170 The Geology of the Gold Fields of Brithh Guiana. 



Half a mile north-east of the exposure of biotite-porphyrite masses 

 of graiiitite are visible in the bed of the river, intersected by a narrow 

 dyke of fine-grained diabase, and changed locally where in contact with 

 the granitite into epidiorite ; these are succeeded about two hundred 

 yards further down the river by a dyke of line-grained diabase striking 

 north-west and south-east. A quarter of a mile Ijelow this granitite is 

 exposed intersected bya dyke of fine-grained diabase trendingwest-north- 

 west and east-south-east ; but the actual junction of the two rocks is not 

 seen, as it has been deeply eioded into by the river. At Wilson's Landing 

 coarse gre}^ granitite is seen in the river followed by exposures of diabase 

 oi medium texture, which near the granitite is altered into proterobase. 

 AVilson's and Caman's Landings are situated at the bases of hills of 

 dark-red clay, the decomposition-product of diabase or of other basic 

 rocks. Below the latter place diabase of medium grain trends south-south- 

 west and north-north-east. In this neighbourhood, ua many places near 

 the banks of the river, ironstone-conglomerates are of very common 

 occurrence. 



Between Caman's Landing and the head of the itabu at Forty Islands, 

 or Itabu Rapids, with the exceptions of a small exposure of quartz- 

 schist, and of others of ironstone-conglomerate, the rocks are hidden by 

 sand-banks. Above the head of the Forty Islands Itabu quartz-schist 

 is exposed in the river for about two hundred yards. Through the 

 itabu and the Forty Islands Rapids quartz-schist is exposed at intervals, 

 but the bed of the stream is generall}^ covered by ironstone-gravel. 

 The quartz-schist, both from above the itabu and from among the 

 islands, is not auriferous. At Forty Islands the river is split up into 

 numerous small and narrow streams, in which the water flows with 

 great velocity, producing a large number of small islets between which 

 the streams are connected by narrow channels. 



Half a mile north-east of the foot of Forty Islands the river makes 

 an S-shaped cvu've, through which it pursues a tortuous course, 

 the southern end of the curve, for about a quarter of a mile, 

 being through hornblende-schist, exposures of which are visible at 

 short intervals. Near Willis s Landing, at the northern end of the 

 curve, masses of very coarse hornblende-granitite occur, followed 

 at about one hundred and fifty yards down the river by a narrow 

 belt of hornblende-schist trending east and west. The hornblende- 

 schist is auriferous, and yields gold at the rate of two pennyweights 

 per ton of the rock. The placers near here are worked in fluviatile 

 sands and gravels deposited at some former period by the river. For 

 half a mile north-east of Willis's Landing the rocks in the river are 

 hidden by sand-banks, while for a similar distance to Stout Fall, 

 exposui'es of coarse granitite occur at intervals. 



At Stout Fall the rapids are over coarse granitite with narrow 

 veins of fine-grained, dark-coloured quartz-porphyry. From here 

 the river pursues a winding course to the north-east, the rocks 

 seen being coarse gneiss and gneissose-granitite. These are crossed 

 by elvans of fine-grained quartz-porphyry, which cause rapids in 



