144 The Geology of the Gold Fields of British Guiana. 



epidiorite are seen in contact with fine-grained porphyrite. The 

 epidiorite contains many small nests of quartz, while the porphyrite is 

 frequently intersected by thin veins of the same mineral. Further 

 exposures of porphyrite occur along the channel of the river to near 

 where its course changes from the south-west to the south-south-west. 



From this point to the Tiboku Cataracts is a distance of about 

 seven miles. Near the turn to the south-south-west, the channel of 

 the river is crossed by a wide dyke, about fifty yards in breadth, of 

 coarse-grained diabase, running north-north-east and south-south-west. 

 About a quarter of a mile above this dyke many rocks are seen, con- 

 sisting of fine-grained granitite-gneiss, while similar rocks, varying in 

 texture from fine to cross-grained, occur for about two miles and a half. 

 Towards the middle of this belt of gneiss the rock is a syenite-gneiss, 

 changing to a hornblende-granitite-gneiss near the southern end. 

 Through the latter rock a narrow vein of fine-grained hornblende- 

 porphyrite is intrusive, and some yards further, a dyke about twenty- 

 five to thirty feet in width of a porphyrite-diorite crosses the river 

 from north-north-west to south-south-east. The dyke, which is intrusive 

 through a coarse-textured hornblende-granitite-gneiss, has an interesting 

 structure ; in the middle parts the very abundant hornblende-pheno- 

 crysts are short and stumpy, and have more the general habit of augite 

 crystals than of hornblende ; whilst near the edges the hornblende has 

 the form of long, relatively narrow, prisms. The outer parts of the 

 dyke, in places, shotv thin veins of epidote traversing the rock. 



For about a mile and a half south of this dyke the rocks are coarse- 

 grained hornblende-granitite-gneiss and finer-textured granitite-gneiss, 

 and are intersected at about a mile from the dyke by a narrow vein of 

 diabase with an east and west trend. Near the south end of the belt 

 of gneiss a very fine-grained gneiss or schist consisting largely of 

 (juartz and muscovite occurs on a small island. 



Beyond the gneissose rocks the course of the river is cut 

 through quartz-porphyrite, and this is the only rock seen below Tiboku 

 Cataracts. These cataracts are over a very broad exposure of a grey 

 quartz-granophyre, the rocks generally being very uniform in character, 

 but at the cataracts they are traversed by some narrow veins of black, 

 very compact basalt. A few narrow veins of quartz, and, as is rather 

 a common occurrence in the porphyrites, some scattered cubes of pyrite 

 occur in the granophyre. 



At Tiboku Cataracts the river bends rather abruptly to the west, 

 and at their head almost suddenly to the northward. 



Above the main cataracts at Tiboku, in the upper rapids on the left 

 bank for about a quarter of a mile, immense blocks of diabase and of 

 concretionary ironstone occur, whilst for about a similar distance the 

 bank above consists of large, smooth, rounded exposures of diabase of 

 from medium to coarse texture. Near the mouth of the Karamang 

 Creek, above the cataracts on the right bank, and again on both banks 

 of the river, diabase is seen for about one hundred yards, while higher 

 up three dykes of diabase are traceable across the river, the rocks on 

 the right bank being alternations of quartz-porphyrite and of diabase. 



