Monthly Archives: October 2003

[ Coral Reefs : Chapter III. Fringing or Shore-Reefs ]

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

Fringing-reefs, or, as they have been called by some voyagers, shore-reefs,
whether skirting an island or part of a continent, might at first be
thought to differ little, except in generally being of less breadth, from
barrier-reefs. As far as the superficies of the actual reef is concerned
this is the case; but the absence of an interior deep-water channel, and
the close relation in their horizontal extension with the probable slope
beneath the sea of the adjoining land, present essential points of
difference.

The reefs which fringe the island of Mauritius offer a good example of this
class. They extend round its whole circumference, with the exception of
two or three parts (This fact is stated on the authority of the Officier du
Roi, in his extremely interesting “Voyage a l’Isle de France,” undertaken
in 1768. According to Captain Carmichael (Hooker’s “Bot. Misc.” volume
ii., page 316) on one part of the coast there is a space for sixteen miles
without a reef.), where the coast is almost precipitous, and where, if as
is probable the bottom of the sea has a similar inclination, the coral
would have no foundation on which to become attached. A similar fact may
sometimes be observed even in reefs of the barrier class, which follow much
less closely the outline of the adjoining land; as, for instance, on the
south-east and precipitous side of Tahiti, where the encircling reef is
interrupted. On the western side of the Mauritius, which was the only part
I visited, the reef generally lies at the distance of about half a mile
from the shore; but in some parts it is distant from one to two, and even
three miles. But even in this last case, as the coast-land is gently
inclined from the foot of the mountains to the sea-beach, and as the
soundings outside the reef indicate an equally gentle slope beneath the
water, there is no reason for supposing that the basis of the reef, formed
by the prolongation of the strata of the island, lies at a greater depth
than that at which the polypifers could begin constructing the reef. Some
allowance, however, must be made for the outward extension of the corals on
a foundation of sand and detritus, formed from their own wear, which would
give to the reef a somewhat greater vertical thickness, than would
otherwise be possible.

The outer edge of the reef on the western or leeward side of the island is
tolerably well defined, and is a little higher than any other part. It
chiefly consists of large strongly branched corals, of the genus Madrepora,
which also form a sloping bed some way out to sea: the kinds of coral
growing in this part will be described in the ensuing chapter. Between the
outer margin and the beach, there is a flat space with a sandy bottom and a
few tufts of living coral; in some parts it is so shallow, that people, by
avoiding the deeper holes and gullies, can wade across it at low water; in
other parts it is deeper, seldom however exceeding ten or twelve feet, so
that it offers a safe coasting channel for boats. On the eastern and
windward side of the island, which is exposed to a heavy surf, the reef was
described to me as having a hard smooth surface, very slightly inclined
inwards, just covered at low-water, and traversed by gullies; it appears to
be quite similar in structure to the reefs of the barrier and atoll
classes.

The reef of Mauritius, in front of every river and streamlet, is breached
by a straight passage: at Grand Port, however, there is a channel like
that within a barrier-reef; it extends parallel to the shore for four
miles, and has an average depth of ten or twelve fathoms; its presence may
probably be accounted for by two rivers which enter at each end of the
channel, and bend towards each other. The fact of reefs of the fringing
class being always breached in front of streams, even of those which are
dry during the greater part of the year, will be explained, when the
conditions unfavourable to the growth of coral are considered. Low
coral-islets, like those on barrier-reefs and atolls, are seldom formed on
reefs of this class, owing apparently in some cases to their narrowness, and
in others to the gentle slope of the reef outside not yielding many fragments
to the breakers. On the windward side, however, of the Mauritius, two or
three small islets have been formed.

It appears, as will be shown in the ensuing chapter, that the action of the
surf is favourable to the vigorous growth of the stronger corals, and that
sand or sediment, if agitated by the waves, is injurious to them. Hence it
is probable that a reef on a shelving shore, like that of Mauritius, would
at first grow up, not attached to the actual beach, but at some little
distance from it; and the corals on the outer margin would be the most
vigorous. A shallow channel would thus be formed within the reef, and as
the breakers are prevented acting on the shores of the island, and as they
do not ordinarily tear up many fragments from the outside, and as every
streamlet has its bed prolonged in a straight line through the reef, this
channel could be filled up only very slowly with sediment. But a beach of
sand and of fragments of the smaller kinds of coral seems, in the case of
Mauritius, to be slowly encroaching on the shallow channel. On many
shelving and sandy coasts, the breakers tend to form a bar of sand a little
way from the beach, with a slight increase of depth within it; for
instance, Captain Grey (Captain Grey’s “Journal of Two Expeditions,” volume
i. page 369.) states that the west coast of Australia, in latitude 24 deg.,
is fronted by a sand bar about two hundred yards in width, on which there
is only two feet of water; but within it the depth increases to two
fathoms. Similar bars, more or less perfect, occur on other coasts. In
these cases I suspect that the shallow channel (which no doubt during
storms is occasionally obliterated) is scooped out by the flowing away of
the water thrown beyond the line, on which the waves break with the
greatest force. At Pernambuco a bar of hard sandstone (I have described
this singular structure in the “London and Edinburgh Phil. Mag.” October
1841.), which has the same external form and height as a coral-reef,
extends nearly parallel to the coast; within this bar currents, apparently
caused by the water thrown over it during the greater part of each tide,
run strongly, and are wearing away its inner wall. From these facts it can
hardly be doubted, that within most fringing-reefs, especially within those
lying some distance from the land, a return stream must carry away the
water thrown over the outer edge; and the current thus produced, would tend
to prevent the channel being filled up with sediment, and might even deepen
it under certain circumstances. To this latter belief I am led, by finding
that channels are almost universally present within the fringing-reefs of
those islands which have undergone recent elevatory movements; and this
could hardly have been the case, if the conversion of the very shallow
channel into land had not been counteracted to a certain extent.

A fringing-reef, if elevated in a perfect condition above the level of the
sea, ought to present the singular appearance of a broad dry moat within a
low mound. The author (“Voyage a l’Isle de France, par un Officier du
Roi,” part i., pages 192, 200.) of an interesting pedestrian tour round the
Mauritius, seems to have met with a structure of this kind: he says
“J’observai que la, ou la mer etale, independamment des rescifs du large,
il y a terre UNE ESPECE D’EFFONCEMENT ou chemin couvert naturel. On y
pourrait mettre du canon,” etc. In another place he adds, “Avant de passer
le Cap, on remarque un gros banc de corail eleve de plus de quinze pieds:
c’est une espece de rescif, que la mer abandonne, il regne au pied une
longue flaque d’eau, dont on pourrait faire un bassin pour de petits
vaisseaux.” But the margin of the reef, although the highest and most
perfect part, from being most exposed to the surf, would generally during a
slow rise of the land be either partially or entirely worn down to that
level, at which corals could renew their growth on its upper edge. On some
parts of the coast-land of Mauritius there are little hillocks of coral-rock,
which are either the last remnants of a continuous reef, or of low
islets formed on it. I observed that two such hillocks between Tamarin Bay
and the Great Black River; they were nearly twenty feet high, about two
hundred yards from the present beach, and about thirty feet above its
level. They rose abruptly from a smooth surface, strewed with worn
fragments of coral. They consisted in their lower part of hard calcareous
sandstone, and in their upper of great blocks of several species of Astraea
and Madrepora, loosely aggregated; they were divided into irregular beds,
dipping seaward, in one hillock at an angle of 8 deg., and in the other at
18 deg. I suspect that the superficial parts of the reefs, which have been
upraised together with the islands they fringe, have generally been much
more modified by the wearing action of the sea, than those of Mauritius.

