef . 
Keeling, south atoll 

north atoll . 

Keffing 



Kemin 
Kennedy 
Keppel . 
Kumi . 



194, 



223 

178 
219 
19 
233 
2 33 
218 
196 
208 
121 
230 



Laccadive group . . . 234 
Ladrones, or Marianas, re- 
cently elevated . . .168 
Ladrones arch. . . . 215 
Lagoon of Keeling atoll . 28 

Lagoons bordered by inclined 
ledges and walls, and theory 
of their formation 48, 132, 133 

of small atolls filled up 

with sediment . 49, 50 
Lagoon-channels within bar- 
rier-reefs . . . 63, 64 
Lagoon-reefs, all submerged 
in some atolls, and rising to 
the surface in others . . 92 
Lancaster reef . . . 194 
Latie ..... 201 
Lauglan islds. . . . 209 
Ledges round certain lagoons, 

48, 133 
Lette 220 

Lighthouse reef . . . 260 
Lloyd, Mr., on corals refixing 

themselves . . . 104 

Loo Choo, recently elevated . 170 
Loo Choo .... 230 
Louisiade .... 209 
Low archipelago, alleged 

proofs of its recent elevation 162 
Low arch. . . . .188 
Lowness of coral-islds. . .118 
Loyalty group . . . 208 



270 



INDEX. 



149 
172 



PAGE 

Lucepara . . . .226 

Lutke, Adm. , on fissures across 
coral-islands , . . 126 

Luzon, recently elevated . 170 

Luzon ..... 228 

Lyell, Mr., on channels into 
the lagoons of atolls . . 47 

on the lowness of their 

leeward sides . . 137 

on the antiquity of cer- 
tain corals . . .96 

— on the apparent con- 
tinuity of distinct coral- 
islands , 

on the recently elevated 

beds of the Red Sea . 

on the outline of the 

areas of subsidence 

Macassar sir. , 

Macclesfield bank . 
Madagascar, quick growth of 

corals at . 

madreporitic rock of 

Madagascar .... 

Madjiko-sima 

Madura {Java) 

Madura {India) . 

Mahlos Mahdoo, theory of 

formation .... 
Malacca, recently elevated . 
Malacca .... 
Malcolmson, Dr., on recent 

elevation of W. coast of India 

on recent elevation of 

Camaran isld. . 



179 

223 
231 

103 
171 

239 
229 
222 
233 

139 
171 

226 

169 

172 
195 



Maiden. .... 
Maldiva atolls, and theory of 
their formation 44, 134, 135, 139 

steepness of their flanks. 39 

growth of coral at . . 103 



Maldiva arch. 
Mangaia isld. 

recently elevated 



Mangaia 

Mangs 

Marianas, recently elevated 



• 234 
. 107 

148, 175 

• 193 
. 216 

16S 



PAGE 

Mariana arch. . . .215 
Mariere . , . .215 

Marquesas arch. . . .191 
Marshall arch. . . . 204 
Marshall isld. . . .216 
Martinique .... 262 
Mart ires . . . .215 
Mary's St. in Madagascar, 

harbour made in reefs . 89 
Mary isld. . . . .196 
Matia, or Aurora . . ,189 
Matilda atoll. . , . 100 
Mauritius, fringing-reefs of . 73 
depths at which corals 



live there . 
recently elevated 



io6, 107 
. 170 
• 235 
. 69 
. 190 
37, 142 
91 



Mauritius 

Maurua, section of 
Maurua 

Menchikoff atoll . 
Mendana arch. 
Menaana isles . . , 208 
Mexico, gulf of . . -255 
Millepora complasata at Keel- 
ing atoll . . . .21 
Mifidoro . . . .228 
Mohilla .... 238 
Molucca islands, recently ele- 
vated 169 

Mopeha. . . . .191 
Moresby, Capt., on boring 

through coral-reefs . . 99 
Morty , . . . .219 
Mosquito coast . . . 260 
Musquillo atoll . . . 141 
Mysol 218 

Namourrek group . .141 
Natunas .... 226 
Navigator arch., elevation of. 168 
Navigator arch. . . . 199 
Nederlandisch . . . 209 
Nelson, Lieut., on the consoli- 
dation of coral-rocks under 
water . . . .98 
theory of coral-forma- 
tions .... 122 






INDEX. 



