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,[5] 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!
[5] “Principles of Geology,” sixth edition, volume iii., page 386.
Mr. Williams[6] 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.”[7] A storm lately entirely swept away two of the Caroline
islands, and converted them into shoals; it partly, also, destroyed two
other islands.[8] 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.”[9] 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,[10] “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:”[11] 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.
[6] Williams’s “Narrative of Missionary Enterprise,” page 31.
[7] Kotzebue’s “First Voyage,” volume iii., page 168.
[8] M. Desmoulins in “Comptes Rendus,” 1840, page 837.
[9] “West of England Journal”, No. I., page 35.
[10] Beechey’s “Voyage to the Pacific,” chapter vii., and Wallis’s
“Voyage in the ‘Dolphin’,” chapter iv.
[11] See an extract from Pyrard’s Voyage in Captain Owen’s paper on
the Maldiva Archipelago, in the “Geographical Journal”, volume ii.,
page 84.
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[12] states, that earthquakes are felt in the Marshall atolls,
which are far from any high land, and likewise in the islands of the
Caroline Archipelago. On one of the latter, namely Oulleay atoll,
Admiral Lutké, as he had the kindness to inform me, observed several
straight fissures about a foot in width, running for some hundred yards
obliquely across the whole width of the reef. Fissures indicate a
stretching of the earth’s crust, and, therefore, probably changes in
its level; but these coral-islands, which have been shaken and
fissured, certainly have not been elevated, and, therefore, probably
they have subsided. In the chapter on Keeling atoll, I attempted to
show by direct evidence, that the island underwent a movement of
subsidence, during the earthquakes lately felt there.
[12] See Chamisso, in Kotzebue’s “First Voyage,” volume iii., pages
182 and 136.
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?
[Illustration]
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.
[Illustration]
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[13] E. and W. 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.
[13] 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.
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.
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.[14]
[14] 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,[15] 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.
[15] 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.
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.[16]
[16] The same remark is, perhaps, applicable to the islands of Ollap,
Fanadik, and Tamatam in the Caroline Archipelago, of which charts are
given in the atlas of Duperrey’s voyage: a line drawn through the
linear reefs and lagoons of these three islands forms a semicircle.
Consult also, the atlas of Lutké’s voyage; and for the Marshall group
that of Kotzebue; for the Gilbert group consult the atlas of
Duperrey’s voyage. Most of the points here referred to may, however,
be seen in Krusenstern’s general Atlas of the Pacific.
_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; and if this can be
satisfactorily shown, not only is the theory confirmed, but as
deductions, the answers are in themselves important. Under this latter
point of view, these questions will be chiefly considered in the
following chapter.[17]
[17] I may take this opportunity of briefly considering the
appearances, which would probably be presented by a vertical and deep
section across a coral formation (referring chiefly to an atoll),
formed by the upward growth of coral during successive subsidences.
This is a subject worthy of attention, as a means of comparison with
ancient coral-strata. The circumferential parts would consist of
massive species, in a vertical position, with their interstices filled
up with detritus; but this would be the part most subject to
subsequent denudation and removal. It is useless to speculate how
large a portion of the exterior annular reef would consist of upright
coral, and how much of fragmentary rock, for this would depend on many
contingencies,—such as on the rate of subsidence, occasionally
allowing a fresh growth of coral to cover the whole surface, and on
the breakers having force sufficient to throw fragments over this same
space. The conglomerate which composes the base of the islets, would
(if not removed by denudation together with the exterior reef on which
it rests) be conspicuous from the size of the fragments,—the different
degrees in which they have been rounded,—the presence of fragments of
conglomerate torn up, rounded, and recemented,—and from the oblique
stratification. The corals which lived in the lagoon-reefs at each
successive level, would be preserved upright, and they would consist
of many kinds, generally much branched. In this part, however, a very
large proportion of the rock (and in some cases nearly all of it)
would be formed of sedimentary matter, either in an excessively fine,
or in a moderately coarse state, and with the particles almost blended
together. The conglomerate which was formed of rounded pieces of the
branched corals, on the shores of the lagoon, would differ from that
formed on the islets and derived from the outer coast; yet both might
have accumulated very near each other. I have seen a conglomerate
limestone from Devonshire like a conglomerate now forming on the
shores of the Maldiva atolls. The stratification taken as a whole,
would be horizontal; but the conglomerate beds resting on the exterior
reef, and the beds of sandstone on the shores of the lagoon (and no
doubt on the external flanks) would probably be divided (as at Keeling
atoll and at Mauritius) by numerous layers dipping at considerable
angles in different directions. The calcareous sandstone and
coral-rock would almost necessarily contain innumerable shells,
echini, and the bones of fish, turtle, and perhaps of birds; possibly,
also, the bones of small saurians, as these animals find their way to
the islands far remote from any continent. The large shells of some
species of Tridacna would be found vertically imbedded in the solid
rock, in the position in which they lived. We might expect also to
find a mixture of the remains of pelagic and littoral animals in the
strata formed in the lagoon, for pumice and the seeds of plants are
floated from distant countries into the lagoons of many atolls: on the
outer coast of Keeling atoll, near the mouth of the lagoon, the case
of a pelagic Pteropodous animal was brought up on the arming of the
sounding lead. All the loose blocks of coral on Keeling atoll were
burrowed by vermiform animals; and as every cavity, no doubt,
ultimately becomes filled with spathose limestone, slabs of the rock
taken from a considerable depth, would, if polished, probably exhibit
the excavations of such burrowing animals. The conglomerate and
fine-grained beds of coral-rock would be hard, sonorous, white and
composed of nearly pure calcareous matter; in some few parts, judging
from the specimens at Keeling atoll, they would probably contain a
small quantity of iron. Floating pumice and scoriae, and occasionally
stones transported in the root of trees (see my “Journal of
Researches,” page 549) appear the only sources, through which foreign
matter is brought to coral-formations standing in the open ocean. The
area over which sediment is transported from coral-reefs must be
considerable: Captain Moresby informs me that during the change of
monsoons the sea is discoloured to a considerable distance off the
Maldiva and Chagos atolls. The sediment of fringing and barrier
coral-reefs must be mingled with the mud, which is brought down from
the land, and is transported seaward through the breaches, which occur
in front of almost every valley. If the atolls of the larger
archipelagoes were upraised, the bed of the ocean being converted into
land, they would form flat-topped mountains, varying in diameter from
a few miles (the smallest atolls being worn away) to sixty miles; and
from being horizontally stratified and of similar composition, they
would, as Mr. Lyell has remarked, falsely appear as if they had
originally been united into one vast continuous mass. Such great
strata of coral-rock would rarely be associated with erupted volcanic
matter, for this could only take place, as may be inferred from what
follows in the next chapter, when the area, in which they were
situated, commenced to rise, or at least ceased to subside. During the
enormous period necessary to effect an elevation of the kind just
alluded to, the surface would necessarily be denuded to a great
thickness; hence it is highly improbable that any fringing-reef, or
even any barrier-reef, at least of those encircling small islands,
would be preserved. From this same cause, the strata which were formed
within the lagoons of atolls and lagoon-channels of barrier-reefs, and
which must consist in a large part of sedimentary matter, would more
often be preserved to future ages, than the exterior solid reef,
composed of massive corals in an upright position; although it is on
this exterior part that the present existence and further growth of
atolls and barrier-reefs entirely depend.
