72 THE PNIEL SERIES 



are well developed in the Pokwani and Taungs anticline, 

 near Mafeking, and again west of Setlagoli, at Botrnan's 

 Poort on the Transvaal border, at Kimberley and at 

 Wit Vlei in Prieska ; they contain boulders of many of 

 the older rocks including the Zoetlief quartz-porphyries. 

 The quartzites are pale -coloured rocks, often ripple- 

 marked, and they may reach a thickness of over 600 

 foot ; some of them are felspathic, and the felspar frag- 

 ments are such as would be derived from a granite 

 country. 



The transition from the mainly sedimentary part of 

 the series to the volcanic beds is sometimes gradual, for 

 lavas and tuffs are found interbedded with the quartzites, 

 the latter becoming less frequent in the higher part of 

 the group. 



The lavas are chiefly blue or green diabases weather- 

 ing with a thin, reddish-brown crust. They are fre- 

 quently amygdaloidal ; the steam-holes are filled with 

 chalcedony, calcite or chlorite. Some of the chalcedony 

 amygdales have a bright pink or a red colour. In some 

 localities, as in the Kimberley mine and on the road to 

 Schmidt's Drift, the amygdales in some layers are very 

 much elongated, they are arranged parallel to each 

 other and perpendicularly, or nearly so, to the base of 

 the lava-flow. Another peculiarity, seen at Barkly 

 West, is the occurrence of vertical cylindrical columns 

 of vesicular lava up to six inches in diameter, in com- 

 pact lava of the same composition. 1 



Plagioclase felspar, chiefly oligoclase and andesine, and 

 augite seem to have been the chief constituents of the 



1 Du Toit, G. M., 1907, p. 13. 



