Chap. IX. 
GEOLOGICAL RECOKD. 
291 
cessive and peculiar marine faunas will probably be 
preserved to a distant age. A little reflection will ex¬ 
plain 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 accumu¬ 
lations 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, sedi¬ 
ment 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 up¬ 
raised, 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 
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