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_." These are of the greatest possible importance to the palaeontologist, as being to a greater or less extent composed of the actual remains of animals or vegetables, and it will therefore be necessary to consider their character and structure in some detail.
By far the most important of the chemically-formed rocks are the so-called _Calcareous Rocks_ (Lat.

_calx_, lime), comprising all those which contain a large proportion of carbonate of lime, or are wholly composed of this substance.

Carbonate of lime is soluble in water holding a certain amount of carbonic acid gas in solution; and it is, therefore, found in larger or smaller quantity dissolved in all natural waters, both fresh and salt, since these waters are always to some extent charged with the above-mentioned solvent gas.

A great number of aquatic animals, however, together with some aquatic plants, are endowed with the power of separating the lime thus held in solution in the water, and of reducing it again to its solid condition.