the dwyka formation 409 



contrast strongly confirms the inference as to the direction of ice-motion, 

 as based on the form of the glaciated floor. We returned to Vryheit on 

 August 27 and to Johannesburg on August 28. 



The Dwyka at Vereeniging. — During the stay of the Association at 

 Johannesburg, Penck and I had, through the courtesy of Dr E. H. Hatch, 

 president of the Geological Society of South Africa, a most interesting 

 morning excursion on September 1 at Vereeniging, where the main line 

 of the Cape Town- Johannesburg railway crosses the Vaal river. Half a 

 mile northeast of the railway bridge there is an important section (figure 

 9) exposed in the river bank, which we saw very well as the river was 

 low at the time of our visit. A small patch of Dwyka is here exposed, 

 with a thickness of 15 or 20 feet, resting unconformably on a local out- 

 crop of the dolomite of the ancient Pretoria series and covered by the 

 horizontal coal-bearing Ecca beds, which occupy most of the country here- 

 about. We could not find any striation or grooving on the underlying 

 dolomite, but it is very probable that further search will be more success- 

 ful than ours. The Dwyka had numerous stones and boulders, up to 



Figure 9. — Local Section on the Bank of the Vaal Rirer at Vereeniging, Transvaal. 



Dolomite on the left ; Dwyka tillite in center ; coal, conglomerate, and shale of the Ecca 

 series on right. Length, about 400 feet. 



4 feet in diameter, many of them well scratched. In its upper part was 

 a layer of sandstone, dipping northeast 15 or 20 degrees. The contact 

 of the Ecca on the Dwyka was not seen, but the relative attitude of the 

 two formations suggested a slight unconformity, possibly due to the orig- 

 inally uneven or sloping surface of the Dwyka rather than to any signifi- 

 cant amount of deformation or erosion. The Ecca included a thin layer 

 of coal covered with two feet of ordinary conglomerate of waterworn 

 pebbles, followed by shales and sandstones. It should be mentioned that 

 this and other conglomerates in the Ecca have been described as "Dwyka" 

 by some observers, thus giving rise to the opinion that coal beds and 

 glacial deposits alternated in this district. Such a conclusion did not 

 seem to us well founded. It is quite possible that the pebbles in this bed 

 of Ecca conglomerate came from the Dwyka sheet near by, but there 

 appears to be no other relation than that between them. The use of the 

 term "conglomerate" as a name for the unstratified sheets of Dwyka, with 

 their blunt-ended, scratched stones, as well as for the layers of normally 

 waterworn pebble beds in the Ecca and other formations, may have con- 

 tributed to the misunderstanding whereby glacial action and coal forma- 



XXXVI— Bull. Geol. Soc. Am., Vol. 17, 1905 



