52 THE LAPSE OF TIME. [Chap. X- 



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 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 

 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 super- 

 imposed 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 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 degrad- 

 ation. 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 pro- 



