On the structure and distribution of coral reefs

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EDITED BY ERNEST RHYS.,

DARWIN’S STRUCTURE AND DISTRIBUTION
OF CORAL REEFS.

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SHEWING THE DISTRIBUTION .OF THE DIFFERENT KINDS OF CORAL REEFS. TOGETHER WITH THE POSITION OF THE ACTIVE VOLCANOS IN THE MAP. (SEE NOZE IN LEFT HAND CORNER)

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45 60

MCrHE STRUCTURE AND
) DISTRIBUTION OF CORAL”
REEFS, BY CHARLES DARWIN.
EDITED, WITH AN INTRODUC-
TION, BY JOSEPH W. WILLIAMS.

MAR 1 7 1993
LIBRARIES

i O.N-D:O Ni:
WALTER SCOTT, 24 WARWICK LANE.
NEW YORK: 3 EAST 147m STREET.

-

THE WALTER SCOTT PRESS, NEWCASTLE-ON-TYNE,

Pb ea POR ve N Orr.

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

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! has tried

1 Natural History Studies,

vi PREFATORY NOTE.

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.

Fic. 2.—BARRIER-REEF,

PREFATORY NOTE. vii

This last example of reef has been compared by Jukes! 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

Fic. 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. xiil.

Vill PREFATORY NOTE.

sounding-lead, at a distance of 2,200 yards from the reef’s 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 ofcoral. 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.’ 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 book 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, toe, 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 ¢Za/even previous to seeing
‘a true coral-reef.’ ‘No other work of mine,’ he says,’ ‘ was

1 Life and Letters of Charles Darwin, 1888, vol. i. p. 70.

PREPATORY 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 dothis 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

6

ae PREFATORY NOTE.

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. Briefly, this theory
is as follows :—That—as the polypes cannot live below a
depth of 100 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, Ist period) between the sea-
level and the 20-fathom line, and then the island gradually

Fic. 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.¢., 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

PREFATORY NOTE. xl

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 dédr7s, 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! 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.

xii PREFATORY 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

Fic. 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

PREPATOR VY NOTE. xiii

portion of the proof-sheets of his book on Axzmal Life which
related to corals.1 (This book was afterwards translated in the
International Scientific Series.) The letter? 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 Cora/ 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 Tn 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 iiber
Korallenriffe einen Irrthume tiber 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.

XIV PREPATORY NOTE:

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! 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 déérzs 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, Waturf. Gesellsch. Wirzburg, 1869-70, p. 157-

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

PREFATORY 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.! Etna and Vesuvius, it is also believed, originated
as submarine volcanoes on a sea-bed which was afterwards
elevated ;? and in the case of the Islands of Santorin and
Thracia, their structure consists of trass, scoriz, 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. Fouqué, 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.’* Many more examples
may be found in our geological text-books.*

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

1 Drasche in Bericht der K. K. Geol, Reithsanstadt, 1875-1876;
also Vélain, Zes Volcans, 1884.

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

$ Compare on the subject: Fritsch, Z. Deztsch. Geol. Ges., xxiii.,
1871, pp. 125-213; Fouqué’s Santorin et ses Eruptions, Paris, 1880;
Geikie, Zext-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’ A/anual of
Geology.

XV1 PREFATORY 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

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

was the case, for example, with Graham’s Island in the Mediter-
ranean! 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; Prévost, Mém. Soc. Géol. France, ii, p. 91; and
Ann. des Sci. Nat., vol. xxiv.

PREFATORY NOTE. Xvil

scoriz 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 talus of its own débrZs, into
a barrier-reef; provided that, darz 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’ 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.

XVili PREFATORY NOTE.

regions, I concluded that shells, the smaller corals, ete,
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’ 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 Sci., vol. xi. p. 107; see also the
‘*Three Cruises of the Blake” (Bull. Mus. Comp. Zool. Harvard
Univ., vol. xiv. 1888).

PREFATORY 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’ 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? 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,* 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 detached 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. Journ. Sct., 3xd ser., xxxiv. p. 161.

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

8 Trans. Roy. Soc. Edinb., xxxii. p. 545; Proc. Roy. Soc. Edind.,
xili. p. 857.

XX PREFATORY 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 Vature (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’ 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. 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.’ ‘ 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 déérzs. Any one who is acquainted with
the structure of coralline rock, knows how such a porous mass

1 Proc. Roy. Sot., xiiii. p. 440, 1888.

PREFATORY NOTE. XXx1

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 madrepforarza flourish in considerable numbers,
but true reef-building species, forites, meandrina, pocillopora,
and various stout species of madrefora 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 stricta,
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 /uugza, 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

XXil PREFATORY NOTE.

which do not rise above the surface, or are awash for the
greater part of their extent at low tides, great quantities of
débris, 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 débrzs 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. Speakers 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

PREFATORY 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 débrzs, 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 mc@an-
drina soon follows, for whilst the surface of the colony grows
the base is dead, is soon riddled by boring sponges, serpule,
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!—a dis-
tinguished authority and student of corals—has advanced his
opinion that ‘all the hypotheses of objection to Darwin’s

1 Am. Journ. Scz., 1885, p. 190.

XXIV PREFATORY NOTE.

theory are alike weak, for all have made these processes
(z.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 decise, 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.

STRUCTURE AND DISTRIBUTION
(OF CORAL-REEFS. ,

PRE PACE.

———9. a=

I SHALL have occasion, in many parts of the following
volume, to acknowledge the valuable information I have
received from several persons; hut 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 [ 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.
SEES ceeeeeee =

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, 1 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
vee7, 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

Fig.

Fig.

Fig.

Fig,

DESCRIPTION OF THE PLATES.

I.—VANIKORO, situated in the western part of the South
Pacific; taken from the survey by Capt. D’Urville in the
Astrolabe; 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.

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.

3.—POUYNIPETE, or SENIAVINE, in the Caroline Archi-
pelago; from the survey by Admiral Lutké.

PLATE II.

J

I.—BOLABOLA, in the Society Archipelago, from the
survey of Capt. Duperrey in the Coguzl/e: 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.

g. 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.

Fig.

Fig.

Fig.

Fig.

Fig.

Fig.

DESCRIPTION OF THE PLATES. 4

3.—HOGOLEU, or ROUG, in the Caroline Archipelago;
taken from the Adlas 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.

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

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.

PEATE Ihe

I.—MAURUA, in the Society Archipelago ; from the survey
by Capt. Duperrey in the Coguzl/e: height of land about
800 feet.

g. 2.—MALDIVA ARCHIPELAGO, in the Indian Ocean; from

the survey by Capt. Moresby and Lieut. Powell.

3.—NEW CALEDONIA, in the western part of the Pacific;
from Krusenstern’s AZz/as, compiled from several sur-
veys; I have slightly altered the northern point of the
reef, in accordance with the AZlas of the Voyage of the
Astrolabe. In Krusenstern’s Ad/as, the reef is repre-
sented by a single line with crosses; I have for the sake
of uniformity added an interior line.

4.—MAHLOS MAHDOO ATOLL, together with Horsburgh
atoll, in the Maldiva Archipelago; irom 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
to 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. Morésby 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 : 3 ‘ : : ves

CHARTER J:
ATOLLS OR LAGOON-ISLANDS.
SECTION I.—DESCRIPTION OF KEELING ATOLL.

Corals on the outer margin.—Zone of Nulliporez.—Exterior
reef. — Islets. — Coral-conglomerate. — Lagoon. — Cal-
careous sediment.—Scari and Holuthurize subsisting
on corals.—Changes in the condition of the reefs and
islets.—Probable subsidence of the atoll.— Future state
of the lagoon , : : i Pes KG) tio). 210)

SECTION II.—GENERAL DESCRIPTION OF ATOLLS.

General form and size of atolls, their reefs and islets.—
External slope.—Zone of Nulliporze.—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 . 5 : : - 36to 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

ie) 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 AL:
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_—Fringine-
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 : : . 7a

CONTENTS. Ti

GCHAPTER: 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 ; 5 : ‘ ate 83 to 96

SECTION II.—ON THE RATE OF GROWTH OF CORAL-
REEFS 5 ; : F : . 96 to 106

SECTION III.—ON THE DEPTHS AT WHICH REEF-BUILD-
ING POLYPIFERS CAN LIVE 3 } . 106 to 114

CEWA PAPERS Ve

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 . - - : . I15 to 150

_ ENCE TO THE THEORY OF THEIR FORMATION, | :

Description of the coloured map.—Proximity of atolls a
barrier-reefs.—Relation in form and position of atoll:
with ordinary islands.—Direct evidence of subsidence __
difficult to be detected.—Proofs of recent elevation —

Absence of active volcanoes in the areas of sub- —
sidence.—Immensity of the areas which have been
oe elevated and have subsided.—Their relation to the ©
present distribution of the land.—Areas of subsidence _
ES elongated, their intersection and alternation with
Ba those of elevation.—Amount and slow rate of the
as subsidence.—Recapitulation . | : . “Fgrtome

mee APPENDIX.

Lee Containing a detailed description of the reefs and islands
ae in Plate V. : ; ; :

General Index ; : ; :

Se
,

ORAL-REEFS

Le

CORATERELES:

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’ 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,

16 CORAL-REEFS.

in the year 1605, exclaim, “C’est une mérueille de voir
chacun de ces atollons, enuironné d’un grand bane de
pierre tout autour, n’y ayant point @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, 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 Coguille, the reef is seen from within,
from one of the high peaks of the island of Bolabola.t

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

=
-
7.

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

CORAL-REEEFS, 17

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

18 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 ~
informatien; and any particular spot may be found oe
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 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.)

GHA Pa RI.

ATOLLS OR LAGOON-ISLANDS.

SECTION First.—KEELING ATOLL.

Corals on the outer margin.—Zone of Nullipore.—Lxteridr reef,—Tslets.
— Coral-conglomeraie.— Lagoon.—Calcareous sediment,—Scart and
Hloluthurie subsisting on corals.—Changes in the condition of thé

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 wataz: where the letter A is placed, the depth is
26 fathoms, and the distance rather more than 150 yards from the edge of the
reef.

> as 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.

D.—A low projecting ledge of brecciated? coral-rock washed by the waves at
bigh 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 Mr. 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 fayourable circum-
stances, afforded by an unusually low tide and smooth
water, to reach the outer margin, where the coral is alive.
I succeeded only twice in gaining this part, and found it
almost entirely composed of a living Porites, which forms
great irregularly rounded masses (like those of an Astreea,
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 complanata.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 JZ.
alcicornis, 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 Porites, apparently of the
same species with that on the outside, is found there,
although it does not seem to thrive, and certainly does
not attain the thousandth part in bulk of the masses
opposed to the breakers.

The 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°.1 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 Zllepora alcicornis were brought up; and on
the arming from an eight-fathom cast, there was a perfect
impression of an Astreea, 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 JZ. corymbosa, were the most abundant. As I searched
in vain in the hollows on the reef and in the lagoon, for
a living specimen of this Madrepore, I 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.

wey

. CORAL-REEFS. 23

is confined to a zone outside, and beneath the surface,
where it must be very abundant. Fragments of the AZ7/e-
pora alcicornis and of an Astrea were also numerous ;
the former is found, but not in proportionate numbers,
in the hollows on the reef; but the Astreea 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 yast 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 Porites and of the Millepora is
dead, three species of Nullipora flourish. One grows in
thin sheets, like a lichen on old trees; the second in stony
knobs, as thick as a man’s finger, radiating from a common
centre ; and the third, which is less common, in a moss-like
reticulation of thin, but perfectly rigid branches! The
three species occur either separately or mingled together;
and they form by their successive growth a layer two or
three feet in thickness, which in some 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 Nulliporz, 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 Nulliporse? 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 knobbed at the extremities. The extremities only are alive and
brightly coloured. The two other species are of a dirty purplish-white.
The second species is extremely hard ; its short knob-like branches are
cylindrical, and do not grow thicker at their extremities.

