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. Shell-fish 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. Close to the seaward edge of the reef, over which, even in calm
weather, a surf almost always breaks, the coral rock is encrusted with
a thick coat of a singular vegetable organism, which contains a great
deal of lime--the so-called _Nullipora_. Beyond this, in the part of
the edge of the reef which is always covered by the breaking waves,
the living, true, reef--polypes make their appearance; and, in
different forms, coat the steep seaward face of the reef to a depth of
100 or even 150 feet. Beyond this depth the sounding-lead rests, not