6); and the matrix may be composed of sand (arenaceous) or of carbonate of lime (calcareous).

In the case of an ordinary sandstone, again, we have a rock which may be regarded as simply a very fine-grained conglomerate or breccia, being composed of small grains of sand (silica), sometimes rounded, sometimes more or less angular, cemented together by some such substance as oxide of iron, silicate of iron, or carbonate of lime.

A sandstone, therefore, like a conglomerate is a mechanically-formed rock, its component grams being equally the result of mechanical attrition and having equally been transported from a distance; and the same is true of the ordinary sand of the sea-shore, which is nothing more than an unconsolidated sandstone.

Other so-called sands and sandstones, though equally mechanical in their origin, are truly calcareous in their nature, and are more or less entirely composed of carbonate of lime.

Of this kind are the shell-sand so common on our coasts, and the coral-sand which is so largely formed in the neighbourhood of coral-reefs.