F. P. Mennell — South African Petrography. 363 



sandstone which rest unconformably on the Bulawayo schists and the 

 marginal portions of the Matopo granite mass. These are evidently 

 of the same age as the coal series and associated beds of the Zambesi 

 Yalley, from which several lavas have been described. A dyke 

 penetrating the sandstone itself at Mafungaburi Peak, Matabeleland, 

 is of the same type, but contains a smaller proportion of glass, while 

 the crystalline constituents are of larger size. The augite is frequently 

 idiomorphic, and is of prior consolidation to the felspar. Good-sized 

 rods of magnetite are abundant, but do not show the definite outlines 

 of the Forest Vale variety. Another rock from the Zimba Eiver, 

 Northern Rhodesia, is holocrystalline and rather coarser-grained. 

 The felspars frequently show pericline as well as albite lamellation, 

 and must be nearly related to anorthite. The augite occasionally 

 shows crystal outline, and microlites occur as inclusions in the 

 felspar, but the latter sometimes penetrates the larger crystals. 

 Eods of magnetite are freely scattered through the mass, and apatite 

 is also present. 



A transition to the ophitic structure is seen in a specimen from the 

 Waterworks, about 4 miles from Bulawayo. It consists of large 

 plates of augite, for the most part in a beautifully fresh condition, 

 and very decomposed felspar. It seems evident that the augite 

 originally tended to be porphyritic, but when the felspar com- 

 menced to crystallize out, crystals continued to grow instead of 

 a second generation forming in the groundmass. This is shown 

 by the fact that the felspars never occur as inclusions in the 

 augite, and seldom penetrate more than a very short distance from 

 the margin of the crystals. If there was originally any glassy 

 matter it is now lost in the confused mixture of decomposition 

 products due to the alteration of the felspar. Ilmenite is abundant, 

 and sometimes shows fine examples of skeleton crystals, while 

 a colourless mineral with very strong refraction and double 

 refraction is probably sphene. 



A beautifully fresh ophitic olivine dolerite occurs near Fort Gwelo, 

 Matabeleland, on the Sebakwe Eoad. The felspar gives the usual 

 lath-shaped sections in most cases, and frequently shows pericline 

 lamellation. The augite is penetrated by the felspar in all directions, 

 and often occurs in aggregates. The olivine is rarely penetrated by 

 the felspar, and shows a rougher surface and higher interference 

 tints than the augite. The cleavage is occasionally shown, and the 

 mineral is traversed by the usual irregular cracks, which are now 

 and then rendered more prominent by incipient serpentinization. 

 A curious feature is the enclosure in the olivine of small crystals of 

 a brownish pleochroic hornblende with well-developed cleavage. 

 A few minute flakes of biotite also occur. Both these minerals are 

 usually associated with grains of magnetite. 



Among the plutonic rocks the granites afibrd examples of a number 



of varieties. The coarse-grained biotite granite of Cape Town has 



been described by Cohen ^ and others, so that it is needless to 



enumerate its normal features. One variety, however, seems worthy 



1 N.J. fill- Mm., 1874. 