2 The Nullipores belong to the Corallinaceze, a group of the sub-class
Carpophycez and the class Algz. Their distinctive characteristic is
the encrustation of the thalli with calcium carbonate, hence their
resemblanc:2 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* 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
Nulliporz on the outer margin, the part most exposed to the
breakers, and this must aes 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 Focz/lopora
verrucosa, which grows in short sinuous plates, or branches,

1 Nipple-shaped.—Ep.

26 CORAL-REEFS.

and when alive is of a beautiful pale lake-red: a Madre-
pora, closely allied or identical with AZ poctiijera, 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 100 to 200, or even 300 yards,
and is strewed with a few large fragments of coral torn up
during gales: it is uncovered only at low water. I could
with difficulty, and only by the aid of a chisel, procure
chips of rock from its surface, and therefore could not
ascertain how much of it is formed by the aggregation of
detritus, and how much by the outward growth of mounds
of corals, similar to those now living on the margin.
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
Nulliporz, 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 frorh 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 ; 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 asa 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! 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 Seriatapora subulata; two species of

1 Resembling spar.—ED.

Aer.
WZ
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MENCHICOFF ATOLL 2

FIG 2X. 258

by
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WE i pi!

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aa Bartholomew. Edin?

oan
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CORAL-REEFS. 40

Porites! 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 $.E. shore of the lagoon, affording a thick
growth Of a Fucus,? on which turtle feed: this mud,
although discoloured by vegetable matter, appears from its
entire solution in acids to be purely calcareous. I have
seen in the Museum of the Geological Society, a similar
but more remarkable substance, brought by Lieut. Nelson
from the reefs of Bermuda, which, when shown to several

1 This Porites has somewhat the habit of P. clayaria, 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.

* A genus of sea-weeds.—ED.

= CORAL-REEFS.

chalk. On the outside of the reef much sediment must be i

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,! 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 Holothurize (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.—Eb.

CORAL-REEFS. 31

srowth 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

2: 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 ‘ 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
Porites with their summits apparently lately killed, and their
sides only three or four inches lower down thickened by a
fresh layer of living coral. But there is a difficulty 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 Nulliporz, and so much

_ of the surface would have been of equal height with the

existing zone of living Nulliporz: 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 Nulliporee,
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 Nulliporze 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,

es 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 of Nulligore.—Conglomerate.—Depth of lagoons.—Sediment.
—Reefs submerged wholly or in part.—Breaches in the reef.—
Ledge-formed shores round certain lagoons.— Conversion of lazoons
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-REEEFS. 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! 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 Lutké’s chart of the
Caroline atolls, we see many instances of the former case;
_and the occurrence of islets, as if placed for beacons, on
the points where there is a gateway or breach through the

* Beechey’s Voyage to the Lactfic and Beering’s Straits, chap, Vili.

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 horseshoe 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 Lutké, 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! 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?
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 (Fzrst 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,

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The slope round Christmas atoll (lat. 1° 4’ N., 157° 45’ W.),
described by Cook,}? 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? 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 too 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. Iam 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.

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.!_ 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? 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 Mémoires pour servir a une description Geolog. de France, tome iv.
p- 216.

- CORAL-REEFS. ai

is much less at the extremities of the more elongated atolls
in the Low Archipelago, than at their sides ; in speaking of
Ducie’s Island he says! the buttress, as it may be called,
which “has the most 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 Vayage, 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. Although Chamisso
does not state that the masses of Nulliporee form points
or a mound, higher than the flat, yet I believe that this
is the case; for Kotzebue,? in another part, speaks of the
rocks on the edge of the reef “as visible for about two
feet at low water,” and these rocks we may feel quite
certain are not formed of true coral.2 Whether a smooth
convex mound of Nullipore, 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 Nullipore.
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 Nullipore are closely related to those
described on the coral-reefs, but I believe are of different species.

* 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 Serpulz, 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 Serpulz hold the same position, relatively to the
action of the sea, with the Nulliporz 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.

hte

CORAL-REEFS. 43

if artificially constructed to protect the margin of Keeling
Island, is of frequent occurrence round atolls, 1 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! a
layer of coarse conglomerate, outside the islets round the
Marshall atolls which ‘‘appears on its upper surface uneven
and eaten away.” From drawings, with appended remarks,
of Diego Garcia in the Chagos group and of several of the
Maldiva atolls, shown me by Captain Moresby,? it is
evident that their outer coasts are subject to the same
round of decay and renovation as those of Keeling
atoll. From the description of the atolls in the Low
Archipelago, given in 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 Kotzebue’s First Voyage, vol. iii. p. 144.

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

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

Where the waves act with unequal force on the two sides
of an atoll, the islets appear to be first formed, and are
generally of greater 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, equaliing 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 T may here observe that on the coast of Brazil, where there is much
coral, the soundings near the land are described by Admiral Roussin,
in the Pilote du Brésil, as siliceous sand, mingled with much finely
comminuted particles of shells and coral. Further in the offing, fora
space of 1,300 miles along the coast, from the Abrolhos Islands to
Maranham, the bottom in many places is composed of ‘‘tuf blanc,
mélé ou formé de madrépores broyés.” This white substance, probably,
is analogous to that which occurs within the above-mentioned lagoons;
it is sometimes, according to Roussin, firm, and he compares it to
mortar.

CORAL-REEFS. 4s

Mourileu atoll,! 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 4o 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 Lutké’s Voyage autour du Monde, vol. ii. p. 201.
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.

ss

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! 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,? 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.
* Beechey’s Voyage, 4to ed., vol. i. p. 189.

a

48 CORAL-REEFS.

where there are coral-reefs; but this is by no means the ©
universal structure in other atolls. Chamisso,* speaking in
general terms of the lagoons in the Marshall atolls, says the
lead generally sinks “from a depth of two or three fathoms
to twenty or twenty-four, and you may pursue a line in
which on one side of the boat you may see the bottom, and
on the other the azure-blue deep water.” ‘The shores of
the lagoon-like channel within the barrier-reef 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,* 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 Popes
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! 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.

869

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 resing
to the surface.—Fosition 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.—Tts submerged condition and extraordinary
structure.

Alihough 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. 51

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! 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 J@/lepora 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 léss 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

3

CORAL-REEPS. Be

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, Phaleedoo, 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 other. 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 too fathoms. Powell’s Island is situated at the
distance of two miles and a half off the northern end of

LLATE IF .

GREAT CHAGOS BANK oe e Ae eer SS.
5 ey = Caer : iN
The shaded parts arc from 010 f™ under water = Sen ee ice Gre Bo

v

2
=
3
=
Ss
=
=

Level of the Sea

15 \ta 20,77

4o to go\ fir’

East & West Section across the Gr. Chagos Bank 76 miler nr length

Fig 2

PEROS BANHOS ATOLL
L1G. 3.

Bartholomew. Edin”

CORAL-REEEFS. 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. 1, in which, for the sake
of clearness, I have had the parts under ten fathoms deep
finely shaded: an east and west vertical section is given in
Fig. 2, in which the yertical scale has been necessarily

bein

CORAL-REEFS. 89

e

Gexacverated. 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 inacircle. ‘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
part, no bottom was found with 190 fathoms; and off
another point, at a somewhat greater distance, there was
none with 210 fathoms. Small steep-sided banks or knolls,
covered with Juxuriantly 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,! “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.

60 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.

CEuAk Lik tk

BARRIER-REEFS.

Closely resemble in general form and structure atoll-reefs.— Width and
depth of the lagoon-channels.—Lreaches through the reef in front of
valleys, and generally on the leeward stde.—Checks to the filling up
of the lagoon-channels.—Size and constitution of the encircled
islands.—Number of tslands within the same reef.—Barrier-reefs
of New Caledonia and Australia.—FPosition of the reef relative to
the slope of the adjocning land.—Lfrobable great thickness of
barrier-7eefs.

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. ee,

reefs of the barrier and atoll classes may be seen in the ~
small, but accurately reduced charts on the Plates, 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? 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 little information regarding the kinds of corals
which live on the outer margin. When I visited the reef
at Tahiti, although it was low water, the surf was too violent
for me to see the living masses; but, according to what I
heard from some 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 Nulliporz, 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. Mem., vol. iii.

CORAL-REEPFS. 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! 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,?
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.

64 CORAL-REEFS.

with true lagoons. In some cases they are open, with a
level bottem 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 chart
of Ulietea, however, there is one sounding laid down of
48 fathoms; at Vanikoro there are several of 54 and one of
564 fathoms (English), a depth which even exceeds by a
little that of the interior of the great Maldiva atolls. Some
barrier-reefs have very few islets on them; whilst others are
surmounted by numerous ones; and those round part of
Bolabola (Plate 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 Facific 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 Hawkesworth’s 4to ed. of Cook’s First
Voyage.

|"
ie

CORAL-REEFS. 68

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. Asa 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
ITI., 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 ae out

7°

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! 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 (Geograph. 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.

oe
ie <

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+ 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

i
is 's
es

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
3032 f6
ep YY | ; i ij
jill | | OO

WRG

B = A = —
Ss,

a. stain nanllr Mh lllintton tera,

South. North,

1. Vanikoro, from the Atlas of the Voyage of the Astrolabe, by D. D’Urville.

2. Gambier Island, from Beechey.

3. Maurua, from the Atlas of the Voyage of the Coquilie, 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,! 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 Tn 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 At/as 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, anda 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,! 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 (/ourn. 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;! and Aitutaki consists of calcareous
rock. We must, therefore, reject these several explanations,
and conclude that the vertical thickness of barrier-reefs,

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

72 CORAL-REEEFS.

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.

7 —

CHAPLER: EIT,

- FRINGING OR SHORE-REEFS.

Reefs of Mauritius.—Shallow channel within the reef.—TIts slow filling
up.— Currents of water formed within it.— Upratsed reefs.— Narrow
Jrinsing-reefs in deep seas.—Reefs on the coast of £. Africa and of
Brazil.—Fringing-reefs in very shallow seas, round banks of
sediment and on worn-down tslands.—Ffringing-reefs affected by
currents of the sea.—Coral coating bottom of the sea, but not
Sorming 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,!
where the coast is almost precipitous, and where, if as is

- 1 This fact is stated on the authority of the Officier du Roi, in
his extremely interesting Voyage a PJsle de France, undertaken in
1768. According to Captain Carmichael (Hooker’s Bot. Mzsc., 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

7

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. Buta
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! 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,? 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

Be oh! oa)

ee ae eee. ee ee ae ee

1 Capt. Grey’s Journal of Two Expeditions, vol. i. p. 369.
2 I have described this singular structure in the Loud. and Edin.
Phil. Mag., October 1841.

CORAL-REEFS. 77

under certain circumstances. To this latter belief I am
led, by finding that channels are almost universally present
within the fringing-reefs of those islands which have 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! of an interesting pedestrian tour round the Mauritius
seems to have met with a structure of this kind: he says,
‘“Vobservai que 1A, ot la mer étale, indépendamment des
rescifs du large, il y a a terre ume 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 elévé de plus de quinze
pieds: c’est une espéce de rescif, que la mer a abandonné:
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 al Isle de France, par un Officier du Rot, part i. pp. 192,
200.

78 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 Astrea 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 18°. 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! 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 IT may give Cuba, as another instance; Mr. Taylor (Loudon's
Mag. of Nat. Hist., vol. ix. p. 449) has described a reef several
miles in length between Gibara and Vjaro, which extends parallel
to the shore at the distance of between half and the third part of a
mile, and 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? 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. 1° 15’ to 1° 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,?
“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 halfa mile. Part of this reef is very symmetrical,
and has a uniform breadth of 200 yards.

The coast of Brazil is in many parts fringed by reefs. Of

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

* 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! 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 4o
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 Brésil, and accompanying hydro-
graphical memoir.

CORAL-REEHFS. 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 FE 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.

