Some of these formations, which are
represented in England by thin beds, are thousands of feet in
thickness on the Continent. Moreover, between each successive
formation, we have, in the opinion of most geologists, enormously long
blank periods. So that the lofty pile of sedimentary rocks in Britain,
gives but an inadequate idea of the time which has elapsed during
their accumulation; yet what time this must have consumed! Good
observers have estimated that sediment is deposited by the great
Mississippi river at the rate of only 600 feet in a hundred thousand
years. This estimate may be quite erroneous; yet, considering over
what wide spaces very fine sediment is transported by the currents of
the sea, the process of accumulation in any one area must be extremely
slow.
But the amount of denudation which the strata have in many places
suffered, independently of the rate of accumulation of the degraded
matter, probably offers the best evidence of the lapse of time. I
remember having been much struck with the evidence of denudation, when
viewing volcanic islands, which have been worn by the waves and pared
all round into perpendicular cliffs of one or two thousand feet in
height; for the gentle slope of the lava-streams, due to their
formerly liquid state, showed at a glance how far the hard, rocky beds
had once extended into the open ocean. The same story is still more
plainly told by faults,--those great cracks along which the strata
have been upheaved on one side, or thrown down on the other, to the
height or depth of thousands of feet; for since the crust cracked, the
surface of the land has been so completely planed down by the action
of the sea, that no trace of these vast dislocations is externally
visible.
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