THE LAPSE OF TIME. 31 5 



diiced a revolution in natunil science, and yet does not 

 admit how vast have been the past periods of lime, may at 

 once close this volume. Not tliat it suilices to study tlie 

 Principles of Geolog-y, or to read special treatises by dltler- 

 ent observers on separate formations, and to mark liow 

 each author attempts to give an inadequate idea of tlie 

 duration of each foj-mation, or even of each stratum. Wc 

 can best gain some idea of past time by knowin.i? tlie agen- 

 cies at work; and learning how deeply the surface of^tiie 

 land has been denuded, and how much sediment lias 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 fur 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 I 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 rouiul a 

 promontory, that the cliffs are at the present time suffering. 

 The appearance of the surface and the vegetation show tliat 

 elsewhere years have elapsed since the waters washed iheir 

 base. 



We have, however^ recently learned from the observa- 



