The retTO(jraphy of the Granitic Rockti. 71 



Inchisions in th" Granitites — In many places the granitite contains 

 masses, varying much in size, of the gneissose rocks through which it 

 has been intruded ; these in no way differ in structure and in composition 

 from the rocks they have been derived from. 



In places, as at Kartabo on the Cuyuni River, masses occur in the 

 granitite which appear to have been derived from clastic rocks. The 

 rock at Kartabo is coarse-textured, and shows in all parts I have 

 examined a more or less laminated structure. Its specific gravity 

 is 2-76. It is made up of a microcrystalline mosaic of quartz, with 

 some plates of water-clear feldspar, and a few showing plagioclastic 

 markings ; traversed by streams of green hornblende lying approximately 

 parallel in their long diameters to one another. A few grains of 

 epidote are also present, and in parts some grains of magnetite are 

 found. 



Quartz-mica-diorite. — This rock occurs in relatively narrow dykes 

 and veins traversing the gneiss. It is a dark grey, usually medium to 

 fine-grained rock, but in places becomes coarse- textured ; its specific 

 gravity vai'ies from 2-81 to 2-94. It is made up of granular to granitic 

 aggregates of plagiociase, principally labradorite, with a few small patches 

 of quartz, in places showing strain-shadows, or, in some specimens being 

 granulitic. Many of the feldspar plates are crowded with minute 

 prisms of epidote, or of zoisite, with a little sericite. The rock contains 

 in abundance large plates and wisps of light-brown biotite, in places 

 changed to chlorite, many irregularly shaped plates of hornblende varying 

 in colour in difi'erent specimens from pale-blue, green, or olive-green, to 

 light-brown, some of the paler ones with many extruded grains of 

 magnetite, a few plates of a colourless augite, and some granules and 

 aggregates of epidote. Small grains of magnetite, of titaniferous iron 

 ore, and of sphene, with minute prisms of apatite, which in some 

 specimens are only sparsely distributed, but in others are relatively 

 abundant, form accessories. 



Quartz-dlorite. — This occurs under similar conditions to those in 

 which the foregoing rock does ; and the two are very closely related. 

 The quartz-diorites are grey to dark-grey in colour, tine to somewhat 

 coarse-grained in texture, and vary in specific gravity from 2"80 to 2"96. 

 They are granular to granitic aggregates of plates of a basic labradorite, 

 usually crowded with minute grains of epidote, of zoisite, and with some 

 flakes of sericite, irregularly bounded areas of interstitial quartz, with 

 more or less idiomorphic plates of varying sizes of green and of bx'own 

 hornblende. Some of the plates of hornblende have a poikilitic struc- 

 ture. In certain specimens the hornblende is either very pale-green or 

 pale-blue in colour, the plates are ragged-edged, in places contain some 

 minute grains of extruded magnetite, and they are accompanied by small 

 aggregates of chlorite, and granules of epidote. In a few specimens 

 pale-brown biotite in flakes, and small plates of colourless augite 

 are sparsely found. The accessory minerals occur in very varying pro- 

 portions, and consist of sphene in granules or in crystals, small grains 

 of magnetite and of titaniferous iron ore, in places with leucoxene, 



