52 THE LAPSE OF TIME. [Chap.X. 



been the past periods of time, may at once close this 

 volume. Not that it^sufhces to study the Principles of 

 Geology, or to read special treatises by different ob- 

 servers on separate formations, and to mark how each 

 author attempts to give an inadequate idea of the dura- 

 tion 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 degra- 

 dation. 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 noth- 

 ing 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, 



