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. 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 one hundred or even one hundred and fifty
feet. Beyond this depth the sounding-lead rests, not upon the wall-like
face of the reef, but on the ordinary shelving sea-bottom. And the
distance to which a fringing reef extends from the land corresponds with
that at which the sea has a depth of twenty or five-and-twenty fathoms.

If, as we have supposed, the sea could be suddenly withdrawn from around
an island provided with a fringing reef, such as the Mauritius,[122]
the reef would present the aspect of a terrace, its seaward face, one
hundred feet or more high, blooming with the animal flowers of the
coral, while its surface would be hollowed out into a shallow and
irregular moat-like excavation.

The coral mud, which occupies the bottom of the lagoon, and with which
all the interstices of the coral skeletons which accumulate to form
the reef are filled up, does 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