‘
na
seit ‘a

o 2 ea

CHAPTER. TV.
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 newto add. The Bermuda
Islands, in 32° 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 28° 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. 3

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

—s ee ee Oe s

1 T 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 2oth 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 Beag/e, 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.t
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,? 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 (Geograph,
Journ., vol. ii. p. 89), that the reefs off Cape St. Anne and the
Sherboro’ Islands were of coral, although the author states that they
are not purely coralline. But I have been assured by Lieut. Holland,
R.N., that these reefs are not of coral, or at least that they do not at
all 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
Holothurize 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

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CORAL-REEFS. Sy

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 areef is most favourable to the growth of coral. The
great mounds of living Porites and of Millepora round
Keeling atoll occur exclusively on the extreme verge of
the reef, which is washed by a constant succession of
breakers ; and living coral nowhere else forms solid masses.
At the Marshall islands the larger kinds of coral (chiefly
species of Astrzea, a genus closely allied to Porites) ‘‘ which
form rocks measuring several fathoms in thickness,” prefer,
according to Chamisso,! the most violent surf. I have
stated that the outer margin of the Maldiva atolls consists
of living corals (some of which, if not all, are of the same
species with those at Keeling atoll), and here the surf is so
tremendous, that even large ships have been thrown, by a
single heave of the sea, high and dry on the reef, all on
board thus escaping with their lives.

Ehrenberg” remarks, that in the Red Sea the strongest
corals live on the outer reefs, and appear to love the surf;
he adds, that the more-branched kinds abound a little way
within, but that even these in still more protected places
become smaller. Many other facts having a similar tend-
ency might be adduced.? It has, however, been doubted

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

2 Ehrenberg, Uler die Natur und Bildung der Corailen Binke im
rothen Meere, p. 49.

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

88 CORAL-REEES.

by MM. Quoy and Gaimard, whether any kind of coral can 4

even withstand, much less flourish in, the breakers of an
open sea:! 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 Annales des Sciences Naturelles, tome vi. pp. 276, 278.—‘** La ou
les ondes sont agitées, les Lytophytés ne peuvent travailler, parce
qu’elles détruiraient leurs fragiles édifices,” ete.

a ; ars
sai ilies neath ail as ene

CORAL-REEEFS. 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,* 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.? 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,? 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 Nitur, etc., etc., p- 46.

2 Tbid., p. 49.

8 Capt. Owen on the Geography of the Maldiva Islands, Geograph.
Journal, vol. ii. p. 88.

== 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.!
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,? 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 ree: 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 (Rezse. 2 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. gI

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.
Inthe 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 Mc//epora 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,! 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; whereas 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
avery long period. With this fact, and with the knowledge,

04 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

4
*
i

CORAL-REEEFS. 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 aréa and those in another.

06 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! 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 anda
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,? with respect to the thickness of
some upraised beds of coral, which they examined at Timor
and some other places. Ehrenberg? saw certain large
massive corals in the Red Sea, which he imagines to be of
such vast antiquity, that they might have been beheld by
Pharaoh ; and according to Mr. Lyell* 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® has adduced the case of

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

2 Annales des Sciences Nat., tom. vi. p. 28.

8 Ehrenberg, zz sz., p. 42.

4 Lyell’s Principles of Geology, book iii. chap. xviii.
5 Beechey’s Voyage to the Pacific, chap. viii. :

4

"l

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! 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.
$72

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,! 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,? to have a similar
structure, and its shores are about forty feet high: some of
the Cook Islands also appear? to be similarly composed.
Capt. Belcher, R.N., in a letter which Capt. Beaufort
showed me at the Admiralty, speaking of Bow atoll, says,
“‘T 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.
8 Williams’s Warrative of Missionary Enterprise, p. 30.

»
¢
4

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 ; 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 Nulliporge cannot increase outwards
without encroaching on the Porites and J/lepora com-
planata, as is now taking place; nor these latter without
encroaching on the strongly branched Madrepora, the JZz//e-
pora alcicornis, and some Astras ; 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

Adele) 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,! ‘‘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.”? 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 7hzrd Voyage, book iii. chap. x.
2 Beechey’s Voyage to the Pacific, chap. vii. and viii.

i

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
Lutké,1—without it could be shown that, in these cases, the
conditions were favourable to the vigorous and unopposed
stowth 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 ufward 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. Lutké’s Voyage autour du Monde. In the group Elato, how-
ever, it appears that what is now the islet Falipi, is called in Cantova’s
Chart, the 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 Nulliporze 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-
pore? 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 diy 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 ‘‘extcrely 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 ‘¢hree 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! 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, LN.,
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 witha
layer of coral, ¢wo 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 fomly fixed by the
secretion of a stony matter, to the bottom.

br gy ig

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.

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. 1, 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 areef, 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. Isaw 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 Coquzile), has remarked, ‘‘que constam-
ment ses valves étaient engagés complétement dans la masse des
Madrepores.”

106 CORAL-REEPFS.

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 which Reef-butlding 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 AZ. 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

ins

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 Astrsea, with rather large! 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 Astrza. Besides these lithophytes,
some fragments of the A/7/epora 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. subu/azta, 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.

108 ; CORAL-REEEFS.

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 JZ. poci/iijera, 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
o to 20 fathoms. Between twenty and thirty-three fathoms
I obtained several soundings, and they all showed a sandy
bottom, with one exception at 30 fathoms, when the arming
came up scooped out, as if by the margin of a large
Caryophyllia. Beyond 33 fathoms I sounded only once;
and from 86 fathoms, at the distance of one mile and a ~
third from the edge of the reef, the arming brought up
calcareous sand with a pebble of volcanic rock. ‘The 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 flourish 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

Ito 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.”! 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, User die Natur, etc., p. 50.

CORAL-REEFS. Ill

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

MM. Quoy and Gaimard! believe that the growth of
coral is confined within very limited depths; and they state
that they never found any fragment of an Astrzea (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 Astrea. J//lepora
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 S7deropora scabra
(Porites of Lamarck) brought up alive from 17 fathoms.
Mr. Couthouy? 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 :° 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. We ought, however,

1 Annales des Sct. Nat., tom. vi.

2 Remarks on Coral Formations, p. 12.

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

4 T 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

=e CORAL-REEFS.

to feel less surprise at this fact, as Caryophyllia alone of the a
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.

Depth in Country and S. |
Name of Zoophyte. | HaAGROIS! eats adc | ee Authority.
Sertularia . : : t : 40 Cape Horn 60° [Where none
Cellaria S : Ditto Ditto is given, the
», A minute scarlet encrust- observation is
ing species, found living . 190 Keeling At. 12° |my own.]
3 An allied, small stony
sub-generic form . 48 S. Cruz Riv. 50°
A coral “allied to Vincularia,
with eight rowsofcells . 40 Cape Horn
Tubulipora, r near to T. patima.; Ditto Ditto
Do. do. 94 East Chiloe 43°
Cellepora, several species, and,
allied sub-generic form... | 40 Cape Horn
Ditto. . ys . | 40and 57 | Chonos Arch. 45°
Ditto . : : : : a 48 | S. Cruz 50°
Eschara . : eRe =F 30 Tierra del Fuego 53°
Ditto . i : : at 48 S. Cruz R. 50°
Retepora | 40 Cape Horn
| ney aoe
Dilton. < 100 C. Good Hope 34° |4 Wate, — nm
Millepora, a strong coral with) ! = Aue
cylindrical branches, of aj! | 2
pink colour, about two | 94and30 |= . Chiloe 43°, Tierra!
inches high, resembling in [| “| del Fuego 53°
the form of its orifices wf '
aspera of Lamarck . H |
| peysnnek in
Coralium .. “ha 120 | Barbary 33° N. etetpon
(ieee 1752.
Antipathes ; é : : ‘ 16 Chonos 45° }
f Cp. Beogi
Te an . : Abrolhos on the | } informed me
Gorgonia (or an allied form) . | 160 {coast of Brazil1s° |) of this fact

in a letter.

Ellis (Wat. Hist. of Coralline, p. 96) states that Ombellularia was
procured in lat. 79° N. stzcking 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. 113

found in Zetland! in Lat. 60° 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. Ata greater depth than 90 fathoms off this
coral island, the bottom was thickly strewed with joints of Halimeda
and small fragments of other Nullipore, but alldead. Captain B. Allen,
R.N., informs me that in the survey of the West Indies it was noticed
that between the depth of 10 and 200 fathoms, the sounding Jead 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 overa wide area. On the beach, within the coral-reefs
of Mauritius, vast quantities of fragments of Nulliporze 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

itA CORAL-REEEFS.

flourish at greater depths than between 20 and 30
fathoms. ;

It has been argued! that reefs may possibly rise from
very great depths through the means of small corals, first
making a platform for the growth of the stronger kinds.
This, however, is an arbitrary 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. 218.

CHAPTER V.

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

The atolls of the larger archipelagoes are not formed on submerged
craters, or on banks of sediment.—Immense areas interspersed with
atolls.— Their subsidence.—The effects of storms and earthquakes
on atolls.—Recent changes in their state.—The origin of barrier-
reefs and of atolls.— Their relative forms.—The step-formed ledges
and walls round the shores of some lagoons.—The ring-formed
reefs of the Maidiva atolis.—The suimerged 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 requtred
by the theory, considered.— The probable composition of the lower
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

116 CORAL-REEFS.

like the northern Maldiva atolls, made up of numerous — I
ring-formed reefs, piaced 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 littleimprobable

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, 1 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 aring. 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 Férst Voyage, vol. ill. p. 331.

GORADREEPS, 17

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 Boney
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

118 | 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 Lutké, regarding the Pacific; and from
Moresby! with respect to the Indian Ocean, that in the
following cases the term “low island” strictly means
land of the height 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? 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 eagle, whilst sailing by, is 114
feet from the szmmit of the trees to the water's edge. This island
appeared to resemble the other atolls of the group.

aa

CORAL-REEFS, 119

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 too 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
of 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 atoH 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

ti alle ek Sle

CORAL-REEFS. 121

reef-building polypifers can exist, and it be assumed that
they can flourish at a depth of even too 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,! 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.
Vv. 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 Odserv., 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 pe/PES to
other genera, and to other families!

4

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,! even in the first
edition of his /7vinciples 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? 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. ili. p. 386.
2 Williams's Varrative 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”! 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.”® 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

are

1 Kotzebue’s Fzrst Voyage, vol. iii. p. 168.
2 M. Desmoulins in Comptes Rendus, 1840, p. 837.
8 West of England Journil, No. 1, 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,! “‘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:”? 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-REEHFS.

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! 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 Lutké, 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, 1 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
136.

CORAL-REEES. 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

Mili
[No. 4.]

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

BB—Shores of the island.

A’A’—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! E. and W. through the highest point
of the encircled island of Bolabola; of which a plan is_
given in Plate IL, Fig. 1. 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 Adas of
the Voyage of the Coguziie. 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 bartrier-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

PAS ee A’
ee eee Peter ee : 29 2
™ tyre ea adr vis SRO ieee a a D gsoerecoES ELS ITT As te
“
; L 2
, La
7%, ,
yj. My =
il = if
Wf" Level of Sea J

[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 trueatoll ; 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 ncticed, 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.

Stepformed ledges round certain lagoons.—If we suppose
an atoll to subside at an extremely slow rate, it 1s 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,

134 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 northern 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; ifso, 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 reejs.—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-REEEFS. 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,! 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

{40 CORAL-REEEFS.

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-REEEFS. 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.!

Trregularly-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 Lutké’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.

142 CORAL-REEFS.

Menchicoff 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 ofa
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-REEEFS. 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 4o 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 ro to 20 fathoms on them; and not far eastward
soundings were struck on a sandy bottom, with between
tro 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

875

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 4o
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 T 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. Ihave 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 scoriz, 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

og

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.