271 



PAGE 

Nelson, Lieut., on the Ber- 
muda islds. . . . 263 
New Britain . . .211 

New Caledonia, steepness of 
its reef . . . .62 

barrier-reef of 7 1 ? J 3 2 

140, 158 
New Caledonia . . . 208 
Netv Guinea (E. end) . .211 
New Gttinea ( W. end) . .217 
New Hanover . . .210 

New Hebrides, recently ele- 
vated .... 168 
New Hebrides . . . 205 
New Ireland, recently elevated 168 
New Ireland . . . 205 
New Na7itucket . . .196 
Nicobar islds. . . .235 
Niouha . . . .201 
Nulliporse at Keeling atoll . 24 

on the reefs of atolls . 42 

on barrier-reefs . . 62 

their wide distribution 

and abundance . . 115 

Objections to the theory of 

subsidence . . . 146 

Ocean islds. . , . 1 98, 204 

Ono 202 

Onouafu .... 202 
Ormuz .... 244 

Oscar group .... 203 
Oscillations of level 173, 174, 182 
Ouallan i or Ualan , .212 

Ouluthy atoll . . .101 

Outong Java . . .210 



Palawan, S. W. coast 

N. W. coast . 

western bank 

Palmerston . 
Palmyra . , 
Paracells . 
Paraquas 
Pat chow 
Pelew islds. . 
Pemba isld. 



, 226 

. 227 
, 232 
. 192 
, 197 
. 231 
, 232 
. 229 
. 214 
singular form of 171 



PAGE 

Pemba ..... 242 
Penrhyn . . . . 194 

Peregrino . -. . .194 
Pernambuco, bar of sandstone 

at . . . . .76 
Persian gulf, recently elevated 172 
Persian gulf . . . . 244 
Pescado .... 195 
Pescadores . . . . 230 
Peyster group . . . 203 

Philip 214 

Philippine arch., recently ele- 
vated .... 170 
Philippine arch. . , . 227 
Phcenix . . . .196 
Piguiram . , , .213 
Pitcairn . , . .189 
Pitt's bank . 144 
Pitt isld. .... 204 

Platte 236 

Pleasant .... 204 
Porites, chief coral on margin 

of Keeling atoll . . 20 

Postillions . . . .223 
Pouynipete . . . .160 

its probable subsidence . 161 

Pouynipete . . . .212 
Prat as shoal . . . .231 
Proby ..... 202 
Providence . . . 236 

Puerto Rico . . . .261 

Pulo Anna .... 215 
Pumice floated to coral-islds. 149 
Pylstaart .... 200 
Pyrard de Laval, astonish- 
ment at the atolls in the 
Indian Ocean . . .15 

Quoy and Gaimard, depths 
at which corals live . .Ill 

description of reefs ap- 
plicable only to fring- 
ing-reefs . , .165 



Range of atolls 
Rapa . 
Pearson 



159 

194 

195 



272 



INDEX. 



Red Sea, banks of rock coated 

by reefs . . . .81 
proofs of its recent eleva- 
tion . . . .171 

supposed subsidence of . 173 

Red Sea .... 245 
Reefs, irregular in shallow seas So 

rising to the surface in 

some lagoons, and all 
submerged in others . 92 

their distribution . . S3 

their absence from some 

coasts . . .84 

Revilla-gigedo . . .187 
Ring-formed reefs of the Mal- 

diva atolls, and theory of 52, 134 
Rodriguez , . . . 235 
Rosario . . . . 216 

Rose isld. . . , . 199 

Rotches .... 204 

Rotoumah .... 203 
Roug . . . . .212 
Rowley shoals . . .221 
Riippell, Dr., on the recent 
deposits of Red Sea . . 171 

