[ Coral Reefs : Chapter I. Atolls or Lagoon-Islands ]

SECTION 1.I.–KEELING ATOLL.

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

(PLATE: UNTITLED WOODCUT, VERTICAL SECTION THROUGH KEELING ATOLL.)

A.–Level of the sea at low water: where the letter A is placed, the depth
is twenty-five fathoms, and the distance rather more than one hundred and
fifty yards from the edge of the reef.

B.–Outer edge of that flat part of the reef, which dries at low water:
the edge either consists of a convex mound, as represented, or of rugged
points, like those a little farther seaward, beneath the water.

C.–A flat of coral-rock, covered at high water.

D.–A low projecting ledge of brecciated coral-rock washed by the waves at
high water.

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.

KEELING or COCOS atoll is situated in the Indian Ocean, in 12 deg 5′ S.,
and longitude 90 deg 55′ E.: a reduced chart of it was made from the
survey of Captain Fitzroy and the Officers of H.M.S. “Beagle,” is given in
Plate I., Figure 10. 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.

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 favourable circumstances, 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 Astraea, but larger) from four to eight
feet broad, and little less in thickness. These mounds are separated from
each other by narrow crooked channels, about six feet deep, most of which
intersect the line of reef at right angles. On the furthest mound, which I
was able to reach by the aid of a leaping-pole, and over which the sea
broke with some 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. (This Millepora (Palmipora of Blainville), as well as the M.
alcicornis, possesses the singular property of stinging the skin where it
is delicate, as on the face and arm.)

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 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
twenty-five fathoms (A in section), beyond which the sides plunge into the
unfathomable ocean at an angle of 45 deg. (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.)
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, however, of
Millepora alcicornis were brought up; and on the arming from an eight-fathom
cast, there was a perfect impression of an Astraea, apparently
alive. I examined the rolled fragments cast on the beach during gales, in
order further to ascertain what corals grew outside the reef. The
fragments consisted of many kinds, of which the Porites already mentioned
and a Madrepora, apparently the M. 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 is confined to a zone
outside, and beneath the surface, where it must be very abundant.
Fragments of the Millepora alcicornis and of an Astraea were also numerous;
the former is found, but not in proportionate numbers, in the hollows on
the reef; but the Astraea I did not see living. Hence we may infer, that
these are the kinds of coral which form the rugged sloping surface
(represented in the woodcut by an uneven line), round and beneath the
external margin. Between twelve and twenty 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 thirteen and sixteen
fathoms. Out of twenty-five soundings taken at a greater depth than twenty
fathoms, every one showed that the bottom was covered with sand; whereas,
at a less depth than twelve fathoms, every sounding showed that it was
exceedingly rugged, and free from all extraneous particles. Two soundings
were obtained at the depth of 360 fathoms, and several between two hundred
and three hundred fathoms. The sand brought up from these depths consisted
of finely triturated fragments of stony zoophytes, but not, as far as I
could distinguish, of a particle of any lamelliform genus: fragments of
shells were rare.

At a distance of 2,200 yards from the breakers, Captain Fitzroy found no
bottom with a line of 7,200 feet in length; hence the submarine slope of
this coral formation is steeper than that of any volcanic cone. Off the
mouth of the lagoon, and likewise off the northern point of the atoll,
where the currents act violently, the inclination, owing to the
accumulation of sediment, is less. As the arming of the lead from all the
greater depths showed a smooth sandy bottom, I at first concluded that the
whole consisted of a vast conical pile of calcareous sand, but the sudden
increase of depth at some points, and the circumstance of the line having
been cut, as if rubbed, when between five hundred and six hundred fathoms
were out, indicate the probable existence of submarine cliffs.

On the margin of the reef, close within the line where the upper surface of
the Porites and of the Millepora is dead, three species of Nullipora
flourish. One grows in thin sheets, like a lichen on old trees; the 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. (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.)
The three species occur either separately or mingled together; and they
form by their successive growth a layer two or three feet in thickness,
which in some cases is hard, but where formed of the lichen-like kind,
readily yields an impression to the hammer: the surface is of a reddish
colour. These Nulliporae, although able to exist above the limit of true
corals, seem to require to be bathed during the greater part of each tide
by breaking water, for they are not found in any abundance in the protected
hollows on the back part of the reef, where they might be immersed either
during the whole or an equal proportional time of each tide. It is
remarkable that organic productions of such extreme simplicity, for the
Nulliporae undoubtedly belong to one of 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 twenty 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 accumulation of rolled fragments. We shall hereafter see
that other coral reefs are protected by a similar thick growth of
Nulliporae on the outer margin, the part most exposed to the breakers, and
this must effectually aid in preserving it from being worn down.