CHAPTER VI.
ON THE DISTRIBUTION OF CORAL-REEFS WITH REFERENCE TO THE THEORY OF
THEIR FORMATION.
Description of the coloured map.—Proximity of atolls and
barrier-reefs.— Relation in form and position of atolls with ordinary
islands.—Direct evidence of subsidence difficult to be detected.—Proofs
of recent elevation where fringing-reefs occur.—Oscillations of
level.—Absence of active volcanoes in the areas of
subsidence.—Immensity of the areas which have been elevated and have
subsided.—Their relation to the present distribution of the land.—Areas
of subsidence elongated, their intersection and alternation with those
of elevation.—Amount and slow rate of the subsidence.—Recapitulation.
It will be convenient to give here a short account of the appended map
(Plate III.) [Inasmuch as the coloured map would have proved too costly
to be given in this series, the indications of colour have been
replaced by numbers referring to the dotted groups of reefs, etc. The
author’s original wording, however, is retained in full, as it will be
easy to refer to the map by the numbers, and thus the flow of the
narrative is undisturbed.]: a fuller one, with the data for colouring
each spot, is reserved for the Appendix; and every place there referred
to may be found in the Index. A larger chart would have been desirable;
but, small as the adjoined one is, it is the result of many months’
labour. I have consulted, as far as I was able, every original voyage
and map; and the colours were first laid down on charts on a larger
scale. The same blue colour, with merely a difference in the depth of
tint, is used for atolls or lagoon-islands, and barrier-reefs, for we
have seen, that as far as the actual coral-formation is concerned, they
have no distinguishing character. Fringing-reefs have been coloured
red, for between them on the one hand, and barrier-reefs and atolls on
the other, there is an important distinction with respect to the depth
beneath the surface, at which we are compelled to believe their
foundations lie. The two distinct colours, therefore, mark two great
types of structure.
The _dark blue colour_ [represented by (3) in our plate] represents
atolls and submerged annular reefs, with deep water in their centres. I
have coloured as atolls, a few low and small coral-islands, without
lagoons; but this has been done only when it clearly appeared that they
originally contained lagoons, since filled up with sediment: when there
were not good grounds for this belief, they have been left uncoloured.
The _pale blue colour_ [represented by (2)] represents barrier-reefs.
The most obvious character of reefs of this class is the broad and
deep-water moat within the reef: but this, like the lagoons of small
atolls, is liable to become filled up with detritus and with reefs of
delicately branched corals: when, therefore, a reef round the entire
circumference of an island extends very far into a profoundly deep sea,
so that it can hardly be confounded with a fringing-reef which must
rest on a foundation of rock within a small depth, it has been coloured
pale blue, although it does not include a deep-water moat: but this has
only been done rarely, and each case is distinctly mentioned in the
Appendix.
The _red colour_ (4) represents reefs fringing the land quite closely
where the sea is deep, and where the bottom is gently inclined
extending to a moderate distance from it, but not having a deep-water
moat or lagoon-like space parallel to the shore. It must be remembered
that fringing-reefs are frequently _breached_ in front of rivers and
valleys by deepish channels, where mud has been deposited. A space of
thirty miles in width has been coloured round or in front of the reefs
of each class, in order that the colours might be conspicuous on the
appended map, which is reduced to so small a scale.
The _vermillion spots_, and streaks (1) represent volcanoes now in
action, or historically known to have been so. They are chiefly laid
down from Von Buch’s work on the Canary Islands; and my reasons for
making a few alterations are given in the note below.[1]
[1] I have also made considerable use of the geological part of
Berghaus’ “Physical Atlas.” Beginning at the eastern side of the
Pacific, I have added to the number of the volcanoes in the southern
part of the Cordillera, and have coloured Juan Fernandez according to
observations collected during the voyage of the _Beagle_ (“Geological
Transactions,” volume v., page 601.) I have added a volcano to
Albemarle Island, one of the Galapagos Archipelago (the author’s
“Journal of Researches,” page 457). In the Sandwich group there are no
active volcanoes, except at Hawaii; but the Rev. W. Ellis informs me,
there are streams of lava apparently modern on Maui, having a very
recent appearance, which can be traced to the craters whence they
flowed. The same gentleman informs me, that there is no reason to
believe that any active volcano exists in the Society Archipelago; nor
are there any known in the Samoa or Navigator group, although some of
the streams of lava and craters there appear recent. In the Friendly
group, the Rev. J. Williams says (“Narrative of Missionary
Enterprise,” page 29) that Toofoa and Proby Islands are active
volcanoes. I infer from Hamilton’s “Voyage in the ‘Pandora’” (Page
95), that Proby Island is synonymous with Onouafou, but I have not
ventured to colour it. There can be no doubt respecting Toofoa, and
Captain Edwards (Von Buch, page 386) found the lava of recent eruption
at Amargura still smoking. Berghaus marks four active volcanoes
actually within the Friendly group; but I do not know on what
authority: I may mention that Maurelle describes Latte as having a
burnt-up appearance: I have marked only Toofoa and Amargura. South of
the New Hebrides lies Matthews Rock, which is drawn and described as
an active crater in the “Voyage of the _Astrolabe_.” Between it and
the volcano on the eastern side of New Zealand, lies Brimstone Island,
which from the high temperature of the water in the crater, may be
ranked as active (Berghaus “Vorbemerk,” II Lief. S. 56). Malte Brun,
volume xii., page 231, says that there is a volcano near port St.
Vincent in New Caledonia. I believe this to be an error, arising from
a smoke seen on the _opposite_ coast by Cook (“Second Voyage,” volume
ii., page 23) which smoke went out at night. The Mariana Islands,
especially the northern ones, contain many craters (see Freycinet’s
“Hydrog. Descript.”) which are not active. Von Buch, however, states
(page 462) on the authority of La Peyrouse, that there are no less
than seven volcanoes between these islands and Japan. Gemelli Creri
(Churchill’s “Collect.” volume iv., page 458), says there are two
active volcanoes in latitude 23° 30′, and in latitude 24°: but I have
not coloured them. From the statements in Beechey’s “Voyage” (page
518, 4to edition) I have coloured one in the northern part of the
Bonin group. M. S. Julien has clearly made out from Chinese
manuscripts not very ancient (“Comptes Rendus,” 1840, page 832), that
there are two active volcanoes on the eastern side of Formosa. In
Torres Straits, on Cap Island (9° 48′ S., 142° 39′ E.) a volcano was
seen burning with great violence in 1793 by Captain Bampton (see
Introduction to Flinders’ “Voyage,” page 41). Mr. M’Clelland (Report
of Committee for investigating Coal in India, page 39) has shown that
the volcanic band passing through Barren Island must be extended
northwards. It appears by an old chart, that Cheduba was once an
active volcano (see also “Silliman’s North American Journal”, volume
xxxviii., page 385). In Berghaus’ “Physical Atlas,” 1840, No. 7 of
Geological Part, a volcano on the coast of Pondicherry is said to have
burst forth in 1757. Ordinaire (“Hist. Nat. des Volcans,” page 218)
says that there is one at the mouth of the Persian Gulf, but I have
not coloured it, as he gives no particulars. A volcano in Amsterdam,
or St. Paul’s, in the southern part of the Indian Ocean, has been seen
(“Naut. Mag.” 1838, page 842) in action. Dr. J. Allan, of Forres,
informs me in a letter, that when he was at Joanna, he saw at night
flames apparently volcanic, issuing from the chief Comoro Island, and
that the Arabs assured him that they were volcanic, adding that the
volcano burned more during the wet season. I have marked this as a
volcano, though with some hesitation, on account of the possibility of
the flame arising from gaseous sources.
