member Lyell's profound remark, that the thickness and extent of
sedimentary formations are the result and measure of the degradation
which the earth's crust has elsewhere suffered. And what an amount of
degradation is implied by the sedimentary deposits of many countries!
Professor Ramsay has given me the maximum thickness, in most cases from
actual measurement, in a few cases from estimate, of each formation in
different parts of Great Britain; and this is the result:--

Feet

Palaeozoic strata (not including igneous beds)..57,154.
Secondary strata................................13,190.
Tertiary strata..................................2,240.

--making altogether 72,584 feet; that is, very nearly thirteen and
three-quarters British miles. 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 p