February lo, 1910] 



NA TURE 



443 



RECENT WORK OF GEOLOGICAL SURVEYS.' 

 II. 

 South Africa and Australasia. 

 T^HE Geological Commission of the Cape of Good Hope 

 ■*■ has issued Sheets 33 and 41 of the geological map 

 01 tlie colony on the scale of 1/238,000, covering the 

 country to the north-east and south-east of Prieska, and 

 just reaching to De .Aar. .A long stretch of the Orange 

 River wanders through Sheet 41, and its excavating action 

 has here and there revealed the rocks below the prevailing 

 Dwyka series. To the north-west the ground rises, and 

 the Transvaal system, with the dolomites, appears in force. 

 Sheet 33 is a remarkable performance considering the 

 small staff of the Survey, and shows the sinuous outcrops 

 Df the Karroo doleritic sheets invading the Ecca shales 

 and the Dwyka series over 3000 square miles of country. 

 Complicated inliers of the older rocks appear in the north- 

 west. The country shown in these maps is described in 

 the .Annual Report of the Geological Commission for iqo8 

 (1909). The ring-like outcrops of the igneous sills in 

 Sheet 33 are explained by the presence 

 of tectonic basins, whereby an escarp- r 

 ment of dolerite, facing outwards, 

 encloses an a^ea of overlying Ecca 

 shales. On p. iii of the report Mr. 

 A. L. du Toit describes the kimber- 

 lite and allied pipes and fissures in 

 the Prieska-Britstown region, giving 

 interesting sections to show the typical 

 dome-formation in the strata where the 

 pipes have broken through. A careful 

 study has been made of the inclusions 

 of garnet-granulite and eclogite that 

 are so characteristic of the pipes, and 

 the author shows that they have been 

 brought up from rocks, possibly of 

 sedimentary origin, that were metamor- 

 phosed at considerable depths. Dr. 

 A. W. Rogers (p. 135) has re-examined 

 the Tygerberg near Prince Albert 

 Village, and continues to maintain, in 

 opposition to Dr. Sandberg, that the 

 structure is that of an ordinary anti- 

 clinal (see Nature, vol. Ixxvi., p. 423). 



The Geological Survey of the Trans- 

 vaal Mines Department shows great 

 vitality. The large areas mapped and 

 reported on in a year may be seen in 

 the maps recording progress, appended 

 to the reports for 1907 and iqoS. In 

 the report for 1907 (received in July, 

 1909) attention is properly directed by 

 the Secretary for Mines to the inter- 

 relation of the so-called scientific and 



economic duties of the Survey. From time to time a brief 

 defence of this kind becomes necessary ; but it requires some 

 delicacy, since few geologists would like to set down in print 

 the extraordinary ignorance of earth-structure prevailing 

 among prospectors and " practical men." Support could 

 always be given, however, to colonial surveyors by those in 

 European countries, who know how small details of dip, 

 unconformity, or rock-sequence shown upon their laborious 

 large-scale maps, or perhaps revealed by pateontological 

 studies, prove again and again of service to inquirers in 

 directing their operations or in diverting them to more 

 profitable fields. Geological surveys play, moreover, a 

 high educational part in explaining the features and 

 historical development of a country, and it should be the 

 pride of a growing nation to allow natural sciences to take 

 the place that has been accorded to dead languages in 

 more conservative lands. In the report for 1907 Mr. 

 A. L. Hall describes the geology of the Haenertsburg 

 goldfields, and shows (p. 53) how the stratified rocks of 

 the Pretoria seriea have been metamorphosed by the Bush- 

 veld granite, yielding every variety of type, from bedded 

 shales with new minerals to strongly marked schists and 

 gneisses. He urges that, while pressures have operated 

 during this contact-alteration, dynamic metamorphism is 

 insufficient to account for the phenomena. The rocks illus- 

 I Continued from p. 382. 



NO. 2102, VOL. 82] 



tiate the spread of material along schistose surfaces per- 

 pendicular to the pressure (Rieke's principle), and the 

 production of minerals of small molecular volume 

 (Lepsius's law of volumes). We should like, however, 

 to hear more of the relation of the foliation-planes of the 

 most altered types of shale to the original bedding, since 

 we beheve that these features are frequently coincident in 

 highly altered contact-rocks. Chromite occurs concentrated 

 in bands in the hypersthenite of the Lydenburg district 

 (p. 59), and similar bands are described by Mr. Humphrey 

 (p. 89) in the norite of Kroonendal, near Rustenburg. 

 .Mr. Hall has been able to extend his conclusions as to 

 the potency of contact-metamorphism to the country south- 

 west of Rustenburg (p. 66). Mr. Kynaston reports on 

 the cassiterite deposits in the Waterberg district, where 

 the hilly ground falls northward to the Limpopo. The 

 ore is found in granite in pipes of circular or oval section, 

 which are formed of highly altered granite, with ore con- 

 centrated towards their margins (pp. 97 and 100). These 

 are compared with smaller occurrences in New South 

 Wales. Our knowledge of Waterberg is extended by 







Kopje of Waterberg Sandsl 



, country wes: of Potgietersrust, Transvaal, 



Messrs. Kynaston and Mellor in the report for 1908. The 

 Waterberg Sandstones, which are somewhat like our Old 

 Red Sandstones, form characteristic tower-like kopjes 

 (Fig. i). The pipes of tin-ore occur on each side of the 

 granite ridge that runs north-west near Zaaiplaats, and 

 are already being well developed (p. 46). In a country 

 where fossiliferous strata are so rare, petrography naturally 

 plays a large part, and the metamorphic zones of both the 

 older and younger granite receive careful description in 

 various papers in this report. The younger granite of 

 Waterberg, a handsomely red rock, yielded boulders to 

 the Upper Waterberg series, but continued to send off- 

 shoots into this series in the Hoekbergen and the Middle- 

 burg district to the south-east. Deposits of manganese, 

 zinc, and lead ore, and of gold are described by Mr. W. A. 

 Humphrey from the Marico district away west; the 

 Malmani goldfield, which has been worked sporadically, 

 lies not far from Mafeking. These reports, which are 

 priced at only 7s. 6d. each, are, as usual, well illustrated 

 by landscapes and by coloured maps and sections. The 

 Survey has also issued a special octavo memoir on the 

 Watei-berg tinfields (1909, price 7s. 6d.), in which the 

 conclusions and illustrative sections are reproduced in a 

 convenient form. The ore-bodies are discussed in detail, 

 and a small example of the remarkable cylindrical pipes 

 is well shown in a photograph in Plate viii. 



