ALKALI ROCKS IN THE TRANSVAAL 765 



quartzite, which are entirely surrounded by rocks belonging to the 

 igneous complex of the Bushveld and most probably were broken 

 up in connection with the intrusion of this complex. 



Dike rocks. — The first nepheline syenite of the Transvaal was 

 discovered by Elie Cohen 1 in 1872, near the Hex River, between 

 Renseburg and Rustenburg. He states that this rock forms the 

 lower parts of the Zwartkoppies, where these hills bend to the 

 northwest. The rocks collected by Cohen were described by E. A. 

 Wulfing 2 in 1886 as porphyritic foyaites in which the nepheline 

 only occurs in the groundmass. 



In 1904 G. A. F. Molengraaff 3 collected nepheline syenites on 

 the farms Elandsheuvel [255] and Tweede Poort [189]; these rocks 

 are porphyritic foyaites in which nepheline occurs as phenocrysts. 

 These rocks are described in the petrographical part of my previous 

 paper. Since that time the sheet Rustenburg (sheet No. 4) has 

 been mapped by the Geological Survey of the Transvaal, and it is 

 shown that several dikes of these porphyritic foyaites intersect 

 the rocks of the Bushveld complex, running from the Pilandsberg 

 in a southeasterly direction. 



Some of them even cut through the Magaliesberg Range to the 

 east of Rustenburg and can be followed still farther to the south. 



A fine-grained red syenitic rock was found by me to the north 

 of the red hill on Rooderand [399], cutting through the norites in a 

 nearly northerly direction. At the boundary of Groenfontein [302] 

 and Bierkraal [545] near the quartzites, a porphyritic syenite was 

 found in the basic rocks. On Plate XIV in the annual report for 

 191 1 of the Geological Survey of South Africa it is shown that 

 several syenitic dike rocks can be followed from the Pilandsberg 

 in a north-northwesterly and northwesterly direction. 



All these dikes cut through the norites and granites of the 

 Bushveld, and they have been intruded after the consolidation of the 

 rocks of the Bushveld igneous complex. 



1 E. Cohen in Berickte iiber die XVI. Versammlung des Oberrheinischen Geologischen 

 Vereins, am 29 Marz, 1883, Stuttgart. 



2 E. A. Wulfing, " Untersuchung eines Nephelinsyenits aus dem mittleren Trans- 

 vaal," Neues Jahrb.f. Min. Geol. u. Pal., 7 Mai, 1888, Bd. II, p. 16. 



3 G. A. F. Molengraaff, "Preliminary Note on the Geology of the Pilandsberg," 

 Trans. Geol. Soc. South Africa, VIII (1905), 208. 



