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.  A recent observation by Sir Wyville Thomson would, however, render it not improbable that some of the great argillaceous accumulations of past geological periods may be really organic.  This distinguished observer, during the cruise of the Challenger, showed that the calcareous ooze which has been already spoken of as covering large areas of the floor of the Atlantic and Pacific at great depths, and which consists almost wholly of the shells of Foraminifera, gave place at still greater depths to a red ooze consisting of impalpable clayey mud, coloured by oxide of iron, and devoid of traces of organic bodies.  As the existence of this widely-diffused red ooze, in mid-ocean, and at such great depths, cannot be explained on the supposition that it is a sediment brought down into the sea by rivers, Sir Wyville Thomson came to the conclusion that it was probably formed by the action of the sea-water upon the shells of Foraminifera.  These shells, though mainly consisting of lime, also contain a certain proportion of alumina, the former being soluble in the carbonic acid dissolved in the sea-water, whilst the latter is insoluble.  There would further appear to be grounds for believing that the solvent power of the sea-water over lime is considerably increased at great depths.  If, therefore, we suppose the shells of Foraminifera to be in course of deposition over the floor of the Pacific, at certain depths they