The woodcut represents a section across one of the islets on the reef, but
if all that part which is above the level of C were removed, the section
would be that of a simple reef, as it occurs where no islet has been
formed. It is this reef which essentially forms the atoll. It is a ring,
enclosing 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 two hundred and fifty to five hundred yards, its surface is
level, or very slightly inclined towards the lagoon, and at high tide the
sea breaks entirely over it: the water at low tide thrown by the breakers
on the reef, is carried by the many narrow and shoal gullies or channels on
its surface, into the lagoon: a return stream sets out of the lagoon
through the main entrance. The most frequent coral in the hollows on the
reef is Pocillopora verrucosa, which grows in short sinuous plates, or
branches, and when alive is of a beautiful pale lake-red: a Madrepora,
closely allied or identical with M. pocillifera, 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 surface varies in
width from one hundred to two hundred, or even three hundred yards, and is
strewed with a few large fragments of coral torn up during gales: it is
uncovered only at low water. I could with difficulty, and only by the aid
of a chisel, procure chips of rock from its surface, and therefore could
not ascertain how much of it is formed by the aggregation of detritus, and
how much by the outward growth of mounds of corals, similar to those now
living on the margin. Nothing can be more singular than the appearance at
low tide of this “flat” of naked stone, especially where it is externally
bounded by the smooth convex mound of Nulliporae, appearing like a
breakwater built to resist the waves, which are constantly throwing over it
sheets of foaming water. The characteristic appearance of this “flat” is
shown in the foregoing woodcut of Whitsunday atoll.

The islets on the reef are first formed between two hundred and three
hundred yards from its outer edge, through the accumulation of a pile of
fragments, thrown together by some unusually strong gale. Their ordinary
width is under a quarter of a mile, and their length varies from a few
yards to several miles. Those on the south-east 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 surface is composed, as
well as the shells mingled with them, almost exclusively consist of those
kinds which live on the outer coast. The highest part of the islets
(excepting hillocks of blown sand, some of which are thirty 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
windward, some being even eight hundred yards in width; but the land thus
added is very low. The fragments beneath the surface are cemented into a
solid mass, which is exposed as a ledge (D of the woodcut), projecting some
yards in front of the outer shore and from two to four feet high. This
ledge is just reached by the waves at ordinary high-water: it extends in
front of all the islets, and everywhere has a water-worn and scooped
appearance. The fragments of coral which are occasionally cast on the
“flat” are during gales of unusual violence swept together on the beach,
where the waves each day at high-water tend to remove and gradually wear
them down; but the lower fragments having become firmly cemented together
by the percolation of calcareous matter, resist the daily tides longer, and
hence project as a ledge. The cemented mass is generally of a white
colour, but in some few parts reddish from ferruginous matter; it is very
hard, and is sonorous under the hammer; it is obscurely divided by seams,
dipping at a small angle seaward; it consists of fragments of the corals
which grow on the outer margin, some quite and others partially rounded,
some small and others between two and three feet across; and of masses of
previously formed conglomerate, torn up, rounded, and re-cemented; or it
consists of a calcareous sandstone, entirely composed of rounded particles,
generally almost blended together, of shells, corals, the spines of echini,
and other such 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 consist of
three closely allied species of true Madrepora in thin branches; of
Seriatapora subulata; two species of Porites (This Porites has somewhat the
habit of P. clavaria, but the branches are not knobbed at their ends. When
alive it is of a yellow colour, but after having been washed in fresh water
and placed to dry, a jet-black slimy substance exuded from the entire
surface, so that the specimen now appears as if it had been dipped in ink.)
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 crowbars a channel of considerable length through
these reefs, in which a schooner, built on the S.E. islet, was floated out.
It is a very interesting circumstance, pointed out to us by Mr. Liesk, that
this channel, although made less than ten years before our visit, was then,
as we saw, almost choked up with living coral, so that fresh excavations
would be absolutely necessary to allow another vessel to pass through it.

