ALKALI ROCKS IN THE TRANSVAAL 761 



The effusive rocks have recently been described in some detail 

 by Humphrey in a paper on the volcanic rocks of the Pilandsberg. 1 

 He divides the rocks into two main groups — the trachytes and the 

 phonolites. The first group contains the effusive representatives 

 of the alkali syenites; the second group, those of the nepheline 

 syenites. An andesitic rock was found on the ridge separating 

 the farm Kafferskraal [890] from Saulspoort [269]. It consists of 

 diallage, diopside, plagioclase, and iron ore in a fine-grained ground- 

 mass, and may be an effusive representative of the diorites. The 

 rock has been classed as leucitophyre. The phenocrysts of ortho- 

 clase are accompanied by phenocrysts of leucite. 



The trachytes attain their greatest development in the eastern 

 portion of the Pilandsberg, on the farms Doornpoort [251] and 

 Vaalboschlaagte [636], where they measure some 5,000 feet in 

 thickness. In this succession the trachytes alternate with red 

 "felsitic" 2 rocks and tuffs, while a thick band of leucitophyres 

 occurs toward the base of the series. These blue-colored leucite- 

 bearing rocks contain phenocrysts of orthoclase and leucite in a 

 groundmass of very finely divided aegirine and feldspar. The 

 phonolites occur in most other parts of the Pilandsberg; they are 

 of a prevailing greenish and bluish color, contrary to the prevailing 

 red of the trachytic series. Typical phonolites on the farm Drie- 

 fontein [888] contain occasional phenocrysts of feldspar in a finely 

 divided groundmass which consists of feldspar, nepheline, and 

 much aegirine. In the neighborhood of Saulspoort is a rock con- 

 taining phenocrysts of sodalite in a cryptocrystalline groundmass. 



Volcanic breccias and tuffs are widely distributed throughout 

 the Pilandsberg. 



The effusive rocks of the isolated mountain at the boundary 

 of Buffelspan [585], Leeuwfontein [429], and Wijdhoek [701] often 

 have a banded appearance, and a beautiful flow structure with 

 parallel arrangement of the feldspar phenocrysts is developed. 

 Well-developed cubes of blue fluorine occur in some of these rocks, 

 while Humphrey mentions the occurrence of leucite crystals. He 



1 "The Volcanic Rocks, etc.," Trans. Geol. Soc. South Africa, 191 2, p. 105. 



2 Felsite is a field term under which Transvaal geologists comprise a great diver- 

 sity of rocks: quartz porphyries, felsites, phonolites, tinguaites, andesites, etc. 



