3G Geology of the Gold Fields of British Guiana. 



of clear microcline, relatively large plates of oligoclase having their 

 lamellae more or less bent or broken, with in a few places a little 

 vermicular micropegmatite ; fairly abundant plates and nests of 

 flakes of brown to greenish-brown biotite, frequently more or less 

 chloritised, and with small plates of apparently original epidote, 

 aggregates of chlorite after biotite, epidote, and sphene with some 

 grains of magnetite. In places, a few irregularly broken pieces of 

 colourless augite with some green hornblende occur. Small crystals, 

 grains, and granular aggregates of sphene are of somewhat frequent 

 occurrence ; whilst a few scattered grains of magnetite, a few minute 

 pi'isms of apatite, and minute crystals of pyrites form unimportant 

 accessories. 



At the Upper Mariwa Cataracts, in the Cuyuni River, at Puruari 

 Creek in the Puruni River, and at Tipuri Point in the Essequibo 

 River, gneiss is found containing granular epidote in considerable 

 abundance. In these rocks the feldspar and quartz are present in 

 clear granulitic mosaics, and in the rock from the latter place granules 

 of melanite (black garnet) are present in abundance. The epidote 

 in them is probably of secondary origin. 



Granitite-gneiss altered by dykes of Basic Hocks. — Where traversed 

 by large dykes and masses of diabase, the gneiss in some cases appdars 

 little affected, whilst in others it has undergone more or less marked 

 changes. For instance, the great belt of diabase which gives rise to 

 the Falls of Oruro Malali on the Demerara River, traverses a country 

 of coarse-textured hornblende-granitite gneiss. The efi'ects of the 

 intrusive mass are distinctly noticeable on the gneiss at about fifty 

 yards from the contact of the rocks, and become very marked upon 

 approaching it. The gneiss gradually assumes a granitoidal character, 

 the foliation becoming obliterated. Close to its contact with the 

 diabase its appearance resembles that of a porphyrite with crystals of 

 orthoclase and oligoclase with rounded outlines lying in a dark- 

 coloured base. Under the microscope the rock does not show any 

 marked signs of alteration in the feldspars, although these are more or 

 less clouded, but the hornblende-plates have been changed to clouded 

 aggreg9.tes of minute specks of augite, with some epidite and zoisite 

 and with small extruded grains of magnetite, whilst the plates of brown 

 biotites are less altered, although they generally contain many extruded 

 grains of magnetite. 



The great dykes of diabase which give rise to the Tinamu and 

 Paiyuka Cataracts of the Cuyuni River, produced during their intrusion 

 into the granitic-gneiss of the district more marked changes than 

 resulted at Oruro-Malali. The gneissose structure of the country is in 

 the vicinity of the dykes obscured or almost obliterated. Where most 

 altered the rock resembles a granite containing small white areas 

 of quartz with abundant dark grey to almost black crystals of feld- 

 spar. Near the dykes the biotite originally present in the gneiss is 

 changed completely into areas of minute grains and dust of magnetite 

 with a little augite scattered through them. Farther from th contact 



