2GS THE LAPSE OF TIME. [Chap. X. 



left projecting. Nothing impresses the mind with the vast duration 

 of time, according to our ideas of time, more forcibly than the con- 

 viction 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 exten- 

 sive areas, and on the other hand the thickness of our sedimentary 

 formal ions. 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 for- 

 merly liquid state, showed at a glance how far the hard, rocky 

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

 lold 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 up- 

 heaval 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 

 i3 externally visible. The Craven fault, for instance, extends for 

 upwards of 30 miles, and along this line the vertical displacement 

 -of the strata varies from GOO to 3000 feet. Professor Ramsay has 

 published an account of a downthrow in Anglesea of 2300 feet; 

 and he informs me that he fully believes that there is one in Merio- 

 nethshire 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 esti- 

 mated one mass of conglomerate at ten thousand feet; and although 

 conglomerates have probably been accumulated at a quicker rate 

 than finer sediments, yet from beins 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. Pro- 

 fessor Ramsay has given me the maximum thickness, from actual 

 measurement in most cases, of the successive formations in different 

 parts of Great Britain ; and this is the result : — 



