ALKALI ROCKS IN THE TRANSVAAL 775 



rocks, as was proved by chemical tests. After treating the powder 

 with hydrofluoric acid, only o . 4 of its weight was evaporated, and 

 a simple calculation makes evident that the Al partly occurs in 

 feldspars, partly in feldspathoids. Microchemically the residue 

 gave a strong soda reaction and a very feeble potash reaction. 

 From this it is evident that "felsites" which are the effusive 

 equivalents of the intrusive rocks are genetically connected with 

 the alkali rocks of the intrusion on Leeuwfontein [320]. 



An exact petrographical examination will greatly assist in the 

 determination of the stratigraphical place of the different "felsites." 

 Identity in age for the felsophyres of the Waterberg district and 

 of the phonolites of Leeuwfontein [320] would seem to be in the 

 highest degree improbable. 



From a petrographical point of view there is much resemblance 

 between the rocks of Leeuwfontein [320] and those of the Pilands- 

 berg; the association of foyaites of varying composition with red 

 syenites and effusive rocks is a common characteristic. The rocks 

 on Leeuwfontein [3 20] near the old dynamite factory are principally 

 red syenites and red hololeucocratic feldspar rocks; in the south- 

 ern part the leeuwfonteinites with accompanying porphyritic equiva- 

 lents occur. The porphyritic rocks sometimes form well-defined 

 dikes. 1 Leeuwfonteinite porphyry and monzonite porphyries are 

 found between Leeuwfontein [320] and Franspoort [426] and along 

 the path to Derde Poort [469]. The numerous varieties of foyaite 

 occur near the boundary of the farms Leeuwfontein [320] and 

 Zeekoegat [287]; they will be described in detail in a forthcoming 

 petrographical paper. The normal foyaite of this region is a coarse- 

 grained, leucocratic, aegirine-amphibole foyaite. In varieties 

 rich in feldspathoids (particularly sodalite) aegirine is the only dark 

 constituent; they pass into rocks which are nearly free from feld- 

 spar (tawites). Rocks very rich in titanite (pienaarites) occur at 

 several places. The rocks on Leeuwfontein [320] differ from those 

 of the Pilandsberg by the absence of rare minerals in the latter 

 rocks. In other nepheline-syenite regions the rare minerals are 



1 The leeuwfonteinites are the same rocks as Henderson's hatherlites (anortho- 

 clase syenites), cf. Henderson, On Certain Transvaal Norites, Gabbros, and Pyroxenites 

 and Other South-African Rocks. They contain much plagioclase and their composition 

 varies between that of the alkali monzonites and that of the alkali syenites. 



