arts
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. To take
another example nearer home, we may find great accumulations of
calcareous matter formed _in place_, by the growth of shell-fish,
such as oysters or mussels; but we can also find equally great
accumulations on many of our shores in the form of "shell-sand,"
which is equally composed of the shells of molluscs, but which is
formed by the trituration of these shells by the mechanical power
of the sea-waves. We thus see that though all these limestones are
primarily organic, they not uncommonly become "mechanically-formed"
rocks in a secondary sense, the materials of which they are composed
being formed by living beings, but having been mechanically
transported to the place where we now find them.

[Illustration: Fig. 11.--Section of Carboniferous Limestone from
Spergen Hill, Indiana, U.S., showing numerous large-sized
_Foraminifera_ (_Endothyra_) and a few oolitic grains; magnified.
(Original.)]

[Illustration: Fig 12.--Section of Coniston Limestone (Lower
Silurian) from Keisler, Westmoreland; magnified. The matrix is
very coarsely crystalline, and the included organic remains are
chiefly stems of Crinoids. (Original.)]

Many limestones, as we have seen, are composed of large and
conspicuous organic remains, such as strike the eye at once.
Many others, however, which at first sight appear compact, more
or less crystalline, and nearly devoid of traces of life, are
found, when properly examined, to be also composed of the remains
of various organisms. All the commoner limestones, in fact, from
the Lower Silurian period onwards, can be easily proved to be
thus _organic_ rocks, if we investigate weathered or polished
surfaces with a lens, or, still better, if we cut thin slices
of the rock and grind these down till they are transparent. When
thus examined, the rock is usually found to be composed of
innumerable entire or fragmentary fossils, cemented together
by a granular or crystalline matrix of carbonate of lime (figs.
11 and 12). When the matrix is granular, the rock is precisely
similar to chalk, except that it is harder and less earthy in
texture, whilst the fossils are only occasionally referable