ALKALI ROCKS IN THE TRANSVAAL 749 



quartzites and the accompanying Daspoort and Timeball quartz- 

 ites here suddenly bend to the southeast. The dip of the strata 

 continues toward the red granites, but the outer ridge of the Maga- 

 liesberg quartzites has been fractured and extended in length; the 

 ridge is broken by "poorten." The inner ridges (Daspoort and 

 Timeball Hill quartzites) were strongly pressed in a direction 

 slightly oblique to the strike of the strata. All this is clearly shown 

 by the grouping of the quartzite hills in the neighborhood of Pre- 

 toria. That the intrusion and dislocations are directly connected 

 is also evident from the study of the zones of contact in the sur- 

 rounding sediments in disturbed and undisturbed regions. 



CONTACT METAMORPHISM 



The quartzites, clay-slates, and "greywackes" of the Trans- 

 vaal system are strongly metamorphosed by the intrusion of the 

 laccolith. The contact phenomena in connection with the laccolith 

 were first mentioned by Molengraaff and later studied in detail 

 by Hall. 1 The quartzites of the Magaliesberg Range are recrys- 

 tallized and consist of more or less hexagonal quartz crystals, which 

 sometimes attain a diameter of more than one centimeter; in the 

 clay slates cordierite, andalusite, and biotite appear. The meta- 

 morphism decreases when the distance to the laccolith increases, 

 but even the rocks of the dolomite series are metamorphosed. 

 Where the Transvaal system is much disturbed and has undoubt- 

 edly been exposed to high pressure, the metamorphosed rocks show 

 a different character in their structure, as well as in their mineralogi- 

 cal composition. These rocks are connected by transitions with 

 the pure contact-rocks of the undisturbed regions. 



Muscovite, glaucophane, and zoisite, which are characteristic 

 for the dynamometamorphic crystalline schists, and in small 

 quantities the contact minerals, cordierite, andalusite, and tour- 

 maline, occur in the metamorphosed rocks. Hall decides upon the 

 contemporaneous action of contact and dynamometamorphism in 

 the disturbed regions, from which it is once more evident that the 

 intrusion was directly connected with the dislocations. 



1 A. L. Hall, "Uber die Kontaktmetamorphose an dem Transvaalsystem im 

 ostlichen und zentralen Transvaal," Tschermaks Min. u. Petr. Mitt., Bd. XXVIII, 

 Heft 1-2 (1909), pp. 115-52. 



