ALKALI ROCKS IN THE TRANSVAAL 753 



The first explorer who mentioned the composition of the Pilands- 

 berg and the surrounding region was Adolf Hiibner, in his descrip- 

 tion of a voyage from Potchefstroom to Inyati. 1 Evidently he 

 had observed the peculiar character of these rocks, because he 

 writes: "Am meisten Beachtung verdient wohl das unten zu 

 beschreibende Gestein der Pilandsberge, welches entschieden als 

 ein basisches plutonisches Gestein zu den Griinsteinen gerechnet 

 werden muss." He crossed the Magaliesberg Range along the 

 Hex River at Olifants Nek and traveled from there to Morgenzon 

 [427] (the same direction which the road from Rustenburg to the 

 Pilandsberg still follows); as the rock of the plain of Rustenburg 

 he mentions a typical medium-grained greenstone, which near the 

 contact with the rocks of the Magaliesberg Range crops out in 

 thick sheets. 2 Then he crossed the Elands River and reached the 

 Pilandsberg, which, he says, consists of mountains of greenstones 

 from about 400 to 600 feet high. The meaning of his sentence, 

 "Die Gesammtheit der Quarze bildet ein wahres Massengebirge," 

 is not very clear. He mentions that a part of the mountains con- 

 sists of a rock which seems to be a "hornblende porphyry," but 

 which shows a syenitic character on closer examination. Because 

 it consists of two minerals, a red "felsite" and a black amphibole, 

 Hiibner says that it is not a normal porphyry, though the red feld- 

 spar (orthoclase) predominates. The amphibole does not form 

 crystals, but shows rather regular forms. As an interesting feature 

 of this perhaps quite new rock, he mentions the numerous inclu- 

 sions of clay slate and granite, which do not show any contact 

 metamorphism. According to Hiibner, this rock would cover a 

 large area in the Pilandsberg. At several places, e.g., behind the 

 negro town on Saulspoort [369] in the northeastern part of the 

 mountains, granites and eruptive breccias, which contain fragments 

 of porphyrite and granite, occur. Because Hiibner mentions 

 that he visited the missionary station on Saulspoort, he perhaps 

 speaks about these rocks, which occur in the neighborhood of 

 syenites with red feldspars. 



'Adolf Hiibner, " Geognostische Reisen in Sud-Africa," Peterm. Geogr. Mitt., 

 XVIII (1872), 424, 426. 



2 These rocks probably are the norites with schistose structure in the margin of 

 the igneous complex of the Bushveld. 



