60 



The Geology of the Gold Fields of British G^iiana. 



Quartz-schist. — Among the schistose rocks bands or belts of 

 cjuartz-schist are not uncommon. Tliese vary in colour from light-grey 

 through red and brown shades to nearly black. In general appearance 

 the quartz-schist resembles quartzite, but has a more or less well- 

 marked foliated structure. Its specific gravity is from 2'61 to 2-64. 

 The quartz-schist usually consists of a micro-mosaic of quartz grains 

 with a few of feldspar, containing a little chlorite, few to abundant 

 very minute grains of magnetite, a few sparsely distributed small grains 

 of epidote, and in places some flakes of white mica. Some specimens 

 contain specks of hematite, and others flakes of limonite instead 

 of grains of magnetite. As a rule, the quartz-schist is not auriferous, 

 but specimens which I obtained from near Markabu Island in the 

 Cuyuni River yielded gold upon assay at the rate of 15 grains per ton 

 of the rock. 



All the rocks of the group under consideration show to a greater 

 or less extent the effects of dynamo-metamorphism, these, in the case 

 of many of the more schistose members, being complicated by later 

 effects due to percolating waters. It is important to note that in 

 this group of rocks the effects have largely been restricted to mechanical 

 ones, to the production of hydrous minerals — for instance, of sericite and 

 chlorite — and to the splitting up of the feldspars with separation of 

 albite and cjuartz, the result of the latter being shown in the production 

 of the micro-mosaic of feldspar and quartz which forms the mass of 

 many of them. 



'J'lIE CHEMICAL AND MINERALOGICAL COMPOSITIONS. 



The following analyses have been made of rocks belonging to this 

 group :— 



MASSIVE ROCKS. 



