CHAPTER XIV. 



THE BRITISH GUIANA, VENEZUELA AND BRAZIL 

 BOUNDARIES. 



The. Britii<h Guiana- Venezuela Boundary. — For about six miles 

 west from Akarabisi, the Cuyuni River, along which the boundary is 

 drawn, traverses a quartz-porphyry and porphyrite district, the rapids 

 at Makapa being over the latter rock. Westward of Makapa, to the 

 mouth of the Wenamu, the Cuyuni traverses the lowest beds of the 

 sandstone-formation, although, in places, it has cut its channel through 

 them into the underlying rocks, as at above Makapa, where these are 

 epidiorites, at Arawatta Island, and near Amakaira Island, where 

 proterobase is exposed, and at Eteringbang Fall, where the rocks consist 

 of f el site. 



In this long stretch of river the lowest Vieds of the sandstone- 

 formation which are exposed consist of pudding-stones and coarse 

 feldspathic grits, composed of re-cemented pebbles, gravels, sands, and 

 silts of quartz-porphyry, porphyrites and felsites. Specimens of these 

 were collected by the Boundary Commissioners from, at, and near the 

 mouth of the Morawan Creek, in the neighbourhood of Eraki Creek, at 

 Arawatta Village, Creek and Island, and at Mekura Rock. In many 

 places the coarse-textured rocks are replaced by fine-grained feldspathic 

 grits and mud-stones, these being of frequent occurrence in the 

 neighbourhood of the Kuruni Creek, and westward from it. These 

 fine-grained rocks are seen at Camp No. 1, at Eraki, near Waka Creek, 

 and near the Maurugaru Creek. Slate Hill, east of the Kuruni Creek, 

 consists of very fine-grained, finely laminated shale, and similar, but 

 not so finely laminated rocks occur near Ekereku. At Kwia-kwia, and 

 at Karapa, these feldspathic rocks are traversed by dykes of fine- 

 grained diabase. 



The rocks at the mouth of the Wenamu River consist of siliceous 

 sandstone of medium texture. Near its mouth are large exposures of 

 an augite-feldspar rock, of the tholeite type. The boundary-line follows 

 the course of the Wenamu River in a southerly direction, and for many 

 miles, to the Kura Falls. The only rocks seen are porphyries, porphy- 

 rites and felsites, which, in many places, and especially between 

 Kukaraima and Tshuau, are intersected by numerous dykes of diabase. 

 From the Kura Falls to the source of the Wenamu, the river tz'averses 

 the sandstone-formation, which, in this neighbourhood, is intersected by 

 numerous dykes of diabase, the latter rock being usually more or less 

 coarse-textured, and tending to a granular structure. 



