ALKALI ROCKS IN THE TRANSVAAL 759 



2. Lujaurites 



a) Eucolite-molengraaffite lujaurites 



b) Eucolite lujaurites 



c) Eucolite-astrophyllite lujaurites 



d) Aenigmatite lujaurites 



e) Lujaurites without eucolite 



3. Aegirine-amphibole foyaites 



4. Aegirine-biotite foyaites 



5. Lujaurite porphyries 



6. Aegirine-nepheline syenite porphyries without foyaitic structure 



7. Aegirine-amphibole-biotite-nepheline syenite porphyries 



8. Tinguaite porphyries 



Syenites: The syenites generally have a red or reddish color; 

 they principally consist of feldspar, while the dark-colored minerals 

 have usually been altered into chlorite. Iron ores, titanite, 

 apatite, and fluorspar are further constituents of these rocks. 



Humphrey 1 mentions that the red syenites of the eastern, 

 western, and central parts of the Pilandsberg have various points 

 of dissimilarity in the hand specimens; those of the east, on Rhen- 

 osterspruit, being almost entirely composed of feldspar, while the 

 other localities furnish rocks in which is much iron ore. The 

 latter rocks are very decomposed. The feldspars are microcline, 

 orthoclase, and anorthoclase. Two analyses of red syenites, which 

 have been published in the Annual Report of the Geological Survey 

 of South Africa for 191 1, show that there are considerable differ- 

 ences in chemical composition between the rocks of this group. A 

 red syenite from Nooitgedacht [748] contains 8 per cent Na 2 and 

 2 per cent K 2 0, while a red syenite from Rhenosterspruit [609] 

 contains 5 per cent Na 2 and 10 per cent K 2 0. The high potash 

 content of some of the syenites tends to connect them with the 

 leucite-bearing effusive rocks, which will be mentioned below. 



These red syenites bound the Pilandsberg complex on the north- 

 western, northern, and southeastern sides. For the greater part 

 they are developed as a massive wall, forming the outermost circle 

 of hills at the periphery of the mountains. The red color of the 

 syenites has given the name to the farm Rooderand [399] and from 



1 W. A. Humphrey, "The Volcanic Rocks, etc.," Trans. Geol. Soc. South Africa, 

 191 2, p. 104. 



