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