Many islands are fringed by reefs quite similar to those of Mauritius (I
may give Cuba, as another instance; Mr. Taylor (“Loudon’s Mag. of Nat.
Hist.” volume ix., page 449) has described a reef several miles in length
between Gibara and Vjaro, which extends parallel to the shore at the
distance of between half and the third part of a mile, and encloses a space
of shallow water, with a sandy bottom and tufts of coral. Outside the edge
of the reef, which is formed of great branching corals, the depth is six
and seven fathoms. This coast has been upheaved at no very distant
geological period.”); but on coasts where the sea deepens very suddenly the
reefs are much narrower, and their limited extension seems evidently to
depend on the high inclination of the submarine slope; a relation, which,
as we have seen, does not exist in reefs of the barrier class. The
fringing-reefs on steep coasts are frequently not more than from fifty to
one hundred yards in width; they have a nearly smooth, hard surface,
scarcely uncovered at low water, and without any interior shoal channel,
like that within those fringing-reefs, which lie at a greater distance from
the land. The fragments torn up during gales from the outer margin are
thrown over the reef on the shores of the island. I may give as instances,
Wateeo, where the reef is described by Cook as being a hundred yards wide;
and Mauti and Elizabeth Islands (Mauti is described by Lord Byron in the
voyage of H.M.S. “Blonde”, and Elizabeth Island by Captain Beechey.), where
it is only fifty yards in width: the sea round these islands is very deep.

Fringing-reefs, like barrier-reefs, both surround islands, and front the
shores of continents. In the charts of the eastern coast of Africa, by
Captain Owen, many extensive fringing-reefs are laid down; thus, for a
space of nearly forty miles, from latitude 1 deg 15′ to 1 deg 45′ S., a
reef fringes the shore at an average distance of rather more than one mile,
and therefore at a greater distance than is usual in reefs of this class;
but as the coast-land is not lofty, and as the bottom shoals very gradually
(the depth being only from eight to fourteen fathoms at a mile and a half
outside the reef), its extension thus far from the land offers no
difficulty. The external margin of this reef is described, as formed of
projecting points, within which there is a space, from six to twelve feet
deep, with patches of living coral on it. At Mukdeesha (latitude 2 deg 1′
N.) “the port is formed,” it is said (Owen’s “Africa,” volume i., page 357,
from which work the foregoing facts are likewise taken.) “by a long reef
extending eastward, four or five miles, within which there is a narrow
channel, with ten to twelve feet of water at low spring-tides;” it lies at
the distance of a quarter of a mile from the shore. Again, in the plan of
Mombas (latitude 4 deg S.), a reef extends for thirty-six miles, at the
distance of from half a mile to one mile and a quarter from the shore;
within it, there is a channel navigable “for canoes and small craft,”
between six and fifteen feet deep: outside the reef the depth is about
thirty fathoms at the distance of nearly half a mile. Part of this reef is
very symmetrical, and has a uniform breadth of two hundred yards.

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The coast of Brazil is in many parts fringed by reefs. Of these, some are
not of coral formation; for instance, those near Bahia and in front of
Pernambuco; but a few miles south of this latter city, the reef follows
(See Baron Roussin’s “Pilote du Bresil,” and accompanying hydrographical
memoir.) so closely every turn of the shore, that I can hardly doubt it is
of coral; it runs at the distance of three-quarters of a mile from the
land, and within it the depth is from ten to fifteen feet. I was assured
by an intelligent pilot that at Ports Frances and Maceio, the outer part of
the reef consists of living coral, and the inner of a white stone, full of
large irregular cavities, communicating with the sea. The bottom of the
sea off the coast of Brazil shoals gradually to between thirty and forty
fathoms, at the distance of between nine and ten leagues from the land.

From the description now given, we must conclude that the dimensions and
structure of fringing-reefs depend entirely on the greater or less
inclination of the submarine slope, conjoined with the fact that
reef-building polypifers can exist only at limited depths. It follows from
this, that where the sea is very shallow, as in the Persian Gulf and in
parts of the East Indian Archipelago, the reefs lose their fringing
character, and appear as separate and irregularly scattered patches, often
of considerable area. From the more vigorous growth of the coral on the
outside, and from the conditions being less favourable in several respects
within, such reefs are generally higher and more perfect in their marginal
than in their central parts; hence these reefs sometimes assume (and this
circumstance ought not to be overlooked) the appearance of atolls; but they
differ from atolls in their central expanse being much less deep, in their
form being less defined, and in being based on a shallow foundation. But
when in a deep sea reefs fringe banks of sediment, which have accumulated
beneath the surface, round either islands or submerged rocks, they are
distinguished with difficulty on the one hand from encircling barrier-reefs,
and on the other from atolls. In the West Indies there are reefs,
which I should probably have arranged under both these classes, had not the
existence of large and level banks, lying a little beneath the surface,
ready to serve as the basis for the attachment of coral, been occasionally
brought into view by the entire or partial absence of reefs on them, and
had not the formation of such banks, through the accumulation of sediment
now in progress, been sufficiently evident. Fringing-reefs sometimes coat,
and thus protect the foundations of islands, which have been worn down by
the surf to the level of the sea. According to Ehrenberg, this has been
extensively the case with the islands in the Red Sea, which formerly ranged
parallel to the shores of the mainland, with deep water within them: hence
the reefs now coating their bases are situated relatively to the land like
barrier-reefs, although not belonging to that class; but there are, as I
believe, in the Red Sea some true barrier-reefs. The reefs of this sea and
of the West Indies will be described in the Appendix. In some cases,
fringing-reefs appear to be considerably modified in outline by the course
of the prevailing currents. Dr. J. Allan informs me that on the east coast
of Madagascar almost every headland and low point of sand has a coral-reef
extending from it in a S.W. and N.E. line, parallel to the currents on that
shore. I should think the influence of the currents chiefly consisted in
causing an extension, in a certain direction, of a proper foundation for
the attachment of the coral. Round many intertropical islands, for
instance the Abrolhos on the coast of Brazil surveyed by Captain Fitzroy,
and, as I am informed by Mr. Cuming, round the Philippines, the bottom of
the sea is entirely coated by irregular masses of coral, which although
often of large size, do not reach the surface and form proper reefs. This
must be owing, either to insufficient growth, or to the absence of those
kinds of corals which can withstand the breaking of the waves.

