Dr. C. Sandberg—The ' Old Granite' of the Tramvaah 553 



a formation older than the "Witwatersrand Beds, because even at 

 the very base of the Hospital Hill series conglomerates are known, 

 the pebbles of which cannot obviously have been derived from the 

 Hospital Hill series by denudation." It has, therefore, I take it, 

 not been the intention of various authors to prove this self-evident 

 ti'uth, but only to try and definitely settle the still open question 

 wliether this older rock is yet represented in the geological sequence 

 of the country, and, if so, what strata should be identified as such. 

 The relative age of the ' old granite ' or ' grey granite ' has been the 

 base and the cornerstone of controversy on this subject, and it 

 therefore becomes imperative to put on record and to continually keep 

 in mind the only reliable mode of determination of the age of eruptive 

 rocks which we yet possess. It would be out of place here to insist 

 upon the origin of abyssal eruptive rocks of the granite family 

 (Rosenbusch's Tiefengesteine). 



The eternal cycle may be considered generally recognised to-day 

 as answering to the formula : Eruptive rocks are in part or in toto 

 the product of the transformation at some considerable depth and 

 under the influence of heat, pressure, time, * agents-mineralisateurs,' 

 etc., of sedimentary strata, which in their turn originated from eruptive 

 rocks by their disintegration and the transport, sorting, and redeposition 

 of the disintegrated elements. 



It is self-evident that in general the basal sedimentary strata are 

 most exposed to this intra-telluric transformation, whilst on the other 

 hand it generally requires the abyssal rocks to get at or near to the 

 surface before their disintegration can take place. Eruptive rock 

 would, therefore, always be restricted to the base of the geological 

 sequence but for the action of mountain-folding forces, and of the 

 corrosive digestion by the unconsolidated magma of the superposed 

 strata, along lines of least resistance, progressing more or less intensely 

 in different parts of the sedimentary envelope. Although consequently 

 eruptive rock is, generally speaking, a direct product of such sedi- 

 mentary rock, older than those indubitably recognisable as such and 

 covering it, it has been universally agreed to assign such geological age 

 to an eruptive rock which corresponds with the time of its consolidation. 

 This geological date is fixed by the age of the youngest sedimentary 

 deposit, traversed, injected, or altered hj the non-consolidated magma ; 

 and as it has now been conclusively proved that the action of an internal 

 magma on the overlying sedimentary strata is most erratic, so that the 

 lower strata might seem not to have been affected at all in one place, 

 whilst in its immediate vicinity conclusive evidence of its intense 

 action on the same and even very much younger strata is abundant, 

 we may not under any circumstance reverse the rule and deduct the 

 relative age of sedimentary strata from their seeming to have been or 

 not to have been affected by a given eruptive rock before its con- 

 solidation. 



Strange as it may seem, this is what has deliberately been done 

 with regard to the question of the basal sedimentary deposit in the 

 Transvaal and South Africa. 



It may be that D. Dorffel started this topsy-turvy way of reasoning. 

 It is certain that Dr. F. H. Hatch and Mr. E. Jorisse'n, relying on the 