The sediment from the deepest parts in the lagoon, when wet, appeared
chalky, but when dry, like very fine sand. Large soft banks of similar,
but even finer grained mud, occur on the S.E. shore of the lagoon,
affording a thick growth of a Fucus, 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
Lieutenant Nelson from the reefs of Bermuda, which, when shown to several
experienced geologists, was mistaken by them for true chalk. On the
outside of the reef much sediment must be formed by the action of the surf
on the rolled fragments of coral; but in the calm waters of the lagoon,
this can take place only 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 finely ground
calcareous matter. This must daily pass from them as the finest sediment;
much also must be produced by the infinitely numerous vermiform and
molluscous animals, which make cavities in almost every block of coral.
Dr. J. Allan, of Forres, who has enjoyed the best means of observation,
informs me in a letter that the Holothuriae (a family of Radiata) subsist
on living coral; and the singular structure of bone within the anterior
extremity of their bodies, certainly appears well adapted for this purpose.
The number of the species of Holothuria, and of the individuals which swarm
on every part of these coral-reefs, is extraordinarily great; and many
shiploads 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 growth of coral-reefs, and that the almost
universal law of “consumed and be consumed,” holds good even with the
polypifers forming those massive bulwarks, which are able to withstand the
force of the open ocean.

This splendid natural feature has to be appreciate as it is not very easy to facilitate a living in a competitive atmosphere like the exposed ocean. In the same manner, we have a complete crypto robot, Ethereum Code to ease the competitive trading platform with expert guidance and easy usage.

Considering that Keeling atoll, like other coral formations, 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 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 foregoing 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 afterward 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 supposition which I must not pass over. If the whole,
or a large part of the “flat,” had been formed by the outward growth of the
margin, each successive margin would naturally have been coated by the
Nulliporae, and so much of the surface would have been of equal height with
the existing zone of living Nulliporae: 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
Nulliporae, perishing as the reef grows outwards, might be worn down by the
surf. If this has not taken place, the reef can in no part have increased
outwards in breadth since its formation, or at least since the Nulliporae
formed the convex mound on its margin; for the zone thus formed, and which
stands between two and three feet above the other parts of the reef, is
nowhere much above twenty yards in width.

Thus far we have considered facts, which indicate, with more or less
probability, the increase of the atoll in its different parts: there are
others having an opposite tendency. On the south-east side, Lieutenant
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
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 Lieutenant Sulivan and myself, from the manner in
which the trees were being washed down, that the surf had lately
recommenced an attack on this line of coast. Appearances indicating a
slight 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
watermark. 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 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 steepness of the submarine flanks, which can be added to
only by sediment derived from the wear and tear of the coral. From the
rapid growth of the coral in the channel cut for the schooner, and from the
several agents at work in producing fine sediment, it might be thought that
the lagoon would necessarily become quickly filled up. Some of this
sediment, however, is transported into the open sea, as appears from the
soundings off the mouth of the lagoon, 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, 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 1.II.–GENERAL DESCRIPTION OF ATOLLS.

