Th<'. Jfazaruni River. 141 



masses near it, consists of a porphyritic granitite witli many large 

 pink phenoci-ysts of alkali-feldspar, and which contains augite and 

 hornblende in ad<Htiou to biotite. The porphyritic rock is traversed 

 by veins of much finer texture having a similar comj)Osition. Near 

 Kartauari Rock the granitite is traversed by a narrow dyke of diabase. 

 Many of the granitoidal masses near here show a somewhat gneissose 

 structure. The Kartauari Cataracts are, in parts, over granitite, and, 

 in others over gneiss, the latter varying from ordinary granitite-gneiss 

 to hornblende-granitite-gneiss, in places with inuch epidote. About half 

 a mile above these cataracts a vein of aplite traverses the gneiss. 

 From Kartauari to Turesi Cataracts, a distance of about three miles, 

 the country is a purely gneissose one, the rocks being varieties of 

 granitite-gneiss, varying much in texture, and in their contents 

 of dark-coloured minerals, some large rocks about a quarter of a mile 

 below Turesi being almost free from them, the little biotite present 

 in them being in small orbicular patches. 



At Turesi, where the river for a short distance both above and 

 below the cataracts is free from islands, so that its full breadth of 

 about half a mile is seen, and for the first time after passing the 

 northern end of Karia Island, a few miles south of Bartica, both its 

 banks are visible at once, the Mazaruni is crossed by a very broad 

 dyke of diabase, which is over two hundred yards in width. The 

 rock is of uniformly medium texture, and it has many remarkable 

 pot-holes, frequently of large size, eaten into it. Both the vertical and 

 the horizontal surfaces of many of the great masses of diabase which 

 stand out from the bulk of the rock are pitted with large and small 

 basin-shaped depressions, which probably result from the rock flaking 

 away from the eflFects of the very great changes in temperature between 

 the afternoon, when the blackish rock becomes too hot in many places 

 to allow the hand to touch it, and after nightfall, when its temperature 

 becomes very rapidly reduced by radiation. The strike of this great 

 dyke is north-east and south-west. The contents of some of the pot- 

 holes in the diabase at Turesi Cataracts were examined, and were found 

 to consist mainly of quartz-gravel and sand, with a few fragments of 

 felsite, and very many grains of ilmenite. Several pot-holes were 

 emptied of their contents, which were washed, and in each case yielded 

 colours of gold. 



Above the cataracts the first great exposures of rock consist of 

 epidiorite passing to hornblende-schist ; above them exposures of 

 epidote-granitite-gneiss occur, followed by many large rocks consisting 

 of amphibolite ; and by masses of augite-granitite-gueiss and of 

 granitite-gneiss, alternating with amphibolite. Above the Turesi 

 Cataracts the river is again crowded with islands. On the right 

 bank, about a mile above the head of the cataracts, a broad exposure 

 of diabase extends for about three hundred yards. South of this the 

 rocks are in low, rounded masses of coarse texture, and consist of 

 quartz-diorite, an epidiorite traversed in places by narrow tongues of 

 diabase. The river is here crossed by a dyke of diabase trending to 

 the south-west from the mass on the right bank. A reef of quartz 



