F. P. MenneM — Bouth African Petrography. 



365 



however, that the most abundant variety is an even-grained rock 

 of typically granitic structure, but entirely destitute of quartz. 

 The felspar is almost exclusively microcline, while hornblende and 

 augite are about equally abundant-. Sphene occurs in very perfect 

 yellowish crystals, while apatite, zircon, and magnetite are also 

 present. An augite-diorite occurs as a basic modification of the 

 mass, but more typical diorites occur as segregations from the 

 Matopo granite near Bulawayo. In these hornblende is the ferro- 

 magnesian constituent, while microcline, oligoclase, aiad orthoclase 

 are present as in the granite itself. Apatite and sphene are very 

 abundant, and often form large though somewhat irregular crystal s>. 

 There are also a few granules of magnetite. 



Fig. 4. 

 Numerous intrusions or segregations of gabbro occur in the very 

 interesting plutonic complex round the Umniati Eiver in Mashona- 

 land. One example from the Goedyema Eoad, north of the Sarui 

 Eiver, is a beautifully fresh rock made up entirely of augite and 

 felspar. The structure is typically granitic and the grain coarse and 

 even. There is not a single grain of magnetite present, and the rock 

 is consequently remarkably light-coloured for so basic a variety. Most 

 of the augite shows diallagic striation and schillerization, due to the 

 presence of two sets of rather large, regularly arranged inclusions. 

 The felspar is evidently near anorthite, giving symmetrical ex- 

 tinctions up to 37°. Specimens from three other localities in the 

 same district have characteristics which seem to be due to the 

 influence of the rocks into which they were intruded. In one from 

 the Sarui Eiver, enstatite is the most abundant ferro-magnesian 

 constituent. It occurs in elongated crystals of considerable size 

 and is partially altered to serpentine. Augite is also common, and 

 often shows good cleavage and a basal striation. This, combined 

 with twinning on the orthopinacoid, gives rise to good examples 

 of the 'herring-bone' structure. Eepeated twinning is also well 



