The depth immediately outside the
barrier, or encircling, reefs, may also be very considerable; but, at
the outer edge of a fringing reef, it does not amount usually to more
than twenty or twenty-five fathoms; in other words, from 120 to 150
feet.
Thus, if the water of the ocean could be suddenly drained away, we
should see the atolls rising from the sea-bed like vast truncated
cones, and resembling so many volcanic craters, except that their
sides would be steeper than those of an ordinary volcano. In the case
of the encircling reefs, the cone, with the enclosed island, would
look like Vesuvius with Monte Nuovo within the old crater of Somma;
while, finally, the island with a fringing reef would have the
appearance of an ordinary hill, or mountain, girded by a vast parapet,
within which would lie a shallow moat. And the dry bed of the Pacific
might afford grounds for an inhabitant of the moon to speculate
upon the extraordinary subterranean activity to which these vast and
numerous "craters" bore witness!
When the structure of a fringing reef is investigated, the bottom of
the lagoon is found to be covered with fine whitish mud, which results
from the breaking up of the dead corals. Upon this muddy floor there
lie, here and there, growing corals, or occasionally great blocks of
dead coral, which have been torn by storms from the outer edge of
the reef, and washed into the lagoon.
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