1886.] Diamond-Mines of South Africa. 65 



outlet given, the whole mass striving for extension, and being only of 

 a very low elasticity, has balanced this pressure by forming a flattened 

 cone. 



For better demonstration of this phenomenon, I may mention an 

 experiment, which is well known to all of us, at least to our lady 

 visitors, although I do not think that the fair experimenters ever 

 became aware of the bearing of their observation. The experiment I 

 speak of, is the baking of a cake in a mould. The bottom and walls 

 of the mould correspond to the underlying blue and the surrounding 

 rocks of the mine, and the crust which is formed at the beginning of 

 the baking process is analagous to the limestone layer which covered 

 once the mine. In both cases there remains inside a mass, elastic to 

 a certain degree, striving for extension and — just as the crust of the 

 cake is almost always lifted at first in the centre, very often cracking 

 thence in several directions — so the decomposing ground of the Kim- 

 berley mine formed the little hillock, the late Colesberg kopje. 



It might be of some interest to estimate the height of this hillock. 

 The increase of volume was found to be w X 25 X f cubic feet. This 

 forms a cone, the base of which is n square feet, its height x feet. 

 As the formula for the contents of a cone is <? = ^-^, we come to the 

 following equation: ^x25xf«=wX f that is, £c= 50. That means 

 to say, the top of the formed cone would have been 50 feet above the 

 surrounding country, if all had happened in a mathematically correct 

 way. The actual height of the hillock was, however, only twelve or 

 fifteen feet, which is not at all surprising, as the conditions under which 

 the formation took place are very different from those to which the 

 blue ground is exposed on the depositing floors, so that the increase of 

 volume will have taken place only in a smaller degree. 



I hope to have succeeded in showing that the little hillock on the 

 top of the mine is not necessarily of volcanic origin, that it is not a 

 cone of ejection, but that it is the result of the forces which were 

 dormant in the blue, and have been set free only to a small extent 

 during the hundred thousands of years elapsed since the time when 

 Griqualand West and the surrounding countries were yet covered by 

 the waters. 