The _uncoloured coasts_ consist, first and chiefly, of those, where
there are no coral-reefs, or such small portions as to be quite
insignificant. Secondly, of those coasts where there are reefs, but
where the sea is very shallow, for in this case the reefs generally lie
far from the land, and become very irregular, in their forms: where
they have not become irregular, they have been coloured. thirdly, if I
had the means of ascertaining the fact, I should not colour a reef
merely coating the edges of a submarine crater, or of a level submerged
bank; for such superficial formations differ essentially, even when not
in external appearance, from reefs whose foundations as well as
superficies have been wholly formed by the growth of coral. Fourthly,
in the Red Sea, and within some parts of the East Indian Archipelago
(if the imperfect charts of the latter can be trusted), there are many
scattered reefs, of small size, represented in the chart by mere dots,
which rise out of deep water: these cannot be arranged under either of
the three classes: in the Red Sea, however, some of these little reefs,
from their position, seem once to have formed parts of a continuous
barrier. There exist, also, scattered in the open ocean, some linear
and irregularly formed strips of coral-reef, which, as shown in the
last chapter, are probably allied in their origin to atolls; but as
they do not belong to that class, they have not been coloured; they are
very few in number and of insignificant dimensions. Lastly, some reefs
are left uncoloured from the want of information respecting them, and
some because they are of an intermediate structure between the barrier
and fringing classes. The value of the map is lessened, in proportion
to the number of reefs which I have been obliged to leave uncoloured,
although, in a theoretical point of view, few of them present any great
difficulty: but their number is not very great, as will be found by
comparing the map with the statements in the Appendix. I have
experienced more difficulty in colouring fringing-reefs than in
colouring barrier-reefs, as the former, from their much less
dimensions, have less attracted the attention of navigators. As I have
had to seek my information from all kinds of sources, and often from
indirect ones, I do not venture to hope that the map is free from many
errors. Nevertheless, I trust it will give an approximately correct
view of the general distribution of the coral-reefs over the whole
world (with the exception of some fringing-reefs on the coast of
Brazil, not included within the limits of the map), and of their
arrangement into the three great classes, which, though necessarily
very imperfect from the nature of the objects classified, have been
adopted by most voyagers. I may further remark, that the dark blue
colour represents land entirely composed of coral-rock; the pale blue,
land with a wide and thick border of coral-rock; and the red, a mere
narrow fringe of coral-rock.
Looking now at the map under the theoretical point of view indicated in
the last chapter, the two blue tints signify that the foundations of
the reefs thus coloured have subsided to a considerable amount, at a
slower rate than that of the upward growth of the corals, and that
probably in many cases they are still subsiding. The red signifies that
the shores which support fringing-reefs have not subsided (at least to
any considerable amount, for the effects of a subsidence on a small
scale would in no case be distinguishable); but that they have remained
nearly stationary since the period when they first became fringed by
reefs; or that they are now rising or have been upraised, with new
lines of reefs successively formed on them: these latter alternatives
are obviously implied, as newly formed lines of shore, after elevations
of the land, would be in the same state with respect to the growth of
fringing-reefs, as stationary coasts. If during the prolonged
subsidence of a shore, coral-reefs grew for the first time on it, or if
an old barrier-reef were destroyed and submerged, and new reefs became
attached to the land, these would necessarily at first belong to the
fringing class, and, therefore, be coloured red, although the coast was
sinking: but I have no reason to believe, that from this source of
error, any coast has been coloured wrongly with respect to movement
indicated. Well characterised atolls and encircling barrier-reefs,
where several occur in a group, or a single barrier-reef if of large
dimensions, leave scarcely any doubt on the mind respecting the
movement by which they have been produced; and even a small amount of
subsequent elevation is soon betrayed. The evidence from a single atoll
or a single encircling barrier-reef, must be received with some
caution, for the former may possibly be based upon a submerged crater
or bank, and the latter on a submerged margin of sediment, or of
worn-down rock. From these remarks we may with greater certainty infer
that the spaces, especially the larger ones, tinted blue in the map,
have subsided, than that the red spaces have remained stationary, or
have been upraised.
_On the grouping of the different classes of reefs_.—Having made these
preliminary remarks, I will consider first how far the grouping of the
different kinds of coral-islands and reefs is corroborative of the
truth of the theory. A glance at the map shows that the reefs, coloured
blue and red, produced under widely different conditions, are not
indiscriminately mixed together. Atolls and barrier-reefs, on the other
hand, as may be seen by the two blue tints, generally lie near each
other; and this would be the natural result of both having been
produced during the subsidence of the areas in which they stand. Thus,
the largest group of encircled islands is that of the Society
Archipelago; and these islands are surrounded by atolls, and only
separated by a narrow space from the large group of Low atolls. In the
midst of the Caroline atolls, there are three fine encircled islands.
The northern point of the barrier-reef of New Caledonia seems itself,
as before remarked, to form a complete large atoll. The great
Australian barrier is described as including both atolls and small
encircled islands. Captain King[2] mentions many atoll-formed and
encircling coral-reefs, some of which lie within the barrier, and
others may be said (for instance between latitude 16° and 13°) to form
part of it. Flinders[3] has described an atoll-formed reef in latitude
10°, seven miles long and from one to three broad, resembling a boot in
shape, with apparently very deep water within. Eight miles westward of
this, and forming part of the barrier, lie the Murray Islands, which
are high and are encircled. In the Corallian Sea, between the two great
barriers of Australia and New Caledonia, there are many low islets and
coral-reefs, some of which are annular, or horse-shoe shaped. Observing
the smallness of the scale of the map, the parallels of latitude being
nine hundred miles apart, we see that none of the large groups of reefs
and islands supposed to have been produced by long-continued
subsidence, lie near extensive lines of coast coloured red, which are
supposed to have remained stationary since the growth of their reefs,
or to have been upraised and new lines of reefs formed on them. Where
the red and blue circles do occur near each other, I am able, in
several instances, to show that there have been oscillations of level,
subsidence having preceded the elevation of the red spots; and
elevation having preceded the subsidence of the blue spots: and in this
case the juxtaposition of reefs belonging to the two great types of
structure is little surprising. We may, therefore, conclude that the
proximity in the same areas of the two classes of reefs, which owe
their origin to the subsidence of the earth’s crust, and their
separation from those formed during its stationary or uprising
condition, holds good to the full extent, which might have been
anticipated by our theory.
