102 Dr. F. H. Hatch— Geological Hidorij oj Souf/i Africa. 



to compute. South of the Band the Klipriversberg Amygdaloid has 

 a thickness of at least 4,000 feet, and adding this to the thickness 

 of the Elsburg Series, we get a total of 7,0U0 to 8,000 feet for the 

 thickness of the Ventersdorp System as developed on the Eand.' 



The Potchefstroom System presents difficulties of another kind, 

 owing to the very variable thickness of its lower members, the Black 

 Eeef and the Dolomite Series. In the Pretoria and "Witwatersraud 

 districts the Black Beef Series is under 100 feet, even sinking to as 

 little as 10 to 20 feet, as has been proved by boreholes on the East 

 Baud - ; whereas at Pilgrims' Best Mr. Thord-Gray ^ has recently 

 estimated its thickness at 1,800 feet, while Mr. Sawyer ^ gives it as 

 1,200 feet at Chuuies Poort. The Dolomite Series has been estimated 

 in the Pretoria district at 5,000 feet by Molengraaff,'' although 

 Mr. Kynaston considers it probable that its thickness does not exceed 

 3,000 feet in that district. In the Makapau Mountains Molengraaff 

 puts it at 4,000 feet, near Godwan at 1,650 feet, in the vicinity of 

 Lydenburg at 2, GOO feet, and south of the Witwatersraud at 2, GOO 

 feet. Mr. Thord-Gray gives its thickness at Elandsfontein in the 

 Lydenburg district as 3,000 feet, while Mr. Holmes informs me that 

 from careful measurements he has made in the Marico district, where 

 the series exists in very regular order in the Dwarsberg, he estimates 

 the thickness of the Dolomite at as much as 7,500 feet, the Black 

 Beef Series being very thin there, not more than 75 feet. • Taking 

 the thicknesses given above, it would appear as if where the basal 

 quartzites are largely developed, as in the Lydenburg district, the 

 Dolomite suffers a corresponding diminution, and where the Dolouiite 

 is thick the Black Beef is poorly developed. Together, the maximum 

 thickness of the two formations probably does not exceed 8,000 feet. 

 In Giiqualand West Messrs. Bogers and Schwarz have found the 

 limestone series, " measured from the quartzites, below which no 

 limestone is seen, to the lowest jaspers or magnetite quartzites of the 

 Griquatown Beds (Pretoria Series)," to have a thickness of 5,000 

 feet, while the underlying quartzite (Black Beef Series) varies from 

 200 to 2,000 feet.« 



The Pretoria Series is estimated by the Transvaal Geological 

 Survey to haA^e a thickness of not less than 10,000 feet.' Mr. Ilolmes 

 has measured this series in the Dwarsberg (Lotteringskop), and 



1 F. 11. Hatch & G. S. Corstorphine, " The Geology of the Bezuideuhout Valley 

 and the Distiict Eas^t of Johauuesburg " : Trans. Geol. See. S. Afr., vol. vii (1904), 

 pp. 97-109. 



* F. H. Hatch, " The Extension of the Witwatersrand Beds eastward under the 

 Dolomite and the Ecca Series of the Southern Transvaal '": Trans. Geol. Soc. S. ASr., 

 vol. vii (1904). p. 63. 



' I. Thord-Gray, " Notes on the Geology of the Lydenburg Goldfields " : Trans. 

 Geol. Soe. S. Afr., vol. viii (190ii), p. 66. 



* A. R. Sawyer, " The Geology of the Transvaal " : North of England Inst. Min. 

 Engineers, 190;"). 



^ G. A. F. Molengraaff, " Geology of the Transvaal " : Johannesburg, 1904. 



^ A. W. Ivogcrs & E. H. L. Schwarz, " Geologv of the Orange Eiver Valley in 

 the Hopetowu and Prieska Districts " : Ann. Eep.^Geol. Comm. Capo Colonv, 1S99, 

 p. 80. 



' Transvaal Geol. Surv. Rep., 1903, p. 40. 



