of the many animals which live upon the reef become imbedded in this
coral limestone, so that a block may be full of shells of bivalves and
univalves, or of sea-urchins; and even sometimes encloses the eggs of
turtles in a state of petrifaction. The active and vigorous growth of
the reef goes on only at the seaward margins, where the polypes are
exposed to the wash of the surf, and are thereby provided with an
abundant supply of air and of food. The interior portion of the reef
may be regarded as almost wholly an accumulation of dead skeletons.
Where a river comes down from the land there is a break in the reef,
for the reasons which have been already mentioned.

The origin and mode of formation of a fringing reef, such as that just
described, are plain enough. The embryos of the coral polypes have
fixed themselves upon the submerged shore of the island, as far out as
they could live, namely, to a depth of twenty or twenty-five fathoms.
One generation has succeeded another, building itself up upon the dead
skeletons of its predecessor. The mass has been consolidated by
the infiltration of coral mud, and hardened by partial solution and
redeposition, until a great rampart of coral rock 100 or 150 feet high
on its seaward face has been formed all round the island, with only
such gaps as result from the outflow of rivers, in the place of
sally-ports.

The structure of the rocky accumulation in the encircling reefs and
in the atolls is essentially the same as in the fringing reef. But, in
addition to the differences of depth inside and out, they present
some other peculiarities. These reefs, and especially the atolls, are
usually interrupted at one part of their circumference, and this part
is always situated on the leeward side of the reef, or that which is
the more sheltered side. Now, as all these reefs are situated within