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
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, enormously long blank periods.  So that the lofty pile of sedimentary rocks in Britain, gives but an inadequate idea of the time which has elapsed during their accumulation; yet what time this must have consumed!  Good observers have estimated that sediment is deposited by the great Mississippi river at the rate of only 600 feet in a hundred thousand years.  This estimate may be quite erroneous; yet, considering over what wide spaces very fine sediment is transported by the currents of the sea, the process of accumulation in any one area must be extremely slow.