362 



F. P. Mennell — 8outh African Petrography. 



completely glassy. It is considerably altered, being practically 

 a ' palagonite,' but the mass of the rock is quite isotropic, though 

 it is crowded with crystallites (longulites) and minute specks of 

 magnetite. It shows occasional rather irregular spherulites. At 

 the Bonsor Mine the glass forms the margin of an intrusion into 

 quartz schist, and becomes rapidly more crystalline and apparently 

 also more basic as it is traced from the contact. About an inch 

 from the edge it is dark brown in colour, and crowded with crystallites 

 and tiny microlites. It shows also idiomorphic phenocrysts of augite 

 and more or less corroded felspar, the latter evidently a basic variety 

 and largely converted into epidote. 



The more crystalline varieties are usually of the granular type 

 and do not as a rule contain olivine. The dykes which pierce the 

 Cape Town granite afford good examples. They consist of lath- 

 shaped felspar, augite, rarely idiomorphic, but of prior consolidation 

 to the felspar, magnetite, ilmenite, and a little interstitial matter. 

 Chlorite occurs as a decomposition product of the augite. The 

 felspar appears to be labrarlorite, and one specimen of the rook gave 

 a silica percentage of 52-41.^ 



A very similar rock, but with a good deal of glassy matter, pene- 

 trates the Matopo granite mass near Forest Vale (Fig. 3). The glass 

 is brown in colour and quite isotropic. It is crowded with little rods 

 of magnetite, very long and thin, while skeleton crystals of felspar are 

 also of frequent occurrence. Magnetite is x&xj abundant all through 

 the rock and is almost invariably idiomorphic, little octahedra being 

 common which are frequently grouped in rows, sometimes apparently 

 along the edges of a cube. The lath-shaped felspars often show 

 bifurcated terminations and inclusions of the groundmass. This 

 rock is near the margin of one of the outliers of Tertiary (?) 



1 Cohen: JST.J. fur Mm., 1874. See also Shaw: Proc. S.A. Phil. Soc, vol. i, 

 pt. 2 (1879), p. 59. 



