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.
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