THE DWYKA FORMATION 411 



dolomite, and these followed unconformable by the Dwyka — all as iden- 

 tified b}^ Doctor Hatch — while the Ecca strata are not far away. It is 

 thus seen that the streams hereabout are cutting through the general 

 cover of Dwyka and Ecca and making a beginning of superposition on 

 the unconformably underlying formations. This process has apparently 

 been advanced to a later stage some 40 miles farther south, where the 

 Yaal river wanders most irregularly on a tilted series of older formations, 

 the Karroo beds that once presumably covered the older rocks in that 

 district having been widely worn off. 



The coal seams of the Ecca are extensively mined at and near Yereeni- 

 ging; one of the fine grained Ecca sandstones is quarried for building 

 stone and is found to contain good prints of ferns (Glossopteris). A 

 shaft is sunk in the Dwyka, and the clay of the tillite, here light bluish 

 gray and imperfectly indurated, is brought up for use in making fire- 

 brick. The waste heaps near the shaft contain many beautifully striated 

 stones of typical glacial form. 



The chief result of this day's excursion was the larger view that we 

 gained of the pre-Dwyka conditions in the northern part of the Dwyka 

 area. The harder members of the Pretoria series northwest of Yereeni- 

 ging rise in rounded hills 300 or 400 feet over the peneplain — one might 

 well say plain — of erosion that has been broadly developed on the weaker 

 beds of the Karroo series. A stronger relief than this must have pre- 

 vailed when the Dwyka was forming, for the hills lost some of their orig- 

 inal height during the accumulation of the later members of the heavy 

 Karroo series, which probably once rose hundreds of feet above the present 

 plain, as well as during the subrecent removal of these higher members; 

 and, moreover, the base of the pre-Dwyka hills is not now seen, being 

 buried under so much of the Karroo series as still remains. Yet the 

 manner in which the pre-Dwyka hilltops come to sight where the streams 

 cut through their Karroo cover suggests very strongly that the pre-Dwyka 

 relief was of rather well subdued form. This is a matter of interest as 

 bearing on the climatic conditions under which the Dwyka ice-sheet was 

 formed. 



The glaciated DwyTca floor at Riverton and Kimberley. — My last sight 

 of the Dwyka was in an excursion energetically planned by Penck for a 

 small party to Riverton, on the Yaal river, about 15 miles northward 

 from Kimberley. The heavy diabase of one of the pre-Karroo formations 

 here appears along the river bank and in a small island within the chan- 

 nel, showing a well rounded, grooved, and striated surface, with patches 

 of typical Dwyka still clinging to it here and there. This important 

 locality was first discovered bv Stow in 1880. I estimated that we could 



