564 ME. D. DEAPEE OS THE OCCUEEENCE OE [Nov. 1 894, 



I have "written this short communication in the hope that those 

 •who have the opportunity of studying the geology of South Africa 

 will in future give particular attention to the ' dolomite,' if they 

 happen to come across it during their travels. 



In conclusion, I wish to record some observations that I have 

 made during my travels in South Africa (extending over a period of 

 thirty years), with regard to the surface-deposits of limestone-tufa, 

 now occupying large areas in the drainage-basin of the Vaal and 

 Orange rivers, and its relation to the dolomite. These beds of 

 travertine are found only on the surface, dotted about the country, 

 sometimes over small areas, but occasionally, as in the districts of 

 Hope-Town, Boshoff, and Jacobsvaal, and in Griqualand West, 1 

 covering almost the entire surface of the country. 



I have never found this tufaceous limestone except in such positions 

 as to render it highly probable that the supply of lime required for 

 its formation was derived from the dolomitic rocks of the Transvaal 

 or of Griqualand West. 



These surface-deposits of carbonate of lime occur at a lower level 

 than the dolomite of the Transvaal, and are never found higher than 

 that rock. Those portions of the Orange Free State and the Trans- 

 vaal that lie higher than the dolomite are devoid of the limestone- 

 tufa ; and the soil of Natal, where the dolomite has not been dis- 

 covered, is so devoid of calcareous matter that wheat will not yield 

 a crop without the application of lime to the arable land. 



The waters flowing from the dolomite of the Transvaal contain 

 so great a quantity of lime that deposits occur wherever they become 

 stagnant ; and the mining town of Johannesburg rejected a scheme 

 for supplying the town with water from Wonderfontein, on account 

 of the quantity of lime held by that water in solution. 



[For the Explanation of Plate XXIII., see p. 560.] 



Discussion (on the preceding two Papers). 



Mr. Rutlet said that the paper on ' The Occurrence of Dolomite 

 in South Africa ' was especially interesting, as it appeared to afford 

 a remarkably good instance of the replacement of limestones by 

 silica, a point which he had already dealt with in a paper recently 

 read before this Society, and in which one of the examples cited 

 was a calcareous, gold-bearing quartzite from Nondweni in Zululand. 

 Mr. Draper's suggestion that the extensive beds of calcareous tufa 

 in South Africa were derived from the waste of the neighbouring 

 dolomites, coupled with the occurrence of chert in those rocks and 

 their graduation into quartzites, seemed to make up a very com- 

 plete account of the changes which these calcareous beds had under- 

 gone. The occurrence of detached nuggets and groups of crystals 

 of gold in a talus on a hillside might perhaps be due to the gold 

 having originally occurred in limestone, which had been subse- 



1 [See G. W. Stow, Quart. Journ. Geol Soc. vol. xxx. (1874) pp. 615-617. — 

 T. R. J.] 



