424 MR. W. GIBSOJS^ ON THE GEOLOGY OF THE GOLD-BEARING 



quartzites and shales approach the gneissic rocks the more schistose 

 they become, and in no case can it be said with certainty that the 

 granite is intruded into the sedimentary beds. 



About six miles west of the Houghton Mine a conspicuous hill 

 rises up from the veldt. It is composed of white schists and quartz- 

 ites having a high dip to the south. Farther north the surface of 

 the country is composed mainly of igneous rocks. 



These igneous rocks are of two dates. The older set have been 

 intensely altered by pressure, and strike roughly east and west. 

 They appear to have been highly basic in character, and many 

 secondary minerals have been formed — chlorite, talc, and asbestos 

 being the commonest. The chlorite occurs in large plates ; the 

 asbestos fibre is short, and not of first-rate quality. 



The newer set also consists of basic rocks, but the strike is here 

 north and south, and the material is unaltered. The principal rock 

 is a diorite, which occurs in dyke-like masses and cuts across the 

 older metamorphosed series. 



2. Relation of the Quartzite and Conglomerate Series to the 

 overlying Strata. 



We have seen that the age of the quartzites and conglomerates 

 can be determined neither from the strata themselves, for they are 

 unfossiliferous, nor from the underlying rocks, as these consist of 

 gneisses and schists. All that can safely be said is that there is a 

 marked unconformity between them and the rocks below ; and it 

 is disappointing to find that in the only locality where any of the 

 overlying rocks have yet been discovered a very great unconformity 

 also intervenes. The age of the conglomerates or reef-beds is there- 

 fore left wholly undetermined. 



The unconformable beds overlying the conglomerate series are 

 the coal-bearing strata of the Boksburg and Heidelberg districts. 

 Good sections can be obtained in any of the coal-pits ;, but the 

 relation of the coal-beds to the conglomerates can be best studied 

 in the inclined shaft of the Cinderella Mine, and in the adjoining 

 property of the ' Blue Skip.' 



The section (see fig. 3, p. 416) displays about 30 feet of white 

 and yellowish- white sandstones, resting nearly horizontally on the 

 edges of the conglomerate-beds which dip 45° S.S.W. That these 

 sandstones belong to the same series of rocks as those shown in the 

 shaft of Wishaw Colliery is sufliciently clear from the contour of the 

 ground and the identity of the beds in the gold-mine and the colliery. 

 The strata in the colliery, lying above and below a seam of coal about 

 11 feet thick, consist of white and yellowish-white sandstones. 



In many places around Boksburg and Heidelberg patches of coal- 

 bearing beds exist, and in some cases are worked for coal. The 

 coal-seams are without underclays ; while olive- white and yellowish- 

 white sandstones form the interstratified layers. Compared with 

 English types the coals are of a very inferior quality, containing a 

 very large percentage of incombustible matter. On the Nigel 