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

[ 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.

[ Coral Reefs : Chapter V. Theory Of The Formation Of The Different Classes Of Coral-Reefs ]

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

The naturalists who have visited the Pacific, seem to have had their
attention riveted by the lagoon-islands, or atolls,–those singular rings
of coral-land which rise abruptly out of the unfathomable ocean–and have
passed over, almost unnoticed, the scarcely less wonderful encircling
barrier-reefs. The theory most generally received on the formation of
atolls, is that they are based on submarine craters; but where can we find
a crater of the shape of Bow atoll, which is five times as long as it is
broad (Plate I., Figure 4); or like that of Menchikoff Island (Plate II.,
Figure 3.), with its three loops, together sixty miles in length; or like
Rimsky Korsacoff, narrow, crooked, and fifty-four miles long; or like the
northern Maldiva atolls, made up of numerous ring-formed reefs, placed on
the margin of a disc,–one of which discs is eighty-eight miles in length,
and only from ten to twenty in breadth? It is, also, not a little
improbable, that there should have existed as many craters of immense size
crowded together beneath the sea, as there are now in some parts atolls.
But this theory lies under a greater difficulty, as will be evident, when
we consider on what foundations the atolls of the larger archipelagoes
rest: nevertheless, if the rim of a crater afforded a basis at the proper
depth, I am far from denying that a reef like a perfectly characterised
atoll might not be formed; some such, perhaps, now exist; but I cannot
believe in the possibility of the greater number having thus originated.

An earlier and better theory was proposed by Chamisso (Kotzebue’s “First
Voyage,” volume iii., page 331.); he supposes that as the more massive
kinds of corals prefer the surf, the outer portions, in a reef rising from
a submarine basis, would first reach the surface and consequently form a
ring. But on this view it must be assumed, that in every case the basis
consists of a flat bank; for if it were conically formed, like a
mountainous mass, we can see no reason why the coral should spring up from
the flanks, instead of from the central and highest parts: considering the
number of the atolls in the Pacific and Indian Oceans, this assumption is
very improbable. As the lagoons of atolls are sometimes even more than
forty fathoms deep, it must, also, be assumed on this view, that at a depth
at which the waves do not break, the coral grows more vigorously on the
edges of a bank than on its central part; and this is an assumption without
any evidence in support of it. I remarked, in the third chapter, that a
reef, growing on a detached bank, would tend to assume an atoll-like
structure; if, therefore, corals were to grow up from a bank, with a level
surface some fathoms submerged, having steep sides and being situated in a
deep sea, a reef not to be distinguished from an atoll, might be formed: I
believe some such exist in the West Indies. But a difficulty of the same
kind with that affecting the crater theory, runners, as we shall presently
see, this view inapplicable to the greater number of atolls.

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

What cause, then, has given to atolls and barrier-reefs their
characteristic forms? Let us see whether an important deduction will not
follow from the consideration of these two circumstances, first, the
reef-building corals flourishing only at limited depths; and secondly, the
vastness of the areas interspersed with coral-reefs and coral-islets, none
of which rise to a greater height above the level of the sea, than that
attained by matter thrown up by the waves and winds. I do not make this
latter statement vaguely; I have carefully sought for descriptions of every
island in the intertropical seas; and my task has been in some degree
abridged by a map of the Pacific, corrected in 1834 by MM. D’Urville and
Lottin, in which the low islands are distinguished from the high ones (even
from those much less than a hundred feet in height) by being written
without a capital letter; I have detected a few errors in this map,
respecting the height of some of the islands, which will be noticed in the
Appendix, where I treat of coral formations in geographical order. To the
Appendix, also, I must refer for a more particular account of the data on
which the statements on the next page are grounded. I have ascertained,
and chiefly from the writings of Cook, Kotzebue, Bellinghausen, Duperrey,
Beechey, and Lutke, regarding the Pacific; and from Moresby (See also
Captain Owen’s and Lieutenant Wood’s papers in the “Geographical Journal”,
on the Maldiva and Laccadive Archipelagoes. These officers particularly
refer to the lowness of the islets; but I chiefly ground my assertion
respecting these two groups, and the Chagos group, from information
communicated to me by Captain Moresby.) with respect to the Indian Ocean,
that in the following cases the term “low island” strictly means land of
the height commonly attained by matter thrown up by the winds and the waves
of an open sea. If we draw a line (the plan I have always adopted) joining
the external atolls of that part of the Low Archipelago in which the
islands are numerous, the figure will be a pointed ellipse (reaching from
Hood to Lazaref Island), of which the longer axis is 840 geographical
miles, and the shorter 420 miles; in this space (I find from Mr. Couthouy’s
pamphlet (page 58) that Aurora Island is about two hundred feet in height;
it consists of coral-rock, and seems to have been formed by the elevation
of an atoll. It lies north-east of Tahiti, close without the line bounding
the space coloured dark blue in the map appended to this volume. Honden
Island, which is situated in the extreme north-west part of the Low
Archipelago, according to measurements made on board the “Beagle”, whilst
sailing by, is 114 feet from the SUMMIT OF THE TREES to the water’s edge.
This island appeared to resemble the other atolls of the group.) none of
the innumerable islets united into great rings rise above the stated level.
The Gilbert group is very narrow, and 300 miles in length. In a prolonged
line from this group, at the distance of 240 miles, is the Marshall
Archipelago, the figure of which is an irregular square, one end being
broader than the other; its length is 520 miles, with an average width of
240; these two groups together are 1,040 miles in length, and all their
islets are low. Between the southern end of the Gilbert and the northern
end of Low Archipelago, the ocean is thinly strewed with islands, all of
which, as far as I have been able to ascertain, are low; so that from
nearly the southern end of the Low Archipelago, to the northern end of the
Marshall Archipelago, there is a narrow band of ocean, more than 4,000
miles in length, containing a great number of islands, all of which are
low. In the western part of the Caroline Archipelago, there is a space of
480 miles in length, and about 100 broad, thinly interspersed with low
islands. Lastly, in the Indian Ocean, the archipelago of the Maldivas is
470 miles in length, and 60 in breadth; that of the Laccadives is 150 by
100 miles; as there is a low island between these two groups, they may be
considered as one group of 1,000 miles in length. To this may be added the
Chagos group of low islands, situated 280 miles distant, in a line
prolonged from the southern extremity of the Maldivas. This group,
including the submerged banks, is 170 miles in length and 80 in breadth.
So striking is the uniformity in direction of these three archipelagoes,
all the islands of which are low, that Captain Moresby, in one of his
papers, speaks of them as parts of one great chain, nearly 1,500 miles
long. I am, then, fully justified in repeating, that enormous spaces, both
in the Pacific and Indian Oceans, are interspersed with islands, of which
not one rises above that height, to which the waves and winds in an open
sea can heap up matter.

