great, from the enormous
degradation of the coast-rocks and from muddy streams entering the
sea. The explanation, no doubt, is, that the littoral and sub-littoral
deposits are continually worn away, as soon as they are brought up by
the slow and gradual rising of the land within the grinding action of
the coast-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 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