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Published online by Cambridge University Press: 01 May 2009
In discussing a paper by Mr. J. S. Curtis on the Witwatersrand Gold Deposits, Mr. George Denny has raised the point that the slates or shales of the Witwatersrand, which especially characterise, 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 Reef 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.
Abstract of a paper read before the South African Association of Engineers, Johannesburg.
page 543 note 2 “The Witwatersrand Ore Deposits and their relation to the various Formations.”
page 544 note 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 Reef at Johannesburg and the granite north of the Houghton Estate ridge, or say between the City and Suburban Township and Orange Grove. In this sense the term is synonymous 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 on the northern slope of Hospital Hill; for instance, in the Show Ground, Braamtontein. 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 would indicate whether they mean to imply the series as a whole, or a particular portion of it.
page 544 note 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 Rand Klipfontein; Mr. D. Wilkinson, who has lent me sections from Cloverfield; and Mr. Dorffel, who has been kind enough to prepare me some sections from boreholes on Brakpan, Cloverfield, Welgedacht, Grootvlei, and other eastern farms.
page 545 note 1 Van Ilise, Ct., “Some Principles controlling the Deposition of Ores”: Trans. Amer. Inst. Min. Eng., vol. xxx (1901), p. 94.Google Scholar
page 546 note 1 GEOL. MAG., 1883, pp. 322–324 (abstract).
page 546 note 2 Quart. Journ. Geol. Soc., vol. liv (1898), p. 81.
page 546 note 3 Sandberger, “Zur Theorie der Bildung der Erzgänge Berg. u. Hüttenm.”: Zeitung, xxxvi, Nos. 44–45.
