Page - 338 - in The Origin of Species

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-