The Forming of the Drahensberg. 67 



It is interesting to note that in India at this same period the lava- 

 flows of the Eajmahal Hills were being poured out. It is interesting 

 again because all over the globe the Jurassic epoch was one during 

 which volcanic activity seems to have been remarkably rare. 



V. — Later History of the Area. 



No sooner had the piling up of the lavas ceased than denudation 

 began to remove the uppermost layers, and small rivers coursed 

 over the plains of volcanic rock. 



It is possible that the mass of lavas was thicker in the centre, 

 thinning away at the sides, so that the formation of the watershed 

 might date from this time. On the other hand, it is equally possible 

 that the watershed was produced at a somewhat later stage. 



Of the physical conditions in the western portion of the Karroo 

 we can only hazard a guess, but it is possible that there was already 

 in that part of the country a well-established river system. 



The folding, which was still going on in the south, gradually 

 extended northwards, and the land surface was thrown into a 

 series of nearly parallel troughs and ridges trending nearly east- 

 north-east. 



The flow of water was naturally along these troughs, and hence 

 the direction of the courses of the Orange River, Kraai River, and the 

 head waters of the Tsomo, Tsitsa, and Kenigha Rivers. 



Apparently, just after these gentle foldings had affected the rocks, 

 the Karroo Dolerites were intruded, invading and penetrating the 

 strata from Natal to Namaqualand, so that dykes of dolerite are even 

 found cutting through the lava-flows. 



It has been suggested that the effect of the dolerite intrusions 

 would be to cause a heating of the strata so that the central portion 

 of the Cape Colony would tend to bulge upwards, but I think that 

 the elevation was produced by the forces which continued to affect 

 the level of the land long after the intrusion of the dolerites, even 

 down to comparatively recent times. 



After these gigantic intrusions had ceased the strata were affected 

 by a series of gentle north and south folds which, to a certain extent, 

 modified the existing river-systems, but those rivers which had 

 already excavated deep valleys were unaffected by the changes of 

 level. The courses of the Indwe, Tsomo, Bashee, and Kenigha 

 Rivers, the Sterk Spruit (Barkly East), Kornet Spruit and Wasch- 

 bank River are thus accounted for. 



Owing to the cross-folding, we get rivers flowing along the dips 

 produced by the compounding of these two sets of folds ; for 