[2] Sailing directions, appended to volume ii. of his “Surveying
Voyage to Australia.”
[3] “Voyage to Terra Australis,” volume ii. page 336.
As groups of atolls have originated in the upward growth, at each fresh
sinking of the land, of those reefs which primarily fringed the shores
of one great island, or of several smaller ones; so we might expect
that these rings of coral-rock, like so many rude outline charts, will
still retain some traces of the general form, or at least general
range, of the land, round which they were first modelled. That this is
the case with the atolls in the Southern Pacific as far as their range
is concerned, seems highly probable, when we observe that the three
principal groups are directed in north-west and south-east lines, and
that nearly all the land in the S. Pacific ranges in this same
direction; namely, N. Western Australia, New Caledonia, the northern
half of New Zealand, the New Hebrides, Saloman, Navigator, Society,
Marquesas, and Austral archipelagoes: in the Northern Pacific, the
Caroline atolls abut against the north-west line of the Marshall
atolls, much in the same manner as the east and west line of islands
from Ceram to New Britain do on New Ireland: in the Indian Ocean the
Laccadive and Maldiva atolls extend nearly parallel to the western and
mountainous coast of India. In most respects, there is a perfect
resemblance with ordinary islands in the grouping of atolls and in
their form: thus the outline of all the larger groups is elongated; and
the greater number of the individual atolls are elongated in the same
direction with the group, in which they stand. The Chagos group is less
elongated than is usual with other groups, and the individual atolls in
it are likewise but little elongated; this is strikingly seen by
comparing them with the neighbouring Maldiva atolls. In the Marshall
and Maldiva archipelagoes, the atolls are ranged in two parallel lines,
like the mountains in a great double mountain-chain. Some of the
atolls, in the larger archipelagoes, stand so near to each other, and
have such an evident relationship in form, that they compose little
sub-groups: in the Caroline Archipelago, one such sub-group consists of
Pouynipète, a lofty island encircled by a barrier-reef, and separated
by a channel only four miles and a half wide from Andeema atoll, with a
second atoll a little further off. In all these respects an examination
of a series of charts will show how perfectly groups of atolls resemble
groups of common islands.
_On the direct evidence of the blue spaces in the map having subsided
during the upward growth of the reefs so coloured, and of the red
spaces having remained stationary, or having been upraised._—With
respect to subsidence, I have shown in the last chapter, that we cannot
expect to obtain in countries inhabited only by semi-civilised races,
demonstrative proofs of a movement, which invariably tends to conceal
its own evidence. But on the coral-islands supposed to have been
produced by subsidence, we have proofs of changes in their external
appearance—of a round of decay and renovation—of the last vestiges of
land on some—of its first commencement on others: we hear of storms
desolating them to the astonishment of their inhabitants: we know by
the great fissures with which some of them are traversed, and by the
earthquakes felt under others, that subterranean disturbances of some
kind are in progress. These facts, if not directly connected with
subsidence, as I believe they are, at least show how difficult it would
be to discover proofs of such movement by ordinary means. At Keeling
atoll, however, I have described some appearances, which seem directly
to show that subsidence did take place there during the late
earthquakes. Vanikoro, according to Chevalier Dillon,[4] is often
violently shaken by earthquakes, and there, the unusual depth of the
channel between the shore and the reef,—the almost entire absence of
islets on the reef,— its wall-like structure on the inner side, and the
small quantity of low alluvial land at the foot of the mountains, all
seem to show that this island has not remained long at its present
level, with the lagoon-channel subjected to the accumulation of
sediment, and the reef to the wear and tear of the breakers. At the
Society Archipelago, on the other hand, where a slight tremor is only
rarely felt, the shoaliness of the lagoon-channels round some of the
islands, the number of islets formed on the reefs of others, and the
broad belt of low land at the foot of the mountains, indicate that,
although there must have been great subsidence to have produced the
barrier-reefs, there has since elapsed a long stationary period.[5]
[4] See Captain Dillon’s “Voyage in search of La Peyrouse.” M. Cordier
in his “Report on the Voyage of the ‘Astrolabe’” (page cxi., volume
i.), speaking of Vanikoro, says the shores are surrounded by reefs of
madrepore, _“qu’on assure être de formation tout-a-fait moderne_.” I
have in vain endeavoured to learn some further particulars about this
remarkable passage. I may here add, that according to our theory, the
island of Pouynipète (Plate I., Figure 7), in the Caroline
Archipelago, being encircled by a barrier-reef, must have subsided. In
the “New S. Wales Lit. Advert.” February 1835 (which I have seen
through the favour of Dr. Lloghtsky), there is an account of this
island (subsequently confirmed by Mr. Campbell), in which it is said,
“At the N.E. end, at a place called Tamen, there are ruins of a town,
_now only_ accessible by boats, the waves _reaching to the steps of
the houses_.” Judging from this passage, one would be tempted to
conclude that the island must have subsided, since these houses were
built. I may, also, here append a statement in Malte Brun (volume ix.,
page 775, given without any authority), that the sea gains in an
extraordinary manner on the coast of Cochin China, which lies in front
and near the subsiding coral-reefs in the China Sea: as the coast is
granitic, and not alluvial, it is scarcely possible that the
encroachment of the sea can be owing to the washing away of the land;
and if so, it must be due to subsidence.
[5] Mr. Couthouy states (“Remarks,” page 44) that at Tahiti and Eimeo
the space between the reef and the shore has been nearly filled up by
the extension of those coral-reefs, which within most barrier-reefs
merely fringe the land. From this circumstance, he arrives at the same
conclusion as I have done, that the Society Islands since their
subsidence, have remained stationary during a long period; but he
further believes that they have recently commenced rising, as well as
the whole area of the Low Archipelago. He does not give any detailed
proofs regarding the elevation of the Society Islands, but I shall
refer to this subject in another part of this chapter. Before making
some further comments, I may observe how satisfactory it is to me, to
find Mr. Couthouy affirming, that “having personally examined a large
number of coral-islands, and also residing eight months among the
volcanic class, having shore and partially encircling reefs, I may be
permitted to state that my own observations have impressed a
conviction of the correctness of the theory of Mr. Darwin.”
