_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.