186 SOURCE OF BOULDERS 



appears therefore to have spread outwards from a centre 

 either in the Northern Transvaal or in Rhodesia. 



The boulders in the tillite of the Transvaal and of 

 Natal confirm the evidence afforded by the glacial 

 striae, for the majority of the inclusions can be matched 

 with rocks found in situ in the Central Transvaal. In 

 Pondoland, while various kinds of granites, gneisses, 

 diabases, quartzites, etc., are abundant as boulders, the 

 jaspers and banded ironstones of the Griqua Town 

 series, which form a small but interesting part of the 

 inclusions in the west and south of the Karroo, are un- 

 represented. 



In Griqualand West the majority of the boulders are 

 diabases and amygdaloids from the Ventersdorp system, 

 a formation which covers an immense area in the South- 

 western Transvaal. With them are quartz-porphyries 

 like those at Klerksdorp, and red granite identical with 

 that of the Bushveld. Wherever the ice has passed 

 over and glaciated an outcrop of a rock with easily 

 recognisable characters the tillite immediately to the 

 south-west contains abundant inclusions of the rock in 

 question. Thus at Delport's Hope the Dwyka is full 

 of angular and partially rounded masses of Black Reef 

 quart zite derived from an outcrop a little to the north. 

 For miles to the south-west of Klokfontein, near Modder 

 River, boulders of rhyolite from that locality are recog- 

 nisable in the tillite. South-west of the Doornberg in 

 Prieska boulders from the Griqua Town beds are 

 abundant, while near Van Wyk's Vlei inclusions from 

 the Matsap beds first make their appearance. 



Serpentines, found in the tillite west of Calvinia, are 