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

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 atolls and the Great Chagos
Bank differ in so many respects, that I shall devote to them, besides
occasional references, a third section of this chapter. Keeling atoll may
be considered as of moderate dimensions and of regular form. Of the
thirty-two islands surveyed by Captain 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 Figure 4, Plate I.), and Clermont Tonnere
has nearly the same proportions. In the Marshall Archipelago (the Ralick
and Radack group of Kotzebue) several of the atolls are more than thirty
miles in length, and Rimsky Korsacoff is fifty-four long, and twenty wide,
at the broadest part of its irregular outline. Most of the atolls in the
Maldiva Archipelago are of great size, one of them (which, 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 Menchikoff Island (See Figure 3,
Plate II.), 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. Captain Beechey (Beechey’s “Voyage to the Pacific and Beering’s
Straits,” chapter viii.) says that in the atolls of the Low Archipelago it
exceeded in no instance half a mile. The description given of the
structure and proportional dimensions of the reef and islets of Keeling
atoll, appears to apply perfectly to nearly all the atolls in the Pacific
and Indian Oceans. The islets are first formed some way back either on the
projecting points of the reef, especially if its form be angular, or on the
sides of the main entrances into the lagoon–that is in both cases, on
points where the breakers can act during gales of wind in somewhat
different directions, so that the matter thrown up from one side may
accumulate against that before thrown up from another. In Lutke’s chart of
the Caroline atolls, we see many instances of the former case; and the
occurrence of islets, as if placed for beacons, on the points where there
is a gateway or breach through the reef, has been noticed by several
authors. There are some atoll-formed reefs, rising to the surface of the
sea and partly dry at low water, on which from some cause islets have never
been formed; and there are others on which they have been formed, but have
subsequently been worn away. In atolls of small dimensions the islets
frequently become united into a single horse-shoe or ring-formed strip; but
Diego Garcia, although an atoll of considerable size, being thirteen miles
and a half in length, has its lagoon entirely surrounded, except at the
northern end, by a belt of land, on an average a third of a mile in width.
To show how small the total area of the annular reef and the land is in
islands of this class, I may quote a remark from the voyage of Lutke,
namely, that if the forty-three rings, or atolls, in the Caroline
Archipelago, were put one within another, and over a steeple in the centre
of St. Petersburg, the whole world would not cover that city and its
suburbs.

The form of the bottom off Keeling atoll, which gradually slopes to about
twenty fathoms at the distance of between one and two hundred yards from
the edge of the reef, and then plunges at an angle of 45 deg into
unfathomable depths, is exactly the same (The form of the bottom round the
Marshall atolls in the Northern Pacific is probably similar: Kotzebue
(“First Voyage,” volume ii., page 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.”) 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 (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.)
informs me that all the soundings, even the deepest, were on coral, but he
does not know whether dead or alive. The slope round Christmas atoll (Lat.
1 deg 4′ N., 157 deg 45′ W.), described by Cook (Cook’s “Third Voyage,”
volume ii., chapter 10.), is considerably less, at about half a mile from
the edge of the reef, the average depth was about fourteen fathoms on a
fine sandy bottom, and at a mile, only between twenty and forty fathoms.
It has no doubt been owing to this gentle slope, that the strip of land
surrounding its lagoon, has increased in one part to the extraordinary
width of three miles; it is formed of successive ridges of broken shells
and corals, like those on the beach. I know of no other instance of such
width in the reef of an atoll; but Mr. F.D. Bennett informs me that the
inclination of the bottom round Caroline atoll in the Pacific, is like that
off Christmas Island, very gentle. Off the Maldiva and Chagos atolls, the
inclination is much more abrupt; thus at Heawandoo Pholo, Lieutenant Powell
(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”, volume v., page 401.) found fifty and sixty fathoms
close to the edge of the reef, and at 300 yards distance there was no
bottom with a 300-yard line. Captain Moresby informs me, that at 100
fathoms from the mouth of the lagoon of Diego Garcia, he found no bottom
with 150 fathoms; this is the more remarkable, as the slope is generally
less abrupt in front of channels through a reef, owing to the accumulation
of sediment. At Egmont Island, also, at 150 fathoms from the reef,
soundings were struck with 150 fathoms. Lastly, at Cardoo atoll, only
sixty yards from the reef, no bottom was obtained, as I am informed by
Captain Moresby, with a line of 200 fathoms! The currents run with great
force round these atolls, and where they are strongest, the inclination
appears to be most abrupt. I am informed by the same authority, that
wherever soundings were obtained off these islands, the bottom was
invariably sandy: nor was there any reason to suspect the existence of
submarine cliffs, as there was at Keeling Island. (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, Captain Beechey
(page 45, 4to edition) 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.) 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
(“Memoires pour servir a une description Geolog. de France,” tome iv., page
216.) 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 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 (Beechey’s “Voyage,” 4to
edition, page 44.) 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 abruptness 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, 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 applicable to the
margin of Keeling atoll. (Kotzebue’s “First Voyage,” volume iii., page
142. Near Porto Praya, in the Cape de Verde Islands, some basaltic rocks,
lashed by no inconsiderable surf, were completely enveloped with a layer of
Nulliporae. 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 Nulliporae are closely related to those described on the
coral-reefs, but I believe are of different species.) Although Chamisso
does not state that the masses of Nulliporae form points or a mound, higher
than the flat, yet I believe that this is the case; for Kotzebue (Kotzebue,
“First Voyage,” volume ii., page 16. Lieutenant Nelson, in his excellent
memoir in the Geological Transactions (volume ii., page 105), alludes to
the rocky points mentioned by Kotzebue, and infers that they consist of
Serpulae, 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,” October 1841). These masses of
Serpulae hold the same position, relatively to the action of the sea, with
the Nulliporae on the coral-reefs in the Indian and Pacific Oceans.), 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 (Captain Moresby, in his valuable paper “on the
Northern atolls of Maldivas” (“Geographical Journal”, volume v.), says that
the edges of the reefs there stand above water at low spring-tides.)
Whether a smooth convex mound of Nulliporae, like that which appears as if
artificially constructed to protect the margin of Keeling Island, is of
frequent occurrence round atolls, I know not; but we shall presently meet
with it, under precisely the same form, on the outer edge of the
“barrier-reefs” which encircle the Society Islands.

