32 Geology of t/ie Gold Fields of British Guiana. 



specific gravity is 2"67, the extremes being 2-75 and 2-58, according 

 to the proportions of biotite ])resent. 



The following are descriptions of typical specimens of the rock : — 



1. G7-anitoidal Variety. — Specimens from Hellgate Falls on the 

 Barima lliver ; Yamatuk and Teboco on the Mazaruni River; from 

 many places sucli as Pigeon Island, Rockstone and Moco-Moco on the 

 Essequibo River ; from the Pomeroon River ; from Devil's Hole 

 Cataracts on the Cuyuni River, and Wallaba on the Demerara River, 

 consist of granitic aggregates of irregular patches of quartz, showing 

 more or less well-marked strain- shadows, or much fractured ; plates of 

 orthoclase-feldspar, in places with a microperthitic structure, irregular 

 patches of water-clear microcline, and plates of oligoclase, sometimes 

 with the lamellae bent or bi'oken, with some of albite, in parts 

 a little vermicular micropegmatite ; some of the feldspar plates are 

 crowded with minute flakes of sericite and prisms of epidote. In places 

 sparse small flakes of muscovite occur, many flakes, wisps and nests of 

 green and olive-green biotite, frequently more or less chloritised, are 

 present ; small plates of green hornblende occasionally occur ; whilst as 

 accessories small grains of garnet, of magnetite, of ilmenite and rarely 

 of anatase, and minute crystals of apatite and zircon are found. 



2. Commencing Granulitic. — The granitoidal gneiss changes gradually 

 and almost imperceptibly into gneiss, the foliation of which in large 

 masses is clearly seen, but which is not well-marked in hand specimens. 



As a rule, its minute structure does not difier greatly from that of 

 the granitoidal, but signs of dynamic metamorphism are more apparent. 

 The feldspar plates frequently show strain shadows, and on their 

 edges signs of becoming granulitic ; the patches of quartz are broken, 

 and are often granular, whilst where they are aflected to a less extent 

 they show well-marked strain-shadows. In parts the striae of the 

 plagioclase-f eld spar are bent or broken, while the biotite-mica in places 

 very markedly shows strain-eifects. 



The following is a general description of the structure of specimens 

 from Poho de Oro in the Amacura River ; Carriage Palls in the 

 Barima River ; Kapasi Channel, Kabowira Cataracts, above Turesi 

 Cataracts and Kartauari Rapids in the Mazaruni River ; Taparoo 

 in the Puruni ; from Souarindo in the Pomeroon River ; from 

 Saxacalli Point and Great Yukuribi Rapids in the Essequibo, from 

 Tiger Creek Falls and Eagle Mountain in the Potaro district, and 

 from Arimu Creek and from above Mopay Rapids in the Cuyuni 

 River. The rocks are granitic aggregates of irregular patches 

 of quartz, more or less granulitic, in places showing well-marked 

 strain-shadows ; plates of orthoclase-feldspar occasionally showing 

 strain-shadows and frequently crowded with sericite and epidote, in 

 places the plates show the structure of microperthite ; some patches of 

 water-clear microline, relatively abundant plates of oligoclase and some 

 of albite, the lamella? of which are frequently bent or broken, whilst 

 micropegmatite is of somewhat rare occurrence. In places a few small 



