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If, as we have supposed, the sea could be suddenly withdrawn from around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.
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