iri4- Tlie Geology of tlie Gold Fields of British Guiana. 



is traversed by dykes of fine-grained diabase. Near the middle of the 

 river, commencins;- at about a mile from Kumaka Landing and extending 

 for about three-quarters of a mile, there are many rocks and small 

 islets, the general trend of which is east and west. The rocks consist of 

 epidiorite and of hornblende-schist. 



At Kumaka Landing a belt of coarse granitite traverses the river, 

 producing a series of islands, while the channels between them being 

 freijuentl}^ interrupted by great masses and boulders of the rock, give 

 rise to the Kumaka Rapids. The islands extend for about three miles. 

 The great masses and boulders of granitite are usually much affected by 

 weathering, and in places where the rock is of a massive structure the 

 surfaces exfoliate in concentric layers, giving rise to rounded masses ; 

 while in others, where the rock exhibits, in common with the granitic 

 belts of the district, a somewhat gneissose structure, the outer portions 

 split off in flat plates, producing angular blocks. From the surfaces of 

 the weathered rocks narrow veins of quartz, usually not more than two 

 inches across, stand out in relief. Near the head of the Kumaka 

 Rapids, two approximately parallel narrow dykes of diabase are 

 visible on the north bank of the river, crossing it from north-north-east 

 to south-south-west. Above the rapids the granitic rocks extend to 

 Kralibu Falls, where they are traversed by a great dyke of diabase 

 from eighty to one hundred feet in width, trending from the northern 

 Ijank towards the middle of the river to the south-south-west, and from 

 there to the southern Ijank to the south ; near the east end of Smith's 

 Post Island the dyke divides, sending off an ofi'-set to the south-west. 

 The off-set cuts into an aplite of which the mass of Smith's Post Island 

 is composed, and exposures of which are seen near its north-east point. 



The south-east end of the island is traversed by a dyke of diabase, 

 more or less altered to epidiorite. From the south-west end of Smith's 

 Post Island the river gradually changes its direction and flows from the 

 south-south-west and later from the south. Many low rocky islets, 

 with a general trend of east and west, are exposed for about a mile or a 

 mile and a half to the south-west of Smith's Post Island, and are 

 composed of epidiorite, while a large mass of the same rock occurs 

 on the south-east bank of the Essequibo about half a mile from the 

 end of the island. 



At the mouth of the Potaro River the Essequibo is crossed from east 

 to west by rocks having a marked schistose structure, and weathering 

 into great slabs. Those exposed on the east bank are much altered by 

 weathering and are practically quartz-schist, while those in the middle 

 of the river, at the east and west ends of the island in the mouth of the 

 Potaro and on the south l)ank of the latter river, near its mouth, are 

 sericite-schists derived from quartz-i:)orphyrite. The schist can be 

 ti-aced to the quartz-porphyrite from which it has been derived and 

 which is seen in places almost unaltered. A highly metamorphosed rock 

 occurs on the west bank of the Essequilio, south of the Potaro mouth, 

 in the form of a fine-grained chloritoid rock with a greasy feel 

 somewhat resembling that of serpentine. In this schistosity has been 

 developed, but not the same extent as in the x'ocks derived horn 

 quartz-porphyrite. 



