The Lower Essequiho River and Cuyuni River District. 129 



Akaiwong or Wakupaiig Cataracts are caused by a broad dyke of 

 diabase from 270 to 300 feet in width, trending through felsitic rocks 

 west-south-west and east-north-east. The rock of the dyke is of 

 medium to fine textui'e. At the upper side of the dyke the junction 

 of the diabase with a much altered, very compact, siHcified felsite can 

 be seen. 



Low ranges of hills trend from the left bank of the river at right 

 angles (east-south-east and west-north-west) to the strike of the dyke. 



Above and below the cataracts are extensive beds of sand and 

 gravel, covered with coatings of ironstone-conglomerate. 



The rocks exposed between Akaiwong Cataracts and near Anaripia 

 Itabu, a distance of about 16 miles by the river, are felsite, quartz- 

 porphyry, and feldspar-porphyrite, the latter in places passing into rocks 

 having the structure of microgranite. Microgranite is the prevalent 

 rock near Anaripia, in Anaripia Itabu, and near Pigeon Island. 



The main channel of the river, when the water is low, is very 

 frequently interrupted by extensive exposures of felsite and porphyrite 

 in great rounded masses, surrounded by weathered slabs ; in other 

 places the felsite has a better developed schistose structure, and occurs in 

 great upstanding slabs ; and in some places the rocks have a slaty 

 appearance, the felsite being either mylonitic or very markedly schistose. 



About 3 miles south-east of the lower end of Anaripia Itabu, in 

 the course of and near the right bank of the river for a distance of 

 about 100 yards, a finely-bedded, fine-grained grit striking north 

 80° east and south 80° west is seen. This grit is a felsitic tuff, and is 

 probably closely connected in origin with the outflows of porphyrites 

 and felsites, some specimens of which upon microscopical examination 

 show signs of having originally been andesitic lavas. 



From a little to the west of the mouth of the Iroma Creek to 

 near Peinkamarika rapids the rocks exposed are more schistose than 

 are those seen between Akaiwong and Iroma, and are fine-grained 

 chlorite-schists. In places the schists are veined with quartz or are 

 replaced by quartz-rock, and occasionally they contain a fair amount 

 of iron-pyrites. 



The hills near the bank of the Cuyuni River in this part of its 

 course are of decomposed felsite with here and there veins of secondary 

 quartz. The gravels of the placers along the St. John's trail consist of 

 quartz with numerous pieces of more or less decomposed felsite. 



Peinkamarika Rapids are over a belt of hornblende-schist. Higher 

 up the river this is succeeded b}^ chlorite-schist which rock extends to 

 near the foot of the Devil's Hole series of cataracts and rapids. 



The Devil's Hole series of cataracts and rapids occurs over an area 

 of gneissose-granitite, granitite-gneiss, and hornblende-granitite-gneiss, 

 with intrusive belts and veins of granitite, of hornblende-granitite, and 

 of aplite. 



The lowest rapid of the series is over a zoisite-hornblende-schist. 

 Above this for about half a mile the rapids are over gneiss of varying 

 texture. The channels of the middle and upper rapids and cataracts 

 are between and over great rounded masses of granitite and of 



K 



