58 THE ORIGIN OF SPECIES 



formations, and to mark how eaoh author attempts to 

 give an inadequate idea of the duration of each forma- 

 tion, 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 deposited. 

 As Lyell has well remarked, the extent and thickness 

 of our sedimentary formations are the result and the 

 measure of the denudation which the earth's crust has 

 elsewhere 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 duration of past time, the monu- 

 ments of which we see all, around us. 



It is good to wander along the coast, when formed of 

 moderately hard rocks, and mark the process of degrada- 

 tion. 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, re- 

 maining 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 bowl- 

 ders, ^11 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, 



