768 H. A. BROUWER 



The aegirine-biotite foyaites in the northwestern part of Boeken- 

 houtfontein [889] contain pegmatitic segregations- and small dikes 

 in which feldspars, feldspathoids, and eucolite mineral and small 

 pale-yellow needles occur. Often they are very rich in aegirine; 

 this agrees with its tardy crystallization in most of the rocks of 

 the region. 



Finally, we found segregations in the lujaurites to the west of 

 the common boundary post of Tusschenkomst [331], Leeuwfontein 

 [429], and Wijdhoek [701]; they consist almost entirely of aegirine 

 spherulites which are up to some centimeters in diameter. 



Segregations rich in fluorine occur in the microfoyaites of 

 Olivenfontein [745], and segregations rich in large aegirine spheru- 

 lites occur in mesocratic foyaites of the valley, running in a north- 

 south direction in the northeastern part of Buffelspan [585]. 



Humphrey mentions the occurrence of very coarse-grained 

 pegmatites with much fluorspar on Doornhoek [134] and beautiful 

 pegmatites, about half a mile from the homestead on Driefontein 

 [888] on the main road to BufTelskloof (219). 



Mechanism of intrusion of the Pilandsberg complex. — Since the 

 rocks of the Pilandsberg complex are younger than the red granites 

 and norites of the Bushveld igneous complex, and since the Pilands- 

 berg is surrounded on three sides by norites and on one side by 

 red granites, it seems to be beyond doubt that the space which is 

 occupied by the Pilandsberg intrusive rocks was occupied, prior 

 to the intrusion, by the norites and red granites of the Bushveld. 



That the removal of the original rocks was not the result of 

 folding is proved by the occurrence of a great number of vertical 

 dikes of vast extension, which are genetically connected with the 

 intrusion. 



The hypothesis that the subsidence of crust blocks elsewhere 

 was the cause of the intrusion of the magma and the hypothesis 

 of laccolithic intrusion seem not to be applicable in the present case. 



As has been stated by Humphrey, there can be no doubt that 

 the Pilandsberg represents the remnant of what was once an impor- 

 tant focus of eruption, and the hypothesis that the intrusive magma 

 has filled up the cavities which were formed by volcanic outbursts 

 of an explosive character seems to be applicable. 



