The Petrography of the Granitic Rocks. 69 



and flakes of sericite. Here and there the plagioclase shows 

 twinning of the pericline type. In some specimens the 

 lamelL-B of the feldspar are bent, curved, or even broken. 



Microcline is present in most of the specimens I have 

 examined, usually in iri^egular areas of later consolidation than 

 the remaining feldspars and the quartz. Quartz occurs in very 

 varying proportions, it is rarely idiomorphic, and generally 

 forms irregular areas, the majority of which show, to a greater 

 or less extent, strain-shadows, whilst patches occur having a 

 granular structure. 



Green, dark-olive-green, and brown biotite is found in very 

 varving proportions in ragged-edged plates and aggregates 

 associated with small granules of epidote, while the latter also 

 occurs in isolated granules and patches. In some specimens 

 irregular patches of chlorite secondary to biotite are noticeable. 

 In places a few flakes of muscovite sparsely occur, generally 

 intergrown with the biotite. Among the rarer and more 

 sparsely distributed constituents are found minute grains of 

 magnetite and of titaniferous iron-ore with leucoxene, small 

 grains, granules and crystals of sphene, small blebs of garnet, 

 a few minute crystals of zircon, and some small prisms of apatite. 

 (h) Pink to red Granitite. — This variety is, as a rule, finer in texture 

 than are the majority of the grey ones, but in places — for 

 instance, at Maripa in the Mazaruni River — it is very coarse- 

 lextured. It is of various shades of pink to red in colour, 

 whilst hand-specimens of the darker varieties show the 

 presence of black mica in abundance. It ranges in specific 

 gravity from 2-60 to 2'79, the lighter kinds generally 

 occurring in relatively narrow veins, the heavier in extensive 

 masses. As in the case of the grey granitite the specific 

 gravity varies with the proportions of biotite the rock contains. 

 In composition it is similar to the grey variety, but orthoclase 

 is usually more abundant and oligoclase less evident in it than 

 they are in the latter. Its red colour is due to the red-coloured 

 crystals of orthoclase it contains. 



(o) Hornhlende-granitite. — The horiablende-granitites are usualh^ 

 pink or reddish rocks with not infrequently patches of green 

 to greenish-grey, whilst in many of them the ferro-magnesian 

 constituents are very conspicuous. The hornblende-granitites 

 vary in specific gravitj' from 2 68 to 2-86, the majority of 

 them falling between 2-73 and 2-78. They closely resemble 

 in composition the granitites, but contain, in addition to 

 biotite, hornblende in greater or less abundance ; the mineral 

 being usually of a green colour, but in some places pale-blue 

 or, very rarely, brown. The hornblende is, as a rule, 

 principally present in aggregates with biotite, some granular 

 epidote, and not infrequently with crystals of sphene. The 

 accessory minerals — magnetite, titaniferous iron ore, sphene, 



