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Show 248 IMPERFECTION OF 'rHE [CIIAP. IX. who can read Sir Charles Lyell's gra;nd ~ork ~n the Pr~nciples of Geology, which the future ~nstonan w1ll.recogn1se as having produced a revolution 1n natural smence, yet does not admit how incomprehensibly vast have been the past periods of time, may at once close this volume. Not that it suffices to study the Principles of Geology, or to read special treatises by different observers on separ~te formations, and to mark how each author attempts to g1ve an inadequate idea of the duration of each formation or even each stratum. A man must for years examine for himself great piles of superimposed strata, and watch the sea at work grinding down old rocks and making fresh sediment, before he can hope to comprehend anything of the lapse of time, the monuments of which we see around us. It is good to wander along lines of sea-coast, when formed of moderately hard roc1\:s, 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 reason to believe that pure water can effect little or 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 ato1n, until reduced in size they can be rolled about by the waves, and then are n1ore 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. CHAP. IX.] GEOLOGICAL RECORD. 249 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 forn1ed 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 accu1nulated. Let him reme1nber 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 fi·om actual measurement, in a few cases from estimate, of each formation in different parts of Great Britain; and this is the result:- Feet. Palreozoi"c strata (not including igneous beds).. .. .. 57,154 Secondary strata .. .. .. .. .. .. .. .. .. .. 13,190 Tertiary strata. .. .. .. .. .. .. .• .. .. .. .. 2,240 -making alt<?gether 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 fonnation, we have, in the opinion of most geologists, enormously long blank Eeriods. So that the lofty pile of sedimentary rocks in Britain, gives but an inadequate ide~ of the time 'vhich has elapsed during their accumulation ; yet what time this must have consumed! Good observers have estimated that sediment is deposited by the great 1\fississippi 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 current!) of the sea, the process of accumulation in any one area must be extremely slow. But the amount of denudation which the strata have in m.any places suffered, independently of the rate of accun1ulation of the degraded matter, probably offers the 11* |