60 THE ORIQII^ OF SPECIES 



face consequently has been gradually lowered, with the 

 lines of harder rock left projecting. Nothing inipresses 

 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 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 upheaval was 

 sudden, or, as most geologists now believe, was slow and 

 effected by many starts, the surface of the land has beenj 

 so completely planed down that no trace of these vast 

 dislocations is externally visible. The Craven fault, for 

 instance, extends for upward of 30 miles, and along this! 

 line the vertical displacement of the strata varies from 

 600 to 3,000 feet. Professor Ramsay has published an 



