ALKALI ROCKS IN THE TRANSVAAL 777 



close behind the zone of ultra-acid rocks which Hall discovered at 

 the boundary between the granites and the basic margin of the 

 igneous complex of the Bush veld. The specimens which Mr. Hall 

 kindly put at my disposal during my stay in the Transvaal are 

 melanocratic lujaurites and lujaurite porphyries, which sometimes 

 show a schistose structure. The colorless minerals are sometimes 

 very subordinate and microscopically the rocks seem to consist 

 almost wholly of fine needle-shaped crystals of aegirine. These very 

 melanocratic lujaurites were rare in the Pilandsberg complex, but 

 seem to cover the greater part of this newly discovered occurrence. 



Foyaites also occur, and it is interesting to find the association 

 of lujaurites with leucocratic feldspathoid rocks {urtites), which 

 consist chiefly of nepheline. The association of lujaurites and 

 urtites in the peninsula of Kola (they received their names from 

 the same place — Lujavr Urt) is also a characteristic of this district. 



An isolated mass of strongly metamorphic limestone is inclosed 

 within the alkali rocks. 



ORIGIN AND AGE OF THE NEPHELINE SYENITES AND ALLIED ROCKS 



It does not seem improbable that the nepheline syenites have 

 originated from the same sources as the granites and norites of the 

 Bushveld. The formation of the basic margin in the main intru- 

 sion of the Bushveld proves that magmatic differentiation took 

 place on a very large scale. Toward the periphery the rocks become 

 more and more basic, while granites occupy the central portion. 

 When tested in detail, the view of general increase of basicity from 

 the center toward the periphery requires modification. Hall 1 has 

 described a zone of ultra-acid rocks with 97 per cent Si0 2 in the red 

 granites close to the boundary with the norites to the west of Lyden- 

 burg. He considers these rocks as a product of extreme differen- 

 tiation, which could take place near the basic margin, when the 

 viscosity of the granitic magma was already strongly increased. 

 That sometimes the acid and basic rocks pass gradually into one 

 another possibly depends on the depth to which the complex has 

 been exposed by erosion. 



1 A. L. Hall, "Note on Certain Widespread Ultra-Acid Rocks Occurring along the 

 Margin of the Bushveld Granite, etc.," Trans. Geol. Soc. South Africa, XIII (1910), 

 p. 10. 



