impossible here to enter into all the necessary details, but I venture
to defy [12] any one to explain in any other manner how it is possible
that numerous islands should be distributed throughout vast areas--all
the islands being low--all being built of corals, absolutely requiring
a foundation within a limited depth from the surface.
Before explaining how atoll-formed reefs acquire their peculiar
structure, we must turn to the second great class, namely,
Barrier-reefs. These either extend in straight lines in front of the
shores of a continent or of a large island, or they encircle smaller
islands; in both cases, being separated from the land by a broad and
rather deep channel of water, analogous to the lagoon within an atoll.
It is remarkable how little attention has been paid to encircling
barrier-reefs; yet they are truly wonderful structures. The following
sketch represents part of the barrier encircling the island of Bolabola
in the Pacific, as seen from one of the central peaks. In this instance
the whole line of reef has been converted into land; but usually a
snow-white line of great breakers, with only here and there a single
low islet crowned with cocoa-nut trees, divides the dark heaving waters
of the ocean from the light-green expanse of the lagoon-channel. And
the quiet waters of this channel generally bathe a fringe of low
alluvial soil, loaded with the most beautiful productions of the
tropics, and lying at the foot of the wild, abrupt, central mountains.
Encircling barrier-reefs are of all sizes, from three miles to no less
than forty-four miles in diameter; and that which fronts one side, and
encircles both ends, of New Caledonia, is 400 miles long. Each reef
includes one, two, or several rocky islands of various heights; and in
one instance, even as many as twelve separate islands. The reef runs
at a greater or less distance from the included land; in the Society
archipelago generally from one to three or four miles; but at Hogoleu
the reef is 20 miles on the southern side, and 14 miles on the opposite
or northern side, from the included islands. The depth within the
lagoon-channel also varies much; from 10 to 30 fathoms may be taken as
an average; but at Vanikoro there are spaces no less than 56 fathoms or
363 feet deep. Internally the reef either slopes gently into the
lagoon-channel, or ends in a perpendicular wall sometimes between two
and three hundred feet under water in height: externally the reef
rises, like an atoll, with extreme abruptness out of the profound
depths of the ocean.
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