The Loiver Essequibo River and Cuyuni River District. 127 



At Tagina Point the river, which is about one-third of a mile wide 

 at Moco-Moco, narrows until it is about 200 yards across. Tagina 

 Point is a boss of fine-grained diabase intrusive through augite-granitite- 

 gneiss. The granitite-gneiss is traversed by veins of coarse-grained 

 pegmatite. 



Above Tagina Point the river runs from the north for about 3 miles 

 until its course is interrupted by the Paiyuka Cataracts. The rocks 

 exposed in its course are near Tagina altered granitite-gneiss, about a 

 mile above feldspathic granite, and near Paiyuka are altered granitite- 

 gneiss. 



The gneiss, about a mile below Paiyuka, contains a mass of pinkish 

 sugary quartz, which is practically barren, yielding only at the rate of 

 one grain of gold to the ton of rock. 



The Paiyuka Cataracts are caused by a broad dyke, about 180 feet 

 in breadth, of a fine-grained diabase trending west-south-west and east- 

 north-east approximately parallel to the Stop-Ofi^ and the Tinamu 

 Dykes. At Paiyuka the junctions of the diabase-dyke with granitite- 

 gneiss are visible both above and below the cataracts. Here the 

 gneiss is of a somewhat finer texture than it is at Tinamu and at 

 Tagina Point, but is altered in the same manner. That from below 

 the cataracts is more affected than that from above, the latter retaining 

 some brown biotite not completely changed to magnetite and augite. 



The rocks above Paiyuka Cataracts beyond the zone of altered 

 granitite are gneissose-granitite and granitite-gneiss, and it is j^robable 

 that the country between Tinamu and Paiyuka is a gneissose one, 

 although the altered rocks do not show signs of foliation. The peculiar 

 appearance of the altered granitite-gneiss in hand-specimens is very 

 marked. At the foot of the Tinamu Cataracts it apj^ears to be a 

 granitoidal rock consisting of small white areas of quartz with abundant 

 dark-gre}'' to almost black crystals of feldspar, whilst the specimens 

 from near the junction with the diabase at the head of the cataracts 

 contain comparatively few of the blackened feldspars but many crystals 

 of white plagioclase-feldspar. 



Near Moco-Moco the rock consists of dark-grey to black feldspar 

 crystals with abundant areas of pinkish-brown feldspar and of glassy 

 quartz. At Tagina Point the granitite-gneiss is coarse-textured, and 

 hand-specimens show signs of parallelism in its structure. The rock 

 consists largely of whitish plagioclase-feldspar, a few plates of dark 

 feldspar, blebs and patches of glassy-white quartz, and dull patches 

 of a greyish-green augite. 



The rock at the foot of Paiyuka Cataracts is very similar in appear- 

 ance to that below Tinamu whilst that from near the junction with 

 the diabase-dyke above Paiyuka is finer-grained, with abundant blebs 

 and patches of glassy-white quartz, the feldspar-crystals generally 

 having a greyish-green tinge, although some are white, and scales of a 

 glistening bronze-coloured mica are scattered here and there in the 

 rock. 



The dark colour of the feldspar-crystals, which causes the gneiss 

 to have in places the appearance of a coarse-textured basic rock — when 



