believes that
there is one in Merionethshire of 12,000 feet; yet in these cases there
is nothing on the surface of the land to show such prodigious movements;
the pile of rocks on either side of the crack having been smoothly swept
away.

On the other hand, in all parts of the world the piles of sedimentary
strata are of wonderful thickness. In the Cordillera, I estimated one
mass of conglomerate at ten thousand feet; and although conglomerates
have probably been accumulated at a quicker rate than finer sediments,
yet from being formed of worn and rounded pebbles, each of which bears
the stamp of time, they are good to show how slowly the mass must
have been heaped together. Professor Ramsay has given me the maximum
thickness, from actual measurement in most cases, of the successive
formations 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,
blank periods of enormous length. So that the lofty pile of sedimentary
rocks in Britain gives but an inadequate idea of the time which has
elapsed during their accumulation. The consideration of these various
facts impresses the mind almost in the same manner as does the vain
endeavour to grapple with the idea of eternity.

Nevertheless this impression is partly false. Mr. Croll, in an
interesting paper, remarks that we do not err "in forming too great a
conception of the length of geological periods," but in estimating them
by years. When geologists look at large and complicated phenomena, and
then at the figures representing several million years, the two produce
a totally different effect on the mind, and the figures are at once
pronounced too small. In regard to subaerial denudation, Mr. Croll
shows, by calculating the known amount of sediment annually brought down
by certain rivers, relatively to their areas of drainage, that 1,000
feet of solid rock, as it became gradually disintegrated, would thus
be removed from the mean