646 



NA TURE 



[October 25, 1906 



GEOLOGICAL STUDIES IN SOUTH AFRICA. 

 T^HE Report of the Geological Survey of the Transvaal 

 -*• Mines Department for 1904 (Pretoria, 1905, price 

 ys. 6d.) is a folio volume, issued at a very moderate 

 price. It contains twenty-three plates, from which our 

 figures are reductions, and two large coloured maps, 

 the latter being conveniently placed in an envelope 





at the end. The cessation of topographical work in the 

 country must in future hamper systematic geological 

 m.ipping, and the Geological Society of South Africa has 

 .ilready approached the Colonial Secretary in Pretoria on 

 the subject (Proc. Geol. Soc, S. Africa, 1906, p. liv). It 

 might be thought that military con- 

 siderations alone would be sufficient to 

 place an accurate map among the first 

 rctiuirements of the colony. 



The director of the Survey, Mi. 

 Kynaston, describes a traverse of the 

 country between Pretoria and Pieters- 

 burg, during which he visited the re- 

 markable Salt Pan, some twentv-five 

 miles N.N.VV. of Pretoria. This lake, 

 which is about as salt as the Dead Sea, 

 lies in a circle of granite hills, 250 feet 

 below their crest, and about 200 feet 

 below the general level of the countr\ 

 on their outer side. Its salts include 

 72.70 per cent, of sodium chloride and 

 2725 per cent, of sodium carbonate. 

 Except that an explosive origin ha> 

 been suggested by Cohen, no adequate 

 explanation of the hollow is as yd 

 forthcoming. Considering, moreover, 

 the antiquity of the last volcanic erup- 

 tions in this area, a crater of explosion 

 ought to have become long ago filled 

 up by products of denudation. One 

 feels tempted to ask if it is possible 

 for the materials filling an igneous 

 neck to sink back long after thov 

 have solidified. Could the fragmenlal 

 materials so common in South African 

 pipes behave in this way? Mr. A. W. 

 Rogers has cited cases where the 

 weathering of these necks has caused hollows at the 

 surface ; but the Salt Pan near Pretoria is 200 feet to 

 250 feet in depth. 



On Mr. Kynaston's retur-n journey from the mouth of 

 the Elands River, the most northerly known outliers of 



the glacial Dwyka conglomerate were traversed (Fig. 2). 

 The boulder-bed, as described in supplementary notes by 

 Mr. Mellor, does not seem more than 50 feet thick, and 

 is associated with sandstones. It was laid down, as in 

 other cases, on a land-surface eroded by streams, and the 

 original topography is now being revealed by the denuding 

 action of the Elands River and other agents. 



.Mr. A. L. Hall (p. 37) describes the 

 geology of the tin fields north-east of 

 Pretoria, where the ore occurs promis- 

 ingly in a rock of greisen type; and 

 Mr. Mellor (p. 45) deals with the pic- 

 turesque area of Rhenoster Kop. The 

 Permian glacial striation in this dis- 

 trict, seen on the uptilted Waterberg 

 Sandstones, has an almost constant 

 direction of S. 33° E. After other 

 papers on special districts, Mr. S. M. 

 Tweddill describes some of the rocks 

 collected, and has been allowed a hand- 

 some series of photographic plates, 

 showing his thin slices in ordinary light 

 and with crossed nicols. The latter 

 figures are produced by the three-colour 

 process, but it is questionable if much 

 is gained by them. Colour-photographs 

 of the sections in ordinary light would 

 probably be more effective, and would 

 equally serve to confirm the author's 

 -determinations. The " acicular crystals 

 of Clay-slate " in the description of 

 Plate XV. a puzzle us not a little, 

 especially as in the text on p. 76 the 

 " stellate forms " are similarly said 

 " to be clay-slate." In a country 

 I where everything depends on field- 



relations, where the scale of pheno- 

 mena is large, where the mention 

 new discovery suggests comparison with some- 

 else a thousand miles away, one probably ex- 

 pects too much from the petrographer. One can imagine 

 the prospector, who has returned bronzed and muscular 

 after his days upon the veld, reading the bare descriptions 



of 

 thing 



^^I^^lH 



dfel^>aLI>»^^— A 



Elands Kv 



, sdowmg toe original boulders, Toitskraal, 



NO. 1930, VOL. 74] 



of rocks with a certain irritation. If they could be inserted 

 in connection with the account of the masses in the field, 

 their true interest would at once appear, for the South 

 African of all men has a pleasurable keenness for geology. 

 This fact is well attested by the publication of the dis- 