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

The many widely-scattered atolls must, therefore, rest on rocky bases. But
we cannot believe that the broad summit of a mountain lies buried at the
depth of a few fathoms beneath every atoll, and nevertheless throughout the
immense areas above-named, with not one point of rock projecting above the
level of the sea; for we may judge with some accuracy of mountains beneath
the sea, by those on the land; and where can we find a single chain several
hundred miles in length and of considerable breadth, much less several such
chains, with their many broad summits attaining the same height, within
from 120 to 180 feet? If the data be thought insufficient, on which I have
grounded my belief, respecting the depth at which the reef-building
polypifers can exist, and it be assumed that they can flourish at a depth
of even one hundred fathoms, yet the weight of the above argument is but
little diminished, for it is almost equally improbable, that as many
submarine mountains, as there are low islands in the several great and
widely separated areas above specified, should all rise within six hundred
feet of the surface of the sea and not one above it, as that they should be
of the same height within the smaller limit of one or two hundred feet. So
highly improbable is this supposition, that we are compelled to believe,
that the bases of the many atolls did never at any one period all lie
submerged within the depth of a few fathoms beneath the surface, but that
they were brought into the requisite position or level, some at one period
and some at another, through movements in the earth’s crust. But this
could not have been effected by elevation, for the belief that points so
numerous and so widely separated were successively uplifted to a certain
level, but that not one point was raised above that level, is quite as
improbable as the former supposition, and indeed differs little from it.
It will probably occur to those who have read Ehrenberg’s account of the
Reefs of the Red Sea, that many points in these great areas may have been
elevated, but that as soon as raised, the protuberant parts were cut off by
the destroying action of the waves: a moment’s reflection, however, on the
basin-like form of the atolls, will show that this is impossible; for the
upheaval and subsequent abrasion of an island would leave a flat disc,
which might become coated with coral, but not a deeply concave surface;
moreover, we should expect to see, in some parts at least, the rock of the
foundation brought to the surface. If, then, the foundations of the many
atolls were not uplifted into the requisite position, they must of
necessity have subsided into it; and this at once solves every difficulty
(The additional difficulty on the crater hypothesis before alluded to, will
now be evident; for on this view the volcanic action must be supposed to
have formed within the areas specified a vast number of craters, all rising
within a few fathoms of the surface, and not one above it. The supposition
that the craters were at different times upraised above the surface, and
were there abraded by the surf and subsequently coated by corals, is
subject to nearly the same objections with those given above in this
paragraph; but I consider it superfluous to detail all the arguments
opposed to such a notion. Chamisso’s theory, from assuming the existence
of so many banks, all lying at the proper depth beneath the water, is also
vitally defective. The same observation applies to an hypothesis of
Lieutenant Nelson’s (“Geolog. Trans.” volume v., page 122), who supposes
that the ring-formed structure is caused by a greater number of germs of
corals becoming attached to the declivity, than to the central plateau of a
submarine bank: it likewise applies to the notion formerly entertained
(Forster’s “Observ.” page 151), that lagoon-islands owe their peculiar form
to the instinctive tendencies of the polypifers. According to this latter
view, the corals on the outer margin of the reef instinctively expose
themselves to the surf in order to afford protection to corals living in
the lagoon, which belong to other genera, and to other families!), for we
may safely infer, from the facts given in the last chapter, that during a
gradual subsidence the corals would be favourably circumstanced for
building up their solid frame works and reaching the surface, as island
after island slowly disappeared. Thus areas of immense extent in the
central and most profound parts of the great oceans, might become
interspersed with coral-islets, none of which would rise to a greater
height than that attained by detritus heaped up by the sea, and
nevertheless they might all have been formed by corals, which absolutely
required for their growth a solid foundation within a few fathoms of the
surface.

It would be out of place here to do more than allude to the many facts,
showing that the supposition of a gradual subsidence over large areas is by
no means improbable. We have the clearest proof that a movement of this
kind is possible, in the upright trees buried under the strata many
thousand feet in thickness; we have also every reason for believing that
there are now large areas gradually sinking, in the same manner as others
are rising. And when we consider how many parts of the surface of the
globe have been elevated within recent geological periods, we must admit
that there have been subsidences on a corresponding scale, for otherwise
the whole globe would have swollen. It is very remarkable that Mr. Lyell
(“Principles of Geology,” sixth edition, volume iii., page 386.), even in
the first edition of his “Principles of Geology,” inferred that the amount
of subsidence in the Pacific must have exceeded that of elevation, from the
area of land being very small relatively to the agents there tending to
form it, namely, the growth of coral and volcanic action. But it will be
asked, are there any direct proofs of a subsiding movement in those areas,
in which subsidence will explain a phenomenon otherwise inexplicable?
This, however, can hardly be expected, for it must ever be most difficult,
excepting in countries long civilised, to detect a movement, the tendency
of which is to conceal the part affected. In barbarous and semi-civilised
nations how long might not a slow movement, even of elevation such as that
now affecting Scandinavia, have escaped attention!