This gentleman believes, that subsequently to the subsidence by
which the atolls in the Low Archipelago were produced, the whole
area has been elevated to the amount of a few feet; this would
indeed be a remarkable fact; but as far as I am able to judge, the
grounds of his conclusion are not sufficiently strong. He states
that he found in almost every atoll which he visited, the shores of
the lagoon raised from eighteen to thirty inches above the
sea-level, and containing imbedded Tridacnae and corals standing as
they grew; some of the corals were dead in their upper parts, but
below a certain line they continued to flourish. In the lagoons,
also, he frequently met with clusters of Madrepore, with their
extremities standing from one inch to a foot above the surface of
the water. Now, these appearances are exactly what I should have
expected, without any subsequent elevation having taken place; and
I think Mr. Couthouy has not borne in mind the indisputable fact,
that corals, when constantly bathed by the surf, can exist at a
higher level than in quite tranquil water, as in a lagoon. As long,
therefore, as the waves continued at low water to break entirely
over parts of the annular reef of an atoll, submerged to a small
depth, the corals and shells attached on these parts might continue
living at a level above the smooth surface of the lagoon, into
which the waves rolled; but as soon as the outer edge of the reef
grew up to its utmost possible height, or if the reef were very
broad nearly to that height, the force of the breakers would be
checked, and the corals and shells on the inner parts near the
lagoon would occasionally be left dry, and thus be partially or
wholly destroyed. Even in atolls, which have not lately subsided,
if the outer margin of the reef continued to increase in breadth
seaward (each fresh zone of corals rising to the same vertical
height as at Keeling atoll), the line where the waves broke most
heavily would advance outwards, and therefore the corals, which
when living near the margin, were washed by the breaking waves
during the whole of each tide, would cease being so, and would
therefore be left on the backward part of the reef standing exposed
and dead. The case of the madrepores in the lagoons with the tops
of their branches exposed, seems to be an analogous fact, to the
great fields of dead but upright corals in the lagoon of Keeling
atoll; a condition of things which I have endeavoured to show, has
resulted from the lagoon having become more and more enclosed and
choked up with reefs, so that during high winds, the rising of the
tide (as observed by the inhabitants) is checked, and the corals,
which had formerly grown to the greatest possible height, are
occasionally exposed, and thus are killed: and this is a condition
of things, towards which almost every atoll in the intervals of its
subsidence must be tending. Or if we look to the state of an atoll
directly after a subsidence of some fathoms, the waves would roll
heavily over the entire circumference of the reef, and the surface
of the lagoon would, like the ocean, never be quite at rest, and
therefore the corals in the lagoon, from being constantly laved by
the rippling water, might extend their branches to a little greater
height than they could, when the lagoon became enclosed and
protected. Christmas atoll (2° N. latitude) which has a very
shallow lagoon, and differs in several respects from most atolls,
possibly may have been elevated recently; but its highest part
appears (Couthouy, page 46) to be only ten feet above the
sea-level. The facts of a second class, adduced by Mr. Couthouy, in
support of the alleged recent elevation of the Low Archipelago, are
not all (especially those referring to a shelf of rock) quite
intelligible to me; he believes that certain enormous fragments of
rock on the reef, must have been moved into their present position,
when the reef was at a lower level; but here again the force of the
breakers on any inner point of the reef being diminished by its
outward growth without any change in its level, has not, I think,
been borne in mind. We should, also, not overlook the occasional
agency of waves caused by earthquakes and hurricanes. Mr. Couthouy
further argues, that since these great fragments were deposited and
fixed on the reef, they have been elevated; he infers this from the
greatest amount of erosion not being near their bases, where they
are unceasingly washed by the reflux of the tides, but at some
height on their sides, near the line of high-water mark, as shown
in an accompanying diagram. My former remark again applies here,
with this further observation, that as the waves have to roll over
a wide space of reef before they reach the fragments, their force
must be greatly increased with the increasing depth of water as the
tide rises, and therefore I should have expected that the chief
line of present erosion would have coincided with the line of
high-water mark; and if the reef had grown outwards, that there
would have been lines of erosion at greater heights. The
conclusion, to which I am finally led by the interesting
observations of Mr. Couthouy is, that the atolls in the Low
Archipelago have, like the Society Islands, remained at a
stationary level for a long period: and this probably is the
ordinary course of events, subsidence supervening after long
intervals of rest.
Turning now to the red colour; as on our map, the areas which have sunk
slowly downwards to great depths are many and large, we might naturally
have been led to conjecture, that with such great changes of level in
progress, the coasts which have been fringed probably for ages (for we
have no reason to believe that coral-reefs are of short duration),
would not have remained all this time stationary, but would frequently
have undergone movements of elevation. This supposition, we shall
immediately see, holds good to a remarkable extent; and although a
stationary condition of the land can hardly ever be open to proof, from
the evidence being only negative, we are, in some degree, enabled to
ascertain the correctness of the parts coloured red on the map, by the
direct testimony of upraised organic remains of a modern date. Before
going into the details on this head (printed in small type), I may
mention, that when reading a memoir on coral formations by MM. Quoy and
Gaimard[6] I was astonished to find, for I knew that they had crossed
both the Pacific and Indian Oceans, that their descriptions were
applicable only to reefs of the fringing class; but my astonishment
ended satisfactorily, when I discovered that, by a strange chance, all
the islands which these eminent naturalists had visited, though several
in number, namely, the Mauritius, Timor, New Guinea, the Mariana, and
Sandwich Archipelagoes, could be shown by their own statements to have
been elevated within a recent geological era.
[6] “Annales des Sciences Nat.” tom. vi., page 279, etc.
In the eastern half of the Pacific, the _Sandwich Islands_ are all
fringed, and almost every naturalist who has visited them, has remarked
on the abundance of elevated corals and shells, apparently identical
with living species. The Rev. W. Ellis informs me, that he has noticed
round several parts of Hawaii, beds of coral-detritus, about twenty
feet above the level of the sea, and where the coast is low they extend
far inland. Upraised coral-rock forms a considerable part of the
borders of Oahu; and at Elizabeth Island[7] it composes three strata,
each about ten feet thick. Nihau, which forms the northern, as Hawaii
does the southern end of the group (350 miles in length), likewise
seems to consist of coral and volcanic rocks. Mr. Couthouy[8] has
lately described with interesting details, several upraised beaches,
ancient reefs with their surfaces perfectly preserved, and beds of
recent shells and corals, at the islands of Maui, Morokai, Oahu, and
Tauai (or Kauai) in this group. Mr. Pierce, an intelligent resident at
Oahu, is convinced, from changes which have taken place within his
memory, during the last sixteen years, “that the elevation is at
present going forward at a very perceptible rate.” The natives at Kauai
state that the land is there gaining rapidly on the sea, and Mr.
Couthouy has no doubt, from the nature of the strata, that this has
been effected by an elevation of the land.
[7] “Zoology of Captain Beechey’s Voyage,” page 176. See also MM. Quoy
and Gaimard in “Annales de Scien. Nat.” tom. vi.
[8] “Remarks on Coral Formations,” page 51.
