364 F. P. Mennell — South African Tetrography. 



of mention, a graphic modification with tourmaline as an abundant 

 accessory. The latter mineral seems to be intergrown with the 

 felspar in a similar way to the quartz. It is a yellowish A'^ariety 

 without marked pleochroism, and the interference tints are not 

 masked, as is often the case, by the strong absorption. It may 

 possibly be regarded as having replaced quartz, but it seems more 

 likely that the intergrowth is more apparent than real, and is due 

 to the penetration of the mineral into cleavage cracks in the already 

 consolidated felspar. A graphic granite from Northern Khodesia, 

 about midway between Kalomo and Monze, may also be noticed. 

 Tlie felspar, which builds pink crystals several inches in length, is 

 microcline. The quartz does not occur in well-defined skeleton 

 crystals, but gives a somewhat irregular wavy longitudinal section 

 and a rounded transverse one. Occasional small flakes of muscovite 

 are present. 



The great granite mass of the Matopos, which forms the backbone 

 of Southern Matabeleland, presents some interesting features. The 

 normal granite in hand-specimens closely resembles the Dartmoor 

 rock (e.g. that of Hay Tor). In a rather fine-grained example 

 from the Khami Valley the dominant felspar is, however, seen under 

 the microscope to be microcline. It is obviously the last product 

 of crystallization, as it even encloses the small patches of micro- 

 pegmatite which occasionally occur. Orthoclase and oligoclase are 

 also present, the former being apparently the felspar of the micro- 

 graphic intergrowths. Biotite is the ferro-magnesian constituent. 

 Sphene and apatite are the accessories. Near Bulawayo, which 

 is situated on the contact zone, the rock assumes a very different 

 aspect. The structure is decidedly gneissic, and the felspars are 

 flesh-coloured. The appearance of foliation is probably due to 

 movement before complete consolidation (Fig. 4). This seems to be 

 implied by the granulation of the quartz and microcline^ and the 

 absence of deformation in the minerals of prior consolidation. It does 

 not do, however, to place too much reliance on this feature, for, as 

 I have previously ^ pointed out, it is precisely these minerals which 

 most readily yield to the effects of pressure, and the quartz may 

 be completely granulitized before the other minerals are appreciably 

 affected. Hornblende is here the dominant ferro-magnesian con- 

 stituent, though biotite is present in most slices. In the pegmatite 

 veins which ramify through the mass and are obviously of late 

 consolidation, microcline takes the place of the other felspars, but 

 in the normal rock orthoclase seems to be rather more abundant 

 and oligoclase frequently forms large pseudo-porphyritic crystals. 

 Sphene, apatite, and zircon are all fairly abundant, the two former 

 building rather large crystals. 



The syenites are represented by what I may term the Hillside 

 intrusion, three miles from Bulawayo, which is of so much interest 

 that I hope to describe it fully in a future paper. I may mention, 



^ See MacMaliou on the gneissose-granite of the Himalayas, Geol. Mag., 1897, 

 pp. 345-355. 



2 " The Copper-bearing Kocks of South Australia " : Brit. Assoc, 1901. 



