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.