The British Guiana, Venezuela and Brazil Boiindnries. 133 



The country to the eastward of the Wenamu is sandstone. The 

 sections exposed on the Paruima Creek show that many bosses, sills, 

 and dykes of diabase occur in it. 



The boundary-line from the source of the Wenamu to Mount 

 Roraima traverses the sandstone-formation. The lower slopes of 

 Roraima consist of a fairly coarse-textured olivine-diabase. This is 

 capped by sandstone, the mountain, apparently, being a dissected-out 

 portion of a former sandstone-plateau, probably upraised, and resting 

 upon a diabase laccolith. 



The Brazilian Boundary and the Sandstone Plateau Districts. — 

 Mounts Kukenaam, Roraima and Yakontipu, and the range of flat- 

 topped mountains extending from Mount Yakontipu to and beyond the 

 source of the Ireng River, are all of similar formation. They rise from 

 an elevated plateau consisting mainly of quartz-porphyry and felsite 

 with intrusions of granite, and, in places, as near the sources of the 

 Kukui and Ireng Rivers, of gabbro. These rocks are traversed by 

 numerous dykes, sills and other intrusive masses of diabase of very 

 varying texture. The lower slopes of these mountains consist of 

 diabase which in some cases, as at Yakontipu, forms the mass of the 

 mountain. The upper slopes and the tops of the mountains consist of 

 horizontally bedded sandstones, which cover and apparently butt 

 ao-ainst the igneous rocks. The beds of sandstone form caps on the 

 tops of the mountains and broad terraces on their slopes. By 

 processes of aerial denudation much of these coverings has been 

 removed, and the remaining portions show great perpendicular escarp- 

 ments, which at Mounts Kukenaam and Roraima rise in abrupt cliffs 

 for heights of about 2,000 feet above their talus-slopes. 



As far as has been ascertained the sandstones show few signs of con- 

 tact metamorphism, but the diabase immediately below the sandstone-cap 

 at Mount Roraima is of very fine textm-e, resembling the chilled outer 

 parts of an intrusive mass, whilst the lower parts of the rock are coarsely 

 crystalline. Probably these mountains are the results of laccolithic 

 intrusions of diabase between the basal quartz-porphyry and the sand- 

 stones, and owe their very remarkable forms to subsequent aerial 

 denudation. 



In places, as in the vicinity of Chimepir and between Orindouk 

 and the Tawailing Mountains, the sandstones show evidence of meta- 

 morphism, being traversed by layers of green and red " jasper " and by 

 narrow veins of white quartz. Elsewhere the sandstone has been 

 changed into a true quartzite or into a micaceous sandstone, an 

 itocolumite closely resembling in hand- specimens a mica schist. 



The sandstones extend southwards to the Tawailing Mountains, 

 northwards to the vicinity of the Cuyuni River between the mouth of 

 the Wenamu River and Makapa, and eastwards to the Merume 

 Mountains near the Mazaruni River, to Amatuk Cataracts on the 

 Potaro River, whilst a narrow tongue of the formation crosses the 

 Essequibo River, in the neighbourhood of Komuti Mountain, and 

 stretches beyond it to the Berbice River in the vicinity of Itabru. 



