in size they can be rolled about by the waves, and then 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.

He who most closely studies the action of the sea on our shores, will, I
believe, be most deeply impressed with the slowness with which rocky coasts
are worn away. The observations on this head by Hugh Miller, and by that
excellent observer Mr. Smith of Jordan Hill, are most impressive. With the
mind thus impressed, let any one examine beds of conglomerate many thousand
feet in thickness, which, though probably formed at a quicker rate than
many other deposits, yet, from being formed of worn and rounded pebbles,
each of which bears the stamp of time, are good to show how slowly the mass
has been accumulated. In the Cordillera I estimated one pile of
conglomerate at ten thousand feet in thickness. Let the {284} observer
remember 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 the 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