730 AIR. R. H. RASTALL ON THE GEOLOGY OF [Nov. I9II, 



regarded as a fracture aod depression occurring in the triangular area 

 between two sets of folds at right angles one to the other. A very 

 similar phenomenon is the well-known area of the Warm Bokkeveld, 

 which is also a depressed region, and immediately north of it is a 

 small patch of Karroo rocks, let down by a fault on the north- 

 eastern side. This fault runs parallel with the Worcester Fault, 

 and is probably due to the same set of causes. 



These folds are obviously due to lateral thrust, since they are 

 clearly asymmetric, but it is still a matter of discussion from which 

 side the greater pressure came. Prof. Schwarz x has put forward 

 the view that the folding of these ranges is due to the intrusion of 

 the vast sheets of dolerite in the Karroo System, thus implying that 

 the maximum pressure came from the interior. This idea is in- 

 genious, but (so far as we know at present) inconsistent with the 

 field evidence. 2 The general structure of the region and the dis- 

 position of the folded chains can be much more easily explained on 

 the supposition of pressure acting from the exterior towards the 

 interior, and consisting of two sets of thrusts : — one towards the 

 north, which produced the east-and-west folds of the Langebergen 

 and other parallel ranges, the Zwartebergen folds of Rogers ; the 

 other towards the east, giving rise to the ranges of the west-coast 

 region, the Cederbergen folds. These two sets of thrusts interfered 

 mutually where they came in contact, in the south-western corner 

 of the Colony, thus producing the fan-shaped arrangement of 

 mountain-chains which is so characteristic a feature. It appears 

 that this fault was formed subsequently to the principal period of 

 folding, and it probably occurred when the pressure was released to 

 a certain extent, thus allowing of a great subsidence. 



This final, post-Enon, movement was evidently accompanied by 

 considerable earth-pressure, since the pebbles of the conglomerate 

 are commonly fractured and indented, and this process presumably 

 took place concurrently with the formation of the fault. 



To sum up : the dominant features of the geology of this region 

 are : — 



(«) The conspicuous folding, of post-Karroo age, in part pre- 

 Cretaceous : this led to the formation of mountain-chains in two 

 sets, east-and-west and north- and-south respectively, with a syn- 

 taxis in the south-western corner of Cape Colony. 



(6) A post-Cretaceous period of movement, which resulted chiefly 

 in a great subsidence to the south of a fault-line cutting obliquely 

 across the syntaxis; accompanying or preceding this were minor 

 lines of crumpling on the downthrow side of the fault, leading to 

 the formation of subsidiary folds striking north-east and south- 

 west, or thereabouts. It is impossible to say with certainty 

 whether the faulting was wholly subsequent to, or in part con- 



1 ' Causal Geology ' London, 1910, pp. 141-43 & fig. 10. 



2 A. W. Rogers & A. L. Du Toit, ' Geology of Cape Colony ' 2nd ed. (19G9) 



pp. 123- 24 & fig. 9. 