CEENE PER NE

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.
— elation in form and position of atolls with ordinary islands,—
Direct evidence of subsidence difficult to be detected.—Froofs of
recent elevation where fringing-reefs occur.— Oscillations of level.—
Absence of active volcanoes tn the areas of subsidence.-—Lmmensity
of the areas which have been elevated and have subsided.— Their
relation to the present distribution of the lund.—Areas of sub-
stdence elongated, their intersection and alternation with those
of elevation.—Amount, and slow rate of the substdence.—Re-
capitulation.

Ir 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
obyious 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 ved 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 4veached in front of riyers 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 T 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), Ihave 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

coe a
» . %,

154 CORAL-REEEFS.

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 coiour 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, 11 Lief. S. 56). Malte
Brun, vol. xli. 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 opfoszte coast by Cook (2%d 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. Descrépt.) 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 Co//ect., 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’
Voyase, p. 41). Mr. M‘Clelland (Report of Committee for investigating
Coal tx 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 Szd/z-
man’s 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 (Hzst. Nat. des Volcans, 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 (Waut. Maz.,
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-REEEFS. 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! 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? has described an atoll-formed reef in
lat. ro°", 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 goo 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 Ausiradzs, vol. li. 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 5$.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

266 - 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 Pouynipéte, 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.—W ith 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,! 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,
“ow 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 Pouynipéte (Plate I., Fig. 3), in the Caroline Archipelago, being
encircled by a barrier-reef, must have subsided. In the Mew 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, zow 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

162 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 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 Iam 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 Tridacnz 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 of Madrepore, with 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

J
:
2

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 the 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

164 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.

- .

\

ty

to 4 a
ope Shik te lel | eel aes ¥

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’* I was astonished to find, for I knew that
they had crossed both the Pacific and Indiain 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? it composes three strata, each
about ten feet thick. Nihau, which forms the northern, as
Hawaii does the southern end of the group (350 miles in
length), likewise seems to consist of coral and volcanic rocks.
Mr. Couthouy® has lately described with interesting details,
several upraised beaches, ancient reefs with their surfaces

1 Annales des Sciences Nat., tom. vi. p. 279, etc.

2 Zoolozy of Capt. Beecheys Voyage, p. 176. See also MM. Quoy
and Gaimard in Aunales des Scien. Nat., tom. vi.

2 Remarks on Coral Formation, p. 51.

166 CORAL-REEFS.

perfectly preserved, and beds of recent shells and corals, at the — 3
islands of .Maui, Morokai, Oahu, and Tauai (or Kauai) in this

group. Mr. Pierce, an intelligent resident at Oahu, is con- Sn’
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 — 4

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,! 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 tnprobability 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
Rey. 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? it is an upraised reef; “and

1 Beechey’s Voyage in the Facific, 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? describes the plants as already grow-
ing out of the dead, but still upright and spreading trees of
coral; and the younger Forster? believes that an ancient
lagoon is now represented by a central plain; here we cannot
doubt that the elevatory forces have recently acted. The
Same conclusion may be extended, though with somewhat less
certainty, to the islands of the Friendly Group, 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.”? 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 atoil with one half originally imperfect, or one
unequally elevated ; and Anamouka, an atoll equally elevated.
This latter island contains‘ 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.

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

4 Jbid., vol. i. p. 235.

168 CORAL-REEFS.

observes, rises above the height of true lagoon-islands), coral-
rock, like that on the beach, was found. In the Mavzgator
Archipelago, Mr. Couthouy? 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. a
quakes are very frequent in this archipelago.

Still proceeding westward we come to the Vew 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, 1 have no infor-
mation ; but at New Ireland, which forms the northern point of
the latter chain, both Labillardiére and Lesson have described
large beds of an apparently very modern madreporitic rock,
with the form of the corals little altered. The latter author?
states that this formation composes a newer line of coast,
modelled round an ancient one. There only remains to be
described in the Pacific, that curved line of fringed islands,
of which the 4/avzanas form the main part. Of these Guam,
Rota, Tiniam, Saypan, and some islets farther north, are
described by Quoy and Gaimard,’? and Chamisso,‘ 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. azs, which lies in the prolonged line

1 Remarks on Coral-Formations, p. 50.

2 Voyage de la Coquille, Part. Zoolog.

3 Freycinet’s Voyage autour du Monde. See also the Hydrographical
Memoir, p. 215.

4 Kotzebue’s Fzrst 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.*

In the East Indian Archipelago, many authors have recorded
proofs of recent elevation. M. Lesson? states, that near Port
Dory, on the north coast of New Guinea, the shores are flanked,
to the height of 150 feet, by madreporitic strata of modern date.
He mentions similar formations at Waigiou, Amboina, Bourou,
Ceram, Sonda, and Timor: at this latter place, MM. Quoy and
Gaimard? have likewise described the primitive rocks, as coated
to a considerable height with coral. Some small islets east-
ward of Timor are said in Kolff’s Voyage* to resemble small
coral islets upraised some feet above the sea. Dr. Malcolmson
informs me that Dr. Hardie found in /ava an extensive for-
mation, containing an abundance of shells, of which the greater
part appear to be of existing species. Dr. Jack® has described
some upraised shells and corals, apparently recent, on Pulo
Nias off Swmatra,; 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®

1 Lutké’s Voyage, vol. ii. p. 304.

2 Partie Zoolog., Voyage de la Coquiille.

3 Ann. des Scien. Nat., tom. 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 (7vans. Astat. 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 (zézd., part
i., p. 131) mounds of shells lying two miles inland on this line of coast.
8 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 PAz/ppines, 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! of the lines of inland reefs, and walls of coral-rock
worn into caves, above the present reach of the waves, at the
Loo Choo Islands, there can be little doubt that they have been
upraised at no very remote period. :
Dr. Davey? describes the northern province of Cey/on 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? 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 madreporitie formation is men-
tioned by M. Cordier in his report to the Institute (May 4th, 1839), on
the voyage of the Chevredte, 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 Scient. 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.

co

CORAL-REEEFS. 171

chapter on fringing-reefs. The northern extremity of J7ada-
gascar is described by Capt. Owen! as formed of madreporitic
rock, as likewise are the shores and outlying islands along an
immense space of Eastern Africa, from a little north of the
equator for 900 miles southward. Nothing can be more vague
than the 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. Riippell? 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 4o 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 Riippell, Reise cx Abyssinien, Band, i. s. 141.

172 CORAL-REEFS.

Dr. Malcolmson (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 4o 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 4o feet, Mr. Burton! found a deposit
replete with existing species of shells, at the height of 200 feet.
In an admirable series of drawings by 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 Guif 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. Ainsworth? 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, Polzt. 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 Przuzczfles, 5th ed., vol. iv. p. 22.—Santa Cruz
and Barbadoes, Prof. Hovey, Sz//cman’s Journ., vol. xxx. p. 74.—St.
Domingo, Courrojolles, Jour. de Phys., tom. liv. p. 106.—Bahamas,
United Service Journal, No, \xxi. 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! have stated that they have observed shells

Beche, Geo/. Man., p. 142.—Cuba, Taylor, in Lond. and Edin. 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 Polynestan 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, War, of Mliss. Enterprise, p. 213; 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 evidences; also Mr. Stutchbury, West of England Journ., No. I,
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-REEEFS.

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
isnot 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,' 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 ‘‘ 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 eevation.—
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-REEEFS.

recently elevated ; and at Kamtschatka! 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.?

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 Descrig. 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 1835, I have shown (Geo/ag. 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 fendemcy 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,' 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 T 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,! 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* may be

1 «*Researches in Physical Geology,” Zransact. 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.

182 CORAL-REEFS.

only with respect to the few last hundred feet of rise. But
with reference to the whole yast 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 only 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 eyery 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

184 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 proxifnity 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
‘praised 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

yolcanic 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.

Rekee END xX

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 Dépot général de la Marine, in 1835.
The names have been altered into English, and the
longitude has been reduced to that of Greenwich. The
colours were first laid down on accurate charts, on a large
scale. The data, on which the 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 Fanama, 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, Revilla-
gigedo, and other neighbouring islands. C/isperton rock,

£88 APPENDIX.

to 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 Zas/er, and
Sala, and Gomez Islands, which do not appear to be
surrounded by reefs.

The Low ArcuHIPELAGo.—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 Ac/eon group in this archipelago
has lately been discovered (Geograph. /Journ., vol. ii. p.
454); it consists of three small and low islets, one of which
has a lagoon. Another lagoon-island has been discovered
(Waut. 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: Z/izabeth 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.
fitcairn 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 Beechey’s
Voyage, p. 164); it is left uncoloured. Gamdier 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 uncdloured.
The Socizety 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. TZafz¢z, 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 (Waut. Mag.,
1836, p. 264) on the N.E. coast of the island, where none
are laid down by Cook. At Zzmeo 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 kesearches, vol. i. p. 18, 12mo edit.). Cook
found deep water (20 fathoms) in some of the harbours
within the reef. Mr. Couthouy, however, states (Aemarks,
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.
Tapamanoa 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 Zeturvoa are situated ; from the description of
them given me by the Rev. J. Williams (the author of the
Narrative of Missionary Enterprise), | 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. J/aztea, 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 AWas of
the Coguille’s Voyage. Maurua is the only one of the
northern islands, in which the water within the reef is not
deep, being only 434 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
Coguille’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 Huahezne 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 Zoubai (Motou-iti of the Coguzlle’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, BeHinghausen,
which is said by Kotzebue (Second Voyage, vol. ii. p. 255)
to be a lagoon island ; J/ophea, which, from Cook’s descrip-
tion (Second Voyage, book iii. chap. i.), no doubt is an atoll;
and the Sec//y Islands, which are said by Wallis ( Voyage,
chap. ix.) to form a group of /ow 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-
cana, Christiana, and Roafoa, 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 Austra Is_.—Falmersion Island
is minutely described as an atoll by Capt. Cook during his
voyage in 1774; coloured blue. <Aztutaki was partially
surveyed by the Beagle (see map accompanying Voyages of
Adventure and Beagle); the land is hilly, sloping gently to
the beach; the highest point is 360 feet; on the southern
side the reef projects five miles from the land: off this
point the Beag/e 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.—Manouat 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 JZ@tzaro 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.MZ.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.—Aézu, 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.—/enoua-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 Coguz//e’s Atlas, a reef is engraved
close to the shore: this isld. is not mentioned in the list
given by Mr. Williams (p. 16) inthe Warrative of Misstonaxy
Lnterprise; nature doubtful. As it is so near Atiu, it has
been unavoidably coloured red.—favotonga ; Mr. Williams
informs me that it is a lofty basaltic isld. with an attached
reef; coloured red.—There are three islands, fouroxzz,
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.—Jd/angaia; height about three hundred feet;
“the surrounding reef joins the shore” (Williams’s Var-
vative, p. 18); coloured red.—fimetara,; 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 (aut. Mag., 1839, p. 738); coloured red.—/urutu ;
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.—Zoubouaz,; in Cook’s chart
(2nd Voyage, vol. il. p. 2) the reef is laid down in part one
mile, and in part two miles from the shore. Mr. Ellis
(Polynes. Res., vol. iil. 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

* Sey ae
ay =, i: 4
are

194 APPENDIX.

coloured blue.—faivaivaz, 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.—Lazcaster
Reef, described in JVaut. Mag., 1833 (p. 693), as an
extensive crescent-formed coral-reef. I have not coloured
it.— Rapa, or Oparree; from the accounts given of it by
Ellis and Vancouver, there does not appear to be any
reef.i—J. de Bass is an adjoining isld., of which I cannot
find any account.—Kemzin Isld. ; Krusenstern seems hardly
to know its position, and gives no further particulars.

ISLANDS BETWEEN ¢he Low and Gilbert Archipelagoes.

