ECONOMIC GEOLOGY 477 



variably the flakes are bent or crumpled. The granites 

 have been considerably affected by earth-movements 

 subsequent to the formation of the pegmatite veins and 

 the mica crystals have in consequence been distorted. 



Molybdenum. Molybdenite, molybdic sulphide, has 

 been found in the granite near Helderberg (Stellenbosch) 

 in quartz veins associated with mispickel, pyrites and 

 chalcopyrite ; also in Namaqualand. 



Nickel Nickel has been found in the Mount Ayliff 

 District in Griqualand East in association with copper 

 (q.v.). 



Petroleum. Indications of mineral oil are not uncom- 

 mon in the northern portion of the Karroo ; the most 

 important feature about their mode of occurrence is 

 that the oily material is invariably found in association 

 with, and even in cavities or vesicles in, dolerite intru- 

 sions ; the same appears to be also the case in the 

 Orange River Colony. This at once suggests that the 

 oil is a secondary product due to distillation by the 

 dolerite, at the time of its intrusion, of coal or carbona- 

 ceous shale below the surface. In Northern Cape 

 Colony such a source for the hydrocarbons is found in 

 the "White Band" of the Dwyka series, for, as has 

 been stated on page 188, this horizon consists of very 

 impure oil shales, the amount of carbonaceous matter in 

 some cases rising to 12 per cent. ; moreover, dolerite 

 has very commonly been injected along these shales. 



Some interesting occurrences of a similar nature 

 have been found in the Stormberg area. For example, 



