376 Transactions of the South African Philosophical Society. 



Karroo and Gouph, districts wliich lie between 1,200 and 4,000 feet 

 above the sea; they then traverse the Zwartebergen, a range of 

 v^r^hich the average height is about 5,000 feet above the sea, and after 

 passing through a mountainous tract between the Zwartebergen and 

 Langebergen they traverse the latter range. 



It is clear that some explanation of this arrangement must be 

 found, for the rivers might have been expected to avoid the Zwarte- 

 bergen and run eastwards to the ocean along the country of 

 moderate elevation that lies north of this mountain belt. 



To enable a further discussion of the river system to be made, 

 some important points in the geological history of the country 

 between the watershed and the coast must be considered. 



It is now known that the anticlines of the Zwartebergen, and in 

 all probability all the other great anticlinal folds which lie east and 

 west on the south of the Karroo, owe their origin to movements in 

 the earth's crust which took place after the lower parts of the rocks 

 of the Karroo System were deposited : this fact is proved by the 

 occurrence of these rocks to the south of the Langebergen between 

 Worcester and Eobertson, where they are involved, together with the 

 Cape System, in the folds produced by those movements, and by the 

 presence of outliers of the lowest division of the Karroo System 

 between the Zwartebergen and Langebergen. 



It will be convenient to call the movements of the earth's crust 

 which folded the rocks in the south of the Colony the Zwartberg 

 movements ; the folds produced by this great crumpling trend 

 nearly east and west. It is important that they should be clearly 

 separated in our minds from the movements which gave rise to the 

 Cederberg anticline near the west coast. The Cederberg anticline 

 trends some degrees west of north, and was in part produced 

 somewhat earlier than the Zwartberg folds, for the Karroo System 

 transgresses unconformably over the rocks affected by the Cederberg 

 movements, but lies conformably upon the rocks affected by the 

 Zwartberg movements. 



It is certain that the Zwartberg movements were completed 

 before the deposition of the Uitenhage Series, for the latter rests 

 uncomformably upon the rocks disturbed by those movements ; 

 whether the Zwartberg folds were produced during or after the 

 deposition of the upper parts of the Karroo rocks is an unsettled 

 question, but it is not unlikely from the fact that the dolerite sheets 

 and dykes so abundant in the Karroo do not occur in the districts 

 where the rocks have been affected to an appreciable extent by the 

 Zwartberg movements, that these movements took place during or 

 shortly before the intrusion of the dolerite. This relationship 



