418 



ME. W. GIBSON ON THE GEOLOGY OF THE GOLD-BEAKING 



River sberg ' Mine, whicli is situated on this probable fault-line, 

 consists of highly schistose and cleaved rocks very much slicken- 

 sided. 



(c) Beds near Germiston. — Near Germiston, a small village S.E. 

 of Johannesburg (see Map, PI. XL), three or four conglomerate- 

 beds are shown in the Spruit near the village.- This stream also 

 cuts through a greyish-black band of volcanic ash about 15 feet 

 thick, which breaks with a highly conchoidal fracture, and in ap- 

 pearance resembles a foetid limestone. The only other locality 

 known to the writer where volcanic ash comes in is near Boksburg 

 (see fig. 3, p. 416). As the ash-band near Germiston lies more than 

 a mile south of the Main Eeef Series, whereas near Boksburg an 

 ash-band closely resembling it, and also associated with similar con- 

 glomerates, lies only a few hundred yards south of what is mapped 

 as Main Reef Series, we have either two separate beds of ash asso- 

 ciated with conglomerate-groups and lying at different horizons, or 

 there is but one bed of ash, and the associated conglomerates are 

 the same. If the latter is the case, are we to suppose that the 

 beds are thinning out towards Boksburg, or that a strike-fault 

 brings the ash -bed at Boksburg nearer to the Main E,eef ? 



Another solution may be suggested, viz., that the conglomerate- 

 beds near Boksburg, and in fact those east of the Simmer and 

 Jack Mine (situated a little west of Germiston), are not the Main 

 E-eef Series which occurs farther north, although they are mapped 

 as such. They do not resemble the Main Reef as it occurs nearer 

 to Johannesburg, but are rather more nearly allied to the Red 

 and Yellow Reefs lying to the south. 



Unfortunately the ground north of Boksburg is covered with 

 grass, and exposures are rare. Taking into consideration, however, 

 the change of strike in the Cinderella Mine, and the appearance of 

 conglomerate-beds identical with the Main Reef Series to the N.N.E. 

 (as shown in the Chimes and Yan Ryn properties), together with 

 the fact that the ash-band is similar to that near Germiston, it 

 seems at least probable that the real Main Reef Series lies farther 

 north of the Cinderella Mine. 



4. The Conglomerates north of the Main Reef Series. 



North of the Main Reef Series conglomerates seem to have been 

 met with in only two places. One or more bands are said to exist 

 north of Durban Roodeport, but these the writer has not examined. 

 An interesting and excellent exposure, however, occurs in an 

 isolated hill behind Jeppe's Township, situated about a mile east of 

 Johannesburg. 



The hill is about a mile and a half long by half a mile wide, and 

 rises to a height of nearly 200 feet above the level of Johannesburg. 

 It is partially cleft in the centre by a gully in which a section 

 (see fig. 4, p. 419) was obtained. Two very marked groups of beds 

 are exposed. The lower one is composed of quartzites of varying 

 hardness, and quartz-schists resting on hardened red shales ; the 



