54 THE LAPSE OF TDIE. [Chap. X. 



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 wliich seem to work so slowly, have pro- 

 duced 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 removed over many 



extensive areas, and on the other hand the thickness of 



our sedimentary formations. I remember having been 



much struck when viewing 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 glance how far 



the hard, rocky beds had once extended into the open 



ocean. The same story is told still more plainly by 



faults, — those great cracks along which the strata have 



been upheaved on one side, or thrown down on the 



other, to the height or depth of thousands of feet ; for 



since the crust cracked, and it makes no great difference 



whether the uphea\'al was sudden, or, as most geologists 



now believe, was slow and effected by many starts, the 



surface of the land has been so completely planed down 



that no trace of these vast dislocations is externally 



visible. The Craven fault, for instance extends for 



