THE KARROO SYSTEM 183 



At several places in the west and south of the Karroo 

 thin beds and thick lenticular masses of white quartzite 

 occur in the tillite. Near Matjesfontein Station several 

 large lenticles of quartzite lie on one horizon ; they are 

 roughly bedded and their dip is the same as that of the 

 enclosing tillite. In the Ceres Karroo near Beukes 

 Fontein, there are also several quartzite lenticles like 

 those at Matjesfontein, but the quartzite is gritty 

 and yellowish in colour, and at its periphery contains 

 boulders. These lenticles are very probably analogous 

 to the interbedded deposits of sands and gravels in the 

 till of the northern hemisphere. 



Some patches of the tillite contain more carbonate of 

 lime than the rest of the rock, and they weather out in 

 the form of spheroidal and lens-shaped lumps that oc- 

 casionally pass into masses large enough to be called 

 lenticular beds. The spheroidal lumps are usually from 

 six to ten inches in diameter and seem to be particularly 

 abundant near Laingsburg and in the Tanqua Karroo, 

 but they have been found in Gordonia and many other 

 districts. The carbonate of lime in these concretions 

 has probably reached its present position by a slow pro- 

 cess of concentration from the surrounding rock, which 

 always contains a certain amount of calcite. 



In the Harts-Vaal Valley and in Gordonia beds of 

 limestone up to eighteen inches in thickness are 

 not uncommonly interbedded with the glacial de- 

 posits. 



Another peculiar variety of deposit is known as gravel 

 Dwyka. It is composed of small fragments of rock, 

 more or less irregular in outline, cemented by a small 



