Such thick and
extensive accumulations of sediment may be formed in two ways; either in
profound depths of the sea, in which case the bottom will not be inhabited
by so many and such varied forms of life as the more shallow seas; and the
mass when upraised will give an imperfect record of the organisms which
existed in the neighbourhood during the period of its accumulation. Or
sediment may be deposited 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 many and varied forms, and
thus a rich fossiliferous formation, thick enough, when upraised, to resist
a large amount of denudation, may be formed.
I am convinced that nearly all our ancient formations, which are throughout
the greater part of their thickness 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 deposited during a downward oscillation of level, and thus gained
considerable thickness.
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