Mr. Williams (Williams’s “Narrative of Missionary Enterprise,” page 31.)
insists strongly that the traditions of the natives, which he has taken
much pains in collecting, do not indicate the appearance of any new
islands: but on the theory of a gradual subsidence, all that would be
apparent would be, the water sometimes encroaching slowly on the land, and
the land again recovering by the accumulation of detritus its former
extent, and perhaps sometimes the conversion of an atoll with coral islets
on it, into a bare or into a sunken annular reef. Such changes would
naturally take place at the periods when the sea rose above its usual
limits, during a gale of more than ordinary strength; and the effects of
the two causes would be hardly distinguishable. In Kotzebue’s “Voyage”
there are accounts of islands, both in the Caroline and Marshall
Archipelagoes, which have been partly washed away during hurricanes; and
Kadu, the native who was on board one of the Russian vessels, said “he saw
the sea at Radack rise to the feet of the cocoa-nut trees; but it was
conjured in time.” (Kotzebue’s “First Voyage,” volume iii., page 168.) A
storm lately entirely swept away two of the Caroline islands, and converted
them into shoals; it partly, also, destroyed two other islands. (M.
Desmoulins in “Comptes Rendus,” 1840, page 837.) According to a tradition
which was communicated to Captain Fitzroy, it is believed in the Low
Archipelago, that the arrival of the first ship caused a great inundation,
which destroyed many lives. Mr. Stutchbury relates, that in 1825, the
western side of Chain Atoll, in the same group, was completely devastated
by a hurricane, and not less than 300 lives lost: “in this instance it was
evident, even to the natives, that the hurricane alone was not sufficient
to account for the violent agitation of the ocean.” (“West of England
Journal”, No. I., page 35.) That considerable changes have taken place
recently in some of the atolls in the Low Archipelago, appears certain from
the case already given of Matilda Island: with respect to Whitsunday and
Gloucester Islands in this same group, we must either attribute great
inaccuracy to their discoverer, the famous circumnavigator Wallis, or
believe that they have undergone a considerable change in the period of
fifty-nine years, between his voyage and that of Captain Beechey’s.
Whitsunday Island is described by Wallis as “about four miles long, and
three wide,” now it is only one mile and a half long. The appearance of
Gloucester Island, in Captain Beechey’s words (Beechey’s “Voyage to the
Pacific,” chapter vii., and Wallis’s “Voyage in the ‘Dolphin’,” chapter
iv.), has been accurately described by its discoverer, but its present form
and extent differ materially.” Blenheim reef, in the Chagos group,
consists of a water-washed annular reef, thirteen miles in circumference,
surrounding a lagoon ten fathoms deep: on its surface there were a few
worn patches of conglomerate coral-rock, of about the size of hovels; and
these Captain Moresby considered as being, without doubt, the last remnants
of islets; so that here an atoll has been converted into an atoll-formed
reef. The inhabitants of the Maldiva Archipelago, as long ago as 1605,
declared, “that the high tides and violent currents were diminishing the
number of the islands” (See an extract from Pyrard’s Voyage in Captain
Owen’s paper on the Maldiva Archipelago, in the “Geographical Journal”,
volume ii., page 84.): and I have already shown, on the authority of
Captain Moresby, that the work of destruction is still in progress; but
that on the other hand the first formation of some islets is known to the
present inhabitants. In such cases, it would be exceedingly difficult to
detect a gradual subsidence of the foundation, on which these mutable
structures rest.

Some of the archipelagoes of low coral-islands are subject to earthquakes:
Captain Moresby informs me that they are frequent, though not very strong,
in the Chagos group, which occupies a very central position in the Indian
Ocean, and is far from any land not of coral formation. One of the islands
in this group was formerly covered by a bed of mould, which, after an
earthquake, disappeared, and was believed by the residents to have been
washed by the rain through the broken masses of underlying rock; the island
was thus rendered unproductive. Chamisso (See Chamisso, in Kotzebue’s
“First Voyage,” volume iii., pages 182 and 136.) states, that earthquakes
are felt in the Marshall atolls, which are far from any high land, and
likewise in the islands of the Caroline Archipelago. On one of the latter,
namely Oulleay atoll, Admiral Lutke, as he had the kindness to inform me,
observed several straight fissures about a foot in width, running for some
hundred yards obliquely across the whole width of the reef. Fissures
indicate a stretching of the earth’s crust, and, therefore, probably
changes in its level; but these coral-islands, which have been shaken and
fissured, certainly have not been elevated, and, therefore, probably they
have subsided. In the chapter on Keeling atoll, I attempted to show by
direct evidence, that the island underwent a movement of subsidence, during
the earthquakes lately felt there.

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

(PLATE: WOODCUT NO. 4.

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

BB–Shores of the island.

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

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

B’B’–The shores of the encircled island.

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

PLATE: WOODCUT NO. 5.

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

CC–The lagoon-channel.

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

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

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

Let us in imagination place within one of the subsiding areas, an island
surrounded by a “fringing-reef,”–that kind, which alone offers no
difficulty in the explanation of its origin. Let the unbroken lines and
the oblique shading in the woodcut (No. 4) represent a vertical section
through such an island; and the horizontal shading will represent the
section of the reef. Now, as the island sinks down, either a few feet at a
time or quite insensibly, we may safely infer from what we know of the
conditions favourable to the growth of coral, that the living masses bathed
by the surf on the margin of the reef, will soon regain the surface. The
water, however, will encroach, little by little, on the shore, the island
becoming lower and smaller, and the space between the edge of the reef and
the beach proportionately broader. A section of the reef and island in
this state, after a subsidence of several hundred feet, is given by the
dotted lines: coral-islets are supposed to have been formed on the new
reef, and a ship is anchored in the lagoon-channel. This section is in
every respect that of an encircling barrier-reef; it is, in fact, a section
taken (The section has been made from the chart given in the “Atlas of the
Voyage of the ‘Coquille’.” The scale is .57 of an inch to a mile. The
height of the island, according to M. Lesson, is 4,026 feet. The deepest
part of the lagoon-channel is 162 feet; its depth is exaggerated in the
woodcut for the sake of clearness.) east and west through the highest point
of the encircled island of Bolabola; of which a plan is given in Plate I.,
Figure 5. The same section is more clearly shown in the following woodcut
(No. 5) by the unbroken lines. The width of the reef, and its slope, both
on the outer and inner side, will have been determined by the growing
powers of the coral, under the conditions (for instance the force of the
breakers and of the currents) to which it has been exposed; and the
lagoon-channel will be deeper or shallower, in proportion to the growth of
the delicately branched corals within the reef, and to the accumulation of
sediment, relatively, also, to the rate of subsidence and the length of the
intervening stationary periods.

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

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

Referring to the diagram shown above, in which the newly formed barrier-reef
is represented by unbroken lines, instead of by dots as in the former
woodcut, let the work of subsidence go on, and the doubly pointed hill will
form two small islands (or more, according to the number of the hills)
included within one annular reef. Let the island continue subsiding, and
the coral-reef will continue growing up on its own foundation, whilst the
water gains inch by inch on the land, until the last and highest pinnacle
is covered, and there remains a perfect atoll. A vertical section of this
atoll is shown in the woodcut by the dotted lines;–a ship is anchored in
its lagoon, but islets are not supposed yet to have been formed on the
reef. The depth of the lagoon and the width and slope of the reef, will
depend on the circumstances just referred to under barrier-reefs. Any
further subsidence will produce no change in the atoll, except perhaps a
diminution in its size, from the reef not growing vertically upwards; but
should the currents of the sea act violently upon it, and should the corals
perish on part or on the whole of its margin, changes would result during
subsidence which will be presently noticed. I may here observe, that a
bank either of rock or of hardened sediment, level with the surface of the
sea, and fringed with living coral, would (if not so small as to allow the
central space to be quickly filled up with detritus) by subsidence be
converted immediately into an atoll, without passing, as in the case of a
reef fringing the shore of an island, through the intermediate form of a
barrier-reef. If such a bank lay a few fathoms submerged, the simple
growth of the coral (as remarked in the third chapter) without the aid of
subsidence, would produce a structure scarcely to be distinguished from a
true atoll; for in all cases the corals on the outer margin of a reef, from
having space and being freely exposed to the open sea, will grow vigorously
and tend to form a continuous ring whilst the growth of the less massive
kinds on the central expanse, will be checked by the sediment formed there,
and by that washed inwards by the breakers; and as the space becomes
shallower, their growth will, also, be checked by the impurities of the
water, and probably by the small amount of food brought by the enfeebled
currents, in proportion to the surface of living reefs studded with
innumerable craving mouths: the subsidence of a reef based on a bank of
this kind, would give depth to its central expanse or lagoon, steepness to
its flanks, and through the free growth of the coral, symmetry to its
outline:–I may here repeat that the larger groups of atolls in the Pacific
and Indian Oceans cannot be supposed to be founded on banks of this nature.

