THE KARROO SYSTEM 171 



gested the term " tillite," a handy word, and one that 

 indicates the glacial origin of the rock. A tillite, there- 

 fore, is a more or less hard rock that was originally a till 

 or glacial boulder-clay. 



The area occupied by the Dwyka formation in Cape 

 Colony can be divided into two distinct portions, the 

 area lying south of latitude 33, where there is a con- 

 formable succession from the base of the Cape system 

 far into the Karroo system, and the area extending 

 northwards from that parallel, in which the Dwyka 

 rests unconformably upon the underlying rocks. 



It is very probable that throughout the northern area 

 the tillite reposes upon an undulating glaciated surface 

 of older rocks, although exposures of such striated 

 floors are not frequent. The reasons are that the 

 Dwyka is usually much hidden under superficial deposits, 

 and that the glaciated surfaces are rapidly destroyed 

 through weathering once the cover of tillite has been 

 stripped off. This is especially true of such rocks as 

 granite, gneiss, and crystalline limestone, but on diabases, 

 amygdaloids, and quartzites the striations are as a rule 

 well preserved. 



Striated floors below the tillite have been found at 

 various places in Prieska and Hopetown, at Kimberley, 

 along the Vaal River above its junction with the Orange, 

 at Vryburg, in the Central and Eastern Transvaal, and 

 in Natal and Zulu! and. The old floor undulates much, 

 and through the cover of Dwyka there frequently pro- 

 ject rounded polished and striated hummocks of rock 

 with their longer axes usually approximately parallel to 