In the southern part of the Low Archipelago, Elizabeth Island is
described by Captain Beechey,[9] as being quite flat, and about eighty
feet in height; it is entirely composed of dead corals, forming a
honeycombed, but compact rock. In cases like this, of an island having
exactly the appearance, which the elevation of any one of the smaller
surrounding atolls with a shallow lagoon would present, one is led to
conclude (with little better reason, however, than the improbability of
such small and low fabrics lasting, for an immense period, exposed to
the many destroying agents of nature), that the elevation has taken
place at an epoch not geologically remote. When merely the surface of
an island of ordinary formation is strewed with marine bodies, and that
continuously, or nearly so, from the beach to a certain height, and not
above that height, it is exceedingly improbable that such organic
remains, although they may not have been specially examined, should
belong to any ancient period. It is necessary to bear these remarks in
mind, in considering the evidence of the elevatory movements in the
Pacific and Indian Oceans, as it does not often rest on specific
determinations, and therefore should be received with caution. Six of
the _Cook and Austral Islands_ (S.W. of the Society group), are
fringed; of these, five were described to me by the Rev. J. Williams,
as formed of coral-rock, associated with some basalt in Mangaia), and
the sixth as lofty and basaltic. Mangaia is nearly three hundred feet
high, with a level summit; and according to Mr. S. Wilson[10] it is an
upraised reef; “and there are in the central hollow, formerly the bed
of the lagoon, many scattered patches of coral-rock, some of them
raised to a height of forty feet.” These knolls of coral-rock were
evidently once separate reefs in the lagoon of an atoll. Mr. Martens,
at Sydney, informed me that this island is surrounded by a terrace-like
plain at about the height of a hundred feet, which probably marks a
pause in its elevation. From these facts we may infer, perhaps, that
the Cook and Austral Islands have been upheaved at a period probably
not very remote.
[9] Beechey’s “Voyage in the Pacific,” page 46, 4to edition.
[10] Couthouy’s “Remarks,” page 34.
_Savage Island_ (S.E. of the Friendly group), is about forty feet in
height. Forster[11] describes the plants as _already_ growing out of
the dead, but still upright and spreading trees of coral; and the
younger Forster[12] believes that an ancient lagoon is now represented
by a central plain; here we cannot doubt that the elevatory forces have
recently acted. The same conclusion may be extended, though with
somewhat less certainty, to the islands of the _Friendly Group_, which
have been well described in the second and third voyages of Cook. The
surface of Tongatabou is low and level, but with some parts a hundred
feet high; the whole consists of coral-rock, “which yet shows the
cavities and irregularities worn into it by the action of the
tides.”[13] On Eoua the same appearances were noticed at an elevation
of between two hundred and three hundred feet. Vavao, also, at the
opposite or northern end of the group, consists, according to the Rev.
J. Williams, of coral-rock. Tongatabou, with its northern extensive
reefs, resembles either an upraised atoll with one half originally
imperfect, or one unequally elevated; and Anamouka, an atoll equally
elevated. This latter island contains[14] in its centre a salt-water
lake, about a mile-and-a-half in diameter, without any communication
with the sea, and around it the land rises gradually like a bank; the
highest part is only between twenty and thirty feet; but on this part,
as well as on the rest of the land (which, as Cook observes, rises
above the height of true lagoon-islands), coral-rock, like that on the
beach, was found. In the _Navigator Archipelago_, Mr. Couthouy[15]
found on Manua many and very large fragments of coral at the height of
eighty feet, “on a steep hill-side, rising half a mile inland from a
low sandy plain abounding in marine remains.” The fragments were
embedded in a mixture of decomposed lava and sand. It is not stated
whether they were accompanied by shells, or whether the corals
resembled recent species; as these remains were embedded they possibly
may belong to a remote epoch; but I presume this was not the opinion of
Mr. Couthouy. Earthquakes are very frequent in this archipelago.
[11] “Observations made during Voyage round the World,” page 147.
[12] “Voyage,” volume ii., page 163.
[13] Cook’s “Third Voyage” (4to edition), volume i., page 314.
[14] Ibid., volume i., page 235.
[15] “Remarks on Coral-Formations,” page 50.
Still proceeding westward we come to the _New Hebrides;_ on these
islands, Mr. G. Bennett (author of “Wanderings in New South Wales”),
informs me he found much coral at a great altitude, which he considered
of recent origin. Respecting _Santa Cruz_, and the _Salomon
Archipelago_, I have no information; but at New Ireland, which forms
the northern point of the latter chain, both Labillardière and Lesson
have described large beds of an apparently very modern madreporitic
rock, with the form of the corals little altered. The latter author[16]
states that this formation composes a newer line of coast, modelled
round an ancient one. There only remains to be described in the
Pacific, that curved line of fringed islands, of which the _Marianas_
form the main part. Of these Guam, Rota, Tiniam, Saypan, and some
islets farther north, are described by Quoy and Gaimard,[17] and
Chamisso,[18] as chiefly composed of madreporitic limestone, which
attains a considerable elevation, and is in several cases worn into
successively rising cliffs: the two former naturalists seem to have
compared the corals and shells with the existing ones, and state that
they are of recent species. _Fais_, which lies in the prolonged line of
the Marianas, is the only island in this part of the sea which is
fringed; it is ninety feet high, and consists entirely of madreporitic
rock.[19]
[16] “Voyage de la ‘Coquille’,” Part. Zoolog.
[17] Freycinet’s “Voyage autour du Monde.” See also the
“Hydrographical Memoir,” page 215.
[18] Kotzebue’s “First Voyage.”
[19] Lutké’s “Voyage,” volume ii., page 304.
In the _East Indian Archipelago_, many authors have recorded proofs of
recent elevation. M. Lesson[20] states, that near Port Dory, on the
north coast of New Guinea, the shores are flanked, to the height of 150
feet, by madreporitic strata of modern date. He mentions similar
formations at Waigiou, Amboina, Bourou, Ceram, Sonda, and Timor: at
this latter place, MM. Quoy and Gaimard[21] have likewise described the
primitive rocks, as coated to a considerable height with coral. Some
small islets eastward of Timor are said in Kolff’s “Voyage,”[22] to
resemble small coral islets upraised some feet above the sea. Dr.
Malcolmson informs me that Dr. Hardie found in _Java_ an extensive
formation, containing an abundance of shells, of which the greater part
appear to be of existing species. Dr. Jack[23] has described some
upraised shells and corals, apparently recent, on Pulo Nias off
_Sumatra;_ and Marsden relates in his history of this great island,
that the names of many promontories, show that they were originally
islands. On part of the west coast of _Borneo_ and at the _Sooloo
Islands_, the form of the land, the nature of the soil, and the
water-washed rocks, present appearances[24] (although it is doubtful
whether such vague evidence is worthy of mention), of having recently
been covered by the sea; and the inhabitants of the Sooloo Islands
believe that this has been the case. Mr. Cuming, who has lately
investigated, with so much success, the natural history of the
_Phillippines_, found near Cabagan, in Luzon, about fifty feet above
the level of the R. Cagayan, and seventy miles from its mouth, a large
bed of fossil shells: these, he informs me, are of the same species
with those now existing on the shores of the neighbouring islands. From
the accounts given us by Captain Basil Hall and Captain Beechey[25] of
the lines of inland reefs, and walls of coral-rock worn into caves,
above the present reach of the waves, at the _Loo Choo_ Islands, there
can be little doubt that they have been upraised at no very remote
period.
[20] Partie Zoolog., “Voyage de la ‘Coquille’.”
[21] “Ann. des Scien. Nat.” tom. vi., page 281.
[22] Translated by Windsor Earl, chapters vi., vii.
