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.
The Craven fault, for instance, extends for upwards of 30 miles, and
along this line the vertical displacement of the strata has varied from
600 to 3000 feet. Professor Ramsay has published an account of a
downthrow in Anglesea of 2300 feet; and he informs me that he fully
believes there is one in Merionethshire of 12,000 feet; yet in these
cases there is nothing on the surface to show such prodigious
movements; the pile of rocks on the one or other side having been
smoothly swept away. The consideration of these facts impresses my mind
almost in the same manner as does the vain endeavour to grapple with
the idea of eternity.
I am tempted to give one other case, the well-known one of the
denudation of the Weald. Though it must be admitted that the denudation
of the Weald has been a mere trifle, in comparison with that which has
removed masses of our pal ozoic strata, in parts ten thousand feet in
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