THE GENESIS OF THE DIAMOND, 199 



and the heavy minerals which are associated Avith them. In the 

 machines in use on the diamond fields we imitate nature in concen- 

 trating the heavy minerals. The light particles are washed away, 

 leaving the concentrates behind. 



The Vaal River diamonds did not have their origin in the Kim- 

 berley mines. The occurrence of well rounded and at times polished 

 l)owlders and small pieces of rock in these mines is proof that other 

 than igneous action was necessary to produce them. 



Professor Bonney says that " the idea that they have been rounded 

 by a sort of cup-and-ball game played by a volcano may be dismissed 

 as practically impossible."" He refers to the Dwyka conglomerate 

 bed as a possible source of these bowlders. A conglomerate bed' com- 

 posed of fragments of quartz, feldspar, chert, shale, quartzite, quartz 

 2)or])hyrv, and other rocks exists in the Kimberley strata between the 

 shale and the melaphyre (or olivine-diabase of Stelzner), and is 

 between 300 and 400 feet below the surface. This conglomerate is 

 from 3 to 10 feet thick, as determined in the various shafts in the 

 Kimberley mines. The rounded stones in the mines did not come 

 from this bed, and are Avholly unlike the stones in the conglomerate. 

 Personally I do not favor the cup-and-ball theory, and would not 

 give it a "second thought were it not for the fact that the diamond- 

 bearing ground as it is found in the mines shows such a mixture with 

 the country rock (shale) that some process of nature must have 

 stirred up and thoroughly mixed the contents of these great craters. 

 I can not comprehend how this result could have been brought about 

 in an igneous volcano. There would have been overflows of the 

 diamond-bearing rock which would have been found in the vicinity 

 of the mines. No such deposits have been foinid, and I do not be- 

 lieve that they exist. 



It is much easier to reconcile existing conditions to the aqueous 

 or mud -volcano theory (especially if the mud was accompanied by 

 large quantities of gases which, on nearing the surface, escaped while 

 the mud receded) than to an igneous theory. 



There must have been innumerable eruptions and explosions to 

 account for the inclusion of the surface shales and fragments of the 

 country rock in the diamond-bearing peridotite. The frequent occur- 

 rence of these eruptions Avould, in a measure, solve the problem as to 

 the manner in which the fragments of rocks var3nng in size from 

 pebbles to bowlders, some with polished surfaces, became, as it were, 

 waterworn. 



The evidence of the movement of the diamond-bearing rock after 

 solidifying is indicated by the slickensides and striated surfaces of 

 the country rocks at their junctions or contacts with the kimberlite. 



o Proceedings of the Royal Society, vol. Ixv., 1899. 



