750 H. A. BROUWER 



ROCK TYPES OF THE BUSHVELD COMPLEX 



i . Red granites. — The acid rocks of the laccolith are amphibole- 

 biotite granites, which are very poor in dark constituents. Prin- 

 cipally they show typical granophyric structure. They differ 

 petrographically from the old granites, in essential features enum- 

 erated by MolengraafL 1 In the red granites muscovite is entirely 

 wanting. 



2. Norites, gabbros, and pyroxenites {with segregations of iron 

 ore). — Nearly everywhere at the periphery of the red granites we rind 

 a zone of basic and ultra-basic rocks. The basic rocks are found 

 near the western and southern part of the Pilandsberg; they 

 accompany the Magaliesberg quartzites in a south-southeasterly 

 direction to the environs of Rustenburg, where they bend to 

 the east in the direction of Pretoria. The Zwartkoppies and 

 Pyramids have been given their respective names from the color 

 and the form of the small hills, which are composed of these rocks. 

 Finally, they are found from the environs of Belfast to those of 

 Piet Potgietersrust; still farther to the north they are in contact 

 with the old granites. Iron ore has been segregated from these 

 basic rocks at several places; lenticular masses of magnetite are 

 developed nearly everywhere around the Bushveld. At some places 

 these masses are thicker than one hundred meters. The iron ore 

 is magnetite, sometimes with chromite. In the norites the per- 

 centage of magnetite goes on increasing, as one approaches the 

 pure magnetite. Ultra-basic pyroxenites and peridotites rill the 

 shallow basin to the West of the Pilandsberg, bounded on the south 

 by the Schurveberg and the Zeerust Hills, on the west by the Marico 

 Hills, and on the north by the Dwarsberg. 



3. Nepheline syenites and syenites. — These rocks, of which the 

 mode of occurrence and the composition are more fully described 

 in the following pages, are uncovered at several places, often near 

 the boundary of norites and granites. 



PNEUMATOLYSIS 



As a result of the cooling down and contraction of the intrusive 

 complex and the pressure upon the surrounding strata, numerous 



1 G. A. F. Molengraaff, Geology of the Transvaal (Johannesburg, 1904), p. 44. 



