ALKALI ROCKS IN THE TRANSVAAL 771 



solidified before the earth's surface was reached, but the Pilands- 

 berg alkali magma must have been very rich in mineralizing agents, 

 which reduced its viscosity, and in such magmas the stoping process 

 may go on when they near the earth surface and until the cover is 

 penetrated. Volcanic outbursts cause an escape of the volatile 

 substances, and the magma becomes more and more viscous, until 

 a new supply of heat and mineralizers from below sets up stoping 

 again. In fact, several rare minerals with a highly complex con- 

 stitution, which are not stable at high magmatic temperatures, 

 occur within the Pilandsberg rocks. Fluorine is a very common 

 constituent, and, as their very name imports, fluorides must have 

 reduced the viscosity considerably. Moreover, fluorine and other 

 minerals in which we find direct evidence of the co-operation of 

 mineralizers are regularly distributed in several rocks of this region, 

 where they crystallized in the last cavities, thus proving that the 

 mineralizing agents in part were regularly distributed until the 

 final consolidation. 



Of course, direct support would be given to the co-operation of 

 overhead stoping if fragments which could only be derived from 

 an original cover of the crystalline rocks were found among the 

 rocks of the Pilandsberg complex, but Humphrey 1 mentions that 

 all the close-grained rocks, which in the hand specimens very much 

 resemble shales, proved themselves under the microscope to be 

 devitrified lavas. 2 Particularly at those places in the northeastern 

 part of the area where the granites are found to within a few hun- 

 dred yards of the Pilandsberg complex it is of great interest to 

 know whether these granites occur in their original position. 3 



Age of the Pilandsberg. — In the neighborhood of the Pilandsberg 

 the rocks which formed the covering of the igneous complex of the 

 Bushveld at the time of its intrusion most probably belonged to the 



1 "The Volcanic Rocks of the Pilandsberg, etc.," Trans. Geol. Soc. South Africa, 

 1912, p. 102. 



2 In my previous paper (Oosprong en samenstelling der Transvaalsche Nephelien- 

 syenieten), p. 17, I mentioned having found shales in the valley to the north of 

 the homestead on Houwater [496], but the rocks were not studied under the 

 microscope. 



3 Cf. also H. A. Brouwer, "On the Formation of Primary Parallel Structure in 

 Lujaurites," Proc. Kon. Akad. Amsterdam, 191 2, p. 734. 