Caroline Isld. (10° S., 150° W.) is described by Mr. F.
D. Bennett (Geograph. Journ., vol. vii. p. 225) as contain-
ing a fine lagoon; coloured blue—A/n?¢ Isld. (11° S.,
151. W.); Krusenstern believes that it is the same with
Peregrino, which is described by Quiros (Burney’s Chron:
fist, 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.—fenrhyn 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.—Starduck 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.—Ma/den Isld. (4° S., 154° W.); in
the same voyage (p. 205) this isld. is said to be of coral-
formation, and no part above 4o 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.—/arvis, or Bunker Isld. (0° 20’ S., 160° W.) is
described by Mr. F. D. Bennett (Geograph. Journ., vol. vii.
p. 227) as a narrow, low strip of coral-formation; not
coloured.—4vook is a small, low isld. between the two
latter; the position, and perhaps even the existence of it
is doubtful ; not coloured.—Fescado and Humphrey Islands ;
I can find out nothing about these islands, except that
the latter appears to be small and low; not coloured.—
feearson, or Grand Duke Alexander’s (10° S., 161° W.);
an atoll, of which a plan is given by Bellinghausen;
blue. — Souvorof Islands (13° S. 163° W.); Admiral
Krusenstern, in the most obliging manner, obtained for
me an account of these islands from Admiral Lazareff
who discovered them. They consist of five very low
islands of coral-formation, two of which are connected by a
reef, with deep water close to it. They do not surround a
lagoon, but are so placed that a line drawn through them
includes an oval space, part of which is shallow; these
islets, therefore, probably once (as is the case with some of
the islands in the Caroline Arch.) formed a single atoll ;
but I have not coloured them.—Danger Isld. (10° 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.—CV/arence 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 /agoons,” 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. Yor& Isld. (8° S., 172° 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, Azzal. Marit., 1829, p. 297); blue—Phenix
Isld. (4° S., 171° W.) is nearly circular, low, sandy, not
more than two miles in diameter, and very steep outside
(Tromelin, Axzzal. Marit, 1829, p. 297); it may be in-
ferred that this isld. originally contained a lagoon, but I
have not coloured it.—Wew Nantucket (0° 15’ N., 174° W.).
From the French chart it must be a low isld.; I can find
nothing more about it or about JZary Isld.; both un-
coloured.— Gardner Isld. (5° S., 174° W.) from its position
is certainly the same as Kemzz 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 Sfectator ; 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., 158° 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 Alem. Supfpl.,
p. 50, also Fanning’s Voyage, p. 233); blue-—Smyth’s 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.—AHawaiz,; in the chart in Freycinet’s
AZlas, 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 Aydrog. 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.—
Morotot, I presume, is fringed: Freycinet speaks of the
breakers extending along the shore at a little distance from
it. From the chart, I believe it is fringed ; coloured red.—
Oahu, Freycinet, in his Hydrog. Memoir, mentions some
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.—Azvoz, 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.—
Oneehow ; 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 G/eds/anes 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 Oceaz 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 Savazz Maouna, Orvosenga, 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 /Varrative, speaks of a reef going round
a small island attached to Oyolava, and returning again to
it: all these islands have been coloured red.—A chart of
ose Island, at the extreme west end of the group, is given
by Freycinet, from which I should have thought that it had
been an atoll; but according to Mr. Couthouy (Remarks,
p. 43), it consists of a reef, only a league in circuit, 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

200 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
Naut. 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.—/y/s/aart Isld.: judging from the »

chart in Freycinet’s Az/as, I should have supposed that it
had been regularly fringed; but as nothing is said in the
f{ydrog. Memoir (or in the voyage of Tasman, the dis-
coyerer) 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 Zoazgee, is represented as fringed;
coloured red.—Zoua. 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 (Odservations, 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 ‘fa 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, Komango, 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.—Zoufoa 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 Zatte 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.

Walls Island, 13° S., 176° W., a chart and view of this
island in Wallis’s Voyage (4to edit.) shows that it is
encircled. A view of it in the Waut. Mag., July 1833,
p- 376, shows the same fact; blue.

202 _ APPENDIX.

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

Fipji or Vit1 Group.—The best chart of the numerous
islands of this group, will be found in the Azas of the
Astrolabe’'s Voyage. From this, and from the description
given in the Aydrog. 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 go 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 (2%d 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 (zid. 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 Oxo, 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

fotoumah, 13° 8.,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., 10° S., 179° E., is described by Mr. G.
Bennett (United Service Journ., 1831, part li. 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, Feyster, and lice Islds. are
figured in Arrowsmith’s chart of the Pacific (corrected to
1832) as atolls, and are said to be very low; blue—
Lederlandisch Is\d. Iam 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 (JZemoir on the FPattfic,
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.—Gvan Cocal
is said in Krusenstern’s AZemoir to be low, and to be
surrounded by a reef; it is small, and therefore probably
once contained a lagoon; uncoloured.—S* Augustin.
From a chart and view of it, given in the Atlas of the
Coguilles 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 Coquilles Voyage, at once shows that it is
composed of ten well-characterised atolls. In D’Urville
and Lottin’s chart, Sydenham is written with a capital letter,
signifying that it is high; but this certainly is not the
case, for it is a perfectly characterised atoll, and a sketch,
showing how low it is, is given in the Coguille’s Aftlas.
Some narrow strip-like reefs project from the southern side
of Drummond atoll, and render it irregular. ‘The southern
island of the group is called Case (in some charts,
Rotches); of this I can find no account, but Mr. F. D.
Bennett discovered (Geograph. Journ., vol. vii. p. 229)
a low extensive island in nearly the same latitude, about
three degrees westward of the longitude assigned to
Rotches, 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.—/2##
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 Atvantic 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 GROoUP.—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 Av/as, and

APPENDIX. 205

in Kotzebue’s Second Voyage. The group consists (with the
exception of two 4##/e 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 Gasfar-Rico, or Cornwallis Isld. in
this group, which is described by Chamisso (Kotzebue’s
First Voyage, vol. ili. 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 4aring Island, of which little is known (see
Krusenstern’s Appendix, 1835, p. 149). I have left it un-
coloured; but Aoston Isld. I have coloured blue, as it is
described (¢ézd.) as consisting of 14 small islands, which, no
doubt, enclose a lagoon, as represented in a chart in the
Coquilles 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 /7irst Voyage, they are certainly low. The
nature, position, and eyen existence, of the shoals and small
islands north of the Marshall group, are doubtful.

New Hepripes.—Any chart, on eyen 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.—Matsthew’s Rock, 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.—Azzatom, the
southernmost of the Hebrides; from a rough woodcut
given in the Unzted Service Journal (1831, part iil. p.
190), accompanying a paper by Mr. Bennett, it appears
that the shore is fringed ; coloured red.—Zauua, 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.—
lmmer is described (Unzted Service Journ., 1831, part iil. ©
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 (Odservations, p. 14), to have been
a low island or even an atoll.—Zzromango Isld.; Cook
(2vd Voyage, vol. ii. p. 45, 4to edit.) speaks of rocks every-
where /izimg 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 Sizgapore Chron., alludes to
the veefs 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 2nd 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.—
Matticollo; 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 (Odservat., 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.—Azuvora and Fentecost
Islds., according to Bougainville, apparently have no reefs ;
nor has the large isld. of S. Espiritu, nor Bugh 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.—Vanzkoro (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.—TZzkop~ia and Fataka
islands appear, from the descriptions of Dillon and
D’Urville, to have no reefs; Anzouda is a low, flat isld.,
surrounded by cliffs (Astrolabe Hydrog., and Krusenstern,
Mem., vol. li. p. 432); these are uncoloured. TZoupoua
(Ozooboa of Dillon) is stated by Capt. Tromelin (Annales
Marit., 1829, p. 289) to be almost entirely included ina
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.—7Zinakoro is a constantly ~
active volcano without reefs.—J/endana Isles (mentioned
by Dillon under the name of JZammee, 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.—Duj’s Islds. compose a small group directed in
a N.W. and S.E. band; they are described by Wilson
(p. 296, JZss. Voy., 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 CAa.Leponia.—The great barrier-reefs on the shores
of this island have already been described (Fig. 3, Plate
III.). They have been visited by Labillarditre, 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 Seaugré) 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, Bamp/on shoal,
Frederic, Vine or Horse-shoe, and Aéer¢ reefs; these have
been coloured dark blue.

LovuIsIADE; 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 Zaugh/an Islds., the reefs
round which are engraved in the atlas of the voyage of the
Astro/abe, 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 We//s reef, described
in G. Hamilton’s Voyage in H.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
Labillardiére, to grow on their shores, this circumstance,
as in the case of the New Hebrides, is a presumption
that they are fringed. I cannot find out anything from
D’Entrecasteaux’s Voyage, regarding the southern islds. of

879

210 APPENDIX.

the group, so have left them uncoloured.—Ma/ay/a Isld. in
a rough MS. chart in the Admiralty has its northern shore
fringed.— Ysadel 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.—CZorseu/ Isld. ; in Bougainville’s
Chart of Chotseul Bay, 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 Coguzlle’s
voyage), and that it was ‘‘garnie d’une bande de récifs 4
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 $.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 Coguille
and Astrolabe. M. Lesson observes that the reefs are open
in front of each streamlet. The Duke of Vork’s Isld. is
also fringed; but with regard to the other parts of ew

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 GRrouPp.—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 Lizabeth isld., which is sur-
rounded by a reef at the distance of a mile; and two miles
eastward of it (Krusenstern, Afpend., 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 arcular-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 Lutké; it contains about forty
groups of atolls, and three encircled islands, two of which
are engraved in Fig. 3, Plate I., and Fig. 3, Plate II. Com-
mencing with the eastern part; the encircling reef round
Calen appears to be only about half a mile from the shore ;
but as the land is low and covered with mangroves (Voyage
autour du Monde, par F. Lutké, vol. i. p. 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 Coguz/le’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.—Pouynipéte, or Sentavine. 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”
(Lutké, vol. ii. p. 4); coloured pale blue.—/ogoleu 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 Wydrog. 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! 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
Lutké), and some even of the very small ones, as
Macaskill and Duperrey, of which plans are given in the
atlas of the Coguzlle’s voyage. ‘There are, however, some
low small islands of coral-formation, namely, Ol/af, Tama-
tam, Bigalt, Satahoual, which do not contain lagoons;
but it is probable that lagoons originally existed, but have
since filled up: Lutké (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 Coguz//e, it might have
been thought that they were not low; but by a comparison
with the remarks of Lutké (vol. ii. p. 107, regarding Bigali)
and of Freycinet ({ydrog. Memoir L’ Uranie 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 Pieuiram, is not
coloured, because I have found no account of it. Vougouor,
or Monte Verdison, which was not visited by Lutké, 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 Lutké’s charts),—a regular atoll.

214 APPENDIX.

described and figured by Mr. Bennett (United Service
Jou: nal, Jan. 1832) as an atoll. All the above-mentioned
islands have been coloured blue.

WESTERN PART OF THE CAROLINE ARCHIPELAGO.—
fais Island is ninety feet high, and is surrounded, as I
have been informed by Admiral Lutké, by a narrow reef
of living coral, of which the broadest part, as represented
in the charts, is only 150 yards; coloured red.—P#ilip

Isld., I believe, is low; but Hunter, in his Azstorical —

Journal, gives no clear account of it; uncoloured.—£iz7 ;
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 Lutké assures me this is not the case: they form
a regular atoll; coloured blue.—Gouap (Zap 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 (Aydrog. Descript. Astrolabe
Voyage, p. 436) for ships within the reef, if a passage could
be found; coloured pale bluee—Gou/ou, from the chart
in the Astro/abe’s atlas, appears to be an atoll. D’Urville
(Aydrog. Descrift., p. 437) speaks of the low islets on
the reef; coloured dark blue.

