Dr. B. F. Rand — 8ome Transvaal Eruptives. 107 



III. — Some Transvaal Eruptives. 

 By R. F. Eand, M.D., F.L.S. 



THE following petrographical notes refer to examples of igneous 

 rocks collected by the writer during a visit to the Transvaal in 

 the Summer of 1904. Thin sections of the rocks have since been 

 made, and it was thought the following condensed accounts of their 

 appearances might perhaps prove a useful record. 



Meg. is prefixed where megascopic characters are noted ; Mic, 

 where the allusion is to microscopic features. The magnification, 

 unless otherwise stated, is one of 60 diameters. 



Nos. 1, 2, 3, 4, 5, and 6 are examples of the granite which emerges 

 two miles or so due north of the Johannesburg P.O. (the point to 

 which distances in the Johannesburg district are referred). The 

 Witwatersrand Series of clastic rocks, the gold-bearers, dip away 

 towards the south, their northern edge forming a bold escarpment 

 overlooking the granite. The specimens, collected from different 

 points, have a general likeness and are grey in colour. 



No. 1. — Locality : small quarry close to the Hyde Park Estate, 

 5|- miles north of Johannesburg. 



Meg. A medium-grained type. Sp. gr. 2'62. 



Mic. Shows the usual granitic type of structure. Quartz bulks 

 largely ; it is traversed by strings and bands of minute bubbles, and 

 includes many fine straight needles, some of which are nearly 

 a millimetre in length. They are probably rutile. The felspars 

 are microcline, plagioclase, and orthoclase in about equal proportions,. 

 All show alteration, but the microcline is in, by far, the freshest 

 condition. The change is principally one of kaolinization, but 

 , amid the cloudiness many fine flakes of muscovite are recognisable. 

 The soda-lime felspar is oligociase, it shows fine albite twinning, 

 and has a mean refractive index slightly lower than that of the 

 adjoining quartz. The orthoclase is in Carlsbad twins and shows 

 zonary decomposition. 



The ferro-magnesian elements are biotite and hornblende in 

 about equal quantities. The biotite is intensely pleochroic, and 

 encloses sphene, sometimes in crystals, but more often in grains. 

 The hornblende is bottle-green in colour, and usually lies in 

 association with the biotite. It frequently encloses epidote, both as 

 crystals and grains. Apatite occurs sparingly, sometimes enclosed 

 in the biotite. A few small zircons are to be seen. Here and there 

 are small patches of a micrographic intergrowth of quartz and 

 felspar, the felspar dominant and quite turbid. 



No. 2. — A fine-grained example from the same quarry as No. 1. 

 Sp. gr. 2-59. 



Mic. The minerals are the same as in No. 1, but the grains are 

 smaller. The difference between the refractive indices of the quartz 

 and the felspars gives the surface of the section a curiously uneven 

 semblance, the quartz appearing to lie at the bottom of shallow 

 hollows. The biotite shows inclusions of sagenitic rutile. 



No, 3. — Locality : Waverley Estate, 5J miles N.N.E. of 

 Johannesburg. 



