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 has no pretension to strict
exactness; 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 {285} 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. Prof. 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 palaeozoic strata, in parts ten thousand feet in thickness, as shown in
Prof. Ramsay's masterly