Sable, He de . . . . 235 
Sahia de Malha . . . 234 
Si!. Pierre . . . .236 
Sala ..... 188 
Salomon arch. . . . 209 
Samoa, or Navigator arch., 

elevation of . . .168 
Samoa arch. . . . 199 

Sand-bars parallel to coasts . 76 
Sandal-wood . . . 220 

Sandwich arch., recently ele- 
vated .... 165 
Sandwich arch. . . . 197 
Sanserot . . . .215 
Santa-Cruz group . . 207 
Savage isld., recently elevated, 

98, 167, 175 

Savage .... 200 

Savu ..... 220 

Saya, or Sahia de Malha . 234 

Scarborough shoal . . . 232 



PAGE 

3° 
211 
191 
149 
221 



Scarus feeding on corals 
Schouten .... 
Scilly ..... 
Scoria? floated to coral-islds. 
Scoffs reef .... 
Sections of islands encircled 
by barrier-reefs . 69, 128 

of Bolabola . . .128 

Sediment in Keeling lagoon 29 

in other atolls 43, 54, 55 

injurious to corals . 89 

transported from coral 

islands far seaward 



Seniavi7ie 

Serajigani 

Seychelles 

Ship-bottom quickly coated 

with coral 
Smyth isld. , 
Society arch., stationary con 

dition of . 

alleged proofs of recent 

elevation 
Society arch. 
Socotra . 
Solor . 
Soolooislds., recently elevated 
Sooloo islds. 
Souvaroff 
Spanish 
Sponge, depths at which found 
Starbuck 
Stones transported in roots of 

trees 
Storms, effects of, on coral 

islds 
Stutchbury, Mr., on the growth 

of an Agaricia 

on upraised corals in 

Society arch 



Subsidence of Keeling atoll 

extreme slowness of 146 

areas of, apparently elon 

gated .... 179 

areas of immense . 178, 179 

great amount of . 181, 190 

Suez, gulf of . . . .251 



149 
212 
219 
235 

104 

197 

162 

174 

189 
244 
222 
170 
227 

195 

215 
112 

194 

149 
124 

105 



174 

42 

182 



INDEX. 



273 



PAGE 

Sulphur islds. . . .216 

Sumatra, recently elevated . 169 
Sumatra . . . .224 

Stcmbawa .... 222 
Surf favourable to the growth 

of massive corals . . 87 

Swallow shoal . . . 232 

Sydney is Id. . . . .196 



Tahiti, alleged proofs of its 

recent elevation 
Tahiti ..... 
Temperature of the sea at the 

Galapagos arch. 
Tenasserim .... 
Tenimber is Id. 

Teturoa .... 
Theories on coral-formations 

115. 

Theory of subsidence, and 
objections to . . 120, 145 

Thickness, vertical, of barrier- 
reefs .... 69, 

Jhomas, St. . 

Tikopia 

Timor, recently elevated 

Timor . 

Timor-latit . 

Tokan-Bessees 

Tongatabou . 

Tonquin 

Toubai 

Toufoa 

Toupoua 

Traditions of change in coral- 
islds. 

Triclacnse embedded in coral- 
rock . 

left exposed in the Low 

arch. . 



Tubular ia, quick growth of 

Tumbelan 

Tumeffe reef 

Ttirtle 



Ualan 



174 
189 

84 
226 
217 
190 

122 



128 
261 
207 
169 
219 
217 
223 
200 

233 
191 
201 
207 

123 

149 



. 162 
. 105 

. 226 
■ 360 
. 202 

. 212 



PAGE 

Vanikoro, section of . .69 
Vanikoro, its state and changes 

in its reefs . . . 161 

Vanikoro .... 207 
Vine reef .... 209 
Virgin Gorda . . .261 
Viti arch. .... 202 
Volcanic islands, with living 
corals on their shores . 84 

matter, probably not 

associated with thick 
masses of coral-rock . 148 
Volcanoes, authorities for their 

position on the map 151, 152, 153 
their presence deter- 
mined by the move- 
ments in progress . 175 

absent or extinct in the 

areas of subsidence I TJ, 178 

Waigiou .... 217 
Wallis isld. . . . .201 

Washington . . . . 197 
Wells' reef .... 209 
Wellstead, Lieut., account of 

a ship coated with corals . 104 
West Indies, banks of sedi- 
ment fringed by reefs . 81 

recently elevated . . 172 

West Indies . . . .252 

Whitsunday isld., view of . 16 

changes in its state . 125 

Williams, Rev. J., on tradi- 
tions of the natives regard- 
ing coral-islds. . . . 123 

on antiquity of certain 

corals . . .107 

Wolchonsky . ., .188 

Wo stock . . . .194 



Xulla islds. . 

