consecutive formations, we may infer that this could nowhere be
ascertained. The frequent
and great changes in the mineralogical composition of consecutive
formations, generally implying great changes in the geography of the
surrounding lands, whence the sediment has been derived, accords with
the belief of vast intervals of time having elapsed between each
formation.
But we can, I think, see why the geological formations of each region
are almost invariably intermittent; that is, have not followed each
other in close sequence. Scarcely any fact struck me more when
examining many hundred miles of the South American coasts, which have
been upraised several hundred feet within the recent period, than the
absence of any recent deposits sufficiently extensive to last for even
a short geological period. 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. 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
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