AND ASSOCIATED EOCKS OF THE SOUTHERN TRANSYAAL. 409 



II. GEOLOGY OF THE WITWATERSEANDT. 



A. The Gold-bearing Conglomerates and Associated !Rocks op 

 THE Johannesburg Area. 



1. The District in General. 



This district embraces an area of about 2000 square miles. It is 

 variously known as the Witwatersrandt, Randt, or Johannesburg 

 Goldfield. 



The chief towns iu its northern portion are Johannesburg, Heidel- 

 berg, and Pretoria ; of these Johannesburg occupies the most central 

 position. The Vaal Eiver forms the southern boundary of the 

 district. East and west the area has no defined limits, but it 

 certainly extends eastwards to where the older strata are concealed 

 by later deposits of Karoo age, and westwards until they are buried 

 beneath igneous matter or lost under the expanse of open veldt. 



As a whole the district may be said to form the northern part of 

 the great central plateau of South Africa. Gradually rising from 

 the Vaal River, its highest point is reached near Johannesburg in an 

 elevation of 5600 feet. Here three parallel ridges extend in an 

 east-and-west direction for about 45 miles, and the high ground thus 

 formed constitutes the watershed between the Limpopo and Yaal 

 E-ivers ; the streams to the north are tributaries of the former 

 river, while those to the south flow mto the Vaal. Most of the 

 stream-courses are mere dry channels during the winter months, 

 but in the summer they become filled with rushing turbid torrents. 



Broadly speaking, the visible rocks in this district consist of a 

 sedimentary series of sandstones, quartzites, and conglomerates, and 

 an igneous and much later series of basic and sub-basic lavas and 

 intrusive sheets. Here and there in the folds of the former set of 

 rocks, shallow deposits of coal-bearing strata have been laid down ; 

 but these do not, so far as the writer's knowledge goes, cover any 

 great extent of surface in the Transvaal, although coal-beds of appa- 

 rently the same age extend over a large area in the Orange Free State. 



The quartzites and conglomerates have generally a southerly dip, 

 with an east-and-west strike. Behind Johannesburg, and again to 

 the south, they constitute two main parallel lines of elevation com- 

 posed of several smaller ridges running east and west. 



The discovery of gold in the conglomerate-beds around Johannes- 

 burg in 1885 has caused this town to spring up in what five 

 years ago was a desert, and has given these strata a world-wide 

 reputation. Since the goldfields have been started mining opera- 

 tions have been extensively carried on along the outcrop of the 

 conglomerate. There are now (1890) over 70 mines, the workings 

 extending in an east-and-west line from Johannesburg for nearly 

 40 miles. The greater number of these mines are situated on what 

 has been named the Main Reef, the course of which is marked all 

 along its outcrop by derricks, hauling machinery, and all the usual 

 adjuncts of mining operations. 



