504 C. B. Honcood 8^ A. Wade— The Old Granites of Africa. 



magnetite-gneiss, mica- schist, and talcose-schist, on the borders of Natal 

 and Zululaud, near Krantzkop, 



With reference to Dr. Yoit's remark that the gneisses and granite 

 are more predominant as one travels northwards towards Central 

 Africa, a paper by Messrs. Preumont & Howe ^ on the Geology of 

 Uelle and Lado Enclave (the north - easternmost provinces of the 

 Congo Eree State, and including the eastern portion of the basin of 

 the Upper jSTile, covering in all an area of some 80,000 square miles) 

 is of great interest. Granite, well-foliated gneisses, mica-schists, and 

 other metamorphic rocks greatly predominate, and are classed as 

 primary rocks. In fact, post-Primary rocks occur only in the extreme 

 western part of the region, about Buta on the River Rubi. The 

 granite is intrusive in the gneisses, quartzites, and mica-schists. 

 Remarkable deposits of haematite and magnetite also occur. From 

 Howe's petrological description of the rocks, the granite and gneisses 

 bear a marked resemblance to the gneisses and Old Granites of the 

 Transvaal. 



The following is his description of microcline-gneiss from the Lado 

 Enclave : — " A cream-coloured, medium-grained, banded rock. The 

 texture, as seen in the section, is granitic ; there is no sign of foliation. 

 Quartz occurs in fair-sized plates, also in blebs within the felspar 

 (corrosion-quartz). Streaks of dusty inclusions are common. The 

 felspars comprise abundant microcline, orthoclase, and plagioclase 

 (oligoclase) ; albitic and periclinic twinning frequently occur together. 

 Some of the felspars have schiller-structure, produced by rows of 

 negative crystals. Micrographic intergrowths of felspar and quartz 

 appear sporadically in small patches. A curious, dark-green, pleo- 

 chroic variety of hornblende occurs in a few patches. Biotite is 

 present, but is sparingly represented ; it is often greenish in colour. 

 A little sphene is associated with the mica. A few small rounded 

 crystals of zircon are present, also short prisms of apatite." 



A microcline-gneiss from between the Rivers Kwado and Aro he 

 described thus: "Microcline in abundant large plates is the pre- 

 dominant felspar ; there is also some plagioclase. Quartz occurs in 

 irregular grains and as bleb-like inclusions in the microcline ; fluid- 

 inclusions are common in the quartz. Both a colourless and a dark- 

 brown mica are present. Epidote is plentiful in rather large pale 

 masses. Zircon and sphene are present, also small patches of chlorite 

 in association with the mica. The foliation is well marked, even in 

 the slide, and is accompanied by an approach to granulation and 

 a sort of mortar-structure." 



From these two descriptions the granites appear to be remarkably 

 similar in petrographical habit to the Old Granites of the Transvaal. 

 The prevalence of microcline-granite-gneisses and microcline-gneiss 

 also suggests great similarity to the Old, Grey, Microcline Granites of 

 South Africa. 



Along the boundary of the Congo State and the Bahr-el-Ghazel 

 a series of very coarse, pegmatoid, gneisses occurs, with large mica, 



1 "The Geolog-y and Petrology of part of the Cono-o Free State," by G. F. J. 

 Preumont & J. A. Howe: Quart. Journ. Geol. Soc, 1905, vol. hi, pp. 641-66. 



