Seite - 336 - in The Origin of Species

336 ORIGIN OF SPECIES 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 separate formations, and to mark how each author attempts to give an inadequate idea of the duration of each formation, or even of each stratum. We can best gain some idea of past time by knowing the agencies at work, and learning how deeply the surface of the land has been denuded, and how much sediment has been de- posited. As Lyell has well remarked, the extent and thick- ness of our sedimentary formations are the result and the measure of the denudation which the earth's crust has else- where 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 dura- tion of past time, the monuments of which we see all around us. It is good to wander along the coast, when formed of mod- erately 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 sur- face and the vegetation show that elsewhere years have elapsed since the waters washed their base. We have, however, recently learnt from the observations of Ramsay, in the van of many excellent observers—of Jukes, Geikie, Croll, and others, that subaerial degradation is a