ECONOMIC GEOLOGY 469 



rock, especially as the intrusion is stated to contain a 

 rhombic pyroxene and therefore to be allied to the 

 norites. The occurrence is of interest in that the ore 

 contains a fair proportion of nickel, as well as a certain 

 amount of platinum. 



At Bekker's Kloof near Cradock there is a vein in 

 dolerite which carries several pennyweights of gold per 

 ton in association with silver and copper. 



Diamonds. The diamondiferous deposits can be divided 

 into two totally distinct classes according as the gem 

 occurs (a) in pipes and fissures filled with blue-ground 

 (kimberlite), or (b) in river-formed gravels. 



(a) The former have already been discussed in chap- 

 ter xi., and to that account there is very little more to 

 add. It has been pointed out that in only an extremely 

 limited proportion of pipes is the diamond present in 

 sufficient quantity to repay the cost of working. It 

 must be emphasised that whereas the minerals garnet 

 and ilmenite, popularly known as " ruby " and " carbon," 

 and mica are essential constituents of the blue-ground 

 everywhere, though in varying proportion, there is 

 strong reason to believe that such is not the case with 

 the diamond, and that the occasional presence of the 

 latter must be considered accidental. There are certain 

 pipes, too, in which the diamond is sparingly present, 

 but the kimberlite may be a "hardibank " and refuse to 

 disintegrate when exposed to the action of the weather. 

 Speaking roughly, this is more commonly the case in the 

 smaller mines. The kimberlite fissures are still less 

 promising in these respects, and only rarely contain 



