TOPOGRAPHY AND CLIMATE IN DWYKA TIME 415 



been created, for the folding that produced them included the Dwyka 

 along with the earlier rock series. The mountains of much earlier forma- 

 tion, indicated b}' the deformation of the Pretoria (Potchetstroom) sys- 

 tem in the Transvaal, had, as already shown, been nearly obliterated be- 

 fore Dwyka time opened. Not only so; the subdued remains of these 

 earlier mountains had been extensively covered with the barren Water- 

 berg sandstones (the supposed northern equivalent of the Table Moun- 

 tain sandstones) in the northern Transvaal before Dwyka time opened, 

 and the horizontal attitude still, as a rule, preserved by the Waterberg 

 series north of the Dwyka area (see Hatch and Corstorphine, pages 180- 

 182), demonstrates that no renewal of mountain-making deformation had 

 taken place in that neighborhood — the apparent source of the Dwyka 

 ice — before the ice-sheet was formed. The observations of Bornhardt 

 (460) in German East Africa and of Passarge (594) in the Kalahari 

 point to the same conclusion. Uplifted areas there must have been, 

 however, somewhere in South Africa then or soon afterward, in order 

 to provide the great bodies of sediment deposited in the heavy Karroo 

 series. 



CLIMATIC CONDITIONS OF DWYKA TIME 



Change of temperature. — If it has been correctly inferred that the 

 Dwyka area and the area next north of it was a region of moderate alti- 

 tude and on the whole of moderate relief in Dwyka time, and that it occu- 

 pied then, as now, a latitude only a few degrees outside of the torrid zone, 

 it follows that the cause of the Dwyka ice-sheet must be searched for in a 

 general lowering of terrestrial temperatures. In no other way can a 

 sufficient snowy precipitation on a lowland in latitude 25 degrees be pro- 

 duced. The reasons that lead to this conclusion are as follows. 



The Dwyka area today occupies the outer part of the southern trade 

 wind belt. It is not reached by the winter rains of the southern sub- 

 tropical belt, which do not extend farther than the southern coastal border 

 of Cape Colony. The interior receives its rainfall chiefly from disturb- 

 ances in the trade wind belt during the summer season. The winters 

 there are prevailingly clear and dry, because of the presence of a seasonal 

 area of high pressure, shown in Bartholemew's Meteorological Atlas as a 

 part of the southern belt of high pressure that encircles the world, in- 

 tensified over the land areas in the colder season. This is an extremely 

 important point in connection with the study of Permian climatic condi- 

 tions, for it shows that the dryness of the winters and the rains of the 

 summers are inseparably associated with persistent elements of the terres- 

 trial wind system. There appears to be no means of modifying this ar- 



