316 THE LAPSE OF TIME. 



tions of Eamsay, in the van of many excellent observers — 

 of Jukes, Geikie, Croll and others, that subaerial degrada- 

 tion is a much more important agency than coast-action, 

 or the power of the waves. The whole surface of the kud 

 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 

 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. Karasay 

 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, can not have 

 been thus formed, for each line is composed of one and the 

 same formation, while 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 denudation better 

 than the sui-rounding 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 worn away through subaerial and littoral action, it 

 is good, in order to appreciate the past duration of time, 

 to consider, on the one hand, the n>asses of rock which 

 have been removed over many extensive areas, and on 

 the other hand the thickness of our sedimentary forma- 

 tions. I remember having been much struck when view- 

 ing volcanic islands, which have been worn by the waves 

 and pared all round into perpendicular cliffs of one or two 

 thousand feet in height; for the gentle slope of the lava- 

 streams, due to their formerly liquid state, showed at a 



