IMPERFECTION OF GEOLOGICAL RECORD 255 



enormously long blank periods. So that the lofty pile 

 of sedimentary rocks in Britain, gives but an inadequate 

 idea of the time which has elapsed during their 

 accumulation ; yet what time this must have consumed ! 

 Good observers have estimated that sediment is de- 

 posited by the great Mississippi river at the rate of 

 only 600 feet in a hundred thousand years. This 

 estimate has no pretension to strict exactness ; yet, 

 considering over what wide spaces very fine sediment 

 is transported by the currents of the sea, the process 

 of accumulation in any one area must be extremely 

 slow. 



But the amount of denudation which the strata have 

 in many places suffered, independently of the rate of 

 accumulation of the degraded matter, probably offers 

 the best evidence of the lapse of time. I remember 

 having been much struck with the evidence of denuda- 

 tion, when viewing volcanic islands, which have been 

 worn by the waves and pared all round into per- 

 pendicular 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 still more plainly told 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, the surface of the land has been so 

 completely planed down by the action of the sea, that 

 no trace of these vast dislocations is externally visible. 



The Craven fault, for instance, extends for upwards 

 of 30 miles, and along this line the vertical displace- 

 ment of the strata has varied from 600 to 3000 feet. 

 Prof. Ramsay has published an account of a downthrow 

 in Anglesea of 2300 feet ; and he informs me that he 

 fully believes there is one in Merionethshire of 12,000 

 feet ; yet in these cases there is nothing on the surface 

 to show such prodigious movements ; the pile of rocks 

 on the one or other side having been smoothly swept 

 away. The consideration of these facts impresses my 



