ALKALI ROCKS IN THE TRANSVAAL 767 



nepheline syenites in the central part of the complex. Dikes of 

 foyaite were found traversing the red syenites of the central part. 



From all that has been said above, it is evident that the foyaites 

 are the youngest rocks of the Pilandsberg complex, while there is 

 good evidence to show that the red syenite is older than some of the 

 effusive rocks. 



Pegmatites. — Dikes of pegmatite, which in many other nepheline 

 syenite regions contain numerous rare minerals, were not met with 

 during my visit. Coarse-grained pegmatitic segregations in the 

 normal-grained rocks are of frequent occurrence, but the rare 

 minerals were not found in much larger crystals than in the normal- 

 grained varieties. 



Pegmatites rich in eucolite are well exposed in lujauritic rocks 

 from the hills to the north of the houses on Driefontein [888]. They 

 consist principally of large crystals of feldspar, green nepheline, 

 long crystals of aegirine, and carmine-red eucolite which is partly 

 altered to catapleiite; they also contain some astrophyllite. The 

 prisms of aegirine are up to 10 centimeters in length, and some- 

 times show a graphic intergrowth with feldspar. Near, and to the 

 west of, the main road to Saulspoort [269] where it crosses the 

 Rhenosterspruit in the northeastern part of Buffelspan [585], we 

 found pegmatites in the aegirine-amphibole foyaites. Feldspars 

 up to 10 centimeters in length, green nepheline, prisms of amphi- 

 bole, and prisms or spherulites of aegirine are the main constituents ; 

 they also contain some fluorine. Amphiboles with a very small 

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

 the plane of symmetry* occur in these rocks. In the southern part 

 of Wijdhoek [701], near, and to the west of, the main road and to 

 the south of the ridge of effusive rocks, we found pegmatites, which 

 are very rich in astrophyllite and spherulites of aegirine, measuring 

 up to several centimeters in diameter. They are found still 

 farther to the southwest on Koedoesfontein [746] in a rivulet which 

 joins the Wolvespruit, where numerous blocks of pegmatites and 

 lujaurites could be collected; some of them are very rich in eucolite. 



1 H. A. Brouwer, "On Zonal Amphiboles in Which the Plane of Optic Axes of the 

 Margin Is Normal to That of the Central Part," Proceed. Kon. Akad. Amsterdam, XVI 

 (1913), 275. 