There appears to be scarcely a feature in the structure of Keeling reef,
which is not of common, if not of universal occurrence, in other atolls.
Thus Chamisso describes (Kotzebue’s “First Voyage,” volume iii., page 144.)
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 (see also
Moresby on the Northern atolls of the Maldivas, “Geographical Journal”,
volume v., page 400.), it is evident that their outer coasts are subject to
the same round of decay and renovation as those of Keeling atoll. From the
description of the atolls in the Low Archipelago, given in Captain
Beechey’s “Voyage,” it is not apparent that 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
leveled the surface so equally. In the Maldiva atolls this is very
conspicuous, and likewise in some of the Caroline and Marshall Islands. In
the former large spaces consist of sand and SOFT CLAY; and Kotzebue 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 Lieutenant Nelson says is called there pipe-clay. (I may here
observe that on the coast of Brazil, where there is much coral, the
soundings near the land are described by Admiral Roussin, in the “Pilote du
Bresil”, as siliceous sand, mingled with much finely comminuted particles
of shells and coral. Further in the offing, for a space of 1,300 miles
along the coast, from the Abrolhos Islands to Maranham, the bottom in many
places is composed of “tuf blanc, mele ou forme de madrepores broyes.”
This white substance, probably, is analogous to that which occurs within
the above-mentioned lagoons; it is sometimes, according to Roussin, firm,
and he compares it to mortar.)