York isld. 
Yucutan, coast of . 



. 218 

. 196 

• 259 



Zones of different kinds 0/ 
corals outside the same 
reefs . . . .91, 101 



883 



On Certain Areas of Elevation and Subsidence in 
the Pacific and Indian Oceans, as deduced from 
the Study of Coral Formations. — A Paper read 
before the Geological Society on May 31st, 1837, by 
Charles Darwin, F.G.S. 



-5 



{Abstract, reprinted '/rem the 11 Proceedings" vol. ii.pp. 552-554.) 

The author commenced by observing on some of the most 
remarkable points in the structure of lagoon islands. He then 
proceeded to show that the lamelliform corals, the only efficient 
agents in forming a reef, do not grow at any great depths ; and 
that beyond twelve fathoms the bottom generally consists of 
calcareous sand, or of masses of dead coral rock. As long as 
lagoon islands were considered tne only difficulty to be solved, 
the belief that corals constructed their habitations (or speaking 
more correctly, their skeletons) on the crests of submarine 
craters, was both plausible and very ingenious ; although the 
immense size, sinuous outline, and great number, must have 
startled any one who adopted this theory. Mr. Darwin remarked 
that a class of reefs which he calls " encircling" are quite, if not 
more, extraordinary. These form a ring round mountainous 
islands, at the distance of two and three miles from the shore; 
rising on the outside from a profoundly deep ocean, and 
separated from the land by a channel, frequently about 200 and 
sometimes 300 feet deep. This structure as observed by Balbi 
resembles a lagoon, or an atoll, surrounding another island. 
In this case it is impossible, on account of the nature of the 
central mass, to consider the reef as based on an external 
crater, or on any accumulation of sediment ; for such reefs 
encircle the submarine prolongation of islands, as well as the 
islands themselves. Of this case New Caledonia presents an 
extraordinary instance, the double line of reef extending 140 



276 CORAL FORMATIONS. 

miles beyond the island. Again the barrier-reef, running for 
nearly iooo miles parallel to the North-East coast of Australia, 
and including a wide and deep arm of the sea, forms a third 
class, and is the grandest and most extraordinary coral forma- 
tion in the world. The reef itself in the three classes, encircling, 
barrier and lagoon, is most closely similar ; the difference 
entirely lying in the absence or presence of neighbouring land, 
and the relative position which the reefs bear to it. The 
author particularly points out one difficulty in understanding 
the structure in the barrier and encircling classes, namely, 
that the reef extends so far from the shore, that a line 
drawn perpendicularly from its outer edge down to the 
solid rock on which the reef must be based, very far exceeds 
that small limit at which corals can grow. A distinct class of 
reefs however exists, which the author calls "fringing reefs," 
which extend only so far from the shore, that there is no 
difficulty in understanding their growth. The theory which 
Mr. Darwin then offered, so as to include every kind of 
structure, is simply that as the land with the attached reefs 
subsides very gradually from the action of subterranean causes, 
the coral-building polypi soon again raise their solid masses to 
the level of the water ; but not so with the land ; each inch lost 
is irreclaimably gone : — As the whole gradually sinks, the 
water gains foot by foot on the shore, till the last and highest 
peak is finally submerged. Before explaining this view in 
detail, the author offered some considerations on the probability 
of general subsidences, — such as the small portion of land in 
the Pacific, where many causes tend to its production, an 
argument first suggested by Mr. Lyell, and the extreme 
difficulty (with the knowledge that corals grow at but limited 
depths) in explaining the existence of a vast number of reefs on 
one level, without we grant subsidence, so that one mountain 
top should be submerged after another ; the zoophytes always 
bringing up their stony masses to the surface of the water. 
Subsidence being thus rendered almost necessary, it was shown 
by the aid of sections, that a simple fringing reef would thus 
necessarily be converted by the upward growth of the coral into 



CORAL FORMATIONS. 277 

one of the encircling order, and this finally, by the disappear- 
ance through the agency of the same movement of the central 
land, into a lagoon island. In the former manner a reef skirting 
a shore would be changed into a barrier extending parallel to, 
but at same distance from, the mainland. 

