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 



