152 



NATURE 



[OctOBER 8, I914 



GEOLOGY IN BRITISH AFRICA. 



MEMOIR No. 6 of the Geological Survey of South 

 Africa, by A. L. Hall, on "The Geologj' of 

 the Murchison Range and District," deals with pic- 

 turesque features of the Drakensberg scarp. Plate xv. 

 finely illustrates the youthful nature of the Grool 

 Letaba River (Fig. i), which is represented (p. 29) 

 as having cut back into the mature drainage-system 

 of the M'Thlapitsi. The banded ironstones near 

 Thabina (p. 66), associated with metamorphosed sedi- 

 ments, are regarded as also of sedimentary origin, 

 although they are now composed of magnetite. We 

 believe that this view must be generally accepted, and 

 it has an obvious bearing on theories of the 

 origin of the sheet-like magnetite ores of Sweden. 

 Pp. 124 to 130 contain an interesting account of the 

 formation of hj^brid rocks between syenite, pyroxenite, 

 and limestone. The limestone now contains olivine, 



Fig. I. — Gorge cut by the Groot Letaba River in its recession from the 

 escarpment ; the stream now receives water captured from the 

 M'i'hlapitiii. 



magnetite, and apatite. The Olifants River mica- 

 fields come within the scope of this memoir (p. 153). 

 We notice (p. 19) that " neck " is used, perhaps un- 

 wisely, in its English form for a notch produced by 

 weathering. 



Sheet 13 of the geological map, and the accompany- 

 ing memoir on "The Geology of the Haenertsberg 

 Goldfields " (1914), are concerned with similar country, 

 and the great scarp of the Black Reef quartzite 

 appears conspicuouslv on the map. The memoir and 

 map (Sheet 12) of the Pilandsberg, a great igneous 

 centre north of the Witwatersrand and Rustenburg, 

 are due to W. A. Humphrey, and direct attention to a 

 region of great petrographic interest. The rocks have 



NO. 2345, VOL. 94] 



been previously described ; they are syenites rich in 

 alkalies, forming a great wall-like ring that sur- 

 rounds a region of tuffs and lavas, also of the alkali 

 type. Aegirine, leucite, and nepheline occur in the 

 volcanic rocks. The ring suggests a "cauldron-sub- 

 sidence," and an oozing up of matter along the mar- 

 ginal crack, as described by E. B. Bailey in the 

 Cruachan area. The circular group of rocks is shown 

 to be later than the Waterberg system, but has no 

 kinship with the Drakensberg outflows. The map 

 of this basal relic, sixteen miles across, forms one of 

 the most striking lessons in volcanic structure with 

 which we are acquainted. 



The annual report of the Geological Survey of South 

 Africa for 1912, published at the close of 1913, contains 

 a summary by the director, H. Kynaston, both in 

 Dutch and English, a memoir by him on the Marico 

 and Rustenburg districts, one on the western Wit- 

 watersrand by E. T. Mellor, and a finely illustrated 

 memoir by A. T. Hall on the country between Belfast 

 and Middelburg. A. L. du Toit reports on Pondoland. 

 It is difficult in a few notes to do justice to the large 

 amount of mineral and petrographic information issued 

 regularly in the Transactions of the Geological Society 

 of South Africa. In vol. xiv. (1912) p. 71, G. S. 

 Corstorphine records a further occurrence of diamond 

 in eclogite from the kimberlite pipe of the Roberts 

 Victor Mine, Orange Free State. He still maintains 

 that such eclogites are segregations from the perido- 

 tite (kimberlite) magma. C. T. Mellor (p. 99) con- 

 tributes a detailed revision of the Lower Witwaters- 

 rand system, with a map and sections. He concludes 

 that the conglomerates indicate a steady progression 

 from deposits on the seaward margin of a delta up to 

 true beaches on a shore. In vol. xv., p. 31, A. W. 

 Rogers publishes his correlation of the Nieuwerust, 

 Malmesbury, and Ibiquas Series of the Cape Province 

 with the Nama system of German South-West Africa. 

 The three series are now shown to have the sequence 

 given above, the Nieuwerust beds being the oldest. P. 

 Range's paper (p. 63) on the topography and geology 

 of the German South Kalahari contains an interest- 

 ing account of the nature of "pans." The author 

 points out that good supplies of water are within reach 

 of the dry lands of the Kalahari, in the Karroo and 

 Nama beds that underlie the calcareous sandstone and 

 dune-sand of the surface. 



The Proceedings of the society include a lengthy 

 discussion of Mr. Mellor's stratigraphical paper, and 

 a presidential address by R. B. Young on the problem 

 of the Rand Banket (191 1, p. xxi.). Dr. Young argues 

 against the detrital origin of the auriferous pyrite in 

 the conglomerates. In vol. xv. (p. 83) he urges that 

 the black colour of the quartz in the banket is due 

 to dark inclusions, and he shows by thin sections how 

 these are commonly related to shattered areas in the 

 pebbles. The colour, then, arose after the vein-quartz 

 had become included in the conglomerate. 



Prof. Schwarz {ihid., vol. xvi., 1913, p. 33) revives 

 interest in the contacts of granite and schist near 

 Cape Town, which were described by Play fair and 

 Basil Hall in 1813, and by Clarke Abel in 1818. He 

 does not mention Darwin's fruitful observations; but 

 he accepts the view that the granite has absorbed 

 the slates, and that the parallel flakes of biotitic 

 matter represent residues from the sedimentary' series. 

 The advance of the granitic matter into the slate has 

 allowed of the development (p. 35) of felspar crystals 

 2 in. long as replacements of digested matter. F. E. 

 Studt provides (p. 41) an Important regional paper on 

 Katanga and Northern Rhodesia, involving a review 

 of South African structure as a whole. He believes 

 that the extensive lava-flows of East Africa were con- 

 nected with subsidences which date back to Middle 



