ALKALI ROCKS IN THE TRANSVAAL 757 



in the central part to green in the margin, while a turning of the 

 plane of optic axes from parallel to the plane of symmetry in the 

 central part to normal to it in the margin was often observed. 

 The extinction angles in sections parallel to [010] are up to 40 . 

 Amphiboles in which the plane of optic axes is normal to the plane 

 of symmetry have also been found in pegmatitic segregations in 

 aegirine-amphibole foyaites on Buffelspan [585]. Their angle of 

 optic axes is very small and the extinction angle b:c is about 14 . 

 In rocks from Wijdhoek [701] many properties of the amphiboles 

 agree with those of the green amphiboles which Ussing 1 described 

 in rocks from Greenland. 



Coarse-grained foyaites are largely developed in the central, 

 and also in the northern, part of the mountains. The hills and a 

 part of the valley on and near Boekenhoutfontein [889] consist of 

 foyaites, which contain aegirine and sometimes are very rich in 

 biotite; more to the south, at Buffelspan [585], coarse-grained 

 aegirine-amphibole foyaites are found. Rocks with the same struc- 

 ture occur in the western part of Houwater [496] and gray foyaites 

 cover a large surface on Schaapkraal [12]. Leucocratic foyaites 

 with aegirine as the only dark constituent are found in the western 

 part of Wijdhoek [701], near the eastern boundary of Tusschen- 

 komst [331]; they are associated with aegirine-amphibole foyaites. 

 They form a complex of isolated small hills in a valley, surrounded 

 by ridges of effusive rocks. The foyaites can be followed to the 

 southern part of Leeuwfontein [429] and to Welgeval [749]. At 

 Wijdhoek [701] they show a considerable amount of variation in 

 structure and composition; we find gray feldspar rocks, which 

 contain biotite as the only dark constituent, varieties which are 

 rich in nepheline, and porphyritic equivalents in which the dark 

 minerals appear as phenocrysts enclosing the elements of the fine- 

 grained groundmass. 



The lujaurites are characterized by their richness in fine needles 

 of aegirine. In a forthcoming petrographical paper this group will 

 be described in detail. Aegirine always predominates; arfved- 



1 N. V. Ussing, " Mineralogisk-petrografiske Undersogelser of Gronlandske 

 Nefelinsyeniter og beslaegtede Bjaergarter," Meddelelser om Gronland, XIV (1894), 