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

We now are able to perceive that the close similarity in form, dimensions,
structure, and relative position (which latter point will hereafter be more
fully noticed) between fringing and encircling barrier-reefs, and between
these latter and atolls, is the necessary result of the transformation,
during subsidence of the one class into the other. On this view, the three
classes of reefs ought to graduate into each other. Reefs having
intermediate character between those of the fringing and barrier classes do
exist; for instance, on the south-west coast of Madagascar, a reef extends
for several miles, within which there is a broad channel from seven to
eight fathoms deep, but the sea does not deepen abruptly outside the reef.
Such cases, however, are open to some doubts, for an old fringing-reef,
which had extended itself a little on a basis of its own formation, would
hardly be distinguishable from a barrier-reef, produced by a small amount
of subsidence, and with its lagoon-channel nearly filled up with sediment
during a long stationary period. Between barrier-reefs, encircling either
one lofty island or several small low ones, and atolls including a mere
expanse of water, a striking series can be shown: in proof of this, I need
only refer to the first plate in this volume, which speaks more plainly to
the eye, than any description could to the ear. The authorities from which
the charts have been engraved, together with some remarks on them and
descriptive of the plates, are given above. At New Caledonia (Plate II.,
Figure 5.) the barrier-reefs extend for 150 miles on each side of the
submarine prolongation of the island; and at their northern extremity they
appear broken up and converted into a vast atoll-formed reef, supporting a
few low coral-islets: we may imagine that we here see the effects of
subsidence actually in progress, the water always encroaching on the
northern end of the island, towards which the mountains slope down, and the
reefs steadily building up their massive fabrics in the lines of their
ancient growth.

We have as yet only considered the origin of barrier-reefs and atolls in
their simplest form; but there remain some peculiarities in structure and
some special cases, described in the two first chapters, to be accounted
for by our theory. These consist–in the inclined ledge terminated by a
wall, and sometimes succeeded by a second ledge with a wall, round the
shores of certain lagoons and lagoon-channels; a structure which cannot, as
I endeavoured to show, be explained by the simple growing powers of the
corals,–in the ring or basin-like forms of the central reefs, as well as
of the separate marginal portions of the northern Maldiva atolls,–in the
submerged condition of the whole, or of parts of certain barrier and
atoll-formed reefs; where only a part is submerged, this being generally to
leeward,–in the apparent progressive disseverment of some of the Maldiva
atolls,–in the existence of irregularly formed atolls, some being tied
together by linear reefs, and others with spurs projecting from them,–and,
lastly, in the structure and origin of the Great Chagos Bank.

STEP-FORMED LEDGES ROUND CERTAIN LAGOONS.

If we suppose an atoll to subside at an extremely slow rate, it is
difficult to follow out the complex results. The living corals would grow
up on the outer margin; and likewise probably in the gullies and deeper
parts of the bare surface of the annular reef; the water would encroach on
the islets, but the accumulation of fresh detritus might possibly prevent
their entire submergence. After a subsidence of this very slow nature, the
surface of the annular reef sloping gently into the lagoon, would probably
become united with the irregular reefs and banks of sand, which line the
shores of most lagoons. Should, however, the atoll be carried down by a
more rapid movement, the whole surface of the annular reef, where there was
a foundation of solid matter, would be favourably circumstanced for the
fresh growth of coral; but as the corals grew upwards on its exterior
margin, and the waves broke heavily on this part, the increase of the
massive polypifers on the inner side would be checked from the want of
water. Consequently, the exterior parts would first reach the surface, and
the new annular reef thus formed on the old one, would have its summit
inclined inwards, and be terminated by a subaqueous wall, formed by the
upward growth of the coral (before being much checked), from the inner edge
of the solid parts of the old reef. The inner portion of the new reef,
from not having grown to the surface, would be covered by the waters of the
lagoon. Should a subsidence of the same kind be repeated, the corals would
again grow up in a wall, from all the solid parts of the resunken reef,
and, therefore, not from within the sandy shores of the lagoon; and the
inner part of the new annular reef would, from being as before checked in
its upward growth, be of less height than the exterior parts, and therefore
would not reach the surface of the lagoon. In this case the shores of the
lagoon would be surrounded by two inclined ledges, one beneath the other,
and both abruptly terminated by subaqueous cliffs. (According to Mr.
Couthouy (page 26) the external reef round many atolls descends by a
succession of ledges or terraces. He attempts, I doubt whether
successfully, to explain this structure somewhat in the same manner as I
have attempted, with respect to the internal ledges round the lagoons of
some atolls. More facts are wanted regarding the nature both of the
interior and exterior step-like ledges: are all the ledges, or only the
upper ones, covered with living coral? If they are all covered, are the
kinds different on the ledges according to the depth? Do the interior and
exterior ledges occur together in the same atolls; if so, what is their
total width, and is the intervening surface-reef narrow, etc.?)

THE RING OR BASIN-FORMED REEFS OF THE NORTHERN MALDIVA ATOLLS.

I may first observe, that the reefs within the lagoons of atolls and within
lagoon-channels, would, if favourably circumstanced, grow upwards during
subsidence in the same manner as the annular rim; and, therefore, we might
expect that such lagoon-reefs, when not surrounded and buried by an
accumulation of sediment more rapid than the rate of subsidence, would rise
abruptly from a greater depth than that at which the efficient polypifers
can flourish: we see this well exemplified in the small abruptly-sided
reefs, with which the deep lagoons of the Chagos and Southern Maldiva
atolls are studded. With respect to the ring or basin-formed reefs of the
Northern Maldiva atolls, it is evident, from the perfectly continuous
series which exists that the marginal rings, although wider than the
exterior or bounding reef of ordinary atolls, are only modified portions of
such a reef; it is also evident that the central rings, although wider than
the knolls or reefs which commonly occur in lagoons, occupy their place.
The ring-like structure has been shown to be contingent on the breaches
into the lagoon being broad and numerous, so that all the reefs which are
bathed by the waters of the lagoon are placed under nearly the same
conditions with the outer coast of an atoll standing in the open sea.
Hence the exterior and living margins of these reefs must have been
favourably circumstanced for growing outwards, and increasing beyond the
usual breadth; and they must likewise have been favourably circumstanced
for growing vigorously upwards, during the subsiding movements, to which by
our theory the whole archipelago has been subjected; and subsidence with
this upward growth of the margins would convert the central space of each
little reef into a small lagoon. This, however, could only take place with
those reefs, which had increased to a breadth sufficient to prevent their
central spaces from being almost immediately filled up with the sand and
detritus driven inwards from all sides: hence it is that few reefs, which
are less than half a mile in diameter, even in the atolls where the
basin-like structure is most strikingly exhibited, include lagoons. This
remark, I may add, applies to all coral-reefs wherever found. The
basin-formed reefs of the Maldiva Archipelago may, in fact, be briefly
described, as small atolls formed during subsidence over the separate
portions of large and broken atolls, in the same manner as these latter were
formed over the barrier-reefs, which encircled the islands of a large
archipelago now wholly submerged.

