THE LATSE OF TIME 337 



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 dur- 

 ing heavy rain, and to a greater extent than might be sup- 

 posed, 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 composed of one and the same formation, whilst our 

 sea-cliffs are everywhere formed by the intersection of vari- 

 ous 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 subae- 

 rial denudation better than the surrounding surface ; this sur- 

 face 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 masses of rock which have been re- 

 moved over many extensive areas, and on the other hand the 

 thickness of our sedimentary formations. I remember hav- 

 ing been much struck when viewing volcanic islands, which 

 have been worn by the waves and pared all round into per- 

 pendicular cVius 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 glance how far the hard, rocky beds 

 had once extended into the open ocean. The same story is 



