An Unrecognised Agent in the Deformation of Bocks. 387 



and dividing by the specific gravity minus 1, we get the following 

 figures : — 



Sandstone, from 1,280 to 8,100 metres, or from "7 to 5 



miles high. 

 Granite, from 6,700 to 12,000 metres, or from 4-1 to 7-5 



miles high. 

 Felsite, from 11,700 to 15,700 metres, or from 7 to 9 miles 



high. 



From this we see that most sedimentary rocks would be unable 

 to support their own weight beyond five miles deep from the earth's 

 surface, while this limit is somewhat deeper in the case of the more 

 compact crystalline rocks. Hence, from the consideration of the 

 crushing strength of rock alone, we get the fact that there is a zone 

 from five to seven miles within the earth's surface in which the weight 

 of the superincumbent material is greater than the cohesion of the 

 constituent particles of the majority of rocks, or in other words, we 

 get deformation of mass at those depths. 



It is a matter of common observation that the actual zone 

 of mass deformation is very much nearer the surface. The 

 creep is mine-levels, where the rock not only bulges in from 

 above, but rises up from below, is known from most deep workings, 

 and in the case of deep bore-holes, a cylinder of rock of the section of 

 the bore gradually rises from the bottom. It is a matter of surprise, 

 however, that the rock in deep gorges does not tend to creep in and 

 close the guUey. In the Zwarteberg range, the valleys of the 

 Buffel's Kiver, Gamka Eiver and Meiring's Poort Eiver are from 

 three to five thousand feet above the level of the stream, which cuts 

 right through the very centre of the range. The crushing strength 

 of some sandstones is far below that requisite to support this weight, 

 and no doubt the soft bands of rock in these Zwarteberg gorges have 

 been squeezed out towards the bottom of the kloof, but just because 

 they are soft rock, the weathering has the more easily attacked 

 them, and hence any squeezing out of the softer layers would 

 be quickly removed by denudation. For the hardest sandstones, 

 however, the limit is far below that requisite to crush the rock, and 

 as the Zwartebergen for the most part consists of hard quartzite, 

 this would stand a crushing strain equal to 10,000 to 12,000 feet of 

 its own material. This line of inquiry is interesting in another way, 

 as it indicates there must be a limit to the height of mountains 

 on this globe, and any theory that brings in a probable existence of 

 mountains of very much greater height than the highest we now 

 have must be a false theory. Dr. Croll, in his works on the glacial 



