464 C. B. Eoncood ^ A. Wade— The Old Granites of Africa. 



constituents as the granite but with granulitic structure. Tourmaline 

 seems to be absent from the Prieska granites. 



" Pegmatite or graphic granite, chiefly composed of an intergrowth 

 of microcline and quartz, forms a large mass in the neighbourhood 

 of Saft Sit Pan. Quartz-porphyries are rather restricted in their 

 occurrence; they have been found only within the granite areas, and 

 are not known to traverse the surrounding rocks in the manner of the 

 quartz-porphyries near Paarl Berg. 



" The granulites of Prieska are abundant and vary greatly in 

 composition. They are fine-grained rocks, usually showing distinct 

 banding on large weathered surfaces, but the banding is often 

 unobservable on a freshly broken surface. They are usually dark 

 in colour, but the more acid or siliceous types are light- coloured. 

 In general appearance they look rather like even-grained quartzites. 

 It is only under the microscope that the distinctive features of the 

 granulites are seen. The most striking character is the uniformity 

 in size of the grains of the various minerals composing the rocks ; 

 another important feature is the almost complete absence of crystalline 

 faces in the minerals, which seem to have separated out in a different 

 manner from that usual in igneous rocks ; enclosures of one mineral 

 by another are abundant, but the enclosed mineral is irregulaidy 

 shaped, usually with a rounded outline. Garnet, which is an 

 important constituent of most of the Prieska granulites, is the only 

 mineral which sometimes shows crj-stal faces, and it very often 

 contains small grains of more than one of the other minerals composing 

 the rock. All the minerals in the granulites are remarkably fresh 

 and free from alteration products. The rocks may be broadly divided 

 into three groups — (1) Granulites made up of the same minerals as 

 the granite and gneiss, viz., quartz, felspar (orthoclase and plagioclase), 

 garnet, and biotite. This seems to be a less abundant rock than those 

 belonging to the two other classes to be mentioned, but on account of 

 its being rather closely related to much of the gneiss, into which it 

 passes by the coming in of a pronounced foliation and the increase 

 in size of some of the felspars, it is easy to overlook small outcrops 

 in the gneiss areas. (2) Hornblende-granulites, composed of quartz, 

 orthoclase, albite, hornblende, biotite, magnetite, garnet, and sphene. 

 The hornblende is a pale, bluish-green variety, different from the 

 hornblende of most of the hornblende schists. Garnet is a less 

 abundant mineral constituent than in the next group. (3) Pyroxene- 

 epidote-granulites, composed of plagioclase, augite, epidote, garnet, 

 magnetite, sphene, and frequently hornblende. The pyroxene is 

 a pale-green or bluish-green monoclinic variety, diopside, and is 

 slightly pleochroic. The abundance of epidote, which often forms 

 a large part of the rock, is very remarkable. 



" The granulites form elongated outcrops in the granite and gneiss, 

 with the longer axes of the areas parallel to the foliation planes of the 

 gneiss ; they have not been found as intrusions in the sedimentary 

 rocks. The nature of their contact with the gneiss has not been made 

 out, as the line of junction of the two rocks is almost invariably 

 concealed under the red sandy soil that the granite and gneiss give 

 rise to. The composition of the pyroxene-granulites must be very 



