The Volcanoes of Grlqualand East. 109 



the south-east side the rivers are crowded together, and their 

 arrangement is quite different from any other group of principal 

 rivers in South Africa ; they bear evidence of having been deflected 

 from their natural course, which was to the south-east, and having 

 been forced to flow across their natural water-parting. The case of 

 the Caledon Kiver, which flows practically on the watershed, is par- 

 ticularly interesting. I think the map showing the courses of the 

 rivers affords very satisfactory evidence of the volcanic range having 

 been thrown across the primitive river system, which means that 

 the volcanic outbursts occurred after that system had been developed, 

 and that the new land of the Karroo, formed off the shores of the 

 old continent, was already dry land at the time. We know from 

 evidence we can get from the Western Province that this river 

 system was instituted in Jurassic times, so here we have a link 

 in the chain of proof that these volcanoes were Upper Jurassic or 

 Cretaceous. 



The second step deals with the deposition of the sediment from 

 the Dwyka Conglomerate upwards, and for this we only have a 

 knowledge of the distribution of a small part of the Cape Colony, 

 and a still less complete knowledge of the distribution in the 

 Transvaal. The Dwyka Conglomerate in the Colony is a deposit 

 containing glaciated or scratched boulders of a number of peculiar 

 types, all of which have now been traced to their origin in the 

 north-west of the Colony, Bechuanaland, and the Transvaal. From 

 which it follows that the glaciers which carried these boulders 

 flowed from the north-west. Mr. Eogers and myself have found 

 in the Colony the actual beds of these glaciers, and have seen the 

 old land- surface over which they flowed. The course of the 

 glaciers was to the south, between S.S.W. and S.S.E., and the 

 old shore-line ran through Prieska in a direction about N.E., 

 passing west of Johannesburg and near Mafeking. Dr. Molengraaff, 

 in Vrijheid in the Eastern Transvaal, has found a similar land- 

 surface, over which the ice moved in a north-westerly direction, 

 so here we have evidence of two shore-lines, separated by the 

 width of the Transvaal, which in Permian times was occupied by 

 an inland sea,'-' 



In the south of the Colony the Dwyka was not deposited on a 

 land-surface, but the mud was laid down under water and the boulders 

 dropped into it from the floating ice that broke off the ends of the 

 glaciers. In this way we are able to get an idea of the trend of 



* The conglomerate in Natal, to the east of the Vryheid occurrence, was also prob- 

 ably formed on land, which leads one to suspect that the country from which the 

 Vryheid glaciers flowed was an island. 



