We have seen that the calcareous or lime-containing rocks are the most important of the group of organic deposits; whilst the siliceous or flint-containing rocks may be regarded as the most important, most typical, and most generally distributed of the mechanically-formed rocks.  We have, however, now briefly to consider certain deposits which are more or less completely formed of flint; but which, nevertheless, are essentially organic in their origin.

Flint or silex, hard and intractable as it is, is nevertheless capable of solution in water to a certain extent, and even of assuming, under certain circumstances, a gelatinous or viscous condition.  Hence, some hot-springs are impregnated with silica to a considerable extent; it is present in small quantity in sea-water; and there is reason to believe that a minute proportion must very generally be present in all bodies of fresh water as well.  It is from this silica dissolved in the water that many animals and some plants are enabled to construct for themselves flinty skeletons; and we find that these animals and plants are and have been sufficiently numerous to give rise to very considerable deposits of siliceous matter by the mere accumulation of their skeletons.  Amongst the animals which require special mention in this connection are the microscopic organisms which are known to the naturalist as Polycystina.  These little creatures are of the lowest possible grade of organisation, very closely related to the animals which we have previously spoken of as Foraminifera, but differing in the fact that they secrete a shell or skeleton composed of flint instead of lime.  The Polycystina occur abundantly in our present seas; and their shells are present in some numbers in the ooze which is found at great depths in the Atlantic and Pacific oceans, being easily recognised by their exquisite shape, their glassy transparency, the general presence of longer or shorter spines, and the sieve-like perforations in the walls.  Both in Barbadoes and in the Nicobar islands occur geological formations which are composed of the flinty skeletons of these microscopic animals; the deposit in the former locality attaining a great thickness, and having been long known to workers with the microscope under the name of “Barbadoes earth” (fig. 15).

[Illustration:  Fig. 15.—­Shells of Polycystina from “Barbadoes earth;” greatly magnified. (Original.)]

[Illustration:  Fig. 16.—­Cases of Diatoms in the Richmond “Infusorial earth;” highly magnified. (Original.)]