766 H. A. BROUWER 



Inside the Pilandsberg several dikes occur. Some dike rocks 

 with the macroscopic appearance of the tinguaites form a band of 

 comparatively high ground, because of their resistance to denuding 

 agencies. On Boekenhoutfontein [889] near the boundary with 

 Kaff erskraal [890] a dike of this kind strikes N. 50 W. ; it measures 

 about 10 meters across and cuts through the foyaites with biotite; 

 the contact with the foyaites is formed by a bent line, as well as 

 the contact of a dike in the southeastern part of Koedoesfontein 



[649L 



Near the boundary of Driefontein [889] and Nooitgedacht [148] 

 I found a tinguaitic dike, 2 meters in diameter, with a sharp contact 

 and a northeasterly strike, cutting through medium-grained 

 nepheline syenites in the valley of a rivulet. Near the contact 

 the rock has a glassy appearance; in the central part the structure 

 is porphyritic. This dike dips steeply to the northeast. At the 

 boundary of Olivenfontein [145] and Rooderand [398], in the valley 

 to the south of the red syenites, a similar dike, which measures 5 

 meters across, is exposed. 



The direction of these dikes agrees nearly with that of the dikes 

 outside the Pilandsberg. In the western rivulet to the north of the 

 houses on Driefontein [888], a tinguaitic dike or segregation, 

 averaging 40 centimeters in width, has a blended contact with the 

 surrounding lujaurites. It is rich in bronze-brown flakes of mica. 

 Near the lujaurites the rock is very rich in aegirine; this mineral 

 is often developed in spherulites which are up to 1 centimeter in 

 diameter. 



According to Humphrey, 1 dikes of foyaite, red syenite, nepheline 

 syenite, and diorite occur in all parts of the Pilandsberg, and, in 

 addition to these, there are many basaltic and tinguaitic varieties 

 occurring in various parts. In the spruit on Saulspoort [269] a 

 dike of red syenite cuts through the effusive rocks. A series of red 

 "felsitic" dikes and blue-black glassy dikes, which were difficult 

 of determination, and dikes of nepheline syenite traverse the red 

 syenites. The dikes of nepheline syenite, which have their greatest 

 development outside the Pilandsberg, seem to disappear into the 



1 "The Geology of the Pilandsberg," Annual Report of the Geol. Survey of South 

 Africa, 1911, p. 85. 



