not proceed from the washing action of
the waves alone; innumerable fishes, and other creatures which prey
upon the coral, add a very important contribution of finely-triturated
calcareous matter; and the corals and mud becoming incorporated
together, gradually harden and give rise to a sort of limestone rock,
which may vary a good deal in texture. Sometimes it remains friable
and chalky, but, more often, the infiltration of water, charged with
carbonic acid, dissolves some of the calcareous matter, and deposits
it elsewhere in the interstices of the nascent rock, thus glueing
and cementing the particles together into a hard mass; or it may even
dissolve the carbonate of lime more extensively, and re-deposit it in
a crystalline form. On the beach of the lagoon, where the coral sand
is washed into layers by the action of the waves, its grains become
thus fused together into strata of a limestone, so hard that they
ring when struck with a hammer, and inclined at a gentle angle,
corresponding with that of the surface of the beach. The hard parts
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 acc