In the case of the barrier-reef of New
Caledonia, which extends for 150 miles beyond the northern point of
the island, 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 farther, 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.
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