550 C.B. Horwood &f A. Wade — The Old Granites of Africa. 



The extinction angles show that albite is the dominant form, though 

 a little lime-soda felspar is present. It is frequently idiomorphic with 

 regard to microcline and orthoclase felspars. The mineral is usually 

 very much altered, so much so that the twin lamellation is frequently 

 obscured. The secondary products include the colourless micaceous 

 mineral sericite, which is often included in minute flakes in the turbid 

 interior of the crystals, or is present along cracks, probably developed 

 by crushing, both in the plagioclase and in the microcline. Small 

 greenish flakes of biotite are sometimes similarly included, whilst 

 lemon-yellow grains of epidote are fairly abundant m the plagioclase. 



A feature to be noted is the presence of clear, fresh, secondary soda- 

 felspar bordering the altered plagioclase in places, practically in optical 

 continuity with the original felspar lamellation (PI. XXVII, Fig. 3), 

 though occasionally the secondary material shows no such lamellation. 

 The microcline contrasts markedly with the other plagioclase felspars 

 by reason of its comparative freshness. It is sometimes quartz-like in 

 its clearness. The typical cross-hatching is frequently beautifully 

 shown. It contains idiomorphic crystals of the other felspars, as well 

 as elongated blebs of quartz and flakes of biotite as inclusions. 

 Sericitization has taken place along strain cracks in the mineral. 



Quartz occurs in colourless irregular grains, sometimes enclosing 

 flakes of biotite, sometimes wrapping round the felspars. Its 

 occurrence inside the microcline simulates graphic structure, whilst 

 micrographic intergrowths with orthoclase felspar are sometimes seen. 

 The grains frequently tail off into or are partly bordered by crushed 

 material. This, accompanied by undulose extinction in both quartz 

 and felspars, shows that the rock has been subjected to severe 

 crushing strains. 



The biotite varies from pale yellow green to green in colour. 

 Longitudinal sections are extremely dichroic : a pale yellowish 

 brown, /3 and 7 green, in unaltered sections. Pleochroic haloes are 

 also common. The mineral has a strong birefringence. It encloses 

 sphene, and sometimes a little sericite is intergrown with it. Epidote 

 is frequently seen in large grains, very intimately associated with the 

 biotite flakes. Chloritization is often seen to have been partly or 

 wholly effected in some of the biotite, with a consequent weakening 

 of the polarization effects. It is mostly seen running through the 

 rock in strings or as groups of flakes enclosing the altered plagioclase 

 crystals. Sometimes it occurs in tiny greenish scales scattered through 

 the much altered felspars. These modes of occurrence, together with 

 its constant association with epidote (PI. XXVII, Pig. 1), suggest that 

 the mineral is in part or even wholly of secondary origin. 



Sphene is unusually abundant. . It is reddish brown in colour, 

 distinctly pleochroic, with high refractive index, strong birefringence, 

 and strong dispersion of the optic axes {p > v). Double wedge-shaped 

 crystals are characteristic, though curiously elongated forms are also 

 present (PI. XXVII, Fig. 2). It belongs to the earlier (porphyritic) 

 generation of minerals present in the rock. Small colourless grains, 

 with high refractive index, are sometimes present, and can be identified 

 as sphene by their strong dispersion. Apatite also belongs to this early 

 generation of minerals, and is a common accessory. It is present as 



