338 ORIGIN OF SPECIES 



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 dif- 

 ference whether the upheaval was sudden, or, as most geolo- 

 gists 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 upwards of 30 miles, 

 and along this line the vertical displacement of the strata 

 varies from 600 to 3000 feet. Professor Ramsay has pub- 

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

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

 Merionethshire of 12,000 feet; yet in these cases there 19 

 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 

 sedimentary strata are of wonderful thickness. In the Cor- 

 dillera I estimated one mass of conglomerate at ten thou- 

 sand 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 thickness, from actual measure- 

 ment in most cases, of the successive formations in different 

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



Feet 



Palaeozoic strata (not including igneous beds) 57.154 



Secondary strata *3.i90 



Tertiary strata 2,240 



— making altogether 72,584 feet; that is, very nearly thirteen 

 and three-quarters British miles. Some of the formations, 

 which are represented in England by thin beds, are thousands 

 of feet in thickness on the Continent. Moreover, between 

 each successive formation, we have, in the opinion of most 

 geologists, blank periods of enormous length. So that the 

 lofty pile of sedimentary rocks in Britain gives but an inade- 

 quate idea of the time which has elapsed during their accu- 



