THE INTRUSIVE DOLEBITES AND ALLIED ROCKS 2G9 



in Fig. 21. As an example of one of the basin-shaped 

 valleys produced by the denudation of this sheet, the 

 Bongolo Valley, close to Queenstown, may be described. 



It is about eight miles in diameter and hemmed in 

 by a nearly circular ridge of dolerite ; the drainage 

 passes through a narrow gap in the south side of the 

 ring. Seen from the outside the Bongolo basin appears 

 as a ridge from 600 to 1,500 feet in height made of nearly 

 horizontal sandstones and shales crowned by a steep 

 palisade of rudely columnar dolerite. In the interior 

 the rim of the basin is formed by a smooth slope of 

 dolerite with an inclination of from 15 to 45 degrees ; 

 the igneous rock dips below the centre of the basin 

 which is occupied by sandstones and shales. The 

 general appearance of one of these basins is very strik- 

 ingly like that of a recent volcanic crater. 



The variation in the level of the Glen Grey sheet, as 

 it is called, is often very considerable, even within a 

 very short distance ; for example, between Hangklip 

 and Lesseyton, a distance of not more than seven miles, 

 the drop is over 3,500 feet. 



The intrusive sills have often produced a certain 

 amount of faulting of the sedimentary rocks ; the amount 

 of the displacement in a few cases is as much as 400 

 feet. 



The sheets are in many cases over 500 feet thick, but 

 the Andriesberg in the south-west corner of Wodehouse 

 and the Wildschuts Berg on the western border of 

 Queenstown are crowned with horizontal masses of 

 dolerite 1,500 and 2,000 feet thick respectively. The 

 inclined portions of the sheets are, as a rule, thinner 



