.

The metamorphism contemplated by the great modern champion of rational
geology is, mainly, that brought about by the exposure of rocks to
subterranean heat; and where no such heat could be shown to have
operated, his opponents assumed that no metamorphosis could have taken
place. But the formation of greensand, and still more that of the "red
clay" (if the _Challenger_ hypothesis be correct) affords an insight into
a new kind of metamorphosis--not igneous, but aqueous--by which the
primitive nature of a deposit may be masked as completely as it can be by
the agency of heat. And, as Wyville Thomson suggests, in the passage I
have quoted above (p. 17), it further enables us to assign a new cause
for the occurrence, so puzzling hitherto, of thousands of feet of
unfossiliferous fine-grained schists and slates, in the midst of
formations deposited in seas which certainly abounded in life. If the
great deposit of "red clay" now forming in the eastern valley of the
Atlantic were metamorphosed into slate and then upheaved, it would
constitute an "azoic" rock of enormous extent. And yet that rock is now
forming in the midst of a sea which swarms with living beings, the great
majority of which are provided with calcareous or silicious shells and
skeletons; and, therefore, are such as, up to this time, we should have
termed eminently preservable.

Thus the discoveries made by the _Challenger_ expedition, like all recent
advances in our knowledge of the phenomena of biology, or of the changes
now being effected in the structure of the surface of the earth, are in
accordance with and lend strong support to, that doctrine of
Uniformitarianism, which, fifty years ago, was held only by a small
minority of English geologists--Lyell, Scrope, and De la Beche--but now,
thanks to the long-continued labours of the first two, and mainly to
those of Sir Charles Lyell, has gradually passed from the position of a
heresy to that of catholic doctrine.

Applied within the limits of the time registered by the known fraction of
the crust of the earth, I believe that uniformitarianism is unassailable.
The evidence that, in the enormous lapse of time between the deposition
of the lowest Laurentian strata and the present day, the forces which
have modified the surface of the crust of the earth were different in
kind, or greater in the intensity of their action, than those which are
now occupied in the same work, has yet to be produced. Such evidenc