SUBMERGED AND DEAD REEFS.

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

As we see that in those parts of the ocean, where coral-reefs are most
abundant, one island is fringed and another neighbouring one is not
fringed; as we see in the same archipelago, that all the reefs are more
perfect in one part of it than in another, for instance, in the southern
half compared with the northern half of the Maldiva Archipelago, and
likewise on the outer coasts compared with the inner coasts of the atolls
in this same group, which are placed in a double row; as we know that the
existence of the innumerable polypifers forming a reef, depends on their
sustenance, and that they are preyed on by other organic beings; and,
lastly, as we know that some inorganic causes are highly injurious to the
growth of coral, it cannot be expected that during the round of change to
which earth, air, and water are exposed, the reef-building polypifers
should keep alive for perpetuity in any one place; and still less can this
be expected, during the progressive subsidences, perhaps at some periods
more rapid than at others, to which by our theory these reefs and islands
have been subjected and are liable. It is, then, not improbable that the
corals should sometimes perish either on the whole or on part of a reef; if
on part, the dead portion, after a small amount of subsidence, would still
retain its proper outline and position beneath the water. After a more
prolonged subsidence, it would probably form, owing to the accumulation of
sediment, only the margin of a flat bank, marking the limits of the former
lagoon. Such dead portions of reef would generally lie on the leeward side
(Mr. Lyell, in the first edition of his “Principles of Geology,” offered a
somewhat different explanation of this structure. He supposes that there
has been subsidence; but he was not aware that the submerged portions of
reef were in most cases, if not in all, dead; and he attributes the
difference in height in the two sides of most atolls, chiefly to the
greater accumulation of detritus to windward than to leeward. But as
matter is accumulated only on the backward part of the reef, the front part
would remain of the same height on both sides. I may here observe that in
most cases (for instance, at Peros Banhos, the Gambier group and the Great
Chagos Bank), and I suspect in all cases, the dead and submerged portions
do not blend or slope into the living and perfect parts, but are separated
from them by an abrupt line. In some instances small patches of living
reef rise to the surface from the middle of the submerged and dead parts.),
for the impure water and fine sediment would more easily flow out from the
lagoon over this side of the reef, where the force of the breakers is less
than to windward; and therefore the corals would be less vigorous on this
side, and be less able to resist any destroying agent. It is likewise
owing to this same cause, that reefs are more frequently breached to
leeward by narrow channels, serving as by ship-channels, than to windward.
If the corals perished entirely, or on the greater part of the
circumference of an atoll, an atoll-shaped bank of dead rock, more or less
entirely submerged, would be produced; and further subsidence, together
with the accumulation of sediment, would often obliterate its atoll-like
structure, and leave only a bank with a level surface.

In the Chagos group of atolls, within an area of 160 miles by 60, there are
two atoll-formed banks of dead rock (besides another very imperfect one),
entirely submerged; a third, with merely two or three very small pieces of
living reef rising to the surface; and a fourth, namely, Peros Banhos
(Plate I., Figure 9), with a portion nine miles in length dead and
submerged. As by our theory this area has subsided, and as there is
nothing improbable in the death, either from changes in the state of the
surrounding sea or from the subsidence being great or sudden, of the corals
on the whole, or on portions of some of the atolls, the case of the Chagos
group presents no difficulty. So far indeed are any of the above-mentioned
cases of submerged reefs from being inexplicable, that their occurrence
might have been anticipated on our theory, and as fresh atolls are supposed
to be in progressive formation by the subsidence of encircling barrier-reefs,
a weighty objection, namely that the number of atolls must be
increasing infinitely, might even have been raised, if proofs of the
occasional destruction and loss of atolls could not have been adduced.

THE DISSEVERMENT OF THE LARGER MALDIVA ATOLLS.

The apparent progressive disseverment in the Maldiva Archipelago of large
atolls into smaller ones, is, in many respects, an important consideration,
and requires an explanation. The graduated series which marks, as I
believe, this process, can be observed only in the northern half of the
group, where the atolls have exceedingly imperfect margins, consisting of
detached basin-formed reefs. The currents of the sea flow across these
atolls, as I am informed by Captain Moresby, with considerable force, and
drift the sediment from side to side during the monsoons, transporting much
of it seaward; yet the currents sweep with greater force round their
flanks. It is historically known that these atolls have long existed in
their present state; and we can believe, that even during a very slow
subsidence they might thus remain, the central expanse being kept at nearly
its original depth by the accumulation of sediment. But in the action of
such nicely balanced forces during a progressive subsidence (like that, to
which by our theory this archipelago has been subjected), it would be
strange if the currents of the sea should never make a direct passage
across some one of the atolls, through the many wide breaches in their
margins. If this were once effected, a deep-water channel would soon be
formed by the removal of the finer sediment, and the check to its further
accumulation; and the sides of the channel would be worn into a slope like
that on the outer coasts, which are exposed to the same force of the
currents. In fact, a channel precisely like that bifurcating one which
divides Mahlos Mahdoo (Plate II., Figure 4.), would almost necessarily be
formed. The scattered reefs situated near the borders of the new
ocean-channel, from being favourably placed for the growth of coral, would,
by their extension, tend to produce fresh margins to the dissevered portions;
such a tendency is very evident (as may be seen in the large published
chart) in the elongated reefs on the borders of the two channels
intersecting Mahlos Mahdoo. Such channels would become deeper with
continued subsidence, and probably from the reefs not growing up
perpendicularly, somewhat broader. In this case, and more especially if
the channels had been formed originally of considerable breadth, the
dissevered portions would become perfect and distinct atolls, like Ari and
Ross atolls (Plate II., Figure 6), or like the two Nillandoo atolls, which
must be considered as distinct, although related in form and position, and
separated from each other by channels, which though deep have been sounded.
Further subsidence would render such channels unfathomable, and the
dissevered portions would then resemble Phaleedoo and Moluque atolls, or
Mahlos Mahdoo and Horsburgh atolls (Plate II., Figure 4), which are related
to each other in no respect except in proximity and position. Hence, on
the theory of subsidence, the disseverment of large atolls, which have
imperfect margins (for otherwise their disseverment would be scarcely
possible), and which are exposed to strong currents, is far from being an
improbable event; and the several stages, from close relation to entire
isolation in the atolls of the Maldiva Archipelago, are readily explicable.

