272 THE ORIGIN OF SPECIES 



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 moder- 

 ately 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 un- 

 dermined, 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! More- 

 over, 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 elsewhere years have elapsed since the 

 waters v/ashed their base. 



We have, however, recently learned 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 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 frag- 

 ments. 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 Hne is composed 

 of ^one and the same formation, while our sea cliffs are every- 

 where 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 com- 



