56 DR. A. HOLMES ON THE PRE-CAMBRLAJST [vol. lxxiv, 



No. 16 illustrates a granite, from the peak of Natupi (Namosa Hills, 

 north-east of Bibawe), intermediate between the type just described and 

 the glomeroplasmatic gneiss. The biotites are here in elongated groups, 

 but otherwise the structure is that of an ordinary granite. Muscovite is 

 present in addition to the usual minerals, and apatite and zircon are well 

 represented as accessories. 



A third example (No. 66) is provided by the rocks on the slope above 

 the Base Camp at Sawa (Bibawe Mountains). This granite shows a slight 

 foliation caused by the grouping of quartz and felspar (microcline and 

 perthite) respectively in elongated aggregates. Biotite, however, does not 

 make up more than 2 per cent, of the rock. Minute inclusions of zircon 

 and rare apatite occur in the felspars, which are somewhat altered by 

 kaolinization along cracks and cleavage-planes. Many of the cracks are 

 also stained a deep red by the deposition in them of haematite. 



The hillsides east of the Sawa Camp were too steep to be 

 ascended, but large blocks which had fallen indicate that the 

 granulitic type passes gradually into a grey porphyritic rock, 

 having phenocrysts of felspar 2 to 5 cms. long' in a coarse-grained 

 (0*5 to 1 cm.) groundmass, which is granitic rather than granu- 

 litic. Grey porphyritic granites have also been recorded by Mr. D. 

 A. Wray from the mountain groups farther west, such as the 

 Inago and Mripa ranges, though he describes these peaks as con- 

 sisting in the main of slightly-foliated biotite -granite passing into 

 biotite- gneiss in the surrounding plains. 



In Nyasaland, still farther west, Dr. A. R. Andrew & the late 

 T. E. Gr. Bailey l described a similar passage from gneisses to 

 granites : — 



' Fine-grained gneisses with platy structure may be traced in the field 

 through every gradation into augen-gneiss, which in turn is found to pass 

 into slightly-foliated syenite or granite. . . . They [the foliated plutonic 

 intrusions] consist, for the most part, of granite or syenite with well- 

 developed porphyritic crystals of felspar. The granites rarely contain 

 much quartz, and resemble the associated syenites in the abundance of 

 microcline, and the occurrence, at times, of perthitic felspars.' 



The Nyasaland gneissose granites appear to differ from those of 

 Mozambique, only in the smaller proportion of quartz jwid in their 

 greater areal extent. 



Prof. J. W. Gregory & Mr. G-. W. Tyrrell 3 have described 

 gneisses from the Angola province of Benguella that bear a close 

 resemblance to the biotite-gneisses of Mozambique described above. 

 After describing the transition from orthogneisses to biotite- 

 schists, Mr. Tyrrell writes (ojj. cit. pp. 5-38-39) : — 



' Another mode of transition is into a group of gneisses with very pronounced 

 cataclastic textures, showing all gradations to mylonite.' 



Several specimens of the gneisses from the Mkwassi River district 

 (along the northern borders of the Ribawe Mountains ; see map, 

 PI. XI) illustrate the beginning of a similar phenomenon which 

 recalls the ' trap-shotten ' structure described bv Sir Thomas 

 Holland.* 



1 Q. J. G. S. vol. lxvi (1910) pp. 194-95. 



2 Trans. Boy. Soc. Edin. vol. li, pt. 3 (1916) pp. 495 & 537. For other 

 references and descriptions of African gneisses and gneissose granites, see C. B. 

 Horwood & A. Wade, Geol. Mag. dec. 5, vol. vi (1909) pp. 455, 497, & 543. 



3 Mem. Geol. Surv. India, vol. xxviii (1900) p. 198. 



