Reviews — Kalgoorlie, W. Australia. 225 



(b) Expelled, by diagenetic settling, crystallization during 

 metamorphism, orogenic stress, the earth's general 

 heat, the heat of orogenic crushing or metamorphic 

 changes, or the heat of igneous intrusion. 

 C. Mixed Types. 



VI. — On the Geology of the Alkali Hocks in the Transvaal. 

 By H. A. Brouwek. Journ. Geol., xxv, p. 741, 1917. 



AFTEB giving a general summary of what is known of the igneous 

 complex of the Bushveld, the author deals with the special 

 types of alkali rocks that occur in the Pilandsberg and Leeufontein, 

 and west of Lydenberg. He suggests that the nepheline syenites and 

 allied rocks were derived from a residual magma left after the 

 differentiation from the parent magma of ultra-acid rocks containing 

 as much as 97 per cent of Si O2. Not only was the residual magma 

 enriched in alkalies and alumina relative to silica, but it also 

 contained a concentration of fluorine and other volatile fluxes. Much 

 more work, however, is needed in this region before either the age of 

 the complex or the genetic relations of its multitude of rock types 

 can be accurately determined. 



VII. — The Geological Features of the "North End", Kalgoorlie. 

 By r. R. Feldtmann. Bull. Geol. Surv. Western Australia, 

 No. 69. pp. 152, with 43 figures and an atlas of 14 plates. 

 Perth, 1916. 



ri'^HE area dealt with in this memoir comprises about 2-^ square 

 J_ miles. It is composed almost entirely of old and highly altered 

 igneous rocks, which can be divided into two main groups, 

 the older and younger greenstones. The older greenstones were 

 originally a large series of basaltic flows, and cover the greater part 

 of the surrounding region. Along a line of weakness striking 

 N.N.E.-S.S.W. in these older rocks a series of dykes, the younger 

 greenstones, were intruded. These show a gradation from basic to 

 more acid composition, and include types ranging from dolerites to 

 albite porphyrites, the latter being the last of the succession. At 

 difli'erent times during the intrusion of these dykes the district M'as 

 subjected to intense pressure in an east and west direction, which 

 resulted in the amphibolitization of the igneous rocks and the 

 production of shear zones and thrust faults. The period of most 

 violent pressure took place after the intrusion of the aibi.te 

 porphyrites. 



Very shortly after this siliceous solutions with vapours of boron, 

 sulphur, and hydrocarbons, or oxides of carbon, were forced along the 

 shear zones, with the formation of jaspers and graphitic schists. 

 These were followed by the gold-bearing solutions. They formed 

 several different mineral deposits ; quartzose and schistose lodes 

 along the strike of the dykes and cross quartz veins striking more 

 or less at right angles to the dykes. The auriferous lodes seem to be 

 genetically connected with the albite porphyrites, and occur most 

 frequently in the neighbourhood of these rocks along the major 



DECADE VI. — vol. V.— NO. V. 15 



