PRE-CAPE ROCKS OF THE NORTH OF THE COLONY 101 



loidal, and is of the andesitic type usual in the Ongeluk 

 group. The line between the blocks and the dark sub- 

 stance separating them is quite sharp. The dark 

 matter is made of decomposition products such as 

 chlorite, epidote and chalcedony, and in it are wedge- 

 shaped masses of vein-quartz which fill up interstitial 

 spaces due to decrease in bulk of the dark rock. 



Peculiarly coloured jaspers or cherts accompany some 

 of the lavas. They are brittle rocks, often with a very 

 brilliant red colour, but some of them are greenish, 

 black, or grey. The black cherts owe their colour to 

 the presence of magnetite, the green to epidote, and the 

 red to hydrous oxides of iron. Yellow garnets occur in 

 these cherts in places. The red jasper is rather like 

 some of the jaspers in the Lower Griqua Town beds, 

 but it has a brighter colour. 



The Ongeluk beds are well over 1,000 feet thick in 

 Hay and Kuruman, but their thickness has not been 

 closely estimated. It is difficult to measure their dip 

 except in such escarpments as are formed by the lower 

 part of the group. 



In the Prieska Division they are found in a narrow 

 syncline, both limbs of which dip westwards, on the 

 farms Uitspansberg and Glenallan, and again farther 

 south on Keikam's Poort. In Hay and Kuruman they 

 occur in several broad shallow synclines, of which the 

 largest is that of Witwater and Ongeluk west of Griqua 

 Town. They have been found as far north as the 

 Kgogole Eiver near the road from Madebing to Heuning 

 Vlei, but west of this region the superficial deposits 

 effectually conceal the underlying rocks. 



