Chapter VII. 

 THE BASIC AND ULTRABASIC INTRUSIVE ROCKS OF AFRICA. 



The basic and ultrabasic intrusive rocks of South Africa belong to several diverse series, of 

 which the most ancient are those occurring in the schistose complex forming the foundation-rocks 

 of the northern part of South Africa, and termed the Swaziland series. Of this the sedimentary 

 portion consists of slates and phyllites, quartzites, and conglomerates, passing into mica-schists. 

 It was invaded by a group of massive basic igneous rocks, which in turn were invaded by granites, 

 now more or less gneissic. The basic rocks form long narrow belts, the structure-planes of which 

 are approximately parallel to those of the associated sediments, and among these the belts of ser- 

 pentine appear to be intrusive into the hornblende- and chlorite-schist with which they are asso- 

 ciated. The whole complex seems to give evidence of primary gneissic structure, due to intrusion 

 under pressure. The granitic masses have invaded this transgressively. The relationships are 

 clearly indicated at Barberton in the eastern Transvaal (Hall 'IS), near Pretoria (Kynaston 

 '06), probably also near Vryburg (Du Toit '05a), and in Southern Rhodesia (Zealley '14). 



The Swaziland series of sediments is followed by the Witwatersrand system of shales, 

 quartzites, and conglomerates with interbedded diabase, which are unconformably overlain by 

 the Ventersdorp series of acid and basic lavas, occasionally with pillowy structure (Du Toit 

 '05) , breccias, tuffs, and conglomerates, and these again are followed by the Transvaal system 

 of conglomerates, dolomite, and the Pretoria series of slates and quartzites with abundant sills 

 of diabase. Grits and sandstones of the Waterberg system (of possibly Devonian age) follow 

 more or less unconformably upon these, the two formations being separated over wide areas 

 by the intrusive rocks of the Bushveld complex. Probably intermediate in age between the 

 Bushveld and Swaziland igneous rocks are those of the Palabora complex to the extreme east 

 of the Transvaal. No detailed account of the tectonic relationship of these can be given; the 

 complex appears to be intrusive into the older Swaziland granites and to consist of granite 

 and syenite, the latter passing by imperceptible gradations into a felspar-less pyroxenite 

 (Hall' 12). 



The Bushveld plutonic complex is one of the most remarkable assemblages of igneous 

 rocks known. (See fig. 14.) Briefly it consists of a great oval mass, 250 miles in length east 

 to west, with a mean width of 60 miles v Its central portion consists of granite, and its marginal 

 portions are largely of norite. All around it the rocks of the Pretoria series dip in toward 

 the center at low angles, while above it the Waterberg rocks lie with comparatively small dis- 

 turbance, and show that they were invaded by the granites. Molengraaff ('01) interpreted the 

 whole as a vast gravitationally differentiated laccolite thrust in between the Pretoria and 

 Waterberg systems. While this conception is fundamentally that now generally accepted, 

 modern work has modified it in many respects. Of this the following is a brief resume: 1 



The abundant sills of dolerite in the Pretoria system beneath the complex are genetically 

 connected with it. Over the greater part of the circumference of the complex, the norites, etc., 

 pass into dolerites of an exactly similar character which form the fine-grained chilled selvage 

 of the complex (often of considerable width) within which the more slowly cooled magma dif- 

 ferentiated (Humphrey '10, Hall '05, Kynaston '04). While the base of the complex is approxi- 

 mately concordant with the stratification of the Transvaal system, if a broad view be taken, a 



1 Grout ('18) considers this complex may be compared with the Duluth and Sudbury complexes to which he ha: applied the term "lopolith" 

 to indicate a generally concordant lenticular mass, with a sunken center, of which the mechanism of intrusion is presumed to be different from that 

 of a laccolite. See p. 53 and also postscript on p. 78. 



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