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. [Footnote 9: "Petrificata montium calcariorum non filii sed parentes sunt, cum omnis calx oriatur ab animalibus."--_Systema Naturae_, Ed. xii., t. iii., p. 154. It must be recollected that Linnaeus included silex, as well as limestone, under the name of "calx," and that he would probably have arranged Diatoms among animals, as part of "chaos." Ehrenberg quotes another even more pithy passage, which I have not been able to find in any edition of the _Systema_ accessible to me: "Sic lapides ab animalibus, nec vice versa. Sic runes saxei non primaevi, sed temporis filiae."] So long as the _Globigerinoe_;, actually collected at the surface, have not been demonstrated to contain the elements of clay, the _Challenger_ hypothesis, as I may term it, must be accepted with reserve and provisionally, but, at present, I cannot but think that it is more probable than any other suggestion which has been made.