Reviews — Geological Structure of Soutlt Africa. 83 



V. — Geological Structure of the Union of South Africa. By 

 A. W. Rogers, Sc.D. pp. 13, with a coloured map. Reprinted 

 from the Official Year Book, 1917. 



IN the space of thirteen pages Dr. Rogers manages to give a very- 

 complete account of the physiography and geology of the Union 

 territories, which cover an area of no less than 476,000 square miles. 

 The account is necessarily much compressed, but still a brief 

 description is given of the characters and distribution of the long 

 succession of rock-formations represented. The latest results of 

 South African stratigraphy are condensed into a tabular form, giving 

 the correlation and thicknesses, so far as known, of all the scattered 

 and puzzling rock-groups which, in the absence of fossils, have 

 given so much trouble to South African geologists. It is interesting 

 to note that the Malmesbury series of the Cape Province is now 

 definitely regarded as equivalent in a general way to the Transvaal 

 or Potchefstroom system, and that the Matsap series of Griqualand 

 West is now placed on the same horizon as the Waterberg, and 

 below the Table Mountain Sandstone. The ages of the different 

 series of igneous intrusions are also indicated, and their close 

 connexion with periods of earth-movement come out clearly in the 

 table. The intrusion of the Bushveld complex is considered to 

 belong to the age of the "Waterberg system. The enormous total 

 thickness of the South African sedimentary rocks is very striking, 

 especially when considered in connexion with the peculiar conditions 

 under which many of them were deposited, as, for example, the 

 Karroo system. Altogether South African geology presents many 

 problems of interest towards the solution of which Dr. Rogers and 

 his colleagues of the Geological Survey have made very notable 

 contributions. 



R. H. R. 



YI. — The Cassiterite Deposits of Tavoy. By J. Coggin Brown. 

 Rec. Geol. Surv. India, vol. xlix, pt. i, pp. 23-33, 1918. 



AT the present time Tavoy is one of the most important wolfram- 

 mining centres of the world, but the occurrence of tin in 

 notable quantities has been somewhat overlooked. Cassiterite occurs 

 as an accessory mineral with wolfram and molybdenite in the 

 granite, and in pegmatite veins and quartz lodes with ores of 

 tungsten, bismuth, iron, copper, arsenic, antimony, lead, and zinc. 



In the lodes cassiterite is almost always in close association with 

 wolfram; in the Paungdaw-Wagon area the mixed concentrates 

 may contain as much as 25 per cent of tin oi - e ; cassiterite tends to 

 be more common than wolfram in the greisens. The methods of 

 lode-working still in use are somewhat primitive, being chiefly 

 " cobbing and panning " : the installation of a magnetic separator at 

 Tavoy has improved the quality of the concentrates shipped. 



The greater part of the output, however, comes from detrital 

 deposits, which are exploited by hydraulic methods, especially by 

 monitors. The richest tin-placers appear to be of sub-recent or late 

 Tertiary age, forming raised river terraces and lake deposits and 



