It Is Equally Improbable That The
Elevatory Forces Should Have Uplifted Throughout The Above
Vast Areas, Innumerable Great Rocky Banks
Within 20 to 30
fathoms, or 120 to 180 feet, of the surface of the sea, and
not one single
Point above that level; for where on the whole
surface of the globe can we find a single chain of mountains,
even a few hundred miles in length, with their many summits
rising within a few feet of a given level, and not one
pinnacle above it? If then the foundations, whence the atoll-
building corals sprang, were not formed of sediment, and if
they were not lifted up to the required level, they must of
necessity have subsided into it; and this at once solves the
difficulty. For as mountain after mountain, and island after
island, slowly sank beneath the water, fresh bases would be
successively afforded for the growth of the corals. It is
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.
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