Reviews — Geology of TransJcei, South Africa. 135 



IV. — The Geology of Part op the Tkanskei. Explanation of 

 Sheet 27, (Cape) Maclear — Umtata. By A. L. du Toit; with 

 an introduction by A. "W". Eogers, Geological Survey, pp. 32. 

 Pretoria, 1917. Price 2s. 6d. 

 rpHIS memoir is to accompany the map on the scale of 3*75 miles 

 JL to an inch, prepared by Dr. du Toit. The map contains a very 

 large amount of detail, considering the character of the country, 

 which includes part of the great Drakensberg escarpment and rises 

 to a height of some 9,000 feet. The area surveyed is entirely com- 

 posed of the rocks of the Karroo system and their accompanying lavas, 

 ashes, and intrusions. The whole of the Karroo system is repre- 

 sented and reaches the great thickness of 14,000 feet, exclusive of 

 the Stormberg lavas, Avhich are about 3,000 feet more. The strata 

 are normal in character and contain plants and reptiles, hence 

 horizons can now be fixed with fair accuracy, since it has been found 

 possible to establish reptile zones in this formation. Some coal- 

 seams of workable thickness are found in the Molteno beds. A con- 

 siderable number of volcanic necks have been located, and the Karroo 

 dolerites have been intruded on an enormous scale, chiefly in the 

 Ecca and Beaufort series. 



Perhaps the most interesting part of the memoir is the description 

 of the copper-nickel betrring area of Insizwa and Tabankulu. The 

 ores occur at the lower contact of great cakes of gabbro-norite, a 

 special phase of the Karroo dolerites, intruded into the lower division 

 of the Beaufort beds. These masses, which are about 3,000 feet 

 thick, have undergone magmatic differentiation by gravity, the 

 lower part being a picrite, followed by olivine-norite and norite ; at 

 the top there is even a little quartz as raicropegmatite. The Insizwa 

 mass is some ten miles in diameter. The ores occur at the base of 

 the intrusion and also to a certain extent in the- country rock close to 

 the contact. The principal minerals are pyrrhotite, chalcopyrite, 

 and pentlandite, with smaller amounts of niccolite and bornite and 

 some oxidized copper and nickel minerals. Platinum has been found 

 by assay up to 1 oz. per ton, but it is not yet known in what form it 

 occurs. There is also a little gold. The ores were undoubtedly 

 formed by differentiation from the norite magma, and the whole 

 occurrence is very similar to the famous nickel deposits of Sudbury, 

 in Ontario. From the geological relations it appears highly probable 

 that the amount of ore will increase in depth when followed below the 

 intrusion. The deposits are now being actively developed, and it 

 seems probable that they will eventually prove to be of great com- 

 mercial value. 



R. H. R. 



y. — Low-temperature Formation op Alealine Felspar in Lime- 

 stone. By R. A. Daly. Proceedings of the National Academy 

 of Sciences, vol. iii, pp. 659-65, 1917. 



AUTHIGEiSriC crystals of felspar, including orthoclase, albite, and 

 perhaps microcline, have been described by various authors in 

 the Jurassic limestones of the Alps and in the Chalk of the Paris 

 Basin. The crystals are well-shaped, but very minute, and are 



