ALKALI ROCKS IN THE TRANSVAAL 755 



graphical part of my first paper. In the winter of 19 10 I visited 

 the occurrences of nepheline syenite in the Transvaal, especially 

 that of the Pilandsberg (during the last two weeks of June). 



After this visit I was able to give a brief description of the 

 general geology of the neighborhood. 1 I showed that these moun- 

 tains consist of red syenites, nepheline syenites, and their por- 

 phyritic and dense equivalents, and, with a sketch map, that the 

 main rock types are disposed in concentric circles. As far as con- 

 cerns the red syenites, I was able to show that they often are covered 

 by the effusive rocks, while they probably were older than the 

 nepheline syenites of the complex. There was clear evidence that 

 the rocks of the Pilandsberg intrusion are younger than the granites 

 and norites of the igneous complex of the Bushveld; dikes, which 

 are genetically connected with the intrusion, cut through the 

 granites and norites. The intrusion of nepheline syenites on 

 Leeuwfontein [320] was shown to be certainly younger than the 

 Waterberg system, because a dike of tinguaite, which has the same 

 chemical composition as the normal foyaites of Leeuwfontein, 

 cuts through Waterberg sandstones and conglomerates on Paarde- 

 fontein [338]. 



The mapping of the Pilandsberg and surrounding area was 

 carried out in the next year by Dr. Humphrey, in company with 

 Dr. P. Wagner, in connection with the work of the Geological 

 Survey of South Africa. 2 Humphrey divides the whole of the rocks 

 into two main groups : the nepheline syenites and phonolites ; and 

 the alkali syenites and trachytes. Each of these groups contains 

 a plutonic and an effusive representative and "the reason for this 

 classification is that the rocks forming the Pilandsberg are the 

 denuded remnants of what was once a stupendous volcano, com- 

 parable in size with the greatest of the present-day active 

 volcanoes." 



General character of rocks. — The foyaites and other allied rocks 

 in Professor Molengraaff's collection have been described by me 



1 H. A. Brouwer, Oorsprong en samenstelling der Transvaalsche Nepheliensyenieten 

 (1910), pp. 12-29. 



2 W. A. Humphrey, "The Volcanic Rocks of the Pilandsberg," Trans. Geol. Soc. 

 of South Africa, August 19, 191 2; "The Geology of the Pilandsberg," Annual Report 

 of the Geol. Survey of South Africa, 191 1, p. 77. 



