October 31, 



1895] 



NA TURE 



639 



Witwatersrand deposits, the development and prospects 

 •of deep-levels, mining practice, surface equipments of the 

 mines, the metallurgical treatment of the ore, economics, 

 mining law and statistics. 



Unfortunately for students of South African geology, 

 much confusion results from the fact that beds of an 

 identical character often receive different names in 

 different localities. The inconvenience of this want of 

 uniformity in the classification of the rock systems will 

 now, it is hoped, be obviated, as the authors' clear 

 exposition of South African stratigraphy cannot fail to be 

 ;<enerall\ accepted. The geology of South Africa is, it 

 may be noted, comparatively simple. The main sub- 

 divisions are ( l ) recent deposits ; (2) the Karroo formation ; 

 (3) the Cape formation, and (5) the South African 

 primaiy formation. The sedimentarj' deposits are under- 

 lain by granites, gneisses, and crystalline schists, which 

 constitute the greater portion of the formation of north- 

 west central .'Vfrica. This primary formation occurs 

 largely in Mashonaland, Matabeleland, and the Mozam- 

 bique, and predominates in the northern and eastern 

 parts of the Transvaal. Lying unconformably on these 

 beds are the shales, sandstones, conglomerates and 

 limestones of the Cape formation, which extend over the 

 southern, western, and middle parts of the Transvaal. 

 They appear to be of an age corresponding with the 

 Devonian and Lower Carboniferous periods of European 

 classification. The Karroo formation, which may possibly 

 be correlated with European Lower Mesozoic formations, 

 has a widespread occurrence in Cape Colony, Natal, the 

 southern Transvaal and the Orange Free State. It 

 derives its importance for the Transvaal from the fact 

 that it carries the coal-seams that have rendered such 

 valuable aid to the development of the auriferous deposits. 

 Lastly, the recent deposits comprise those of alluvial 

 and ;eolian origin, together with the curious surface 

 material to which thcj authors apply the somewhat mis- 

 leading name of " lateritc.' This material is widely 

 distributed throughout the Transvaal. The gold of the 

 Witwatersrand is obtained entirely from beds of con- 

 glomerate, known as "banket," carried by the Cape 

 formation. These are composed mainly of pebbles of 

 white or grey quartz embedded in a matri.x consisting 

 originally of sand, but now completely cemented to an 

 almost homogeneous material by a later deposition of 

 c|uartz. The pebbles as a rule do not carrj' any gold, the 

 mineralisation being confined to the matrix. The average 

 total yieUl of the conglomerate stamped last year was 

 I3'i6 dwts. of fine gold per ton. With regard to the 

 origin of the ore-bodies, the authors enumerate the 

 various hypotheses without giving their support to any 

 one of them. They have, however, been unable to find 

 any evidence in favour of the idea locally prevalent that 

 the dykes met with, have acted bencfically on the banket 

 in their immediate neighbourhood in regard to gold 

 contents. Tetrologically the dykes belong to the group 

 of dark-coloured greenstones, among which the authors 

 have recognised the following types : diabase, olivine- 

 diabase, bronzite-diabase, epidiorite, gabbro and olivine- 

 norite. 



One of the most interesting chapters in the book is 

 that on the development and prospects of deep-levels. 

 As the bedded character of the banket deposits became 

 NO. 1357, VOL. 52] 



known, and as the persistency in depth and the uniformity 

 in the gold-contents became established by deep bore- 

 holes, companies were organised to work the deep-seated 

 portions of the beds. In discussing the depth at which 

 the main bed will be found, the authors bring forward 

 evidence to show that a ver)' important flattening of the 

 bed takes place. They therefore take a more optimistic 

 view of the future of the gold-mining industry than that 

 taken by other writers. The most important problem 

 that presents itself is to ascertain the limit in depth to 

 which mining may profitably be carried. The limiting 

 factors are increased temperature, excessive initial 

 expenditure, and increase of working costs. The rise in 

 temperature with increasing depth must, the authors 

 think, be ascribed almost entirely to secular causes. 

 Unfortunately very few experiments have been made to 

 gauge the rate of increase. Mr. Hamilton Smith in 1894 

 made some determinations of the water in the Rand 

 Victoria borehole at a depth ot 2500 feet, the results 

 indicating an increase of i' K. for every 82 feet. Some 

 rough determinations, too, have been made by Mr. A. Y. 

 Crosse at the Ferreira and Crown Deep shafts. In view 

 of the scientific interest and commercial importance of 

 the matter, it is to be hoped that an accurate determina- 

 tion of the temperature will be made at the bottom of 

 the borehole which is now being put down to intersect 

 the main bed at a depth of 3500 feet. At present, ex- 

 perience in other countries is the only available guide, 

 and it is to be regretted that such results collated by the 

 authors are very incomplete. A table of temperatures in 

 some deep European and American mines is given 

 (p. 104), but this, being disfigured by gaps and misprints, 

 such as St. Andre for St. Andreasberg, Prizebram for 

 Przibram, Sanson for Samson, Lambert for Charlcroi, 

 does not carry much weight. Nor are the shafts of 

 the Michigan copper mines fair illustrations to select, 

 inasmuch as the coolness of the rock is undoubtedly due 

 to the proximity of the cold waters of Lake Superior. 

 The authors' statement that at the Calumet and Hecla 

 shaft, Michigan, there is a rise of only 4^ F. in a depth of 

 4400 feet, is certainly inaccurate. The temperature 

 determination must have been influenced by the fact that 

 compressed-air rock-drills are m use at that mine. The 

 ice-cold exhaust would lead to erroneous results. The 

 usual geothermic gradient is 50 to 55 feet for an increase of 

 temperature of 1° F., and the lowest recorded is that of 100 

 feet to r F., at the Lake Superior copper mines. It would 

 appear, therefore, that in assuming it to be somewhat less 

 t han this in the Rand, the authors are taking too optimistic 

 a view, more especially as Mr. Crosse's determination 

 (p. 103) of bb-f F. at 825 feet, and 707° F- at 1030 feet, 

 indicate the normal gradient of 50 feet to 1° F. In the 

 discussion of this important subject, the authors might 

 have referred with advantage to Kocbrich's 3S7 deter- 

 minations of temperature in the Schladebach borehole. 

 These arc of special importance, as they were taken at 

 fifty-eight points at equal distances of 30 metres down to 

 the greatest depth yet attained of 1716 metres. The 

 result of this investigation was that the gradient was 

 found to be 4609 metres for 1° R. 



The chapters describing mining practice, surface equip- 

 ment, and the metallurgical treatment of the ore, occupy 

 more than a third of the volume. Admirably illustrated by 



