38 Geology of the Gold FipldA of Britiftlt Guiana. 



magnetite, with some minute prisms of apatite form unimportant ac- 

 cessories. The country rock of the Aruka in the North-Western district 

 is a gneiss which, near to the masses of epidiorite and hornblende-schist 

 traversing it, has the character of a granitite passing into a 

 biotite-schist. These varieties differ only in tlie arrangement of tlie 

 biotite ; in the granitite it is in irregularly scattered small l>rown plates 

 of varying shapes, while in the schist it forms lath-shaped crystals 

 airanged in the rock in streams parallel to their long diameters. The 

 mass of these rocks is a mosaic of water-clear plagioclase-feldspar with 

 (juartz, some sparse plates of muscovite, a few irregular grains of epidote, 

 scarce grains of pyrite, and some minute prisms of apatite. 



The lower rapids in the Waini River are over a dark-grey, highly 

 contorted mica-schist traversed by narrow veins of muscovite-granite. 

 The mica-schist is made up of very numerous elongated prisms of dark 

 brown biotite with some of muscovite arranged in curving more or less 

 parallel streams traversing a rather coarse-textured granulitic mosaic 

 of quartz with some water-clear feldspar. A few small granules of 

 garnet and some minute grains of zircon in the biotite form 

 unimportant accessories. 



The specific gravity of the biotite-schists varies greatly according to 

 the proportion of biotite present, the range being from 2-63 to 2'95. 



Aluscovite-schist. — This appears to be of rare occurrence. I have 

 found specimens in two localities only — in the Black Creek, a tribu- 

 tary of the Groete Creek, and near Teboco in the Mazaruni River. 

 The former is a white glistening rock having a specific gravity of 2"65, 

 and consists of a granulitic mosaic of water-clear feldspar, some 

 granules of which show the striag of plagioclase, and quartz ; with 

 abundant plates, varying greatly in size, of muscovite. The latter rock 

 is similar in appearance but grey in colour, and its specific gravity is 

 2-62. It is made up of a mosaic of granules of quartz with granules 

 of much altered and sericitised feldspar, in which are many laths 

 of muscovite, some ghosts of flakes of biotite, and relatively 

 abundant minute grains of magnetite. In some parts of this rock 

 the quartz is in patches which are much fractured but are not granulitic. 



Pyroxene-gneiss. — Foliated rocks containing as their principal 

 ferro-m:ignesian constituents either augite or other pyroxene are of 

 very rare occurrence in the districts I have visited. The only place 

 where I have found a pyroxene-gneiss is at Haiowa Rapids on the 

 Essequibo River. It is a fairly compact, well-foliated, dark-coloured, 

 rock, of specific gravity 2-92, and consists of allotriomorphic granules 

 of colourless augite and some of enstatite in a mosaic of grains of 

 water-clear plagioclase-feldspar, with a few of quartz ; it also 

 contains a few plates of pale green hornblende, some granular epidote 

 in places, and some grains of sphene and small prisms of zoisite. 



Pegmatite. — The gneiss is traversed in places by very numerous 

 veins of pegmatite. These are either made up of very large plates of 

 oligoclase with some of orthoclase showing more or less well-marked 



