F. P. Mennell — Minerals in 8. African Granites. 347 



appreciable difference from 120°, though only two of the lateral 

 faces appear as a rule to be normal to the basal plane. They 

 appear, in fact, to possess orthorhombic symmetry, and to be really 

 bounded by the pyramid, brachypinacoid, and basal plane. 



Several of the granites from Northern Ehodesia contain orthite 

 (allanite). This mineral occurs in yellowish - brown idiomorphic 

 crystals or rounded grains, the colour being sometimes very 

 irregularly distributed. The pleochroism is not strong, and the 

 double refraction as a rule scarcely exceeds that of quartz. A rock 

 from the Jibuyi Eiver contains numerous crystals about '5 to 

 1-5 mm. in length, occasionally twinned, and showing inclusions 

 of zircon, apatite, and magnetite. The orthite is usually surrounded 

 by epidote with more or less irregular outlines. The two minerals 

 do not extinguish simultaneously, but that they are intergrown in 

 definite crystallographic relation is shown by the fact that if the 

 outline of suitable orthite crystals is taken as indicating the 

 orientation of the surrounding epidote, the latter is found to give 

 straight extinction. A biotite-hornblende granite from Kalomo 

 shows zoned orthite surrounded by epidote with good crystal 

 outline, the latter being enclosed in turn in biotite. Minute crystals 

 of orthite also occur enclosed in the mica, where they give rise to 

 halos which resemble but are not quite so strongly pleochroic as those 

 which usually surround zircon. Epidote is extraordinarily abundant, 

 and there is a good deal of sphene. A gneissose rock from the 

 Wankie District of Matabeleland, with orthoclase crystals several 

 inches across, presents some special points of interest. Little pink 

 garnets and minute brown granules, which are seen under the 

 microscope to be orthite, can be detected by the unaided eye. The 

 garnet is in the larger grains, but the orthite is much more abundant. 

 It is rather more strongly pleochroic than in the rocks previously 

 mentioned. It shows zonary banding, and is frequently surrounded 

 by biotite, but no epidote is present. The rock contains much 

 apatite. All these orthite-bearing rocks have a distinctly gneissic 

 aspect, which is sufficient to suggest a secondary or metamorphic 

 origin for the orthite, even apart from its association with epidote. 

 The presence of garnet appears to point in the same direction. It 

 may be remarked, however, that all the rocks are exceptionally 

 fresh, while those from Northern Ehodesia contain micropegmatite^ 

 a fact which appears absolutely conclusive as to their igneous origin. 

 The fact of the epidote being idiomorphic towards the mica points, 

 moreover, to its primary nature, as do the inclusions of apatite and 

 magnetite, with zircon as well in the case of the orthite, which 

 alone encloses recognizable crystals of that mineral. It may also 

 be mentioned that the fine-grained modifications of the Jibuyi rock 

 contain correspondingly small crystals of orthite, and we seem 

 accordingly driven to regard both orthite and epidote as normal 

 products of the consolidation of a molten magma. 



