544 Dr. F. H. Hatch — Witwatersrand Beds, Transvaal. 



but are not confined to, the Lower Witwatersrand Beds (Hospital 

 Hill Series),^ are not of sedimentary, but of igneous origin. He 

 relies chiefly on the fact that he has observed in places that a so-called 

 band of slate cuts across the bedding of the quartzites. I think 

 •we are all prepared to agree with him that, where he can point out 

 that this occurs, the rock which traverses the bedded formation must 

 be an igneous intrusion ; but such cases are rare. In the vast 

 majority of sections where the slates are exposed, they are found to 

 occur truly bedded, and in conformable relation with the quartzites 

 with which they are associated. It seems to me that, after all, this 

 is in the main a petrological question, which can be easily settled by 

 the examination of the rocks in question under the microscope. 

 With this end in view I have examined a number of thin sections 

 of these rocks, in all cases prepared from the cores of boreholes, on- 

 account of the difficulty of obtaining near the surface specimens 

 sufficiently fresh and unweathered for microscopic examination ; and 

 I have selected geological horizons which are well known on the 

 Witwatersrand. They are (1) the band of slates which occurs in the 

 neighbourhood of the Bird Eeef Series, and (2) the slates which 

 occur in the footwall of the Main Reef itself, in both cases in the 

 eastern portion of the Witwatersrand, as at Van "Ryn and Geduld. 

 In all I have examined thirty sections of slates.^ 



A surprising fact is at once revealed by the microscope, viz., 

 that the slates, both in point of composition and structure, are 

 closely related to the quartzites with which they are associated. 

 We find that both quartzites and slates are composed of angular 

 to subangular fragments of quartz imbedded in a microcrystalline 

 to cryptocrystalline ground-mass. There are occasional flakes of 

 a yellowish-green pleochroic mica, and iron pyrites occurs abundantly 

 as an accessory constituent. Where the slates have a greenish tinge, 



1 There is evidently some confusion existing among writers on "Witwatersrand 

 geology as to the use of the terms "Hospital Hill Series," "Hospital Hill Shales 

 or Slates." Personally I have always used the expression "Hospital Hill Series" 

 to include the whole of the slates and quartzites which occur in alternating belts 

 between the outcrop of the Main Eeef at Johannesburg and the granite north of the 

 Houghton Estate ridge, or say between the City and Suburban To-miship and Orange- 

 Grove. In this sense the term is sjnionymous with " Lower "Witwatersrand Beds," 

 as used above. Others, no doubt, use the term "Hospital Hill Shales or Slates " to 

 denote one particular belt of striped ferruginous shales which in Johannesburg is- 

 characteristically developed ou the northern slope of Hospital Hill ; for instance, in 

 the Show Ground, Braamfontein. It is not so clear what is meant when the name 

 " Hospital Hill Shales" is applied to ferruginous slates in other parts of the country 

 (e.g. in the Heidelberg district), for although such beds belong no doubt to the 

 Lower "Witwatersrand Beds or Hospital Hill "Series," it is by no means certain 

 that they can be correlated with that particular slate belt which occurs on the 

 northern slope of Hospital Hill in Johannesburg ; for banded ferruginous slates are 

 not confined to one horizon in the Lower 'Witwatersrand Beds. Confusion would be 

 avoided if writers woidd indicate whether they mean to imply the series as a whole, 

 or a particular portion of it. 



2 I have to thank Mr. Holford, who has courteously placed at my disposal his 

 collection of rock- sections from borehole cores from the Boksburg Gold Mines and 

 Eand Klipfontein ; Mr. D. Wilkinson, who has lent me sections from Cloverfield ; 

 and Mr. Dorifel, who has been kind enough to prepare me some sections from 

 boreholes on Brakpan, Cloverfield, "Welgedacht, Grootvlei, and other eastern farms. 



