470 GEOLOGY OF CAPE COLONY 



diamonds. From another point of view, however, the 

 kimberlite dykes are of value ; wells or bore-holes sunk 

 on them rarely fail to obtain supplies of water satis- 

 factory both as to the quantity and quality, and this is 

 especially valuable since they are numerous in an area 

 that has a poor water supply, and one in which dolerite 

 dykes are neither abundant nor satisfactory as water 

 carriers. 



(6) The workable diamondiferous gravels are con- 

 fined to the valley of the Vaal Eiver from Christiana 

 down to its junction with the Orange. Similar gravels 

 exist along the Orange at various points below Hope- 

 town, but they have in nearly every case been converted, 

 through the deposition of carbonate of lime, into com- 

 pact conglomerates. The same is the case to a con- 

 siderable degree with a patch of gravels on the Kaap 

 Plateau at Mahura Muthla between Vryburg and 

 Kuruman, 1 in which the diamond has been found. The 

 Vaal River gravels 2 are found at various levels, occa- 

 sionally as much as 400 feet above the bed of the 

 present river, and often at a distance from it of several 

 miles. 



The principal "diggings" are at Klipdam and Wind- 

 sorton and between Barkly West and Delport's Hope. 

 In spite of the erratic yields and the smaller size of the 

 stones there is a fair output annually, the " river stones," 

 as they are called, being on the whole of a better quality 

 and commanding a much higher price than those from 

 the pipes. Much of the deposits has been worked over 

 more than once, but there still remain untouched 

 1 G, C., xi., pp. 76-77. 3 Ibid., p. 171, 



