st-waves.

We may, I think, safely conclude that sediment must be accumulated in
extremely thick, solid, or extensive masses, in order to withstand the
incessant action of the waves, when first upraised and during subsequent
oscillations of level. Such thick and extensive accumulations of sediment
may be formed in two ways; either, in profound depths of the sea, in which
case, judging from the researches of E. Forbes, we may conclude that the
bottom will be inhabited by extremely few animals, and the mass when
upraised will give a most imperfect record of the forms of life which then
existed; or, sediment may be accumulated to any thickness and extent over a
shallow bottom, if it continue slowly to subside. In this latter case, as
long as the rate of subsidence and supply of sediment nearly balance each
other, the sea will remain shallow and favourable for life, and thus a
fossiliferous formation thick enough, when upraised, to resist any amount
of degradation, may be formed.

I am convinced that all our ancient formations, which are rich in fossils,
have thus been formed during subsidence. Since publishing my views on this
subject in 1845, I have watched the progress of {292} Geology, and have
been surprised to note how author after author, in treating of this or that
great formation, has come to the conclusion that it was accumulated during
subsidence. I may add, that the only ancient tertiary formation on the west
coast of South America, which has been bulky enough to resist such
degradation as it has as yet suffered, but which will hardly last to a
distant geological age, was certainly deposited during a downward
oscillation of level, and thus gained considerable thickness.

All geological facts tell us plainly that each area has undergone numerous
slow oscillations of level, and apparently these oscillations have affected
wide spaces. Consequently formations rich in fossils and sufficiently thick
and extensive to resist subsequent degradation, may have been formed over
wide spaces during periods of subsidence, but only where the supply of
sediment was sufficient to keep the sea shallow and to embed and preserve
the remains before they had time to decay. On the other hand, as long as
the bed of the sea remained stationary, _thick_ deposits could not have
been accumulated in the shallow parts, which are the most favourable to
life. Still less could this have happened during the alternate periods of
elevation; or, to