CHAPTEK XVII. 



THE POTARO AND KURIBRONG RIVERS. 



The Potaro River. — -The Potaro River, from its junction with the 

 Essequibo to the mouth of its tributary, the Kuribrong, pursues a 

 westerl}^ and easterly course, and from the Kuribrong to the Kaieteur 

 Pall a south-westerly and north-easterly one. 



The sericite-schist rocks described as occurring in the Essequibo, 

 near its junction M'ith the Potaro, are also exposed in the latter river, near 

 its mouth, on both banks, on the small island near its mouth, and at 

 short intervals for about two miles up the river. On the southern bank, 

 near the mouth, is an exposure where the sericite-schist is in contact 

 with a chloritoid rock. The degree of schistosity in the sericitic rocks 

 varies greatly ; in places they are almost massive, while in others the 

 schistosity is developed to a marked extent, and they resemble soft sliales 

 or slates. About tlu'ee and a half miles from the mouth of the Potaro, 

 near the entrance of its tributary, Tiger Creek, compact porphyrite takes 

 the place of sericite-schist and is seen at intervals for about a mile 

 along the course of the river on both banks, and in the river. About 

 half a mile west of Tiger Creek there is an exposure of diabase on the 

 southern bank. Near Bucktown, about a mile east of Tumatumari 

 Cataracts, porphyrite is exposed, and below the foot of the cataracts 

 there are large exposures of massive ((uartz-porphyry near the south- 

 westeni bank, whilst others protrude through the sand of the extensive 

 sandbanks on the north-eastern side of the river. 



Tumatumari Cataracts ai'e caused by a great d^'ke of diabase of 

 very varying texture which crosses tlie river with a trend of east- 

 north-east and west-south-west, the dyke being not less than five 

 hundred yards across. The mass of the rock is coarse in texture, in 

 places almost approaching a gabbro in structure, while its margins 

 ai-e comj^act and fine-grained. It is intersected by later d3^kes and 

 tongues of diabase and of basaltic diabase, or tholeite, running, as a rule, 

 east and west. One of these, a coarse diabase, to the south of the 

 cataracts, is about twenty-five feet in thickness ; another, near the 

 middle of them, is about a foot across and traverses both the diabase 

 and the (|uartz-p(jrphyry at the foot of the cataracts ; whilst between them 

 is a third which consists of compact basaltic diabase, eight to ten inches 

 across, having a columnar structure at right angles to its walls. These 

 dykes evidently were injected into cracks in the main mass after its 

 consolidation. The fine-grained diabase on the north-west of the head 

 of the cataracts passes into a compact epidiorite, proljably l)y ccmtact 

 action with the acidic rocks throu2;h Yvlrich they were intruded. About 



