Next, imagine sponges, Radiolaria, Foraminifera, and diatomaceous plants, such as those which now exist in the deep-sea, to be introduced:  they would be distributed according to the same laws as at present, the sponges (and possibly some of the Foraminifera), covering the bottom, while other Foraminifera, with the Radiolaria and Diatomacea, would increase and multiply in the surface waters.  In accordance with the existing state of things, the Radiolaria and Diatoms would have a universal distribution, the latter gathering most thickly in the polar regions, while the Foraminifera would be largely, if not exclusively, confined to the intermediate zone; and, as a consequence of this distribution, a bed of “chalk” would begin to form in the intermediate zone, while caps of silicious rock would accumulate on the circumpolar regions.

Suppose, further, that a part of the intermediate area were raised to within two or three hundred fathoms of the surface—­for anything that we know to the contrary, the change of level might determine the substitution of greensand for the “chalk”; while, on the other hand, if part of the same area were depressed to three thousand fathoms, that change might determine the substitution of a different silicate of alumina and iron—­namely, clay—­for the “chalk” that would otherwise be formed.

If the Challenger hypothesis, that the red clay is the residue left by dissolved Foraminiferous skeletons, is correct, then all these deposits alike would be directly, or indirectly, the product of living organisms.  But just as a silicious deposit may be metamorphosed into opal or quartzite, and chalk into marble, so known metamorphic agencies may metamorphose clay into schist, clay-slate, slate, gneiss, or even granite.  And thus, by the agency of the lowest and simplest of organisms, our imaginary globe might be covered with strata, of all the chief kinds of rock of which the known crust of the earth is composed, of indefinite thickness and extent.

The bearing of the conclusions which are now either established, or highly probable, respecting the origin of silicious, calcareous, and clayey rocks, and their metamorphic derivatives, upon the archaeology of the earth, the elucidation of which is the ultimate object of the geologist, is of no small importance.

A hundred years ago the singular insight of Linnaeus enabled him to say that “fossils are not the children but the parents of rocks,"[9] and the whole effect of the discoveries made since his time has been to compile a larger and larger commentary upon this text.  It is, at present, a perfectly tenable hypothesis that all siliceous and calcareous rocks are either directly, or indirectly, derived from material which has, at one time or other, formed part of the organized framework of living organisms.  Whether the same generalization may be extended to aluminous rocks, depends upon the conclusion to be drawn from the facts respecting the red clay areas brought to light by the Challenger.  If we accept the view taken by Wyville Thomson and his colleagues—­that the red clay is the residuum left after the calcareous matter of the Globigerinoe ooze has been dissolved away—­then clay is as much a product of life as limestone, and all known derivatives of clay may have formed part of animal bodies.