ed. As Lyell has
well remarked, the extent and thickness of our sedimentary formations
are the result and the measure of the denudation which the earth's crust
has elsewhere undergone. Therefore a man should examine for himself the
great piles of superimposed strata, and watch the rivulets bringing down
mud, and the waves wearing away the sea-cliffs, in order to comprehend
something about the duration of past time, the monuments of which we see
all around us.

It is good to wander along the coast, when formed of moderately hard
rocks, and mark the process of degradation. The tides in most cases
reach the cliffs only for a short time twice a day, and the waves eat
into them only when they are charged with sand or pebbles; for there is
good evidence that pure water effects nothing in wearing away rock. At
last the base of the cliff is undermined, huge fragments fall down, and
these remaining fixed, have to be worn away atom by atom, until after
being reduced in size they can be rolled about by the waves, and then
they are more quickly ground into pebbles, sand, or mud. But how often
do we see along the bases of retreating cliffs rounded boulders, all
thickly clothed by marine productions, showing how little they are
abraded and how seldom they are rolled about! Moreover, if we follow for
a few miles any line of rocky cliff, which is undergoing degradation,
we find that it is only here and there, along a short length or round
a promontory, that the cliffs are at the present time suffering. The
appearance of the surface and the vegetation show that elsewhere years
have elapsed since the waters washed their base.

We have, however, recently learned from the observations of Ramsay, in
the van of many excellent observers--of Jukes, Geikie, Croll and
others, that subaerial degradation is a much more important agency than
coast-action, or the power of the waves. The whole surface of the land
is exposed to the chemical action of the air and of the rainwater,
with its dissolved carbonic acid, and in colder countries to frost; the
disintegrated matter is carried down even gentle slopes during heavy
rain, and to a greater extent than might be supposed, especially in
arid districts, by the wind; it is then transported by the streams
and rivers, which, when rapid deepen their channels, and triturate the
fragments. On a rainy day, even in a gently undulating country, we see
the effects of subaerial degradation in the muddy rills which