Mr. Darwin then showed that there existed every inter- 
mediate form between a simple well -characterised encircling 
reef, and a lagoon island ; that New Caledonia supplied a link 
between encircling and barrier reefs ; that the different reefs 
produced by the same order of movement were always in juxta- 
position, of which the Australian barrier associated with 
encircled islets and true lagoons, affords a good example. He 
then proceeded to show that within the lagoon of Keeling 
Island, proofs of subsidence might be deduced from many 
falling trees and a ruined storehouse ; these movements appear- 
ing to take place at the period of bad earthquakes, which 
likewise affect Sumatra, 600 miles distant. It was thence 
inferred as probable, that as Sumatra rises (of which proofs are 
well known to exist), the other end of. the level sinks down ; 
Keeling Island thus acting as an index of the movement of the 
bottom of the Indian Ocean. Again at Vanikoro, where the 
structure indicates, according to the theory, recent subsidence, 
violent earthquakes are known lately to have occurred. 

The author then removed an apparent objection to the 
theory, namely, that subsidence would form a disc of coral, but 
not a cup-shaped mass or lagoon, by showing that corals which 
grow in tranquil water are very different from those on the out- 
side, and less effective ; and that as the basin became shallower 
they are subject to various causes of injury. The lagoon never- 
theless is constantly filling up to the height of lowest water 
spring tides (the utmost possible limit of living coral), and in 
that state it long remains, for no means exist to complete the 
work. Mr. Darwin then proceeded to the main object of the 
paper, in showing that as continental elevations act over wide 
areas, so might we suppose continental subsidences would do, 
and in conformity to these views, that the Pacific and Indian seas 
could be divided into symmetrical areas of the two kinds ; the 



278 CORAL FORMATIONS. 

one sinking, as deduced from the presence of encircling and 
barrier-reefs, and lagoon islands, and the other rising, as known 
from uplifted shells and corals, and skirting reefs. The absence 
of lagoon islands in certain wide tracts, such as in both the 
West and East Indies, Red Sea, etc., was thus easily explained, 
for proofs of recent elevation are there abundant. In a like 
manner, in very many cases where islands are only fringed with 
reefs, which according to the theory had not been subsiding, 
actual proofs of elevation were adduced. Mr. Darwin remarked 
that, excepting on the theory of the configuration of reefs being 
determined by the order of movement, the circumstance that 
certain classes which are characteristic and universal in some 
parts of the sea, being never found in others, is quite anomalous, 
and has never been attempted to be explained. 

Mr. Darwin then pointed out the above areas, both in the 
Pacific and Indian Oceans, and deduced the following as the 
principal results : — ist. That linear spaces of great extent are 
undergoing movements of an astonishing uniformity, and that 
the bands of elevation and subsidence alternate. 2. From an 
extended examination, that the points of eruption all fall on the 
areas of elevation. The author insisted on the importance of 
this law, as thus affording some means of speculating, wherever 
volcanic rocks occur, on the changes of level even during 
ancient geological periods. 3. That certain coral formations 
acting as monuments over subsided land, the geographical 
distribution of organic beings (as consequent on geological 
changes as laid down by Mr. Lyell) is elucidated, by the 
discovery of former centres, whence the germs could be dis- 
seminated. 4. That some degree of light might thus be 
thrown on the question, whether certain groups of living beings 
peculiar to small spots are the remnants of a former large 
population, or a new one springing into existence. Lastly, when 
beholding more than a hemisphere, divided into symmetrical 
areas, which within a limited period of time have undergone 
certain known movements, we obtain some insight into the 
system by which the crust of the globe is modified during the 
endless cycle of changes. 



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ROMANCE OF KING ARTHUR. 

ENGLISH OPIUM-EATER. 

LANDOR'S CONVERSATIONS. 

PLUTARCH'S LIVES. 

