280 AGE OF THE INTRUSIONS 



the eruption of the volcanic rocks, yet nevertheless there 

 seems to be a very close connection between the two 

 groups of rocks, indicating a common origin ; the one 

 consolidated below ground and the other at the surface. 



It is difficult to form a satisfactory estimate of the 

 thickness of rock cover at the time of the intrusion of 

 any particular sheet, but a minimum estimate can be 

 made in the case of some of the sheets under the 

 Drakensbarg in Elliot. The dykes here ascend to an 

 altitude of about 8,000 feet above sea level, while sheets 

 and dykes are exposed on the low ground not far away at 

 an altitude of about 3,500 feet, e.g., Lady Frere and Eng- 

 cobo. The thickness of cover must have been at least 

 4,500 feet without reckoning the thickness of volcanic 

 beds subsequently removed by denudation. 



The question of the origin of the dolerite intrusions 

 and the means whereby they were able to force their 

 way between and through the sedimentary rocks is 

 at present beyond our knowledge. During the Storm- 

 berg period there must have been an enormous mass of 

 basic material lying at an unknown depth beneath the 

 surface of the South African region ready to burst its 

 bonds and rise towards the surface when favourable 

 conditions prevailed. What these conditions were is at 

 present a subject for speculation rather than for state- 

 ment. It may be noted that the mountain building in 

 the south and south-west had probab]y then reached or 

 passed its maximum , and that the great forces exerted 

 cannot but have influenced the fluid or potentially fluid 

 rock-magma. The avoidance by the dolerites of those 

 portions, which were being compressed and folded, 



