35*^ OKICIN OF RAND liAXKFTS. 



mountains of Cape Colony, and the inference, therefore, is, that 

 the Witwatersrand Y^eds have heen intensely folded. 



The Transvaal laccolite or, as it is often called, the igneous 

 complex of the Bushveld, forms a solid mass of igneous rock 

 actually 255 miles^ long from east to west ; it is of an average of 

 80 miles broad, but in the centre, on the north, there is a great 

 extension, making the total north and south length only a little 

 shorter than the east and west one. It is composed of granite 

 with dark red felspar ; hence the frequent use of the term red 

 granite, to indicate this rock in contrast to the older grey granite 

 All round the rim of the main portion, excluding the northern 

 extension, there is a broad selvage of basic rocks, principally 

 norite. The basic rocks vary in basicity through olivine rocks to 

 actual masses of magnetite and chromite, and the granites, also, 

 vary in acidity to rocks composed of quartz with only a small 

 proportion of felspar, the ultra-acid rocks. The body of the 

 laccolite rests on the rocks of the Transvaal System, the Black 

 Reef c[uartzites below, the Dolomite above, and the highly 

 ferruginous sediments of the Pretoria Series on top. Into these 

 latter there have been intruded a vast series of sills or dykes 

 running concordantly with the bedding of the sedimentary rocks ; 

 these are of diabase or altered dolerite. 



The arrangement of igneous rocks in this great laccolite is 

 in accordance with similar occurrences elsewhere; that is to say, 

 Rosenbusch's law of decreasing basicity in the order of con- 

 solidation of the minerals and the rock types composed of the 

 minerals, is reversed. As a matter of fact, Rosenbusch's law 

 only obtains in Plutonic masses, and then by no means universally ; 

 in masses of fluid rock crystallising untler dyke conditions the 

 basic rocks appear always at the edge, and .the centre, which 

 remained liquid long after the outside had consolidated, crystal- 

 lises as acid rock. It seems possible that this differentiation in 

 the magma is not original, but that the outer edge of the liquid 

 mass incorporates portions of the surrounding rocks ; in the 

 present case the vast addition of lime, magnesia and iron to the 

 liquid magma by the incorporation of the Dolomite and Pretoria 

 Beds would seem more naturally to account for the basic and 

 ultra-basic selvage, the more so as where the granite impinges on 

 the siliceous lavas or the sandstones of the Waterberg. there no 

 basic selvage makes its appearance. The inten&e viscosity of 

 liquid siliceous magmas would prevent the assimilated lime, 

 magnesia and iron salts from mixing with the more siliceous 

 interior, and the lower melting-point of the basic rocks would 

 cause that portion rich in these bases to crystallise readily, so 

 that on this view the sharp demarcation between the acid centre 

 and basic selvage can very well be explained ; better, in fact, than 

 on the magmatic segregation view. 



The question arises, if the basic selvage were derived from 

 the assimilation of the sedimentary rocks by the siliceous igneous 