RELIGIO MEDICI, &c. 

SHELLEY'S LETTERS. 

PROSE WRITINGS OF SWIFT. 

GREAT ENGLISH PAINTERS. 

LORD BYRON'S LETTERS. 

ESSAYS BY LEIGH HUNT. 

MARCUS AURELIUS. 

WHITE'S SELBORNE. 

DEFOE'S SINGLETON. 

MAZZINI'S ESSAYS. 

PROSE WRITINGS OF HEINE. 

REYNOLDS* DISCOURSES. 

PAPERS OF STEELE AND ADDISON 



BURNS'S LETTERS. 

VOLSUNGA SAGA. 

SARTOR RESARTUS. 

SENECA'S MORALS. 

LIFE OF LORD HERBERT. 

ENGLISH PROSE. 

IBSEN'S PILLARS OF SOCIETY. 

IRISH FAIRY AND FOLK TALES. 

EPICTETUS. 

ESSAYS OF DR. JOHNSON. 

ESSAYS OF WILLIAM HAZLITT. 

LANDOR'S PENTAMERON, &c. 

POE'S TALES AND ESSAYS. 

VICAR OF WAKEFIELD. 

POLITICAL ORATIONS. 

CHESTERFIELD'S LETTERS. 

STORIES FROM CARLETON. 

JANE EYRE. 

ELIZABETHAN ENGLAND. 

WRITINGS OF THOMAS DAVIS. 

S PENCE'S ANECDOTES. 

MORE'S UTOPIA. 

SADI'S GULI3TAN. 

ENGLISH FOLK AND FAIRY TALES. 

NORTHERN STUDIES. 

FAMOUS REVIEWS. 

ARISTOTLE'S ETHICS.- 



Edited by Ernest Rhys. 

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Edited by Ernest Rhys. 

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A NEW COMPREHENSIVE EDITION OF THE POETS. 

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CHRISTIAN YEAR. By Rev. John Keble, M.A. 

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WORDSWORTH. Edited by A. J. Symington. 

BLAKE. Edited by Joseph Skipsey. 

CHATTERTON. Edited by John Richmond. 

BURNS. Songs. Edited by Joseph Skipsey. 

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MARLOWE. Edited by Percy E. Pinkerton. 

KEATS. Edited by John Hogben. 

HERBERT. Edited by Ernest Rhys. 

VICTOR HUGO. Translated by Dean Carrington. 

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OWEN MEREDITH. Selected by M. Betham-Edwards. 

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GREAT WRITERS. 

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LIFE 

LIFE 

LIFE 

LIFE 

LIFE 

LIFE 

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LIFE 

LIFE 

LIFE 

LIFE 



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LIVES GREAT AND SIMPLE. 
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OUR AMERICAN COUSINS. 
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OUR QUEEN. 
CONDUCT AND DUTY. 
NEW WORLD HEROES. 
LIFE OF GARIBALDI. 
LIFE OF ROBERT MOFFAT. 
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RICHARD COBDEN. 
JOHN BRIGHT. 
LIFE OF H. W. BEECHER. 
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LIFE OF GRACE DARLING. 
MEMORABLE ENGINEERS. 
VICAR OF WAKEFIELD. 
BOOK OF MARTYRS. 



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UNA MONTGOMERY. By — Jones. 

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OLD CURIOSITY SHOP. By Charles Dickens. 

PICKWICK PAPERS. By Charles Dickens. 

NICFIOLAS NICKLEBY. By Charles Dickens. 

OLIVER TWIST. By Charles Dickens. 

MARTIN CHUZZLEWIT. By Charles Dickens. 

SKETCHES BY BOZ. By Charles Dickens. 

RODERICK RANDOM. By Tobias Smollett. 

PEREGRINE PICKLE. By Tobias Smollett. 

IVANHOE, By Sir Walter Scott. 

KENILWORTH. By Sir Walter Scott. 

JACOB FAITHFUL. By Captain Marryat. 



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PAUL CLIFFORD. 

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ALICE; OR, THE MYSTERIES. 

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LAST OF THE MOHICANS. 

POOR JACK. 

JANE EYRE. 

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