34 CONTACT METAMORPHISM 



Only the extreme southern end of the VanKhyn'sDorp- 

 Namaqualand granite mass has as yet been examined in 

 detail. 1 The rock is principally a biotite granite with 

 microcline felspar but it passes into a gneiss ; there are 

 many other types represented such as augite and hypers- 

 thene-granulites, amphibolite, etc., while in the Lang- 

 berg region the bulk of the gneiss is garnetiferous. 

 Around Bitter Fontein there are some peculiar schists' 

 veined by the granite and faulted on the east against 

 the Ibiquas beds. They include mica-schists, quartz- 

 schists and a peculiar sillimanite-cordierite gneiss. 



Contact Metamorphism. 



The granite has in nearly every case produced con- 

 siderable mineralogical changes in the surrounding 

 rocks. The result varies considerably in intensity and 

 nature, depending chiefly upon the character of the rock 

 invaded. Highly quartzitic rocks are the least affected, 

 and the alteration seems to increase with the clay con- 

 tent of the original slate ; hence at some distance from 

 the granite, traces of metamorphism can only be recog- 

 nised in certain of the more susceptible slaty beds. 



Around the Paarl, Stellenbosch and Somerset West 

 granites the clay-slates become spotted at a distance of 

 about 300 yards from the contacts ; along the Cape Town 

 granite, however, the belt of "spotting" is fully a mile 

 and a quarter wide. The spots become larger and more 

 numerous as the contact is approached, at the same 

 time very minute flakes of red-brown mica are abund- 

 antly developed. The spots are found in thin sections 



1 G. C., ix., pp. 19-23. 



