360 F. P. Mennell — South African Petrogra^ihy. 



micrographic intergrowths of quartz and felspar ; indeed, under 

 a higli power most of them are resolvable into micropegmatite. 

 Large flakes of biotite occasionally occur, and smaller, sometimes 

 idiomorphic, crystals of hornblende are also present. Both ferro- 

 magnesian minerals are somewhat decomposed, chlorite and calcite 

 occurring as alteration products. The felspars are beautifully zoned 

 and for the most part very fresh. They frequently form a nucleus 

 for the spherulites, or the latter may diverge from their angles. 



Another type of granophyre occurs in Northern Ehodesia (British 

 Central Africa), near the junction of the Chibwe and Zambesi Elvers. 

 It shows good-sized phenocrysts of felspar, and smaller ones of quartz, 

 set in a purplish groundmass. It bears a considerable resemblance 

 to the well-known ' quartz porphyry ' which occurs so abundantly 

 as boulders in the New Red Breccia near Teignmouth in South 

 Devon. The groundmass is, however, of a finely granular type, and 

 is by no means so red. Both quartz and felspar occur in idiomorphic 

 but corroded crystals, and the latter seems almost entirely referable 

 to microcline, though decomposition has obscured the cross-hatching 

 in many cases. Magnetite and sphene are abundant accessories, 

 while there are also occasional prisms of zircon. 



At Francistown, in Bechuanaland, occurs a granophyre in which 

 hornblende is the principal porphyritic constituent. A second 

 generation belongs to the groundmass, which is, however, composed 

 principally of quartz and felspar with abundant apatite and sphene. 

 The quartz, though not thoroughly idiomorphic, occurs in grains 

 and granular aggregates, and almost invariably presents convex 

 boundaries to the felspar. Orthoclase predominates, but twinned 

 plagioclase is also present. 



A rock which has quite a granitic appearance in hand-specimens 

 occurs at the Dopodge Eiver near Waukie, in Northern Matabele- 

 land. It shows large porphyritic felspars, which are seen under 

 the microscope to be orthoclase. They are frequently twinned on 

 the Carlsbad plan, and often show rounded inclusions of quartz. 

 Plagioclase is also present. Both muscovite and biotite occur, 

 sometimes as aggregates, and the latter is largely chloritized. The 

 groundmass, which does not make up a very large bulk of the rock, 

 is partly microgranitic and partly micropegmatitic. In the latter 

 case the quartz is of what has been called the 'vermicular' type, 

 being rounded and showing a wavy outline in longitudinal sections. 



The designation felsophyre has been adopted instead of the very 

 ambiguous term felsite for the dyke rocks corresponding to the 

 syenites. A number of the ' amygdaloidal diabases ' and ' felsites ' 

 of Cape Colony no doubt belong to this division, but without 

 chemical analyses it is difficult to assign them to their proper 

 position. A large intrusion near Belingwe, in Matabeleland, may, 

 however, be referred here with some certainty (Fig. 2). It consists 

 of idiomorphic felspars (orthoclase and oligoclase) imbedded in 

 a ' felsitic ' groundmass, stained with chlorite, representing the 

 original ferro-magnesian mineral. The orthoclase is identified by 

 the simultaneous straight extinction of the two halves of a Carlsbad 



