THE INTRUSIVE DOLERITES AND ALLIED ROCKS 283 



while thin intrusions of the igneous rock are altered to 

 the material known as " white-trap ". The hydrocar- 

 bons produced by the distillation of coal and carbonace- 

 ous shale may in part enter the molten rock and be 

 retained in it upon its solidification. It is therefore 

 not unusual to find in a dolerite intrusion traversing 

 carbonaceous shales, cavities or vesicles containing oil 

 or tarry matter ; the sediments at the contacts may also 

 be impregnated with bituminous material. 



The numerous petroleum " indications " in Central 

 Cape Colony are almost without exception associated 

 with dolerite intrusions and seem to be due to the de- 

 structive distillation of carbonaceous Karroo shales, 

 notably the " White band " of the Dwyka series. It is 

 therefore a matter of regret that so much capital and 

 labour is being spent upon these indications, owing to 

 the true nature of the occurrences not being realised. 



A dolerite intrusion not infrequently produces a 

 columnar structure in the adjoining sedimentary rock. 

 A very marked example of this is shown in Plate XX., a 

 view of the junction of a thick dolerite sheet with the 

 Dwyka tillite on the farm DwdS Douw in the Doom 

 Eiver Valley, Calvinia. The rough-looking rock in the 

 upper part of the cliff is the dolerite, and the columnar 

 rock forming a vertical face fifteen feet high is the 

 tillite. The lower end of the columnar layer is sharply 

 marked, and below it the tillite is the usual sandy mud- 

 stones containing numerous boulders of many varieties 

 of rocks. The photograph was taken at too great a 

 distance from the cliff to allow the boulders exposed on 

 the joint faces to be seen. The joints that divide the 



