F. P. Mennell — South African Petrogmphy. 361 



twin, while the oligoclase gives a maximum extiuction angle of about 

 10°, and is nearly always twinned on the pericline as well as the 

 albite type. The prism faces are well developed, especially in the 

 orthoclase, and the crystals are usually elongated in the direction of 

 the vertical axis. A few grains of ilmenite occur in the groundmass. 

 Another rock from Belingwe may be referred to the porphyrites. 

 It is very similar to the last in appearance and structure, but is 

 obviously more basic. Andesine is the predominant porphyritic 

 felspar, and patches of chlorite appear to represent a ferro-magnesian 

 constituent which was also possibly porphyritic. Numerous granules 

 of leucoxene indicate the presence of original ilmenite, and apatite 

 also occurs in shortish prisms. Epidote and calcite are decom- 

 position products. 



Fig. 2. 



Quite another type of porphyrite is rej)resented by a specimen from 

 the Modder Eiver. It consists of an aggregate of fair-sized lath- 

 shaped felspars, with elongated crystals of augite, largely chloritized, 

 granules of ilmenite, small but numerous, and a good deal of 

 glass. The last is of quite a bright green colour and perfectly 

 isotropic, but it encloses numbers of minute colourless spherulites, 

 which never give a black cross in polarized light, but show an 

 extinction curve exactly similar to the '• brushes ' of a biaxial 

 interference figure. The felspar seems to be mostly andesine. Like 

 many of the South African dyke rocks this example is amygdaloidal. 

 The amygdules are almost spherical as a rule, and are made up 

 of calcite and quartz, sometimes intermixed, with a border of 

 feebly pleochroic chlorite. Crystals of pyrites occur as inclusions 

 in the calcite. 



We now come to the dolerites, which present great diversity of 

 structure. Glassy examples occur at the Criterion Mine not far 

 from Bulawayo and at the Bonsor Mine in the Selukwe district of 

 Matabeleland. At the former locality the intrusion appears to be 



