Along the whole west coast, which is
inhabited by a peculiar marine fauna, tertiary beds are so scantily
developed, that no record of several successive and peculiar marine
faunas will probably be preserved to a distant age. A little
reflection will explain why along the rising coast of the western side
of South America, no extensive formations with recent or tertiary
remains can anywhere be found, though the supply of sediment must for
ages have been 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.
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