Chapter 1 Section I.III

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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 in the kindest manner.

The Maldiva Archipelago is 470 miles in length, with an average breadth of
about 50 miles.  The form and dimensions of the atolls, and their singular
position in a double line, may be seen, but not well, in the greatly
reduced chart (Figure 6) in Plate II.  The dimensions of the longest atoll
in the group (called by the double name of Milla-dou-Madou and
Tilla-dou-Matte) have already been given; it is 88 miles in a medial and
slightly curved line, and is less than 20 miles in its broadest part.
Suadiva, also, is a noble atoll, being 44 miles across in one direction, and
34 in another, and the great included expanse of water has a depth of between
250 and 300 feet.  The smaller atolls in this group differ in no respect from
ordinary ones; but the larger ones are remarkable from being breached by
numerous deep-water channels leading into the lagoon; for instance, there
are 42 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 II., Figure 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 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 (Captain Moresby informs me that Millepora complanata is one of the
commonest kinds on the outer margin, as it is at Keeling atoll.) within
which there is a flat surface of coral rock; of this flat, sand and
fragments have in many cases accumulated 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 contingent on the marginal channels or breaches
being wide; and, consequently, on the whole interior of the atoll being
freely exposed to the waters of the open sea.  When the channels are narrow
or few in number, although the lagoon be of great size and depth (as in
Suadiva), there are no ring-formed reefs; where the channels are somewhat
broader, the marginal portions of reef, and especially those close to the
larger channels, are ring-formed, but the central ones are not so; where
they are broadest, almost every reef throughout the atoll is more or less
perfectly ring-formed.  Although their presence is thus contingent on the
openness of the marginal channels, the theory of their formation, as we
shall hereafter see, is included in that of the parent atolls, of which
they form the separate portions.

The lagoons of all the atolls in the southern part of the Archipelago are
from ten to twenty fathoms deeper than those in the northern part.  This is
well exemplified in the case of Addoo, the southernmost atoll in the group,
for although only nine miles in its longest diameter, it has a depth of
thirty-nine 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 Captain 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 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 Archipelago the winds, during
the monsoons, blow during nearly an equal time from opposite quarters, and
although, as I am informed by Captain Moresby, the westerly winds are the
strongest, yet the islets are almost all placed on the eastern side of the
northern atolls, and on the south-eastern side of the southern atolls.
That the formation of the islets is due to detritus thrown up from the
outside, as in the ordinary manner, and not from the interior of the
lagoons, may, I think be safely inferred from several considerations, which
it is hardly worth while to detail.  As the easterly winds are not the
strongest, their action probably is aided by some prevailing swell or
current.

In groups of atolls, exposed to a trade-wind, the ship-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 fragments on the reef and to its more perfect continuity on
one side of the atoll than on the other, have concurred, but this has not
been the case with the Maldivas; for we have seen that the islets are
placed on the eastern or south-eastern sides, whilst the breaches in the
reef occur indifferently on any side, where protected by an opposite atoll.
The reef being more continuous on the outer and more exposed sides of those
atolls which stand near each other, accords with the fact, that the reef of
the southern atolls is more continuous than that of the northern ones; for
the former, as I am informed by Captain 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 hand, several islets, and even
some of those which are believed to be very old, are now fast wearing away.
The work of destruction has, in some instances, been completed in ten
years.  Captain Moresby found on one water-washed reef the marks of wells
and graves, which were excavated when it supported an islet.  In South
Nillandoo atoll, the natives say that three of the islets were formerly
larger: in North Nillandoo there is one now being washed away; and in this
latter atoll Lieutenant 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 Lieutenant 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 disseverment of a single one.  Male consists of
three perfectly characterised atolls, of which the shape and relative
position are such, that a line drawn closely round all three, gives a
symmetrical figure; to see this clearly, a larger chart is required than
that of the Archipelago in Plate II.; the channel separating the two
northern Male atolls is only little more than a mile wide, and no bottom
was found in it with 100 fathoms.  Powell's Island is situated at the
distance of two miles and a half off the northern end of Mahlos Mahdoo (see
Figure 4, Plate II.), 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
(Figure 4., Plate II.) of Mahlos Mahdoo, and the line of unfathomable water
be followed, no one will hesitate to consider it as one atoll.  But a
second look will show that it is divided by a bifurcating 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.  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 halves; 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 II., Figure 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 Figure 2, in which the vertical scale has been
necessarily exaggerated.  Its longest axis is ninety nautical miles, and
another line drawn at right angles to the first, across the broadest part,
is seventy.  The central part consists of a level muddy flat, between forty
and fifty fathoms deep, which is surrounded on all sides, with the
exception of some breaches, by the steep edges of a set of banks, rudely
arranged in a circle.  These banks consist of sand, with a very little live
coral; they vary in breadth from five to twelve miles, and on an average
lie about sixteen fathoms beneath the surface; they are bordered by the
steep edges of a third narrow and upper bank, which forms the rim to the
whole.  This rim is about a mile in width, and with the exception of two or
three spots where islets have been formed, is submerged between five and
ten fathoms.  It consists of smooth hard rock, covered with a thin layer of
sand, but with scarcely any live coral; it is steep on both sides, and
outwards slopes abruptly into unfathomable depths.  At the distance of less
than half a mile from one part, no bottom was found with 190 fathoms; and
off another point, at a somewhat greater distance, there was none with 210
fathoms.  Small steep-sided banks or knolls, covered with luxuriantly
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 (Figure 1, Plate II.), although reduced to
so small a scale, without at once perceiving that the Great Chagos Bank is,
in the words of Captain Moresby (This officer has had the kindness to lend
me an excellent MS. account of the Chagos Islands; from this paper, from
the published charts, and from verbal information communicated to me by
Captain Moresby, the above account of the Great Chagos Bank is taken.),
"nothing more than a half-drowned atoll."  But of what great dimensions,
and of how extraordinary an internal structure?  We shall hereafter have to
consider both the cause of its submerged condition, a state common to other
banks in the group, and the origin of the singular submarine terraces,
which bound the central expanse: these, I think, it can be shown, have
resulted from a cause analogous to that which has produced the bifurcating
channel across Mahlos Mahdoo.

Chapter I, Section 1.1

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.

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.