58 The Geology of th' Gold Fields of British Guiana. 



The following are descri[)ti<)ns of some of the more characteristic 

 samples : — 



[a) Srhintft from Quartz-povplhij^'y, Granophyre and Quartz- 

 porphyriti'. — These more or less well-foliated rocks are usually 

 light-coloured to grey <»r greenish-grey in hue, though some are 

 dark-g!-ey. Their specific gravity varies from 2'60 to 2'78. 

 They are made up of a micro-granular mosaic of feldspar and 

 (juartz, with varying proportions of sericite and subordinate 

 quantities of chlorite, ejndote and actiiiolite. In those which 

 show a well-developed schistose structure sericite is present in 

 abundance in veins, films and streaks. In many specimens 

 blebs of quartz are noticeable, some of them with more or less 

 corroded outlines. These blebs usually show strain-shadows, 

 whilst not unfrequently in places they are much fractured. 

 Phenocrysts of feldspar are noticeable in many specimens, some 

 showing the strife of plagioelase whilst a few consist of 

 microcline. As a rule, the feldspars are clouded with sericite, 

 zoisite and epidote, while in places their phenocrysts show 

 strain-shadows, or are elongated and otherwise distorted. As 

 minor accessories, wisps of biotite or of chlorite after V)iotite 

 occur, whilst in some specimens large flakes of much-altered 

 biotite are present, and more or less fractured or elongated, small 

 crystals of spheue and minute grains of magnetite are found. 

 Carbonates are of frequent occurrence as alteration-products, 

 and grains of pyrite are occasionally seen. 



(/>) Schists from Feldspar-porphyrite, Pegmatite and Felsite. — 

 Where the metamorphic forces have been most intense in their 

 actions schists, consisting essentially of sericite and quartz, with 

 some albite, have resulted from tliem. Excellent examples of 

 these occur at the Government Station at Arawak Matope on 

 the Cuyuni River, near the mouth of the I'otaro River on its 

 left bank, and near Arakaka on the Barima River. 



They are silvery-white, very fissile, soft schists, but in places 

 are stained grey, or are rust-coloured, Their specific gravity is 

 aljout 2"75. They are made up of a colourless grauulitic mosaic 

 of quartz, with some water-clear feldsjiar, which is traversed by 

 streams and wisps of sericite, and with here and there small 

 patches of chlorite. A few grains of epidote and some sparsely 

 scattered minute grains of magnetite are found. In places the 

 rock contains partings of limonite. 



The ct)mmoner types of the schists are of very varying 

 colours — for instance, pale-grey to dark-grey, or almost black, 

 cream-coloured, bluish-grey, bluish-purple, ochrey-yellow, red, 

 purplish-red, l)uff or brown ; in hand-specimens some of them 

 appear to be compact, but the majority show more or less 

 clearly their fissile structure. They vary in specific gravity 

 from 2'76 in the lighter-coloured ones to 2"78 in the darker 

 ones. They consist of a microgranular mosaic of feldspar with 



