Vol. 54.] WITWATEKSEAND AND OTHER DISTRICTS IN S. TEANSVAAL. 75 



II. The Arch^an Eocks. 

 Distribution. 



As already pointed out, these rocks protrude in a few places 

 through the sedimentary beds which cover the greater part of the 

 area under consideration. In the Northern Transvaal and Rhodesia 

 they bulk largely, consisting there of granites, gneisses, and basic 

 crystalline schists derived from the deformation of igneous rocks. ^ 

 But in the Southern Transvaal their appearance is quite local and 

 of limited extent. Besides the occurrence of basic eruptive rocks 

 (in part gabbro) north of the Magaliesberg, three principal masses 

 of the Archaean rocks crop out. The largest of these is that lying 

 between Pretoria and Johannesburg, its southern margin being 

 immediately north of the Witwatersrand range. This mass has an 

 extent from north to south of about 18 miles, and of about 28 miles 

 from east to west. The granitic rocks of which it is composed 

 form no conspicuous or striking outcrop, the surface of the ground 

 being either flat or of a gently undulating or rolling character. 

 Next in importance is the mass which crops out on the Yaal Biver 

 some 28 miles S.E. of Potchefstroom. The greatest portion of this 

 mass lies on the Pree State side of the river ; but the Transvaal 

 portion is about 10 miles long by 3| miles wide. The outcrop is 

 of a broken character near the Yaal, but on the Pree State side 

 the. hills diminish as they recede from the river. 



The third mass, measuring 8 miles long by 7 broad, is situated 

 on the farms Prischgewaagd, Uitkyk, Langzeekoegat, Witkop, and 

 Palmietfontein, etc., about 40 miles south-east of Johannesburg. 

 Here, also, the surface of the country is flat or gently rolling. 



Description of the Arclisean Rocks. 



These ancient crystalline rocks do not constitute a single granite 

 massif in any of the occurrences mentioned ; they are rather 

 members of an igneous complex of rocks of varied composition, 

 although rocks of ultra-acid composition are rare. A series of these 

 rocks collected by Cohen in 1872-73 has been described by P.Dahms.^ 

 He mentions the following types : — biotite-granite (Mlilier's Parm, 

 south-west of Pretoria), amphibole-granite (Bietfontein, near 

 Tokeskey Biver), granite-porphyry (Miiller's Parm), mica-syenite- 

 porphyry (between Grobler's and Miiller's Parm). 



Molengraaff ^ is of the opinion that the predominant rock in the 



^ J. Gotz, ' Untersuchung einer Gesteins-suite aus der Gegend der Goldfelder 

 von Marabastad, im nordliclien Transvaal,' Neues Jabrb. Beilage-Bd. iv (1886) 

 p. 110 ; and J. A, Chalmers & P. H. Hatch, ' Notes on the Geology of Mashona- 

 land & Matabeleland,' Geol. Mag. 1895, p. 193. 



^ ' Ueber einige Eruptivgesteine aus Transvaal in Sud-Afrika,'Neues Jabrb. 

 Beilage-Bd. vii (1891) p. 90. 



^ ' Beitrag zur Geologie der Umgegend der Goldfelder auf dem Hoogeveld in 

 der siidafrikanischen Eepublik,' Neues Jahrb. Beilage-Bd. ix (1894-95) p. 184. 



