386 Transactions of the Soutli African Philosophical Society. 



modus operandi of mountain building by studying how these small 

 grains or pebbles respond to stresses. 



The first and most important agent in deforming rocks is pressure, 

 either lateral, due to tangential stresses, or vertical, due to the dead 

 weight of material. 



The usual method of testing the strength of rocks is to subject 

 a small block of the substance to immense pressure in a crushing 

 machine, and noting the number of kilograms to the square centimetre 

 required to break it. The following figures " will give some idea of 

 the weight required to break a small cube under ordinary atmospheric 

 conditions : — 



Sandstone, from 120 to 897 kilograms. 

 Granite, from 1,234 to 2,036 

 Felsite, from 2,092 to 3,026 



Knowing the specific weight of the various rocks, it is very easy 

 to calculate the greatest height of a column whose foot can still bear 

 the weight of the superincumbent mass without breaking. We 

 find the following are the heights of these columns : — 



Sandstone, from 620 to 4,400 metres. 

 Granite, from 4,300 to 7,600 

 Felsite, from 7,800 to 11,000 



If a column of rock of the particular specific gravity and crushing- 

 strength, were to be made higher than these figures, the foot of 

 it w^ould give way. In Nature, however, the rocks are permeated 

 with water, and it is a remarkable fact that the crushing strength is 

 always less, sometimes considerably less, when the specimen tested 

 is soaked in water ; for instance, in the particular rocks taken in the 

 above table the differences are : — 



Sandstone, 58 kilograms, or 6% less weight required. 

 Granite, from 7 to 54 kilograms, or -06 to 2% less. 

 Felsite, from 141 to 934 kilograms, or 7 to 13% less. 



I shall return to this point afterwards ; what I want to get at now 

 is the height of a column of rock which, under natural circumstances 

 in the earth's crust, would, by its own weight, crush the rock at the 

 foot. The water in the interstices would buoy up the rock, and the 

 effective weight of the column would be, not the weight of the 

 material in air, but in water. Taking the new crushing values, 



* These figures are taken from Hermann's " Steinbruchindustrie Sachsens," 

 Berlin, 1899. 



