Dr. R. F. Rand — Some Transvaal Eruptkes. 113 



Meg. Holocryatalline, bluish-black rock, of fine grain. Sp. 

 gr. 2-95. 



Mic. The ground-mass is made up of numerous short laths of 

 plagioolase (with albite twinning) and small irregular grains of 

 augite and olivine ; the olivine grains being the larger. These are 

 largely decomposed into a dark, greenish serpentine. Amidst the 

 ground-mass lie larger elements — (a) groups of large plagioclase 

 crystals (labradorite) ; (6) occasional large irregular grains of 

 olivine ; (c) crystalline patches of magnetite ; (d) irregular green 

 patches of serpentine succeeding olivine; (e) a few long, lath-like 

 crystals of twinned augite, sometimes showing erosion. The rook is 

 referable to the olivine-basalts. 



Beyond Pretoria. 



No. 20. — Locality : from summit of range of hills about 10 miles 

 north of Pretoria, via Wonderboom Poort, the Poort being a defile 

 in the Magaliesberg Eange where river, road, and rail jostle each 

 other in their passage through. The range from which the rock was 

 collected runs nearly parallel with the Magaliesberg. " Pyramids " 

 vicinity. 



Meg. A holocrystalline rock of medium grain, dark grey and 

 granitoid. Sp. gr. 2-92. 



Mic. A rock of simple constitution, its elements consisting of 

 augite, frequently diallagic ; and labradorite ; with the allotriomorphio 

 arrangement of the granular rocks. The proportion is about one- 

 third augite to two-thirds labradorite. A gabbro of simple type. 

 The augite is much decomposed and is often turbid. It contains 

 grains of magnetite as decomposition-product. Little patches of 

 chlorite are sometimes included, and, in places, one sees a marginal 

 alteration of the augite sending invading strings of chlorite into the 

 surrounding felspar. The labradorite is comparatively fresh, showing 

 broad albite lamellae, and, quite rarely, pericline striation. 



No. 21. — Locality : outcrop upon the flats midway between 

 Wonderboom Poort and the range from which No. 20 was collected. 

 It probably constitutes part of the same rock-mass. 



Meg. A very coarse-grained, holocrystalline rock consisting of 

 large black shimmering crystals set in a white matrix, showing 

 frequent cleavage-faces. Sp. gr. 2-9. 



Mic, Large crystals of augite, twinned, with basal cleavage, giving 

 the herring-bone structure, are conspicuous. The crystals frequently 

 show a marginal alteration into brown hornblende. Grouped with, 

 the augite are large crystals of hypersthene, partially converted into 

 bastite. Like the augite it too shows a marginal conversion into 

 brown hornblende. The hypersthene is rather more abundant than 

 the augite. The felspar (labradorite) is more decomposed than 

 that of No. 20. It shows much granular opacity. Combinations 

 of Carlsbad, albite, and pericline twinning occur. The crystals are 

 frequently crowded with long needle-like inclusions, whose regular 

 disposition in planes shows an approach to schiller - structure. 

 These inclusions sometimes occupy patchy areas in the interior of 



DBCAOE V. — VOL. II. — NO. III. 8 



