34 Geology of the Gold Fields of Jiritislt. Guiana. 



quartz showing struiu-shatkiwis, or in places broken and showing signs 

 of becoming granulitic ; large clouded plates of orthoclase-feldspar, 

 some of which show the structure of microperthite, abundant patches 

 of water-clear microcline, and many of oligoclase, in a few places with 

 abundant minute crystals of epidote and flakes of sericite included in 

 them, with some small patches of albite and some vermicular 

 micropegmatite. Small flakes of muscovite are of rare occurrence ; 

 plates of dark-green biotite, which in places are more or le:s chloritised 

 or completely changed to green chlorite, are present in varying (juantity, 

 frequently forming aggregates with flakes and patches of green horn- 

 blende (some of the plates of the latter contain extruded grains of 

 magnetite) ; the aggregates of biotite and hornblende are in places 

 accompanied by apparently original crj^stals of epidote ; small grains of 

 augite are present but rarely ; epidote is present in small quantity as 

 aggregates and in iri-egular grains. 8phene is of rather common 

 occurrence in small crystals and in granules, whilst grains of magnetite, 

 of ilmenite and garnet, and minute crystals of apatite, zoisite and zircon 

 are unimportant accessories ; in places strain-phenomena are noticealjle 

 in the feldspar plates and in those of biotite as well as in the quartz. 



At Matope Cataracts, at Takkari Rapids, and at Devil's Hole in the 

 Cuyuni River ; at Itaki Rapids in the Mazaruni River ; at Stop Falls in 

 the Puruni River ; at the Kuribrong Falls in the Potaro District ; and 

 in the Groete Creek of the lower Essequibo River hornblende-granitite- 

 gneiss is present, in which the foliation is fairly well-marked. As a 

 rule the ferro-magnesian minerals are present in somewhat greater 

 abundance in those than in the granitoidal vai'ieties, and the fissile 

 structure is due to parallelism in their arrangement. The rocks are 

 composed of irregular areas of granular quartz, the larger fragments of 

 which in places show strain-shadows ; plates of orthoclase usually 

 clouded with minute flakes of sericite and of epidote, and in a 

 few specimens small irregular patches of microcline, many plates of 

 oligoclase in places crowded with minute crystals of e2:)idote, and having 

 their lamellae not infrequently bent or bi'oken. Some of the feldspar- 

 plates are in parts converted into a quartz-feldspar micromosaic, and, 

 generally speaking, the efiects of dynamic metamorphism are very 

 noticeable in the feldspar of this class of gneiss. Abundant plates of 

 olive-green and of green hornblende are found which in places are bent 

 or broken or contain extruded gi-ains of magnetite ; in parts, but 

 rarely, plates of colourless augite are present in small quantity ; dark 

 brownish-green or green biotite is present either as plates, frequently 

 bent and showing well-marked strain-effects, scattered through the 

 rock or forming streams encircling eyes of feldspar ; the biotite is in 

 places more or less chloritised or completely altered into chlorite. 

 Small plates and grains of epidote, grains of magnetite and of sphene 

 and minute crystals of apatite and of zircon form unimportant 

 accessories. The rocks are in places traversed by many thin veins of 

 epidote with some calcite. 



Occasionally the hornblende-granitite-gneiss approaches in structure 

 to a schist. Specimens of this variety have been obtained from Juanita 



