SOME ASPECTS OF GEOLOGIC TIME 255 



Other evidence of long-continued erosion is found in the 

 existence of " faults." In times of terrestrial upheaval, the crust 

 of the earth has been twisted in all directions. Strata, laid down 

 horizontally in the bed of the ocean, are upheaved into gigantic 

 folds. Locally, the series will break. Younger strata, in the 

 course of time, will be thrust upwards over older formations, 

 and the consequent " faults " often imply a vertical displacement 

 of many thousands of feet. Where, as is usually the case, the 

 fault has been subject to subsequent erosion, so that there is 

 no trace of it in the conformation of the country, and its presence 

 is only indicated by the juxtaposition of strata of different ages, 

 we have definite evidence of prolonged denudation. The depth 

 of the fault is shown by comparing the structure of the strata on 

 opposite sides, and we are able to infer that the total erosion 

 has been much greater than the thickness of the fault. The 

 ground on the lower side must also have been eroded, and the 

 depth of the fault merely shows the excess of the erosion of the 

 upper over the lower levels. 



One striking example we owe to the researches of Prof. Judd. 

 He has shown that, at Movern in Scotland, since the Miocene 

 epoch, a fault of no less than 2,000 feet has been formed, and the 

 upper side has been denuded so that Miocene basalts lie against 

 Silurian gneiss. Assuming the erosion on the upper side of the 

 fault to be twice as rapid as on the lower side, 4,000 feet will 

 have been removed. At Prof. Sollas' rate of denudation, this 

 would take more than 10,000,000 years. Allowing every possible 

 weight to the advocates of a minimum of geologic time, we could 

 indicate a minimum of 5,000,000 years for Pleistocene, Pliocene, 

 and a small fraction of the Miocene. 



Many other instances have been brought forward by the late 

 James Croll. Near Dunbar, there is a fault of no less than 

 15,000 feet, eroded between the Silurian and the Carboniferous. 

 In the Appalachians, a region has been eroded to the extent of 

 no less than 35,000 feet. Nearly 10,000 feet of strata have 

 been removed between the Millstone Grit and the Permian. 



Present knowledge, as yet, does not allow us, from such data 

 as these, to make definite numerical conclusions, but here is a 

 method of research which should be developed by geologists. 

 If they can first find the rate of erosion under a great variety of 

 conditions, and then discover the extent of erosion and the con- 

 ditions under which it took place in particular instances, during 



