Where Banks Or Sediments Have Accumulated Near To
The Surface, As In Parts Of The West Indies, They Sometimes
Become Fringed With Corals, And Hence In Some Degree Resemble
Lagoon-Islands Or Atolls, In The Same Manner As Fringing-Reefs,
Surrounding Gently Sloping Islands, In Some Degree Resemble
Barrier-Reefs.
No theory on the formation of coral-reefs can be considered
satisfactory which does not include the three great
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classes. We have seen that we are driven to believe in the
subsidence of those vast areas, interspersed with low islands,
of which not one rises above the height to which the wind and
waves can throw up matter, and yet are constructed by animals
requiring a foundation, and that foundation to lie at
no great depth. Let us then take an island surrounded by
fringing-reefs, which offer no difficulty in their structure;
and let this island with its reefs, represented by the unbroken
lines in the woodcut, slowly subside. Now, as the island
sinks down, either a few feet at a time or quite insensibly,
we may safely infer, from what is known 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 inner 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 reef; and a ship is anchored in the
lagoon-channel. This channel will be more or less deep,
according to the rate of subsidence, to the amount of sediment
accumulated in it, and to the growth of the delicately branched
corals which can live there. The section in this state resembles
in every respect one drawn through an encircled island: in fact,
it is a real section (on the scale of .517 of an inch to a mile)
through Bolabola in the Pacific. We can now at once see
why encircling barrier-reefs stand so far from the shores
which they front. We can also perceive, that a line drawn
perpendicularly down from the outer edge of the new reef,
to the foundation of solid rock beneath the old fringing-reef,
will exceed by as many feet as there have been feet of
subsidence, that small limit of depth at which the effective
corals can live: - the little architects having built up their
great wall-like mass, as the whole sank down, upon a basis
formed of other corals and their consolidated fragments.
Thus the difficulty on this head, which appeared so great,
disappears.
If, instead of an island, we had taken the shore of a continent
fringed with reefs, and had imagined it to have subsided,
a great straight barrier, like that of Australia or New
Caledonia, separated from the land by a wide and deep channel,
would evidently have been the result.
Let us take our new encircling barrier-reef, of which the
section is now represented by unbroken lines, and which, as
I have said, is a real section through Bolabola, and let it go
on subsiding. As the barrier-reef slowly sinks down, the
corals will go on vigorously growing upwards; but as the
island sinks, the water will gain inch by inch on the shore -
the separate mountains first forming separate islands within
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one great reef - and finally, the last and highest pinnacle
disappearing. The instant this takes place, a perfect atoll
is formed: I have said, remove the high land from within an
encircling barrier-reef, and an atoll is left, and the land has
been removed. We can now perceive how it comes that
atolls, having sprung from encircling barrier-reefs, resemble
them in general size, form, in the manner in which they are
grouped together, and in their arrangement in single or
double lines; for they may be called rude outline charts of
the sunken islands over which they stand. We can further
see how it arises that the atolls in the Pacific and Indian
Oceans extend in lines parallel to the generally prevailing
strike of the high islands and great coast-lines of those
oceans. I venture, therefore, to affirm, that on the theory of
the upward growth of the corals during the sinking of the
land, [13] all the leading features in those wonderful
structures, the lagoon-islands or atolls, which have so long
excited the attention of voyagers, as well as in the no less
wonderful barrier-reefs, whether encircling small islands or
stretching for hundreds of miles along the shores of a
continent, are simply explained.
It may be asked, whether I can offer any direct evidence
of the subsidence of barrier-reefs or atolls; but it must be
borne in mind how difficult it must ever be to detect a
movement, the tendency of which is to hide under water the part
affected. Nevertheless, at Keeling atoll I observed on all
sides of the lagoon old cocoa-nut trees undermined and falling;
and in one place the foundation-posts of a shed, which
the inhabitants asserted had stood seven years before just
above high-water mark, but now was daily washed by every
tide: on inquiry I found that three earthquakes, one of them
severe, had been felt here during the last ten years. At
Vanikoro, the lagoon-channel is remarkably deep, scarcely
any alluvial soil has accumulated at the foot of the lofty
included mountains, and remarkably few islets have been
formed by the heaping of fragments and sand on the wall-like
barrier reef; these facts, and some analogous ones, led
me to believe that this island must lately have subsided and
the reef grown upwards: here again earthquakes are frequent
and very severe. In the Society archipelago, on the
other hand, where the lagoon-channels are almost choked up,
where much low alluvial land has accumulated, and where in
some cases long islets have been formed on the barrier-reefs
- facts all showing that the islands have not very lately
subsided - only feeble shocks are most rarely felt.
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