AND ASSOCIATED EOCKS Or THE SOUTHERN TRANSVAAL. 405 



I. INTRODUCTION. 



(1) General Shetch of the Geology of the Southern Transvaal. 



The rocky floor of the Transvaal is very distinct from that of 

 South Africa in general. Travelling for several hundred miles 

 through the northern parts of Cape Colony and the Orange Free 

 State, such strata as are seen consist of horizontal sandstones and 

 shales of Karoo and Stormberg age. These are injected and over- 

 flowed in numberless directions by masses and thin sheets of igneous 

 matter, while worn-down stumps of volcanic necks can be frequently 

 recognized. But in the Transvaal the nature of the strata under- 

 goes a complete change. Instead of flat-lying sandstones and shales 

 of the wide-spreading Karoo formation, we find twisted and highly 

 inclined quartzites and conglomerates of undetermined geological 

 age. The igneous rocks occur more in the form of bosses than sheets, 

 and make rugged hills with precipitous slopes which are almost bare 

 of vegetation. 



At first sight the geological structure of the southern part of the 

 Transvaal appears quite simple, the impression being that the rocks 

 merely form an ordinary basin. So far, however, from this 

 simplicity being real, the geological structure of the country is in 

 fact highly complex, and is rendered still more difficult to unravel 

 from its being apparently impossible to find any distinctive petro- 

 logical band, available as a means of mapping the ground, or any 

 group of fossiliferous strata, which might be used as a stratigraphical 

 index. The difficulties of mapping the district are also increased by 

 the fact that most of the country is covered by a deep red surface- 

 layer, which is often many feet thick, and entirely conceals any out- 

 crops over large areas. So far as is at present known the rocks are 

 wholly UD fossiliferous ; while they are highly faulted, and in many 

 cases sheared and overthrust. 



The dominant rocks of the Southern Transvaal are sandstones, 

 quartzites, and conglomerates, with here and there a few bands of 

 shales and slates. A large portion of the country is also occupied 

 by a later intrusive set of igneous rocks of basic and sub-basic types. 

 Sometimes however — as near Vredefort and to the north of Johan- 

 nesburg — the crystalline floor of South Africa, with its granites, 

 gneisses, and schists, comes to the surface from below the stratified 

 formations already noticed (see Map, PI. XI.). These crystalline 

 rocks, together with a dark-coloured limestone, become the chief 

 geological features in the Northern Transvaal, and, unlike the same 

 rocks in the south, rise up into hills of moderate height, particularly 

 round Zoutpansberg. 



The area more particularly described in the present paper is that 

 lying immediately to the east and west of the town of Johannesburg. 

 This area will be regarded as constituting the typical district for 

 the development of the stratigraphy of the Transvaal. The rocks 

 are well displayed both in the numerous mines and in natural and 

 artificial cuttings ; and the writer has had abundant opportunities. 



