250 THE LAPSE OF TIME. [CHAP. X. 



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 bo 

 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 

 Ramsay, in the van of many excellent observers of Jukes, 

 Geikie, Croll, and others, that subaerial degradation is a much 

 more important agency that 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. Ramsay and Whitaker 

 have shown, and the observation 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 com- 

 posed of one and the same formation, whilst our sea-cliffs are 

 everywhere formed by the intersection of various formations. 

 This being the case, we are compelled to admit that the escarp- 

 ments owe their origin in chief part to the rocks of which they 

 are composed having resisted subaerial denudation better than 

 the surrounding surface ; this surface consequently has been 

 gradually lowered, with the lines of harder rock left projecting. 

 Nothing impresses the mind with the vast duration of time, 

 according to our ideas of time, more forcibly than the conviction 

 thus gained that subaerial agencies which apparently have so 

 little power, and which seem to work so slowly, have produced 

 great results. 



When thus impressed with the slow rate at which the land is 

 away through subaerial and littoral action, it is good, ia 



