We
have to remember, however, that though the limestones, both ancient
and modern, that we have just spoken of, are truly organic, they
are not necessarily formed out of the remains of animals which
actually lived on the precise spot where we now find the limestone
itself. We may find a crinoidal limestone, which we can show to
have been actually formed by the successive growth of generations
of sea-lilies _in place_; but we shall find many others in which
the rock is made up of innumerable fragments of the skeletons
of these creatures, which have been clearly worn and rubbed by
the sea-waves, and which have been mechanically transported to
their present site. In the same way, a limestone may be shown
to have been an actual coral-reef, by the fact that we find in
it great masses of coral, growing in their natural position,
and exhibiting plain proofs that they were simply quietly buried
by the calcareous sediment as they grew; but other limestones
may contain only numerous rolled and water-worn fragments of
corals. This is precisely paralleled by what we can observe in
our existing coral-reefs. Parts of the modern coral-islands and
coral-reefs are really made up of corals, dead or alive, which
actually grew on the spot where we now find them; but other parts
are composed of a limestone-rock ("coral-rock"), or of a loose
sand ("coral-sand"), which is organic in the sense that it is
composed of lime formed by living beings, but which, in truth,
is composed of fragments of the skeletons of these living beings,
mechanically transported and heaped together by the sea.
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