90 DR. F. H. HATCH — GEOLOGY OF THE [Feb. 1 898, 



5. The Magaliesbeeg and Gatseand Series. 



The Magaliesberg and Gatsrand Series consists of quartzites, flag- 

 stones, and shales with interbedded sheets of volcanic rock. As in 

 the case of the Hospital Hill Series, the harder rocks (quartzites 

 and flagstones) crop out in the hills, while the softer rocks (shales 

 and decomposed volcanic rocks) occur in the intervening valleys. 

 Eeferring again to a north-and-south section, these beds (immedi- 

 ately overlying the Dolomite Formation) constitute the crest of the 

 great anticlinal arch, the northern limb being formed by the 

 Magaliesberg and the southern by the Gatsrand. The Magaliesberg 

 rocks were thought by Schenck to be related to the Hospital Hill 

 Series north of Johannesburg. Molengraaff' held the same view, 

 correlating the Magaliesberg Beds (which he called Pretoria Beds ^) 

 with the Hospital Hill Series, although he correctly placed the 

 Gatsrand Beds above the Dolomite.^ 



Penning,^ however, recognized their true relation, and connected 

 the Magaliesberg Beds (his ' Megaliesberg Formation ') with the 

 Gatsrand Beds (his 'Klip River Series'). He estimates the whole 

 series, including the interstratified traps, at 18,000 feet thickness, 

 and with this figure my estimate agrees very closely. 



Distribution. 



This formation occurs, as already shown, in two distinct outcrops. 

 The northern belt embraces the Magaliesberg and Witwatersberg 

 hill-ranges, a width of some 10 miles, while the southern belt 

 extends from the Gatsrand range southward for a distance of 

 15 miles, a large portion of this area being occupied by contem- 

 poraneous volcanic flows of immense thickness. The beds are well 

 seen on the road between Potchefstroom and Klerksdorp, striking 

 south-west, towards the Yaal River. 



The eastern continuation of the Gatsrand Hills makes a sudden 

 bend at a point 12 miles south of Johannesburg and, striking then 

 south-west, crosses the Vaal River west of Yereeniging. 



Description of the Rocks. 



The quartzites are of a similar type to the other quartzitic 

 members of the Cape System, that is, they consist of grains of quartz 

 cemented firmly together by redeposited silica. The shales consist 

 chiefly of clay; they are closely associated with basic volcanic 

 rocks, and it is probable that they owe their origin partly to the 

 decomposition of the latter, partly to the original association of the 

 flows with deposits of volcanic ash or tufl" (schaalstein). Mineral 

 deposits of galena and zinc-blende are found in the shales. Shaly 

 flagstones, intermediate in petrological character between the 

 quartzites and the shales, also occur, especially in the Gatsrand 

 range. 



1 Neues Jahrb. Beilage-Bd. ix (1895) p. 205. ^ ^i^^ ^ 230. 



^ Quart. Journ. Geol. Soc. toI. xlvii (1891) p. 456. 



