164 The Geology of the Gold Fields of British Guiana. 



coloured quartz-diabase having a micro pegmatitic structure. In this 

 dyke the accumulations of the ferro-magnesian minerals and iron-ores to 

 the edges of tlie dyke, and of the more acidic constituents to the middle 

 of the rock mass, are very marked. Half a mile above the diabase dyke 

 exposures of schistose quartz-porphyry and of sericite-schist occur, which 

 are intersected by a narrow dyke of diabase. The schists continue at 

 intervals for about six hundred yards, where three narrow dykes of 

 diabase are intrusive through them, the dykes trending to the north- 

 east. For about a mile further up the course of the river the rocks 

 exposed are, as a rule, schistose-quartz-porphyry, with here and there 

 quartz-boulders, only one exposure of diabase occurring. For the 

 succeeding mile or mile and a half, up to the foot of the great series of 

 the Kuribrong Cataracts, many exposures of diabase of varying 

 texture occur, their general trend being north-east and south-west. 

 The Kuribrong Cataracts commence about four and a half miles from 

 the mouth of the river, and extend along its course round a great loop 

 for six or seven miles, the distance as the crow flies between the lowest 

 and the uppermost fall being only a little over a mile. The river 

 pursues a very tortuous course round the loop, traversing fine-grained 

 gneiss, and during its passage through the narrow clefts in the rock it 

 gives rise to eight small cataracts and to numerous rapids. None of 

 the cataracts are of any i^reat fall, but owing to the narrowness of the 

 confined channel the water rushes over and through them with very 

 great force. The gneiss is an epidote-granitite-gneiss at the foot of the 

 cataracts, and is granitite-gneiss at the middle and top ; it is traversed 

 by numerous small elvans of felsite and cpiartz-porphj^ry, and here 

 and there by thin veins and stringers of quartz, of which the 

 thickest noticed, about eighteen inches across, is at the top of the 

 series. The quartz of this vein contains only traces of gold. 



Towards the middle of the series of cataracts the gneiss is traversed 

 by two dykes of diabase, one being about five feet across with about one 

 and a half inches of altered contact rock at its edges, whilst the other, 

 situated a little lower down the river, where it gives rise to a cataract, 

 is a great dyke fully fifty yards across ; it is of fairly coarse texture 

 towards its middle parts and is very compact near its junction 

 with the gneiss. C. B. Brow^n describes, on p. 200 of the " Geological 

 Reports," a curious triangular block at the end of one of the masses of 

 rock in this dyke to which it is joined by a thin neck. This block 

 occupies at present the same position as it did at the time of his 

 visit, whilst lower down a similar block, weighing proljably not less 

 than half a ton, lies in the river, having fallen from the main mass, the 

 ]iarrf)w neck of which shows a clean, comparative!}" fresh fracture. 



The chief features of the Kuribrong River in its reaches below these 

 cataracts is the prevalence of schistose quartz-jjorphyries and sericite- 

 schists, and of the boulders of quartz which in places obstruct its course. 

 The district through which the river runs above the cataracts is mainly 

 a granitic one until it reaches the sandstone-formation, and does not 

 seem likely to be of value as a gold-producing one. 