Where the waves act with unequal force on the two sides of an atoll, the
islets appear to be first formed, and are generally of greater continuity
on the more exposed shore. The islets, also, which are placed to leeward,
are in most parts of the Pacific liable to be occasionally swept entirely
away by gales, equalling hurricanes in violence, which blow in an opposite
direction to the ordinary trade-wind. The absence of the islets on the
leeward side of atolls, or when present their lesser dimensions compared
with those to windward, is a comparatively unimportant fact; but in several
instances the reef itself on the leeward side, retaining 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 I., Figure 9) in the Chagos
group, with Mourileu atoll (Frederick Lutke’s “Voyage autour du Monde,”
volume ii., page 291. See also his account of Namonouito, below, and the
chart of Oulleay in the Atlas.) in the Caroline Archipelago, and with the
barrier-reef (Plate I., Figure 8) 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 submerged part is nine miles in length, and lies at an average
depth of about five fathoms; its surface is nearly level, and consists of
hard stone, with a thin covering of loose sand. There is scarcely any
living coral on it, even on the outer margin, as I have been particularly
assured by Captain Moresby; it is, in fact, a wall of dead coral-rock,
having the same width and transverse section with the reef in its ordinary
state, of which it is a continuous portion. The living and perfect parts
terminate abruptly, and abut on the submerged portions, in the same manner
as 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
Archipelago, both these modifications of the reef concur; it consists of a
great flat bank, with from twenty to twenty-five fathoms water on it; for a
length of more than forty 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 described. The Speaker’s Bank offers an excellent example of
this structure; its central expanse, which is about twenty-two fathoms
deep, is twenty-four miles across; the external rim is of the usual width
of annular reefs, and is well-defined; it lies between six and eight
fathoms beneath the surface, and at the same depth there are scattered
knolls in the lagoon. Captain Moresby believes 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 structure wholly obliterated. It appears from the survey
of Freycinet, that there are banks of this kind in the Caroline
Archipelago, and, as is reported, in the Low Archipelago. When we discuss
the origin of the different classes of coral formations, we shall see that
the submerged state of the whole of some atoll-formed reefs, and of
portions of others, generally but not invariably on the leeward side, and
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. 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 (“Principles
of Geology,” volume iii., page 289.) 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 considerable 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 (Beechey’s “Voyage,” 4to edition, volume i., page
189.), 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 where there are
coral-reefs; but this is by no means the universal structure in other atolls.
Chamisso (Kotzebue’s “First Voyage,” volume iii., page 142.), speaking in
general terms of the lagoons in the Marshall atolls, says the lead
generally sinks “from a depth of two or three fathoms to twenty or
twenty-four, and you may pursue a line in which on one side of the boat you
may see the bottom, and on the other the azure-blue deep water.” The shores
of the lagoon-like channel within the barrier-reef at Vanikoro have a similar
structure. Captain Beechey has described a modification of this structure
(and he believes it is not uncommon) in two atolls in the Low Archipelago,
in which the shores of the lagoon descend by a few, broad, slightly
inclined ledges or steps: thus at Matilda atoll (Beechey’s “Voyage,” 4to
edition, volume i, page 160. At Whitsunday Island the bottom of the lagoon
slopes gradually towards the centre, and then deepens suddenly, the edge of
the bank being nearly perpendicular. This bank is formed of coral and dead
shells.), 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 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 deg S.) is described by Chamisso
(Kotzebue’s “First Voyage,” volume iii., page 221.) 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-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.

(DESCRIPTION OF THE PLATES.

PLATE II.–GREAT CHAGOS BANK, NEW CALEDONIA,MENCHIKOFF ATOLL, ETC.

FIGURE 1.–GREAT CHAGOS BANK, in the Indian Ocean; taken from the survey by
Captain Moresby and Lieutenant 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 four to ten fathoms; the banks
bounded by the dotted lines lie from fifteen to twenty fathoms beneath the
surface, and are formed of sand; the central space is of mud, and from
thirty to fifty fathoms deep.

FIGURE 2.–A vertical section, on the same scale, in an eastern and western
line across the Great Chagos Bank, given for the sake of exhibiting more
clearly its structure.

FIGURE 3.–MENCHIKOFF ATOLL (or lagoon-island), in the Marshall
Archipelago, Northern Pacific Ocean; from Krusenstern’s “Atlas of the
Pacific;” originally surveyed by Captain Hagemeister; the depth within the
lagoons is unknown.

FIGURE 4.–MAHLOS MAHDOO ATOLL, together with Horsburgh atoll, in the
Maldiva Archipelago; from the survey by Captain Moresby and Lieutenant
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 not be introduced; the nautical mark (dot over a dash)
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.

FIGURE 5.–NEW CALEDONIA, in the western part of the Pacific; from
Krusenstern’s “Atlas,” compiled from several surveys; I have slightly
altered the northern point of the reef, in accordance with the “Atlas of
the Voyage of the ‘Astrolabe’.” In Krusenstern’s “Atlas,” the reef is
represented by a single line with crosses; I have for the sake of
uniformity added an interior line.

FIGURE 6.–MALDIVA ARCHIPELAGO, in the Indian Ocean; from the survey by
Captain Moresby and Lieutenant Powell.)

SECTION 1.III.–ATOLLS OF THE MALDIVA ARCHIPELAGO–GREAT CHAGOS BANK.

Maldiva Archipelago.–Ring-formed reefs, marginal and central.–Great
depths in the lagoons of the southern atolls.–Reefs in the lagoons all
rising to the surface.–Position of islets and breaches in the reefs, with
respect to the prevalent winds and action of the waves.–Destruction of
islets.–Connection in the position and submarine foundation of distinct
atolls.–The apparent disseverment of large atolls.–The Great Chagos
Bank.–Its submerged condition and extraordinary structure.

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

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