We might go even further, and assert as not improbable, that the first
formation of the Maldiva Archipelago was due to a barrier-reef, of nearly
the same dimensions with that of New Caledonia (Plate II., Figure 5), for
if, in imagination, we complete the subsidence of that great island, we
might anticipate from the present broken condition of the northern portion
of the reef, and from the almost entire absence of reefs on the eastern
coast, that the barrier-reef after repeated subsidences, would become
during its upward growth separated into distinct portions; and these
portions would tend to assume an atoll-like structure, from the coral
growing with vigour round their entire circumferences, when freely exposed
to an open sea. As we have some large islands partly submerged with
barrier-reefs marking their former limits, such as New Caledonia, so our
theory makes it probable that there should be other large islands wholly
submerged; and these, we may now infer, would be surmounted, not by one
enormous atoll, but by several large elongated ones, like the atolls in the
Maldiva group; and these again, during long periods of subsidence, would
sometimes become dissevered into smaller atolls. I may add, that both in
the Marshall and Caroline Archipelagoes, there are atolls standing close
together, which have an evident relationship in form: we may suppose, in
such cases, either that two or more encircled islands originally stood
close together, and afforded bases for two or more atolls, or that one
atoll has been dissevered. From the position, as well as form, of three
atolls in the Caroline Archipelago (the Namourrek and Elato group), which
are placed in an irregular circle, I am strongly tempted to believe that
they have originated by the process of disseverment. (The same remark is,
perhaps, applicable to the islands of Ollap, Fanadik, and Tamatam in the
Caroline Archipelago, of which charts are given in the atlas of Duperrey’s
voyage: a line drawn through the linear reefs and lagoons of these three
islands forms a semicircle. Consult also, the atlas of Lutke’s voyage; and
for the Marshall group that of Kotzebue; for the Gilbert group consult the
atlas of Duperrey’s voyage. Most of the points here referred to may,
however, be seen in Krusenstern’s general Atlas of the Pacific.)

IRREGULARLY FORMED ATOLLS.

In the Marshall group, Musquillo atoll consists of two loops united in one
point; and Menchikoff atoll is formed of three loops, two of which (as may
be seen in Figure 3, Plate II.) are connected by a mere ribbon-shaped reef,
and the three together are sixty miles in length. In the Gilbert group
some of the atolls have narrow strips of reef, like spurs, projecting from
them. There occur also in parts of the open sea, a few linear and straight
reefs, standing by themselves; and likewise some few reefs in the form of
crescents, with their extremities more or less curled inwards. Now, the
upward growth of a barrier-reef which fronted only one side of an island,
or one side of an elongated island with its extremities (of which cases
exist), would produce after the complete subsidence of the land, mere
strips or crescent or hook-formed reefs: if the island thus partially
fronted became divided during subsidence into two or more islands, these
islands would be united together by linear reefs; and from the further
growth of the coral along their shores together with subsidence, reefs of
various forms might ultimately be produced, either atolls united together
by linear reefs, or atolls with spurs projecting from them. Some, however,
of the more simple forms above specified, might, as we have seen, be
equally well produced by the coral perishing during subsidence on part of
the circumference of an atoll, whilst on the other parts it continued to
grow up till it reached the surface.

THE GREAT CHAGOS BANK.

I have already shown that the submerged condition of the Great Chagos Bank
(Plate II., Figure 1, with its section Figure 2), and of some other banks
in the Chagos group, may in all probability be attributed to the coral
having perished before or during the movements of subsidence, to which this
whole area by our theory has been subjected. The external rim or upper
ledge (shaded in the chart), consists of dead coral-rock thinly covered
with sand; it lies at an average depth of between five and eight fathoms,
and perfectly resembles in form the annular reef of an atoll. The banks of
the second level, the boundaries of which are marked by dotted lines in the
chart, lie from about fifteen to twenty fathoms beneath the surface; they
are several miles broad, and terminate in a very steep slope round the
central expanse. This central expanse I have already described, as
consisting of a level muddy flat between thirty and forty fathoms deep.
The banks of the second level, might at first sight be thought analogous to
the internal step-like ledge of coral-rock which borders the lagoons of
some atolls, but their much greater width, and their being formed of sand,
are points of essential difference. On the eastern side of the atoll some
of the banks are linear and parallel, resembling islets in a great river,
and pointed directly towards a great breach on the opposite side of the
atoll; these are best seen in the large published chart. I inferred from
this circumstance, that strong currents sometimes set directly across this
vast bank; and I have since heard from Captain Moresby that this is the
case. I observed, also, that the channels or breaches through the rim,
were all of the same depth as the central lagoon-like space into which they
lead; whereas the channels into the other atolls of the Chagos group, and
as I believe into most other large atolls, are not nearly as deep as their
lagoons: for instance at Peros Banhos, the channels are only of the same
depth, namely between ten and twenty fathoms, as the bottom of the lagoon
for a space about a mile and a half in width round its shores, whilst the
central expanse of the lagoon is from thirty-five to forty fathoms deep.
Now, if an atoll during a gradual subsidence once became entirely
submerged, like the Great Chagos Bank, and therefore no longer exposed to
the surf, very little sediment could be formed from it; and consequently
the channels leading into the lagoon from not being filled up with drifted
sand and coral detritus, would continue increasing in depth, as the whole
sank down. In this case, we might expect that the currents of the open
sea, instead of any longer sweeping round the submarine flanks, would flow
directly through the breaches across the lagoon, removing in their course
the finer sediment, and preventing its further accumulation. We should
then have the submerged reef forming an external and upper rim of rock, and
beneath this portion of the sandy bottom of the old lagoon, intersected by
deep-water channels or breaches, and thus formed into separate marginal
banks; and these would be cut off by steep slopes, overhanging the central
space, worn down by the passage of the oceanic currents.

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

This great atoll-formed bank appears to be in an early stage of
disseverment; should the work of subsidence go on, from the submerged and
dead condition of the whole reef, and the imperfection of the south-east
quarter a mere wreck would probably be left. The Pitt’s Bank, situated not
far southward, appears to be precisely in this state; it consists of a
moderately level, oblong bank of sand, lying from 10 to 20 fathoms beneath
the surface, with two sides protected by a narrow ledge of rock which is
submerged between 5 and 8 fathoms. A little further south, at about the
same distance as the southern rim of the Great Chagos Bank is from the
northern rim, there are two other small banks with from 10 to 20 fathoms on
them; and not far eastward soundings were struck on a sandy bottom, with
between 110 and 145 fathoms. The northern portion with its ledge-like
margin, closely resembles any one segment of the Great Chagos Bank, between
two of the deep-water channels, and the scattered banks, southward appear
to be the last wrecks of less perfect portions.

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

OBJECTIONS TO THE THEORY OF THE FORMATION OF ATOLLS AND BARRIER-REEFS.

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

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