TJifi Essequibo River. 155 



On the west bank of the Essequibo River, about one and a half miles 

 from the mouth of the Potaro and north of Yesi Island, are large 

 exposures of quartz-porphyrite, whilst similar ones occur on the east bank 

 opposite to the island. Rounded masses of fine-grained gneiss, much 

 altered by weathering, occur on the east bank of the river near the 

 south of Yesi Island, and are traversed by a dyke of compact, dark-grey 

 quartz-porphyrite which weathers into angular blocks. The dyke gives 

 rise to small rapids in the eastern channel of the river. South of Yesi 

 Island the rocks have a more or less marked schistose structure, and 

 are derived from porphyrites, felsites, and quartz-porphyrites. 



About a mile north of the mouth of the Konawaruk Creek a narrow 

 dyke of diabase crosses the river from east to west, and half a mile south 

 of this another one trends north-east by east and south-west by west. 

 Exposures of quartz-porphyrite occur towards the east bank of the 

 river south of the latter dyke. North of the mouth of the Konawaruk 

 Creek a broad dyke of diabase of medium texture crosses the river 

 striking south-west by west and north-east by east, giving rise to the 

 Lower Konawaruk Rapids. South of the Konawaruk three dykes of 

 similar rock to that of the one to the north and parallel to it in ti-end 

 ai-e the causes of the upper rajDids. About half a mile south 

 from here, in the middle of the west channel of the river near the north 

 end of Benhori-Bumoco Island, there is a pointed mass of a fine-grained, 

 somewhat schistose, cpiartz-porphyrite ; and about a mile and a 

 quarter south-east of this the river is crossed by a narrow dyke of 

 epidiorite striking north-east through the island towards the east bank 

 of th,' river, while at and near the south-east end of the island the rocks 

 consist of greyish fine-grained quartz-porphyrite. Somewhat north-east 

 of these, on the east bank of the river, is an exposure of a very dark- 

 groy, compact quartz-porphyrite weathering like granite, and becoming 

 somewhat schisto.se near the island w-here it gives rise to small rapids. 

 The quartz porphyrite is traversed by thin veins of quartz, a somewhat 

 unusual feature in this class of rocks in this district. 



About a mile south-east of the southern end of Benhori-Bumoco 

 Island the river is crossed from east to west by a dyke of coarse- 

 textured diabase, followed at about fifty yards by a narrower one 

 parallel to it. South of these diabase dykes there is an exposure of 

 gneiss on the east bank of the river. Two miles south-east of Benhori- 

 Bumoco Island the course of tlie river is interrupted for about a mile 

 by the "Waraputa Rapids and Cataracts. The river, which runs from 

 these rapids to the mouth of the Konawaruk in a north-westerly 

 direction, for a long distance south of Waraputa pursues a northerly 

 course. The western rapids are caused by confused masses of coarse, 

 more or less gneissose hornblende-granitite, the east or main fall being 

 over the same rock, through which a dyke of light- coloured diabase 

 (altered in places by contact to epidiorite) passes with an east and west 

 trend at the foot of the fall. The intrusive lock is covered in places with 

 rude prehistoric figures of the kind known in the colony as Timehri 

 writings. About two hundred yards north of the fall a very dense, 

 almost flint-like, compact, fine-grained quartz-porphyrite traverses the 



