748 H. A. BROUWER 



separate volcanic series, which is younger than the Transvaal 

 system and older than the Waterberg system and the main lacco- 

 lithic intrusion. 



ACCOMPANYING DIKES AND INTRUSIVE SHEETS 



Except the basic rocks, which alternate with the shales and 

 quartzites of the Pretoria series and which are perhaps genetically 

 connected with the intrusion, numerous syenite dikes cut through 

 the Pretoria series and the Dolomite series, and intrusive sheets 

 of red and grey syenite are found in the dolomites. The well- 

 known dike of porphyritic nepheline syenite of the station Wonder- 

 fontein in the Potchefstroom district can be followed over Breedts 

 Nek in the Magaliesbergen as far as the nepheline syenites of the 

 Pilandsberg. 



At the contact of the intrusive sheets, which have a thickness of 

 three to forty meters, the syenite is finer-grained to microcrystal- 

 line, and the dark dolomite has been changed into white marble. 



TECTONIC CHANGES CONNECTED WITH THE INTRUSION 



The study of the Transvaal system in the neighborhood of the 

 laccolith proves that there are numerous dislocations directly 

 connected with the intrusion. 1 



The strata sank under the weight of the intrusive mass; this 

 explains the increasing of the dip, when the distance from the 

 complex decreases, and also explains why the complex is sur- 

 rounded by a ridge of harder sediments, which dip toward the 

 central part. In the neighborhood of Pretoria and from there to 

 the west, as far as Rustenburg, we see the ridge of the Magaliesberg 

 quartzites uninterrupted, the strata dipping toward the intrusive, 

 complex. 



To the east of Pretoria is a series of step faults, which can be 

 followed easily in parallel ridges, which consist of quartzites of the 

 Pretoria series. 



We see the dislocations in a remarkable manner where the 

 periphery of the complex forms a re-entering angle as, for instance, 

 at Franspoort east of Pretoria. The ridge of the Magaliesberg 



1 This question was discussed in detail by Molengraaff; cf. Geology of the Trans- 

 vaal (Johannesburg, 1904), p. 50. 



