Chap. x. The Lapse of Time. 267 



It is good to wander along the coast, when formed of 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 boulders, 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 promontory, that the cliffs are at the present time suffering. 

 The appearance of the surface and the vegetation show that else- 

 where years have elapsed since the waters washed their base. 



We have, however, recently learnt 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 im- 

 portant 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 countries 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 fragments. On a rainy 

 day, even in a gently undulating country, we see the effects of 

 subaerial degradation in the muddy rills which flow down every 

 slope. Messrs. Eamsay and Whitaker have shown, and the ob- 

 servation is a most striking one, that the great lines of escarpment 

 in the Wealden district and those ranging across England, which 

 formerly were looked at as ancient sea-coasts, cannot have been 

 thus formed, for each line is composed of one and the same forma- 

 tion, whilst our sea-cliffs are everywhere formed by the intersection 

 of various formations. This being the case, we are compelled to 

 admit that the escarpments owe their origin in chief part to the 

 rocks of which they are composed having resisted subaerial denu- 

 dation better than the surrounding surface ; this surface conse- 

 quently has been gradually lowered, with the lines of harder rock 



