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 petrification.
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.
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