No explanation of the origin of flint, however, can be satisfactory, unless it embraces the origin of chert in almost all great limestones from the Silurian upwards, as well as the common phenomenon of the silicification of organic bodies (such as corals and shells) which are known with certainty to have been originally calcareous.] In addition to deposits formed of flint itself, there are other siliceous deposits formed by certain _silicates_, and also of organic origin.

It has been shown, namely--by observations carried out in our present seas--that the shells of _Foraminifera_ are liable to become completely infiltrated by silicates (such as "glauconite," or silicate of iron and potash).

Should the actual calcareous shell become dissolved away subsequent to this infiltration--as is also liable to occur--then, in place of the shells of the _Foraminifera_, we get a corresponding number of green sandy grains of glauconite, each grain being the _cast_ of a single shell.

It has thus been shown that the green sand found covering the sea-bottom in certain localities (as found by the Challenger expedition along the line of the Agulhas current) is really organic, and is composed of casts of the shells of _Foraminifera_.

Long before these observations had been made, it had been shown by Professor Ehrenberg that the green sands of various geological formations are composed mainly of the internal casts of the shells of _Foraminifera_, and we have thus another and a very interesting example how rock-deposits of considerable extent and of geological importance can be built up by the operation of the minutest living beings.
As regards _argillaceous_ deposits, containing _alumina_ or _clay_ as their essential ingredient, it cannot be said that any of these have been actually shown to be of organic origin.