[23] “Geolog. Transact.” 2nd series, volume i., page 403. On the
Peninsula of Malacca, in front of Pinang, 5° 30′ N., Dr. Ward
collected some shells, which Dr. Malcolmson informs me, although not
compared with existing species, had a recent appearance. Dr. Ward
describes in this neighbourhood (“Trans. Asiat. Soc.” volume xviii.,
part ii., page 166) a single water-worn rock, with a conglomerate of
sea-shells at its base, situated six miles inland, which, according to
the traditions of the natives, was once surrounded by the sea. Captain
Low has also described (Ibid., part i., page 131) mounds of shells
lying two miles inland on this line of coast.
[24] “Notices of the East Indian Arch.” Singapore, 1828, page 6, and
Append., page 43.
[25] Captain B. Hall, “Voyage to Loo Choo,” Append., pages xxi. and
xxv. Captain Beechey’s “Voyage,” page 496.
Dr. Davy[26] describes the northern province of _Ceylon_ as being very
low, and consisting of a limestone with shells and corals of very
recent origin; he adds, that it does not admit of a doubt that the sea
has retired from this district even within the memory of man. There is
also some reason for believing that the western shores of India, north
of Ceylon, have been upraised within the recent period.[27] _Mauritius_
has certainly been upraised within the recent period, as I have stated
in the chapter on fringing-reefs. The northern extremity of
_Madagascar_ is described by Captain Owen[28] as formed of madreporitic
rock, as likewise are the shores and outlying islands along an immense
space of _Eastern Africa_, from a little north of the equator for nine
hundred miles southward. Nothing can be more vague than the expression
“madreporitic rock;” but at the same time it is, I think, scarcely
possible to look at the chart of the linear islets, which rise to a
greater height than can be accounted for by the growth of coral, in
front of the coast, from the equator to 2° S., without feeling
convinced that a line of fringing-reefs has been elevated at a period
so recent, that no great changes have since taken place on the surface
of this part of the globe. Some, also, of the higher islands of
madreporitic rock on this coast, for instance Pemba, have very singular
forms, which seem to show the combined effect of the growth of coral
round submerged banks, and their subsequent upheaval. Dr. Allan informs
me that he never observed any elevated organic remains on the
_Seychelles_, which come under our fringed class.
[26] “Travels in Ceylon,” page 13. This madreporitic formation is
mentioned by M. Cordier in his report to the Institute (May 4th,
1839), on the voyage of the “Chevrette”, as one of immense extent, and
belonging to the latest tertiary period.
[27] Dr. Benza, in his “Journey through the N. Circars” (the “Madras
Lit. and Scient. Journ.” volume v.) has described a formation with
recent fresh-water and marine shells, occurring at the distance of
three or four miles from the present shore. Dr. Benza, in conversation
with me, attributed their position to a rise of the land. Dr.
Malcolmson, however (and there cannot be a higher authority on the
geology of India) informs me that he suspects that these beds may have
been formed by the mere action of the waves and currents accumulating
sediment. From analogy I should much incline to Dr. Benza’s opinion.
[28] Owen’s “Africa,” volume ii., page 37, for Madagascar; and for S.
Africa, volume i., pages 412 and 426. Lieutenant Boteler’s narrative
contains fuller particulars regarding the coral-rock, volume i., page
174, and volume ii., pages 41 and 54. See also Ruschenberger’s “Voyage
round the World,” volume i., page 60.
The nature of the formations round the shores of the _Red Sea_, as
described by several authors, shows that the whole of this large area
has been elevated within a very recent tertiary epoch. A part of this
space in the appended map, is coloured blue, indicating the presence of
barrier-reefs: on which circumstance I shall presently make some
remarks. Rüppell[29] states that the tertiary formation, of which he
has examined the organic remains, forms a fringe along the shores with
a uniform height of from thirty and forty feet from the mouth of the
Gulf of Suez to about latitude 26°; but that south of 26°, the beds
attain only the height of from twelve to fifteen feet. This, however,
can hardly be quite accurate; although possibly there may be a decrease
in the elevation of the shores in the middle parts of the Red Sea, for
Dr. Malcolmson (as he informs me) collected from the cliffs of Camaran
Island (latitude 15° 30′ S.) shells and corals, apparently recent, at a
height between thirty and forty feet; and Mr. Salt (“Travels in
Abyssinia”) describes a similar formation a little southward on the
opposite shore at Amphila. Moreover, near the mouth of the Gulf of
Suez, although on the coast opposite to that on which Dr. Rüppell says
that the modern beds attain a height of only thirty to forty feet, Mr.
Burton[30] found a deposit replete with existing species of shells, at
the height of 200 feet. In an admirable series of drawings by Captain
Moresby, I could see how continuously the cliff-bounded low plains of
this formation extended with a nearly equable height, both on the
eastern and western shores. The southern coast of Arabia seems to have
been subjected to the same elevatory movement, for Dr. Malcolmson found
at Sahar low cliffs containing shells and corals, apparently of recent
species.
[29] Rüppell, “Reise in Abyssinien,” Band i., s. 141.
[30] Lyell’s “Principles of Geology,” 5th edition, volume iv., page
25.
The _Persian Gulf_ abounds with coral-reefs; but as it is difficult to
distinguish them from sand-banks in this shallow sea, I have coloured
only some near the mouth; towards the head of the gulf Mr.
Ainsworth[31] says that the land is worn into terraces, and that the
beds contain organic remains of existing forms. The _West Indian
Archipelago_ of “fringed” islands, alone remains to be mentioned;
evidence of an elevation within a late tertiary epoch of nearly the
whole of this great area, may be found in the works of almost all the
naturalists who have visited it. I will give some of the principal
references in a note.[32]
[31] Ainsworth’s “Assyria and Babylon,” page 217.
[32] On Florida and the north shores of the Gulf of Mexico, Rogers’
“Report to Brit. Assoc.” volume iii., page 14.—On the shores of
Mexico, Humboldt, “Polit. Essay on New Spain,” volume i., page 62. (I
have also some corroborative facts with respect to the shores of
Mexico.)—Honduras and the Antilles, Lyell’s “Principles,” 5th edition,
volume iv., page 22.—Santa Cruz and Barbadoes, Prof. Hovey,
“Silliman’s Journal”, volume xxxv., page 74.—St. Domingo,
Courrojolles, “Journ de Phys.” tom. liv., page 106.—Bahamas, “United
Service Journal”, No. lxxi., pages 218 and 224. Jamaica, De la Beche,
“Geol. Man.” page 142.—Cuba, Taylor in “Lond. and Edin. Mag.” volume
xi., page 17. Dr. Daubeny also, at a meeting of the Geolog. Soc.,
orally described some very modern beds lying on the N.W. parts of
Cuba. I might have added many other less important references.
It is very remarkable on reviewing these details, to observe in how
many instances fringing-reefs round the shores, have coincided with the
existence on the land of upraised organic remains, which seem, from
evidence more or less satisfactory, to belong to a late tertiary
period. It may, however, be objected, that similar proofs of elevation,
perhaps, occur on the coasts coloured blue in our map: but this
certainly is not the case with the few following and doubtful
exceptions.
