The Fetrografhy of the, Fundamental Gneissose Complex. 35 



on the Amucura lliver, Wanaparn ou the Barima River, Arimu Creek 

 in the Cuyuni River, Parawakas Rapids on the Mazaruni and at Itaka 

 on the Lower Essequibo. These have the following structures : — 



The rock is made up of a micromosaic of quartz and feldspar, some 

 of the granules of the latter showing the characteristics of microcline 

 and others of oligoclase ; whilst in places a few scattered small plates 

 of muscovite are present. Streams of more or less elongated 

 plates of pale bluish-green hornblende traverse the rock, and are 

 accompanied by wisps of biotite usually more or less chloritised ; rarely 

 a few granules of augite are present ; some granular epidote is a 

 common accompaniment of the hornblende ; small grains of sphene, 

 and of melanite and rarely minute crystals of apatite form accessories. 



Epidoie-granite-gneiss. — A noticeable feature in several of the 

 specimens of the gneiss is the occurrence of epidote in the form of 

 small prisms in some of the feldspars, and also as small plates 

 surrounded by hornblende or biotite, in such a manner as to indicate 

 that the mineral is an original product of the solidification of the 

 magma. The epidote-granitite-gneiss is usually either grey or greenish- 

 grey in colour, and varies from almost granitoidal rocks to others more 

 or less well-foliated. The specific gravity of this variety of gneiss 

 varies from 2 '70 to 2-86, the mean being 2' 78. 



Gneiss containing small prisms of epidote in the feldspars occurs at 

 the Kuribrong Falls in the Potaro district, near Turesi and Kartauari 

 in the Mazaruni River, and in the Arimu Creek of the Cuyuni River ; 

 and has the following general composition : — 



A granitic aggregate of irregular areas of granulitic quartz ; plates 

 of orthoclase-feldspar in places clouded with films of sericite, and 

 larger ones of oligoclase crowded with minute crystals of epidote and 

 of zoisite, the lamelite of the feldspar plates being frequently bent or 

 broken. A few small flakes of muscovite, and more abundant plates 

 of greenish biotite, more or less chloritised, or in parts completely 

 changed into chlorite are found. In places some granules of colourless 

 augite occur, and some small plates of green hornblende, which, in 

 places, is more or less chloritised, whilst small granules of secondary 

 epidote are scattered through the altered hornblende more or less 

 abundantly. Small grains of magnetite and of sphene, and some 

 minute crystals of zircon are present. The rock is in places traversed 

 by thin veins of aggregates of epidote and chlorite. 



The variety containing small plates and crystals of epidote 

 surrounded by biotite or hornblende occurs at the Towakaima Creek 

 and Falls on the Barama River ; on the lower part of the Kamaria 

 road on the Cuyuni River ; near Turesi, Kartauari, and south of 

 Tupeku on the Mazaruni River ; and in the Black Creek, a tributar}' 

 of the Groete Creek, in the Lower Essequibo district. This variety 

 consists of a granitic aggregate of irregular areas of quartz showing 

 strain-shadows ; plates of orthoclase-feldspar in places with abundant 

 inclusions of small crystals of muscovite, but more usually clouded 

 by minute inclusions of sericite and of epidote, a few irregular patches 



