346 F. P. Mennell — Minerals in S. African Granites. 



of Cape Town itself is remarkably rich in accessories. Tourmaline, 

 in particular, is very abundant in places. In thin sections it is 

 ef a yellowish brown colour frequently bordered and zoned with 

 pale blue, while some crystals show alternate bands of yellow and 

 brown. Cordierite also appears to be sometimes present in the 

 normal granite. It may be quite fresh and almost indistinguishable 

 from quartz, while in other cases it is entirely replaced by the 

 yellowish micaceous 'pinite' pseudomorphs. The intermediate 

 stages of the alteration are well shown, while it is interesting to 

 note that it seems to have crystallized sometimes before and 

 sometimes after the felspar. The mineral is most often found in 

 the basic patches of the granite, which are no doubt derived from 

 the fusion and subsequent recrystallization of fragments of the 

 adjacent slate, and are especially interesting. Some are largely 

 made up of andalusite in good crystals or somewhat rounded grains, 

 with abundant strongly pleochroic biotite, and a certain amount of 

 quartz, felspar, muscovite, tourmaline, cordierite, and apatite. Both 

 the patches and the normal rock also contain numerous small 

 zircons, generally as inclusions in the biotite, where they give 

 rise to the usual intensely pleochroic ' halos.' 



In the Tati district of Bechuanaland granite occurs as a modi- 

 fication of the prevailing syenite, and is chiefly remarkable for the 

 amount of apatite it contains. I have seen crystals of this mineral 

 from Tati nearly an inch in diameter ; but in the rocks I have 

 examined they are of purely microscopic dimensions. They show 

 not only the usual cross-fracture, but also complete dislocation of 

 single crystals into a number of separate fragments divided by 

 portions of the enclosing quartz or felspar. Sphene is abundant 

 in this rock. It surrounds the iron ores in whitish granular 

 aggregates, which, unlike the variety leucoxene, are more or less 

 transparent and show brilliant interference tints when the sections 

 are sufficiently thin. 



The granites of Ehodesia are notable for the abundance of 

 microcline, which is frequently the dominant, and sometimes the 

 only, felspar. The now well-known Matopo granite is typically 

 composed of microcline, quartz, and biotite, with minute granule& 

 of magnetite and sparingly distributed but large crystals of 

 broAvnish sphene. The Bulawayo margin of the mass is a horn- 

 blende granite with microcline, oligoclase, and orthoclase as the 

 felspars. The accessories include large crystals of apatite, abundant 

 yellowish sphene, a little magnetite, and sometimes a good deal of 

 pale yellow epidote. The last-named mineral occurs in druses, and 

 forms veins running through the granite in places. Large plates of 

 biotite are found in the pegmatitic modifications, with microcline, 

 etc. Fluor, malachite, chessylite, and chrysocolla occur lining- 

 cracks and joints, while molybdenite is found associated with a little 

 secondary copper pyrites, etc., in a quartz segregation vein near 

 Glenville, about three miles from Bulawayo. This mineral is in 

 good crystals, thin hexagonal plates looking as if bounded by the 

 prism and basal plane. Careful measurements of the angles give no 