The entire area of the Red Sea appears to have been upraised within a
modern period; nevertheless I have been compelled (though on
unsatisfactory evidence, as given in the Appendix) to class the reefs
in the middle part, as barrier-reefs; should, however, the statements
prove accurate to the less height of the tertiary bed in this middle
part, compared with the northern and southern districts, we might well
suspect that it had subsided subsequently to the general elevation by
which the whole area has been upraised. Several authors[33] have stated
that they have observed shells and corals high up on the mountains of
the Society Islands,—a group encircled by barrier-reefs, and,
therefore, supposed to have subsided: at Tahiti Mr. Stutchbury found on
the apex of one of the highest mountains, between 5,000 and 7,000 feet
above the level of the sea, “a distinct and regular stratum of
semi-fossil coral.” At Tahiti, however, other naturalists, as well as
myself, have searched in vain at a low level near the coast, for
upraised shells or masses of coral-reef, where if present they could
hardly have been overlooked. From this fact, I concluded that probably
the organic remains strewed high up on the surface of the land, had
originally been embedded in the volcanic strata, and had subsequently
been washed out by the rain. I have since heard from the Rev. W. Ellis,
that the remains which he met with, were (as he believes)
interstratified with an argillaceous tuff; this likewise was the case
with the shells observed by the Rev. D. Tyerman at Huaheine. These
remains have not been specifically examined; they may, therefore, and
especially the stratum observed by Mr. Stutchbury at an immense height,
be contemporaneous with the first formation of the Society Islands, and
be of any degree of antiquity; or they may have been deposited at some
subsequent, but probably not very recent, period of elevation; for if
the period had been recent, the entire surface of the coast land of
these islands, where the reefs are so extensive, would have been coated
with upraised coral, which certainly is not the case. Two of the
Harvey, or Cook Islands, namely, Aitutaki and Manouai, are encircled by
reefs, which extend so far from the land, that I have coloured them
blue, although with much hesitation, as the space within the reef is
shallow, and the outline of the land is not abrupt. These two islands
consist of coral-rock; but I have no evidence of their recent
elevation, besides, the improbability of Mangaia, a fringed island in
the same group (but distant 170 miles), having retained its nearly
perfect atoll-like structure, during any immense lapse of time after
its upheaval. The Red Sea, therefore, is the only area in which we have
clear proofs of the recent elevation of a district, which, by our
theory (although the barrier-reefs are there not well characterised),
has lately subsided. But we have no reason to be surprised at
oscillation, of level of this kind having occasionally taken place.
There can be scarcely any doubt that Savage, Aurora,[34] and Mangaia
Islands, and several of the islands in the Friendly group, existed
originally as atolls, and these have undoubtedly since been upraised to
some height above the level of the sea; so that by our theory, there
has here, also, been an oscillation of level, —elevation having
succeeded subsidence, instead of, as in the middle part of the Red Sea
and at the Harvey Islands, subsidence having probably succeeded recent
elevation.
[33] Ellis, in his “Polynesian Researches,” was the first to call
attention to these remains (volume i., page 38), and the tradition of
the natives concerning them. See also Williams, “Nar. of Missionary
Enterprise,” page 21; also Tyerman and G. Bennett, “Journal of
Voyage,” volume i., page 213; also Mr. Couthouy’s “Remarks,” page 51;
but this principal fact, namely, that there is a mass of upraised
coral on the narrow peninsula of Tiarubu, is from hearsay evidence;
also Mr. Stutchbury, “West of England Journal,” No. i., page 54. There
is a passage in Von Zach, “Corres. Astronom.” volume x., page 266,
inferring an uprising at Tahiti, from a footpath now used, which was
formerly impassable; but I particularly inquired from several native
chiefs, whether they knew of any change of this kind, and they were
unanimous in giving me an answer in the negative.
[34] Aurora Island is described by Mr. Couthouy (“Remarks,” page 58);
it lies 120 miles north-east of Tahiti; it is not coloured in the
appended map, because it does not appear to be fringed by living
reefs. Mr. Couthouy describes its summit as “presenting a broad
table-land which declines a few feet towards the centre, where we may
suppose the lagoon to have been placed.” It is about two hundred feet
in height, and consists of reef-rock and conglomerate, with existing
species of coral embedded in it. The island has been elevated at two
successive periods; the cliffs being marked halfway up with a
horizontal water-worn line of deep excavations. Aurora Island seems
closely to resemble in structure Elizabeth Island, at the southern end
of the Low Archipelago.
It is an interesting fact, that Fais, which, from its composition,
form, height, and situation at the western end of the Caroline
Archipelago, one is strongly induced to believe existed before its
upheaval as an atoll, lies exactly in the prolongation of the curved
line of the Mariana group, which we know to be a line of recent
elevation. I may add, that Elizabeth Island, in the southern part of
the Low Archipelago, which seems to have had the same kind of origin as
Fais, lies near Pitcairn Island, the only one in this part of the ocean
which is high, and at the same time not surrounded by an encircling
barrier-reef.
_On the absence of active Volcanoes in the areas of subsidence, and on
their frequent presence in the areas of elevation._—Before making some
concluding remarks on the relations of the spaces coloured blue and
red, it will be convenient to consider the position on our map of the
volcanoes historically known to have been in action. It is impossible
not to be struck, first with the absence of volcanoes in the great
areas of subsidence tinted pale and dark blue,—namely, in the central
parts of the Indian Ocean, in the China Sea, in the sea between the
barriers of Australia and New Caledonia, in the Caroline, Marshall,
Gilbert, and Low Archipelagoes; and, secondly, with the coincidence of
the principal volcanic chains with the parts coloured red, which
indicates the presence of fringing-reefs; and, as we have just seen,
the presence in most cases of upraised organic remains of a modern
date. I may here remark that the reefs were all coloured before the
volcanoes were added to the map, or indeed before I knew of the
existence of several of them.
The volcano in Torres Strait, at the northern point of Australia, is
that which lies nearest to a large subsiding area, although situated
125 miles within the outer margin of the actual barrier-reef. The Great
Comoro Island, which probably contains a volcano, is only twenty miles
distant from the barrier-reef of Mohila; Ambil volcano, in the
Philippines, is distant only a little more than sixty miles from the
atoll-formed Appoo reef: and there are two other volcanoes in the map
within ninety miles of circles coloured blue. These few cases, which
thus offer partial exceptions to the rule, of volcanoes being placed
remote from the areas of subsidence, lie either near single and
isolated atolls, or near small groups of encircled islands; and these
by our theory can have, in few instances, subsided to the same amount
in depth or area, as groups of atolls. There is not one active volcano
within several hundred miles of an archipelago, or even a small group
of atolls. It is, therefore, a striking fact that in the Friendly
Archipelago, which owes its origin to the elevation of a group of
atolls, two volcanoes, and, perhaps, others[35] are known to be in
action: on the other hand, on several of the encircled islands in the
Pacific, supposed by our theory to have subsided, there are old craters
and streams of lava, which show the effects of past and ancient
eruptions. In these cases, it would appear as if the volcanoes had come
into action, and had become extinguished on the same spots, according
as the elevating or subsiding movements prevailed.
[35] See the note at p. 121, on the authorities for colouring the
volcanoes in the appended map.