No Theory On The Formation Of Coral-Reefs Can Be Considered
Satisfactory Which Does Not Include The Three Great
[Picture]
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.
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Words from 194434 to 194941
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