PELEW IsLps.—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, 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. — Guakan.
Almost the whole of this island is fringed by reefs, which
extend in most parts abouta 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 ona large scale; coloured red.—
Rota. ‘‘Lrile est presque entitrement entourée des récifs”
(p. 212, Freycinet’s Aydrog. 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 Medinilia 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 Memotr. ‘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.—/Zangs, however, is described (by Freycinet, p. 219,
ffydrog.) from some Spanish charts, as formed of small
islands placed “au milieu des nombreux récifs;” and as
these reefs in the general chart of the group do not project
so much asa 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 Islas. 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. — feel Js/d. 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.—Grampus Js/d. to the eastward,
does not appear (Meare’s Voyage, p. 95) to have any reefs,
nor does Rosario Ls/d. (from Lutké’s chart), which lies to
the westward. Respecting the few other islds. in this part
of the sea, namely the-Su/shur Jsi/ds., 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 Enp oF New Guinea.—fort Dory. From the
charts in the Voyage of the Coguille, 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.—
Waigtou. 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.—Azou, the plan of this group,
given in the atlas of the voyage of the Astrolabe, shows
that it is an atoll; and, from a chart in Forrest’s Voyage, it
appears 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 Avrzu,
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 Keffing are surrounded
by very extensive reefs, projecting into deep water, which,
as in the last case, I strongly suspect belong to the barrier
class; but I have not coloured them. From the south side
of Keffing, the reefs project five miles (Windsor Earl’s
Sailing Direct. for the Arafura Sea, p. 9).

CERAM.—In various charts which I have examined,
several parts of the coast are represented as fringed by
reefs.—AZanifa 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.—ouvrou ;
parts of this island appear fringed by coral-reefs, namely,
the eastern coast, as seen in Freycinet’s chart; and Caye
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 wateron them. In several charts, portions of
the islands forming the AMBoINA Group are fringed by reefs;
for instance, /Voessa, Harenca, and Ucasier, 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 XwZ/a 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.—J/yso/ 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.— Guede, 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.—
Gilofo. Ina chart published by Dalrymple, the numerous
islands on the western, southern (Batchian and the Strazt of
Patientia), and eastern sides appear fringed by narrow reefs ;
these reefs, I suppose, are of coral, for it is said in Malte
Brun (vol. xii. p. 156), “Sur les cotes (of Batchian) comme
dans les plupart des iles de cet archipel, il y a de rocs
de médrepores d’une beauté et d’une variété infinies.”
Forrest, also (p. 50), says Seland, near Batchian, is a little
island with reefs of coral; coloured red.—dorty 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.—Ce/eves. 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 Strazts of Limbe, and parts of the adjoining
shore, appear to be fringed: the east side of the bay
of JZanado, has deep water, and is fringed by sand and
coral (Astro/, 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 Sevangant,
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

=49 ar. =
ae

aie

,

220 _ APPENDIX.

islands is taken from Capt. D. Kolffs Voyage, in 1825,
translated by Mr. W. Earl, from the Dutch.—Ze/fe has
“reefs extending along shore at the distance of half a mile
from the land.”—JZoa has reefs on the S.W. part—Lakor
has a reef lining its shore; these islands are coloured red.—
Still more eastward, Zuaz 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 Z7mor, 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.—Zzmor;
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 Aydrog. 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 or AUSTRALIA.—It appears, in Capt. King’s
Sailing Directions (Varrative 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. 28° S. on west coast) have lately been
surveyed by Capt. Wickham (as described in Waut. Mag.,
©1841, p. 511): they lie on the edge of a steeply shelving
bank, which extends about 30 miles seaward, along the
whole line of coast. The two southern reefs, or islands,
enclose a lagoon-like space of water, varying in depth from
5 to 15 fathoms, and in one spot with 23 fathoms. The
greater part of the 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.—fow/ley Shoals. ‘These lie some way from the
N.W. coast of Australia: according to Capt. King (Varra-
tive of Survey, vol. 1. 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” (vincitles of Geolog., book iii.
chap. xvili.); coloured blue.—Scot?s 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 Flores, 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 SaZy Bay. From the same authority it appears
(p. 610) that reefs fringe the island of Zzmor- 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.—AaZ/y Isld. Ina Dutch MS. chart on
a large scale of Java, which was brought from that island
by Dr. Horsfield, who had the kindness to show it me at
the India House, its western, northern, and southern
shores 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 Font 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; there is a depth between 30 and 4o 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 Sesorrowodee Bay, and the extreme S.E. and E.
portions are likewise fringed by coral-reefs; all the above-
mentioned places coloured red. .

Macassar Str.; 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 aré 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 Fosiilions,
Kalatoa, Tokan-Lessees, 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 SAonin, 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 Bouton which, according to an
old chart, is formed by a reef, parallel to the shore, with
deep water within; and in the voyage of the Coguzl/e, 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, Zxgano ls/d. 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) ; coloured red.—fat Js/d. (3° 51'S.) is sur-
rounded by reefs of coral, partly dry at low water (Hors-
burgh, vol. ii. p. 96).—Z7ieste [sland (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.—/ulo 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. 228

coral-bank, which stretches about forty yards from the shore,
is steep to all round: ina chart, also, which I have seen,
the island is represented as regularly fringed.—Pulo Mintao
is lined with reefs on its west side (Horsburgh, vol. ii.
p. 107).—fulo 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 (WVo/ices of
the Indian Arch., published at Singapore, p. 105).— Paulo
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 JVotices 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 (Aiistory 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. .
Nicopar IsLanps.—The chart represents the islands of
this group as fringed by reefs. With regard to Great
LVicobar, Capt. Moresby informs me, that it is fringed by
reefs of coral, extending between 200 and 300 yards from
the shore. The JVorthern 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 Aszatic
880

226 APPENDIX.

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

The coast of A/a/acca, Tene and the coasts north-
ward, appear in the greater part to be low and muddy:
where reefs occur, as in parts of A/alacca 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 Va‘uzas and the Anambas Islds.
are represented in the charts on a large scale, published in
the Azas of the Voyage of the Favourite, as fringed by reefs
of coral, with very shoal water within them.—Zwmbelan and
Bunoa Islds. (1° N.) are represented in the English charts as
surrounded by a very regular fringe-—.S2 Barbes (0 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 Plo Leaz, 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 Pa/azwan, are fringed by very distant coral
reefs; thus the reefs in the case of Ba/avac 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,

4 ova
‘ A
oa ‘ n af
ey
~~

APPENDIX. 224

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; angoo/aran, although ten miles
long, entirely consists of a bed of coral-rock (WVotices of £.
Lntian Arch., p. 58): I believe from Horsburgh that the
island is low; not coloured.—Zahow dank, 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 Cavillz, 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 evena
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 Isid. is said, by Horsburgh (vol. ii. p. 436), to be
“lined by a reef.”—Zuzon: 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 haye ©
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 J/asbaze, and nearly the ~

APPENDIX. 229

whole of Sool, 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.

Basuyan Istanps.—Horsburgh says (vol. ii. p. 442),
coral-reefs line the shores of the harbour in Fuga; and
the charts show there are other reefs about these islands.
Camiguin has its shore in parts lined by coral-rock
(Horsburgh, p. 443); 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 Mapjiko-Sima Groups. /atchuson has been described
by Capt. Broughton (Voy. to the NV. Facific, 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

230 APPENDIX.

reef; but it does not appear that the water is deép 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.—TZayfzx-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 Aumz (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 Cuoo. ‘The greater
part of the coast of this moderately hilly island is skirted
by reefs, which do not extend far from the shore, and
which do not leave a channel of deep water within them, as
may be seen in the charts accompanying Capt. B. Hall’s
voyage to Loo Choo (see also remarks in Appendix, pp. xxl.
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.
Pp. 450) extend far out from them, it is impossible to draw
any inferences regarding the nature ofthe 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 faracells 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, ete.) are so arranged round a moderately
shallow space, as to appear as if they had once formed
one large atoll.—Zombay 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.
Pp. 332) on its west side; it is very steep outside.— Descovery
Shoa/, 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. i. p. 333) of only twenty yards
from the reef, soundings could not be obtained. The
Paracells are coloured blue.—J/acclesfield Bank: this is
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. It 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: Swal/ow —
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.”—a/flMoon 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.—Bombay Shoa/
appears (Horsburgh, vol. ii. p. 432) “to be a basin of
smooth water surrounded by breakers.” These three shoals
I have coloured blue-—The /Paraguas 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 Palawan. 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, Zonguin,
and Cochin-China, forming the western boundary of the China
Sea, appear to be without reefs: with regard to the two last-
mentioned coasts, I speak after examining the charts on a
large scale in the atlas of the voyage of the Favourite.

InDIAN OcEAan.— South Keeling atoll has been specially
described ; nine miles north of it hes North Keeling, a very
small atoll, surveyed by the Geag/e, 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—CrYLON: a space about
eighty miles in length of the S.-western and southern shores
of these islands has been described by Mr. Twynam (/Vauwt.
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 A/adura.

234 APPENDIX.

CuHaGos, 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 Cagos and J/addiva groups are entirely
formed of great atolls, or lagoon-formed reefs, surmounted
by islets. In the Zaccadive 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 de Mala bank consists of a series of narrow banks,
with from 8 to 16 fathoms on them ; they are arranged ina
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.—// 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 tome. 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 outline, however, of the land appears to be
(Life of Sir J. Makintosh, vol. ii. p. 165) hilly and rugged.
I am unable to decide whether these reefs belong to the
barrier class, as seems probable from their great extension,
or to the fringing class ; uncoloured.— Bourbon. The
greater part of the shores of this island are without reefs ;
but Capt. Carmichael (Hooker’s Got. 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 4o

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 Mahé and the adjoining islands of St Anne and Cer7,
are regularly fringed by coral-reefs. A portion of the S.E.
part of Curzeuse sid., the N., and part of the S.W. shore of
Fraslin Isid., and the whole west side of Dégue Jsid.,
appear fringed. From a MS. account of these islands by
Capt. F. Moresby, in the Admiralty, it appears that Se/houetie
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. Amirantes 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 désris 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—F/atte, Alphonse,
Coetiv:, 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, /van de
NVova, 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 this 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 P—uncoloured.

Comoro Group.—Mayotia, according to Horsburgh
(vol. i. p. 216, 4th edit.), is completely surrounded by a
reef, which runs at the distance of three, four, and in some
places even five miles from the land; in an old chart,
published by Dalrymple, a depth in many places of 36 and

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
(War., 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.—/Zofzl/a; on the S. side of this island
there is anchorage, in from 30 to 45 fathoms, between a reef
and the shore (Horsburgh, vol. 1. p. 214); in Capt. Owen’s
chart of Madagascar, this island is represented as encircled ;
coloured blue.—Gveat Comoro TIsid. 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 sZore, 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. 230

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 S# Augustine's Bay appears fringed; but Zudlear
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
uncoloured. From /az 22° 16’ fo 21° 37’, the shore is
fringed by coral-reefs (see Lieut. Boteler’s (Varraizve, 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 mileand 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 ova (in lat. 17° 8.) 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 Warrenda and Passandava

240 APPENDIX.

Bays (14° 40’) and the islands in front of Radama
ffarbour 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 Zamatave, in
18° 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 Weoncy, of the above line of coast
might have been inferred. Lieut. Boteler (vol. i. p. 155)
speaks of ‘the reef surrounding the island of S# Mary’s at
a small distance from the shore.” Ina 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 /Varratve ;
and that by Lieut. Boteler. At A/ukdeesha (10° 1' N.) there
is a coral-reef extending four or five miles along the shore
(Owen’s WVar., 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 /uda, a little S. of the
equator, to Zamoo (in 2° 20'S.) ‘‘the coast and islands are
formed of madrepore” (Owen’s WVarrative, vol. i. p. 363).
The chart of this part (entitled Dundas Jslds.) 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 (Vaz, vol. i. p. 369) describes the
muddy bay of atta, 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
eleyation 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 haye
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° 5'5.). The island which forms the harbour,
‘“‘is surrounded by cliffs of madrepore, capable of being
rendered almost impregnable” (Owen’s /Vav., yol. 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.—feméa
(5° 8.) is an isld. of coral-formation, level, and about 200
feet in height (Owen’s Var., vol. 1. 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 5S. of —
Zanzibar, there are some banks parallel to the coast, which

APPENDIX. 243

T should have thought had been formed of coral, had it not
been said (Boteler’s WVar., vol. ii. p. 39) that they were
composed of sand; not coloured.—Latham’s Bank is a
‘small island, fringed by coral-reefs ; but being only ten feet
high, it has not been coloured.—A/onjeea 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.—Aee/wa. 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 Zindy and Monghow (9° 59’ and
to 7S.) has the same structure ; coloured red.— Querimba
Lslands (from 10° 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 (Var, 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.—J/onadbila
(13° 32’ S.). In the plan of this harbour, the headlands —
outside are fringed by reefs apparently of coral; coloured
red.— Mozambique (150° S8.). 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 JVar. (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 Orvmuz and Larrack
appear so regularly fringed, that I have coloured them red.
There are certainly no atolls in the Persian Gulf. The
shores of Jimmaum, 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. 125) to be without any reefs. I believe

APPENDIX. 245

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

RED SEeA.—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,
Uber die Néatur der Corallen-Binken 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 10 on the eastern side, there is a piece of
coast, with very deep water close to it, without any reefs,
but not differing in 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 ona
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 Dka/ac 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, their shores are fringed by living coral-reefs.

