108 Dr. R. F. Rand — Some Transvaal Eruptwes. 



Meg. A medium-grained type. Sp. gr. 2 66. 



Mic. The quartz is of the usual kind, but there is an absence of 

 the rutile needles noted in No. 1. The felspars are microcline, 

 orthoclase, and plagioclase, as before. Hornblende is present, but 

 no biotite. It is pale green in colour, and carries, as inclusions, 

 granules of sphene, small crystals of magnetite, and a little apatite. 



Accessories. — (a) Sphene, in good crystals of a yellowish-brown 

 colour and carrying numerous magnetite crystals as inclusions. 

 The sphene crystals are sometimes a millimetre in length, so that 

 with a little scrutiny they may be made out in the hand-specimen. 

 (b) Pyrites, in crystal-groups and sometimes in grains, sporadic in 

 its distribution, (c) Apatite, in fairly large crystals. 



Muscovite, calcite, and epidote all occur sparingly, as decomposition- 

 products. Micrographic intergrowth of quartz and felspar is 

 present, but in less degree than in No. 1. 



No. 4. — Locality : Norwood, 4 miles north-east of Johannesburg 

 and about f mile from the escarpment of the Witwatersrand Series. 



Meg. Of rather coarser grain than that of the preceding 

 numbers. Sp. gr. 2-66. 



Mic. Shows quartz as in No. 1. The felspars are the same, the 

 orthoclase showing very good zoning. Hornblende and biotite 

 occur, the former preponderating. Mixed up with the hornblende, 

 at whose expense it appears to have grown, is a large quantity of 

 epidote, sometimes showing good crystal outlines. Sphene occurs 

 in good crystals, which occasionally show excellent cleavage-cracks. 

 It is sometimes enclosed among the hornblende-clusters. Apatite as 

 in previous examples. 



No. 5. — Locality : a small quarry near stream which escapes 

 towards the granite below Sans Souci. About 2| miles N.N.W. of 

 Johannesburg and close to the Witwatersrand Series escarpment. 



Meg. The rock is of medium grain, its minerals tending towards 

 directional arrangement. Sp. gr. 2*76. 



Mic. The quartz is in smaller grains. It appears to have suffered 

 some comminution. The felspar is striped, much decomposed, and 

 largely predominates over the quartz. The hornblende is in large 

 individuals, of greenish colour and irregular outline; the coloration 

 is patchy, and the interior of the crystal often shows bleaching. 

 Pleochroism is strong, yellow to indigo-blue. Sphene, epidote, and 

 apatite are commonly enclosed within the hornblende-groups, the 

 epidote abundantly. A few zircons are noticeable. 



No. 6. — Aplitic vein from the same quarry as No. 5. 



Meg. Milk-white in colour and holocrystalline. Sp. gr. 262. 



Mic. Consists almost exclusively of quartz and felspar. Quite 

 rarely a little epidote is to be seen, decomposed at core into calcite ; 

 a shred or two of hornblende ; and small stray crystals of pyrites. 

 Between crossed nicols one sees a pell-mell of quartz and felspathic 

 fragments, the quartz often in large irregular grains, and, grouped 

 around them, are smaller fragments of plagioclase, mostly with 

 their long axes in rude parallelism. In certain cases the felspar 

 has undergone extreme destruction and is reduced to a granular 



