774 H. A. BROUWER 



which from Rustenburg strike in a northeasterly direction, bend 

 again to the west, and the nepheline syenite intrusions to the north- 

 east of Pretoria are found where the ridges of quartzite bend to the 

 southeast. The foyaite intrusion on Franspoort [426] is entirely 

 surrounded by Magaliesberg quartzite and is clearly intrusive in 

 them. Near the intrusion on Leeuwfontein [320], which borders 

 "felsites" only on the north, the Magaliesberg quartzites cover a 

 large surface in consequence of numerous faults. Following the 

 valley of the Pienaars River to the north, we see a succession of 

 red "felsites" with an approximate east-west strike and a varying 

 dip to the north, alternating with eruptive breccias, conglomerates, 

 and basic effusive and dike rocks. More to the north, on Roode- 

 plaat [314], they are covered by shales and syenitic rocks of doubtful 

 age, and on Paarderfontein [338], at a great distance from the 

 foyaite, dikes of tinguaitic and andesitic character, which in part 

 are connected with the intrusion on Leeuwfontein [329], cut through 

 the sandstones and conglomerates of the Waterberg system. The 

 chemical composition of a tinguaite of Paardenfontein [338] closely 

 agrees with that of the normal foyaites on Leeuwfontein [320]. 

 The small differences are similar to those which characterize the 

 nepheline syenites and accompanying tinguaite dikes from other 

 regions. 



An interesting dike of basic camptonite has recently been 

 described by Wagner. 1 It occurs on Buffelspruit [1920] in the 

 Waterberg district and cuts through Waterberg sandstone. This 

 proves again that the intrusion of nepheline syenite with which 

 the dike most probably is connected is younger than the Waterberg 

 sandstones. 



The "felsites" are the effusive equivalents of the intrusive 

 rocks on Leeuwfontein [320]. The liebenerite porphyries, which in 

 the southern part of Roodeplaat are exposed over a long distance 

 in the valley of the Pienaars River, show the same characteristics 

 as the liebenerite porphyries of Alno and the Tyrol. But also the 

 dense weathered rocks of which the mineralogical composition 

 could not be recognized under the microscope belong to the alkali 



1 P. A. Wagner, "Note on an Interesting Dyke Intrusion in the Upper Waterberg 

 System," Trans. Geol. Soc. South Africa, 191 2. 



