It follows from this that every bed of any mechanically-formed
rock is the measure and equivalent of a corresponding amount of
destruction of some older rock. It is not necessary to enter
here into a minute account of the subdivisions of these rocks, but
it may be mentioned that they may be divided into two principal
groups, according to their chemical composition. In the one group
we have the so-called _Arenaceous_ (Lat. _arena_, sand) or
_Siliceous_ Rocks, which are essentially composed of larger or
smaller grains of flint or silica. In this group are comprised
ordinary sand, the varieties of sandstone and grit, and most
conglomerates and breccias. We shall, however, afterwards see
that some siliceous rocks are of organic origin. In the second
group are the so-called _Argillaceous_ (Lat. _argilla_, clay)
Rocks, which contain a larger or smaller amount of clay or hydrated
silicate of alumina in their composition. Under this head come
clays, shales, marls, marl-slate, clay-slates, and most flags
and flagstones.
B. CHEMICALLY-FORMED ROCKS.--In this section are comprised all
those Aqueous or Sedimentary Rocks which have been formed by
chemical agencies. As many of these chemical agencies, however,
are exerted through the medium of living beings, whether animals
or plants, we get into this section a number of what may be called
"_organically-formed rocks_.
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