these remaining fixed, have to be worn away, atom by atom, until
reduced 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. Let
him 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
Pal ozoic 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
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