THE IMPERFECTION OF THE GEOLOGICAL RECORD 273 



posed, having resisted suba^rial denudation better than the sur- 

 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 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 been so completely planed down that no trace of these 

 vast dislocations is externally visible. The Craven fault, for in- 

 stance, extends for upward of thirty miles, and along this line 

 the vertical displacement of the strata varies from 600 to 3,000 

 feet. Professor Ramsay has published an account of a down- 

 throw in Anglesea of 2,300 feet; and he informs me that he fully 

 believes that there is one in Merionethshire of 12,000 feet; 

 yet in these cases there is nothing on the surface of the land to 

 show such prodigious movements; the pile of rocks on either side 

 of the crack having been smoothly swept away. 



On the other hand, in all parts of the world the piles of sedi- 

 mentary strata are of wonderful thickness. In the Cordillera, I 

 estimated one mass of conglomerate at 10,000 feet; and although 

 conglomerates have probably been accumulated at a quicker 

 rate than finer sediments, yet from being formed of worn and 

 rounded pebbles, each of which bears the stamp of time, they 

 are good to show how slowly the mass must have been heaped 

 together. Professor Ramsay has given me the maximum thick- 

 ness, from actual measurement in most cases, of the successive 