From the account given by Ruppell? 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, Rezse tn Abyssinzen, Band. i., s. 247. ? Lbid., 8. 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 (Uéer de,
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 Red 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), ©

3
3

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. 22° to 24°.—This part of the
coast (north of the space coloured blue on the map) is
fronted by an irregularly shelving bank, from about to 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 may 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.

Lastern 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 southern 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 InpDizs.—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 to 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. 256

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,! 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. ili. 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? 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, 1841, 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 (Sz//zman’s 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 Gakama 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.—Cusa: 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 ‘‘ daxo,” 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. WVar.,
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 Shoals (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 /s/
of Fines 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, Fess. Vaz, 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. 250

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, 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. TZurneffe 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 WVorthern Triangles, on which there is an islet be-
tween 70 and 8o feet, and to Cozumel 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 AZosguzto coast, there are between
lat. 12° and 16° some extensive banks (already mentioned,
Pp. 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.—/amaica :
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.—5Sz 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.
—/fuerto. 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 S# Thomas is fringed ; most of
the Virgin Gorda Islands, as I am informed by Mr.
Schomburgk, are fringed; the shores of Axegada, 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 Savxta Cruz appears in the Danish survey
to be fringed (see also Prof. Hovey’s account of this island,
in S¢lliman’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 (Descrig. Lles 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 fifteen 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 Bermupa Isi~anps 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, zézd., 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,

IN DEX.

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 : ; 8I
A brolhos (Australia) e222

Absence of coral-reefs from

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

reefof . a Ge)

Madreporitic rock of 171
Africa, east coast . - 240
Age ofindividual corals 95, 109
Aiou . - , : Bedi)
Attutakt 192
Aldabra 227
Alert reef. » 209
Alexander, Grand Dike. oie 195

Allan, Dr., on Holuthuriz
feeding on corals 30

on quick growth of corals
at Madagascar . 103, 104

—— on reefs affected by cur-
rents . , : Od
Alloufatou . : 5 1 202
Alphonse. . : « 236
Amargura . 4 » » 201
Amboina . 218
America, west coast 187, 188
Amirantes » 236
Anachorites . ea 2ur
Anambas 226
Anamouka, description of 167
Anamouka . : , sa 2OL
Andamanislds. . ° 225
Antilles ; : 5 ZOLA 202
Appoo reef . : - 225

PAGE
Arabia Felix 2 : 240
Areas, great extent of, inter-
spersed with low islands 118, 119
of subsidence and of
elevation 178, 179

—— of subsidence appear to
be elongated .

—— of subsidence alternat-
ing with areas of ele-
vations : e's

Arru group . ° “

Arzobispo .

Ascidia, depth at which found

Assomption . : .

Astova . 5 ° °

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 . I15, 116,

120, 121, 122, 181, 182

—— of irregular forms 36, 37,

38, 141, 142

—— steepness of their flanks 38, 39

—— width of their reef and

islets : ok RTS S

their lowness . I17, 118

—— lagoons 41, 42, 43

—— general range - 157, 158

—— with part of their reef
submerged, and theory

of . 44, 45, 130, 137

266

PAGE
Augustine, St. . ; 203
Auroraisland, an upraisedatoll

107, 118, 175
Aurora : 9 |
Austral islands, recently ele-
vated : » 166,174, 175
Australislands . . . 192
Australia, N.W. coast . {220
Australian barrier-reef . 67, 158
Australian barrier . 208
Babuyan group : . 220
Bahama banks . 255, 257
Balabac : - 226
Bally 222
Baring - - 205
Barrier-reef of Australia 67, 158
of New Caledonia 71

Barrier-reefs, breachesthrough 129
not based on worn down

margin ofrock . — 270

on banks of sediment 70 j

on submarine craters 71

steepness of their flanks 62, 63
their probable vertical
thickness 70, 128
theory of their formation
127,120, ee 130

Bampton shoal
Banks islas. . . 207

Banks in the West In: lies 252
Bashee islds. . 22G
Bass isla. . 194
Batoa . 202
Beaupré reef : 208
Beechey, Captain, obligations
of the author to = 235
on submerged reefs 40
accountof Matildaisland Io0o
Belcher, Captain, on boring
through coral-reef . 98
Bellinghausen IQI
Bermuda ts/ds. 263
Beveridge reef 200
Blizch 207
Bolabola, view of. 17
Bombay shoal 232
Bonin Bay . - 224

INDEX.

:

Bonin group

Borings through coral-reefs 98, 99
Borneo, W. coast, recently ele-

vated 169, 170
pe E. coast . 223
S. W. and W. coast 226, 227
— JV. coast : aoe
western bank Pee
Boscawen + 201
Boston . » 205
Bouka . » 210
Bourbon ; «235
Bourou : : . 278
Bouton . 224
Brazil, fringing- reefs on coast
of 80
Breaches through barrier-reefs 129
Brook : : : - Os
Bunker : : : » 195
Bunoa . : : . 226
Byron . ; : : » 204
Cagayanes ., , » 227
Candelaria . : » 210
Cargados Carajos . 2a
Caroline arch, . 2i2
Caroline zsld, . 194
Carteret shoal 5 217
Caryophyllia, pan at which
it lives . 108
Caviliz . 225
Cayman tsld, . 260
Celebes . a . 219
Ceram . . 218
Ceylon, recently elevated , 1762)
Ceylon - | eae
Chagos Great Bank, descrip-
tion and theory of 58, 59, 142, 143
Chagos group A : - 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 . - . nore ae
— of atolls : - . 124

INDEX. 267
PAG
Channels leading into the | Corals merely coating the bot-
lagoons of atolls 45, 46, 138! tomofthesea . OL O2
the Maldiva atolls 51, 54 | ——— standing exposed in the
through barrier-reefs . 129 Low arch. . : LOZ
Chase . . : : 204 | Corallian sea » 158
China sea . . 231 | Coraliian sea . 208
Christmas atoll . 99, 164 | Cornwallis * 205
Christmas atoll . 118 | Cosmoledo 237)
Christmas tsld, (Indian Ocean) 233 | Couthouy, Mr., alleged proofs
Clarence. : : - 195 of recent elevation of the
Clipperton rock - 187 Low arch. : - 162
Cocos, or Keeling atoll . eeir9 on coral-rock at Mangaia
Cocos (or Keeling) 233 and Aurora islds, . 107
Cocos isld. (Pacific) 187 on external ledges round
Cochin China, encroachments coral-islands Behave
of the sea on the coast 161 remarks confirmatory of
Cochin China 233 the author’s theory 162
Coetzvi . 236 | Crescent-formed reefs 142
Comoro group . 237 | Cuba 257
Composition of coral forma- Cuming, Mr., on "the recent
tions. . 148, 149 elevation of the Philippines 170
Conglomerate coral- rock on :
- ee eis ; : ie Dangerous, or Low arch. 188
A ack 7 148 Danger zslds. : 195
= Depths at which reef- building
Cook islds., recently elevated, Is i 6
: 63 174 corals live : 10
Cok isis. g 192 at Mauritius, the Red
Coral-blocks bored by \ vermi- Pee zune win ne Weare, 15
form animals - 30, 149 gare h hel il d
Coral-reefs, their distribution a “ic ech ee “ge pa ae
and absence from certain Dien Oe aie eons 6
Passe 83 aac group . 24
destroye d by Tooce ae ae Diego Garcia, slow srowth of
ment . 89 reef . ° 93
Coral-rock at Keeling atoll 27, 28 eens ei the larger
| oe ee ; 7 groups of atolls . 116, 119
eect 149 Disseverment of the Maldiva
Corals dead but upright in Fos Fie MACOS 2)e Bd asien) "50
ié@eling lacoon - 31 Distribution of coral-reefs 83
depths at which they LEG Sh een
poe 107, 108 Dory, Port, recently elevated . 169
:: ~ 2 :
off Keeling atoll . 22 Duff Port ; ; ‘ Ae
— killed bya short exposure 20 Dies : ae
living in the lagoon of
Keeling atoll 28, 29
quick growth of, in Keel- | Lap 214
ing lagoon. - ; 20 Earthquakes at Keeling atoll. 34

268 INDEX
PAGE PAGE
Earthquakes in groups of Flint . : - 194
atolls - . i25, 126 | Flores . ~ 222
in Navigator arch. . 168 | Florida : 256
East Indian arch., recently | Folger . . : 216
elevated . - : . 169 | Formosa. 229
Easter . 188 | Forster, theory of coral- forma
Echequier 211! tions : : : ~ 122
Ehrenberg, on the banks of | Frederick reef . 209
the Red Sea. . 81, 245 | Freewsll 217
on depths at which corals Friendly group recently ele-
live in the Red Sea. II0 vated g * 167, 177,
on corals preferring the Friendly arth . : - 200
surf, . 87 /| Fringing-reefs, absent where
on the antiquity of certain coast precipitous 73> 74
corals. 90 breached in front of
Eimeo . d . 189 streams . 90
Elevated reef of Mauritius 77 | —— described by MM. “Quoy
Elevations, recent proofsof . 164 and Gaimard . - 165
immense areas of . 178, 179 not closely attached to
Eltvt . 2aa shelving coasts » 75
Elizabeth isld. : - 98 of east coast of Africa . 79
recently elevated . 165, 176 | of Cuba “ : sees
Elizabeth isla, . 188 | of Mauritius . ° 73
Lillice group . . 203 | on worn down banks of
Encircled islds., their height. 66 rock . - (eo
geol. composition 68, 70, 71 on banks of sediment SI
Eoua, description of . Srey their appearance when
Loua - 200 elevated 77
Erupted matter probably not their growth influenced
associated with thick masses by currents , onvor
ofcoral-rock . . 149 | —— by shallowness of sea 80
FaIs, recently elevated. 168, 175 | Galapagos arth. . : - 188
WATS. - 2T4. | Galega . : - 236
fanning 3 £0741 Gambier islds. “7 section of. 69
Farallon de Medinilla ‘ 215 | Gambzer zslds. ; . - 189
Farsan group - . 246, 247 | Gardner : ee . 196
Fataka. . ° : . 207 | Gaspar-Rico. . » 205
Fijt arth. . 202 | Geological composition of
Fish, feeding on corals - =z. 30 coral-formations 148,
Fish killed in Keeling lagoon Gilbert arch.
by heavy rain 36 | Gzlolo . °
Fissures across coral-islds. 126 | Glortosa .

Fitzroy, Capt.,on a submerged

shed at Keeling atoll. .
on an inundation in the
Low arch. .

