i 1)8 Tkansactions. 



found ill considerable quantities in a coarse conglomerate on the 

 banks of the Vaal River. The stones in this conglomerate are all 

 water-worn and about the size of a hen's egg. They appear to 

 begin suddenly at Warrentou and Sixteen streams, and are found 

 along its course for a considerable distance. Probably the river 

 has cut into some mine similar to those at Kimberley, and the 

 diamonds have been washed out of it. It is worth noting that at 

 Warrentou this conglomerate is 60 or even 100 feet above the pre- 

 sent bed of the river. 



A very remarkable point about the occurrence of these mines 

 consists in their being distributed along a narrow belt of country. 

 This runs N.N.E. by S.S.W., and is about 80 miles long and 2 or 

 3 miles broad. Such a distribution may perhaps point to a line 

 of weakness, along which volcanic craters were formed. It is now 

 generally admitted that the diamond mines are simply volcanic 

 necks or pipes, and they appear to occur, so far as I could judge, 

 about the epoch of the Kimberley shales. 



The following sections shew the rocks encountered in the 

 shafts where records were kept : 



The sections of De Beer's are peculiarly interesting, as they 

 shew that the dolerite thins out as one proceeds away from the 

 mine. This is also shewn, though not so well, at the Central 

 Mine, as the bed of dolerite there is 6 feet thicker as exposed at 

 the edge of the mine than it is where encountered by the shaft at 

 some little distance from the edge. This thinning out of the 

 dolerite, and especially its upward course from the mine, as well 

 as the fact that it did not extend over the blue ground, tend to 

 prove that it proceeded from the openings now filled by diamondi- 

 ferous earth. The black shale below is also hardened (as one 

 would expect), though I could not see the junction to tell if there 

 was a special hardening there. I may mention here that the edges 

 of the black shale are in the Central and Du Toit's Pan inclined 

 upwards at an angle of some 4-5°. This is clearly shewn also at 



