Nothing impresses the mind with the
vast duration of time, according to our ideas of time, more forcibly than
the conviction thus gained that subaerial agencies, which apparently have
so little power, and which seem to work so slowly, have produced great
results.
When thus impressed with the slow rate at which the land is worn away
through subaerial and littoral action, it is good, in order to appreciate
the past duration of time, to consider, on the one hand, the masses of rock
which have been removed over many extensive areas, and on the other hand
the thickness of our sedimentary formations. I remember having been much
struck 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 told still more plainly 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, and it makes no great difference whether
the upheaval was sudden, or, as most geologists now believe, was slow and
effected by many starts, the surface of the land has been so completely
planed down that no trace of these vast dislocations is externally visible.
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