Gloucester isld.
Glover reef.
Gomez . -
Gouap . °

Huaheine; alleged proofs of

its recent elevation . ly
Huaheine , . : LOE
Humphrey . . . - 195
Hunter = : - 202
Hurricanes, effects of, on

coral-islds, : . 124
lmmaum - 244
Lndependence : 203
India, west coast, recently

elevated : : . 170
India . » 245

Irregular reefs in shallow seas
Islets of coral-rock, their for-

mation ; F x
their destruction in

80
26

the Maldiva atolls 56
Jamaica 201
jsarvis . » 195
Java, recently elevated . . 169
java . : s222
Johnston isld, 1 197
Juan de Nova 227

INDEX. 269
PAG PAGE
Goulou “ Bi Juan de Nova (Madagascar) . 239
Grampus . San} eae . 216
Gran Cocal . 203 | Kalatoa : : 228
Great Chagos Bank, descrip- Kamtschatka, proofs of its
tion and ‘theory of 58, 142 recent elevation. ° . 178
Grey, Capt., on sandbars 76 | Karkalang : “| 219
Grouping of the different Keeling atoll, section of reef . 19
classes of reefs . : . 157 | Keeling, south atoll 238
Guedes C : 27 north atoll A ADR.
Keffing ° SEZs
HALL, Capt. B., on Loo Choo 170 | Kemzn : 194, 196
Harvey islds., recently ele- Kennedy » 208
vated : . 174| Keppel. 6 i
_ Height of encircled islds, 66 | Kumz . 230
Hermites o 25
Larvey or Cook isids. . 192 | Laccadive group oF
Hogoleu 212 | Ladrones, or Marianas, re-
Holuthuriz feeding on coral . 30 cently elevated . - 168
Honden isld., height of . 118 | Ladrones arch. . « 215
Honduras, reef off ° - 259 | Lagoon of Keeling atoll BS
florn . : » 202 | Lagoons bordered by inclined
Houtman’ s A brolhos 221 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
SEGUE 6 201
Lauslan islds. 209
Ledges round certain lagoons,

48, 133
LECILEN ee : . ° 220
Lighthouse reef 260
Lloyd, Mr., on corals refixing

themselves ‘ 104
Loo Choo, neon elevated . 170
Loo Choo ° 230
Louisiade 209
Low archipelago, alleged

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

270
PAGE
Lucepara “226
Lutké, Adm., on fissures across
coral- islands - - . 126
Luzon, recently elevated . 170
Luzon . - : ‘ 220
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 . . :
on the apparent con-
tinuity of distinct coral-

islands . 149
on the recently elevated
beds of the Red Sea . 172
on the outline of the
areas of subsidence 179
Macassar str. $ 223
Macclesfield bank . - au
Madagascar, quick growth of
corals at . . - 103
madreporitic rock of 17I
Madagascar . : é - 239
Madjtko-sima . . » 229
Madura(Java) . - 222
Madura (lna7a) : a2ee
Mahlos Mahdoo, theory of
formation . - 139
Malacca, recently elevated EZ E
Malacca : - - 226
Malcolmson, Dr., on recent
elevation of W. coastof India 169
on recent elevation of
Camaranisld. . ory2
Malden. : 195
Maldiva atolls, and theory of
their formation 44, 134, 135, 139
steepness of their flanks. 39
growth of coral at. + 03
Maldiva arch. : =o MMT W284
Mangaia isld. = ° - 107
recently elevated . 148, 175
Mangaia : : 193
Mangs : : - 216
Marianas, recently elevated . 168

INDEX.

PAGE
Mariana arch. ; 205
Martere : 5 . « 295
Marquesas arch. ; . I9QI
Marshallarcth . : . 204
Marshall isid. - . 216
Martinique . . : - 262
Martires : i
Mary’s St. in Madagascar,
harbour made in reefs . 89
Mary tsld. . a . 196
Matia, or Aurora. . 189
Matilda atoll, - ; . 100
Mauritius, fringing-reefs of . 73
depths at which corals
live there . « 106, 107
recently elevated . = :L¥Ox
Mauritius . : ° - 235
Maurua, section of - 69
Maurua . < . « 590
Menchikoff atoll « ) BFS
Mendana arch . in TOE
Menaana isles : 3 s 208
Mexico, gulf of . 255
Millepora complanata at ‘Keel-
ing atoll . ; - eon
Mindoro : : : 3 2ooe
Mohilla 2338
Molucca islands, recently ele-
vated. - : eae . 169
Mopeha. ; Pee ie ii
Moresby, Capt., on boring
through coral-reefs . 98
Morty , - + ° 219
Mosquito coast - . 260
Musguillo atoll . - < 5ae
Mysol . ; . : 218
NAMOURREK group. I4I
Natunas : i226
Navigator arch., elevation of. 168
Navigator arch. . : - 199
Nederlandisch . = « 29>
Nelson, Lieut., on the consoli-
dation of coral-rocks under
water : 98
= DD coral-forma-
tions . : . agze

INDEX.

PAGE
Nelson, Lieut., on the Ber-
muda islds. ° ° 32603
New Britain : 5 WEI
New Caledonia, steepness of
m@esreef .. . . 5 (2
barrier-reef of 71, 132
140, 158
New Caledonia . “4 38208
New Guinea (£. end) eats
New Guinea (W, end) . SkOr7,
New Hanover 210

New Hebrides, recently ele-

vated “ : ° , 168
New Hebrides 205
New Ireland, recently elevated 168
New Treland : » 205
New Nantucket . 196
Nicobar islds. aan2 25
Niouha - 201

Nullipore at Keeling atoll. 24
on the reefs of atolls 42
on barrier-reefs 62

their wide distribution
and abundance . é

115

OBJECTIONS to the theory of

subsidence ‘ 146
Ocean tslds. . - 1098, 204
Ono . . : : 2 202
Onouafu - ‘ ‘ » 202
Ormuz ; ° : » 244
Oscar group . 77202

Oscillations of level 173, 174, 182

Ouallan, or Ualan . 212
Ouluthy atoll : 5 - IOI
Outone Java : : - 210
Palawan, S.W. coast , , 226

N.W, coast . ; 227,

western bank ‘ 1 232
Palmerston . ‘ » 192
Palmyra . » » 197
Paracells ., 1 231
Paraquas . 1 232
Patchow . , » 229
Pelew isids. . - 214
Pemba isld., singular form of 171

271
PAGE
Pemba . 242
Penrhyn 194
Feregrino 194
Pernambuco, bar of sandstone
atin 76
Persian gulf, ‘recently elevated 19/22
Persian gulf . 5 244
Pescado : 195
Pescadores 230
Peyster group 203
Philip - 214
Philippine arch. = recently ele-
vated : 4 . 170
Philippine arch. . ‘ 227
Phenix ‘ 196
Piguiram . ; - 213
Pitcairn 189
Pitt’s bank . T44
Pitt isld. 204
Platte . 236
Pleasant E204
Porites, chief coral on margin
of Keeling atoll : 20
Postitlions ; : ; 1228
Pouynipéte . 160
its probable subsidence . 161
Pouynipete 212
Pratas shoal . 231
Proby . 5 13202
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 . 4 ef EG
Quoy and Gaimard, depths
at which corals live . . ‘Il
description of reefs ap-
plicable only to ws
ing-reefs_ , , 165
RANGE ofatolls , : > 159
Rapa , ° : ' » 194
Rearson ‘ : ‘ « 195

272

PAGE

Red Sea, banks of rock coated
By recis.. 3

proofs of its recent eleva-
tion .

supposed subsidence of .

heed Sea

Reefs, irregular in shallow seas

rising to the surface in
some lagoons, and all

Saya, or Sahia de Matha
Scarborough shoal. -

submerged in others . 92
—— their distribution . 83
—— their absence from some
coasts - 84
Revilla-gicedo ; 187
Ring- formed reefs of the Mal-
diva atolls, and theory of 52, 134
_ Rodriguez . ° : » 235
Rosario : : . 256
Rose zsld. : - 199
Rotches : : : - 204
Rotoumah . : - 203
Roug 212
Rowley shoals ; 221
Riippell, Dr., on the recent
deposits of Red Sea . ay
Sable, zle de» 23
Sahia de Matha 234
Sha LETTE 236
aie - . 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. -SEO7
Sanserot A W2Es
Santa-Cruz group . . 207
Savage isld., recently elev ated,
98, 167, 175
Savace - , : 200
Savu . » 220
; eK

a)232

INDEX.

PAGE
Scarus feeding on corals ae
Schouten - : - a De
Sczlly - IQI
Scoriz floated to “coral: islds. 149

Scott’s reef. « S2BE

Sections of ade encircled
by barrier-reefs . 69, 323
of Bolabola . 2 Es

Sediment in Keeling lagoon 29

in other atolls 43, 54, 55
injurious to corals . 89
transported from coral-
islands far seaward . 149
SEuiavine . : : +, Bae
Sevangant . : ° . 219
Seychelles - 35
Ship-bottom quickly coated
with coral - 104
Smyth tsld. « . . 197
Society arch., stationary con-
dition of 162
alleged proofs of recent
elevation . . - 174
Soczety arch. . 189
Socotra . e - 244
Solor . ° 222
Sooloo islds. recently elevated 170
Sooloo zslds. x 2 227
| SouvaroffT : . 195
Spanish : : : +255
Sponge, depths at which found

Starbuck.
Stones transported i in roots of

trees 149
Storms, effects of, on coat
islds. 124

| Stutchbury, Mr., onthegrowth

' Ofan Agaricia : :

on upraised corals in

Society arch. .

Subsidence of Keeling atoll .

extreme slowness of 146,

| areas of, apparently elon-

gated. : . -

areas of immense . 178,

great amount of . I8I,

| Suez, gulf of . -

INDEX.

PAGE

Sulphur zslds. . 216
Sumatra, recently elevated 169
Sumatra 224
Sumbawa . 222
Surf favourable to the growth
of massive corals ‘ 87
Swallow shoal 232
Sydney isla. . 196
TAHITI, alleged proofs of its
recent elevation . 174
Tahitz . 189
Temperature of the sea at the
Galapagos arch. 84
Tenasserim . ee
Tenimber isla. Pas ily)
Teturoa 190
Theories on coral- formations
EES, 122
Theory of subsidence, and
objections to i20;, 145
Thickness, vertical, of barrier-
reefs . : 69, 128
Thomas, St. . 261
Tikopia 3 207
Timor, recently elevated 169
Ger ; ‘ P 219
Timor-laut . 217
Tokan-Bessees 223
Tongatabou . 200
Tonquin : 5 PBB
Toubaz ; : 191
Toufoa 201
Toupoua 207
Traditions of change i in coral-
islds. : 123
Tridacnz embedded i in ‘coral-
rock . 149
left exposed i in the Low
arch. . soz
Tubularia, quick growth of . 105
Tumbelan . - 226
LTurneffe reef : 360
Turtle ‘ - 202
Ualan . : ; ; 212

273

PAGE

VANIKORO, section of . 69
Vanikoro, its state andchanges

in its reefs : : . 161

Vanikoro . : : 207

Vine reef. 209

Virgin Gorda : ; 5
Viti arch.
Volcanic islands, with ‘living
corals on their shores :
matter, probably not
associated with thick
masses of coral-rock .
Volcanoes, authorities for their
position on themap 151, 152,
their presence deter-
mined by the move-
ments in progress :
absent or extinct in the
areas of subsidence 177,

Wazgtou : ° : :
Wallis isld. . : - °
Washington . : ° °
Wells’ reef - : :
Wellstead, Lieut., account of
a ship coated with corals .
West Indies, banks of sedi-
ment fringed by reefs
recently elevated .
West Indies . 5
Whitsunday isld., view of
—— changes in its state
Williams, Rev. J., on tradi-
tions of the natives regard-
ing coral-islds.
——on antiquity of certain
corals - .
Wotchonsky .
Wostock

Xulla islds. . : 5

York tsld.
Yucutan, coast of .

ZONES of different kinds of
corals outside the same
reefs . : - 2 OW,

833

aul25

123

. 107
. 188

194

. 218

. 196
25g

TOL

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.

(Abstract, reprinted from the“ Proceedings, vol. tt. Dp. 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 the 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 1000 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 :—1st. 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

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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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