PRE-CAPE ROCKS OF THE NORTH OF THE COLONY 69 



crystals of enstatite, or pseudomorphs after that mineral. 

 The more basic lavas are andesites. The matrix of the 

 devitrified rhyolites sometimes shows perlitic cracks, a 

 feature characteristic of glassy lavas, and flow structure 

 is often well developed. The largest area of these beds 

 is in the Vryburg division, and extends from near the 

 farm England to Zoetlief, a distance of twenty-eight 

 miles. Arkose and quartzitic flagstones occur at the 

 base of the series in this area, though they are only 

 seen in wells ; the lavas are porphyries of various kinds 

 representing rhyolites, trachytes, and andesites. Above 

 the lavas there are again sedimentary rocks, acid tuffs 

 and breccias, quartzites, flagstones, shales, thin lime- 

 stones and chert, with which lava-flows are occasionally 

 interbedded. The flagstones are often finely ripple- 

 marked. The flagstones of Zoetlief and the Groot 

 Chwaing inlier are used for fencing poles and building 

 purposes. 



Inliers, surrounded by Pniel lavas and sand, of ripple- 

 marked flagstones and shales succeeded by acid lavas 

 occur at Groot Chwaing, Karree Bult, and Schat Kist 

 in Vryburg. On the Motiton Eeserve there is an out- 

 'lier of these rocks nine miles long and three wide in 

 the granite area ; the lowest beds seen are grey flag- 

 stones and they are overlain by quartz-porphyries. 



Five miles north of Mafeking there is a small area of 

 Zoetlief lavas, tuffs, cherts and quartzites, overlain un- 

 conformably by Pniel beds. 



The working of the mines at Kimberley has proved 

 the presence of 1,300 feet of Zoetlief beds lying nearly 

 horizontally on an old granite surface. In the Kimber- 



