538 MR G. W. TYRRELL. 



I. The Gneisses and Schists. 



The gneisses are predominantly quartzo-felspathic varieties with biotite as the 

 principal ferro-magnesian constituent. They are medium- to coarse-grained, pink or 

 grey, well-foliated rocks, with the biotite aggregated into thick clots or folia, or less 

 often evenly spangling the rock. Hornblendic gneisses occur, but are much less 

 common than the biotite-bearing varieties. 



In thin section the biotite-gneisses are coarse-grained aggregates of quartz and 

 potash-felspar with folia of biotite, and show marked dynamo-metamorphic effects. 

 The larger quartz crystals are broken up into a comparatively coarse granular mass 

 of angular fragments ; but the felspars have only suffered a fine peripheral granula- 

 tion, the products of which, mingled with quartz, form a kind of groundmass in 

 which the larger, more or less rounded, "eyed" crystals of felspar are set. The 

 felspar is prevailingly microcline. Orthoclase is subordinate, and a little oligoclase 

 is occasionally present. The felspars show the effects of crushing in the irregularity, 

 curvature, and fracture of their twin striations. The biotite is of the common yellow 

 variety, and occurs in large clots or folia consisting of small flakes felted together 

 with minute grains of quartz, epidote, and occasionally a little muscovite. Felspar 

 is absent from these aggregates. 



The above description refers particularly to a rock from the Bui River, S. of 

 Ochilesa (l85), # which may be taken as typical of the. predominant biotite-gneisses. 

 Other rocks of the same type occurring at Bailundo (15^) are very rich in "gitter" 

 microcline. Rocks from the Lengwe Gorge (105, 106, 109) are decidedly richer in 

 soda-lime felspars (oligoclase or albite-oligoclase), and show a development of minute, 

 colourless, euhedral crystals within the felspars. These appear to be of two or three 

 different kinds. Some are clearly small zircons, but others are referred to clinozoisite 

 and epidote. In some types (e.g. (156), W. branch of Lovule River), the foliation is 

 not so definite as in the above-described rocks. The texture is evenly granular and 

 the biotite is uniformly distributed. The granulitic texture becomes more pronounced 

 in a rock from the Benguella Railway, 55 kilometres, E. of San Pedro (114). In this 

 specimen the biotite occurs as thin leaves which appear as narrow bands of biotite 

 flakes in thin section, and produce a bacillar type of foliation. This rock has some 

 resemblance to the Moine gneisses of the Scottish Highlands. Biotite becomes very 

 scarce and the cataclastic texture very prominent, in a pink, granulitic, quartzo- 

 felspathic gneiss from Kambengi (195). 



The rocks described above are all orthogneisses derived by dynamo-metamorphism 

 from ordinary biotite-granites. These rocks grade into biotite-schists, e.g. at 55 kilo- 

 metres, Benguella Railway (115). The texture becomes more even and finer-grained, 

 the amount of quartz and biotite increases, whilst that of felspar diminishes, in com- 

 parison with the gneisses described above. Another mode of transition is into a 



* The numbers refer to the registration numbers of the rocks in Professor Gregory's collection. 



