Thus, if the water of the ocean should 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;[121] 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. Shellfish and worms of various kinds abound;
and fish, some of which prey upon the coral, sport in the deeper pools.
But the corals which are to be seen growing in the shallow waters of the
lagoon are of a different kind from those which abound on the outer edge
of the reef, and of which the reef is built up.
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