If The Sea Had Formerly Eaten Deeply Into The Islands,
Before They Were Protected By The Reefs, Thus Having
Left A Shallow Ledge Round Them Under Water, The Present
Shores Would Have Been Invariably Bounded By Great Precipices,
But This Is Most Rarely The Case.
Moreover, on this
notion, it is not possible to explain why the corals should
have sprung up, like a wall, from the extreme outer margin
of the ledge, often leaving a broad space of water within,
too deep for the growth of corals.
The accumulation of a
wide bank of sediment all round these islands, and generally
widest where the included islands are smallest, is highly
improbable, considering their exposed positions in the central
and deepest parts of the ocean. In the case of the barrier-reef
of New Caledonia, which extends for 150 miles beyond
the northern point of the islands, in the same straight line
with which it fronts the west coast, it is hardly possible to
believe that a bank of sediment could thus have been
straightly deposited in front of a lofty island, and so far
beyond its termination in the open sea. Finally, if we look
to other oceanic islands of about the same height and of similar
geological constitution, but not encircled by coral-reefs,
we may in vain search for so trifling a circumambient
depth as 30 fathoms, except quite near to their shores; for
usually land that rises abruptly out of water, as do most of
the encircled and non-encircled oceanic islands, plunges
abruptly under it. On what then, I repeat, are these barrier
reefs based? Why, with their wide and deep moat-like channels,
do they stand so far from the included land? We shall
soon see how easily these difficulties disappear.
We come now to our third class of Fringing-reefs, which
will require a very short notice. Where the land slopes abruptly
under water, these reefs are only a few yards in width,
forming a mere ribbon or fringe round the shores: where
the land slopes gently under the water the reef extends
further, sometimes even as much as a mile from the land;
but in such cases the soundings outside the reef always show
that the submarine prolongation of the land is gently inclined.
In fact, the reefs extend only to that distance from the shore,
at which a foundation within the requisite depth from 20 to
30 fathoms is found. As far as the actual reef is concerned,
there is no essential difference between it and that forming
a barrier or an atoll: it is, however, generally of less width,
and consequently few islets have been formed on it. From
the corals growing more vigorously on the outside, and from
the noxious effect of the sediment washed inwards, the outer
edge of the reef is the highest part, and between it and the
land there is generally a shallow sandy channel a few feet in
depth. 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.
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