TEE LAPSE OF TIME. 315 



duced a revolution in natural science, and yet does not 

 admit how 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 difEer- 

 ent observers on sepai'ate formations, and to mark how 

 each author attempts to give an inadequate idea of the 

 duration of each foi'mation, or even of each stratum. We 

 can best gain some idea of past time by knowing the agen- 

 cies 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 some- 

 thing about the duration of past time, the monuments 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, 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 re- 

 treating cliffs rounded boulders, all thickly clothed 

 by marine productions, showing how little they are 

 abraded, and how seldom they are rolled about! More- 

 over, 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 



go 



We have, however, recently learned from the observa- 



