Dr. R. F. Rand — 8o7ne Transvaal Erupt ives. 115 



having definite outline. The greater stability of the augite, aS 

 compared with the enstatite, is very noticeable, this last showing 

 advanced bastitic change. The felspar crystals are in good condition. 

 They consist of labradorite. Many show beautiful zoning, the 

 zones varying in constitution, and usually following the rule of 

 increase in acidity on passing from core to crust. Sometimes, 

 however, relatively basic and acid zones are seen to regularly 

 alternate. Ilmenite occurs, sparingly, in regular plates, and there 

 are a few scattered irregular grains of magnetite. It is interesting 

 to compare this rock with Nos. 20 and 21 from beyond Pretoria. 



No. 26. — Locality : steep, rocky hill behind the old town of 

 Klerksdorp, west of the Schoon spruit. 



Meg. A rock with dark-blue, stony ground in which lie opaque, 

 white, quadrangular crystals and glassy - looking blebs. The 

 structure is typically porphyritic. The weathered surface is 

 scoriaceous-looking, owing to the weathering out of the porphyritic 

 crystals ; it is inclined to rustiness in colour. Sp. gr. 2-56. 



- Mic. The ground-mass is micro-felsitic, with some residual glass. 

 Its more transparent portions show vague and ghost-like crystalline 

 outlines, as though the excessively fine granules were on the way 

 to crystalline form. The phenocrysts are : (a) Quartz, its crystal 

 outlines rounded off by the magma ; the crystals cracked and 

 showing occasional inclusion of the ground-mass. (6) Felspar : 

 much decomposed, shows albite twinning, sometimes of exceeding 

 fineness ; this is sometimes associated with pericline twinning. It 

 may be regarded, provisionally, as an acid-oligoclase. The edges 

 show irregular corrosion, and there are inlets of the ground-mass 

 within the crystals, (c) Orthoclase : in simple Carlsbad twins, 

 clouded like the foregoing, the crystal edges slightly rounded, but 

 not worm-eaten-looking as in the case of the plagioclase. {d) Nearly 

 opaque crystals of hornblende ; relics of magmatic corrosion. They 

 are recognisable by the shape of their cross-sections and cleavage- 

 fragments, (e) Pseudomorphs of green chlorite, following augite. 

 These show a narrow, dark resorption-border. Specks of magnetite 

 and grains of calcite ai-e to be seen in the chlorite. The rock is 

 interesting as a relatively acid type of eruptive, and may be classed 

 as a quartz-porphyry. 



No. 27. — Locality : about eight miles N.N.E. of Klerksdorp. 



Meg. A bluish-black rock of fine-grained groundwork, which 

 includes many fine, angular, glassy fragments. Sp. gr. 2*68. 



Mic. The rock consists of crystalline fragments and lapilli 

 cemented together by a glassy, tufaceous matrix. The crystalline 

 fragments consist chiefly of quartz ; they are sometimes rounded, 

 but more often are irregular in shape and sharply angular. The 

 fragments contain the strings of inclusions of the type seen in 

 granitic quartz. Here and there are crystalline aggregates of pyrites, 

 and fragments of microcline and plagioclase. The lapilli vary 

 a good deal in size. A piece of one large one flanks the whole 

 section. It consists of an interlacing framework of plagioclase bars, 

 whose meshes are filled by opaque, unindividualised, glassy matter. 



