CHAPTER X. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.
3. ON THE POORNESS OF PALAEONTOLOGICAL COLLECTIONS. (continued)
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 poorly 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 the 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, 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, as well as the subsequent subaerial degradation. 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.
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