296 THE LAPSE OF TIME. 



elsewhere undergone. Therefore a man should examine fos 

 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 monuments of which we see all 

 around us. 



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

 moderately hard rocks, and mark the process of degradation 

 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 bowlders, all 

 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, we find that it is 

 only here and there, along a short length or round a prom- 

 ontory, 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 

 base. 



We have, however, recently learned from the observations 

 of Kamsay, in the van of many excellent observers — of 

 Jukes, Geikie, Croll, and others, that subaerial degradation 

 is a much more important agency than coast-action, or the 

 power of the waves. The whole surface of the land is 

 exposed to the chemical action of the air and of the rain- 

 water, with its dissolved carbonic acid, and in colder coun- 

 tries to frost ; the disintegrated matter is carried down even 

 gentle slopes during heavy rain, and to a greater extent than 

 might be supposed, especially in arid districts, by the wind ; 

 it is then transported by the streams and rivers, which, 

 when rapid, deepen their channels, and triturate the frag- 

 ments. On si rainy day. even in a gently undulating coun- 

 try, we see the effects of subaerial degradation in the muddy 

 rills which flow down every slope. Messrs. Ramsay and 

 Whitaker have shown, and the observation is a most strik- 

 ing one, that the great lines of escarpment in the Wealde^ 



