The Mazaruni River. 143 



At the lower end of the Marabisi Channel, or still-water, the course 

 of the river is fairly free from islands, and not interrupted by rapids, 

 its width being about six to seven hundred yards. In places, when 

 the water is low, large sandbanks are exposed in the channel. The 

 rocks, at the eastern end of the channel are fine-grained hornblende- 

 porphyrite and epidi'orite traversed by a diabase dyke, the rock of 

 which is of medium texture and is in part altered to proterobase. The 

 direction of this dyke is north-east and south-west. The epidiorite is 

 succeeded by a greenish-coloured chlorite-schist, with abundant small 

 cubical crystals of pyrite disseminated through it. Near the south-west 

 end of an island, about half a mile east of the mouth of the Issano Creek, 

 there is a (piartz-reef, traversing greenish-coloured well-foliated chlorite- 

 schist, of which about sixty yards in length is seen trending generally 

 nurch 20° west, and south 20° east. The main reef varies in width 

 from three to six feet, and on the shore of the island thickens out to 

 from ten to twelve feet, whence it throws ofi' two branches, one 

 to the south-east about eighteen inches across, and one to about 

 south 40° east of nearly the same thickness. While crossing the 

 northern channel of the river the reef splits, the main part of it 

 continuing to strike to north 20° west and the smaller off-set to 

 a little west of north. The quartz of these reefs and veins is glassy 

 and is veined, parallel to the walls of the reefs, by thin layers of 

 greenish-black tourmaline. Samples of quartz from dijBferent parts of 

 the reef were assayed and yielded at the rate of three pennyw^eights 

 of gold to the ton of the rock. Several exposures of the chloritic rock, 

 more or less markedly foliated, occur between this island and the mouth 

 of the Issano Creek. 



For some distance above the mouths of the Issano and of the 

 Marabisi Creeks many large sandbanks are exposed, at times, 

 along the course of the river, especially near its right bank. 

 About two miles above Issano Creek masses of quartz-schist strike 

 across the river fi'om the north-west to the south-east ; other masses 

 are seen about three hundred yards from the first, and a third lot is 

 visible about five hundred yards from the second. The quartz-schist is 

 probably a local modification of a quartzose-felsite. Between the 

 second and third exposures of quartz-schist rocks occur on the left bank 

 of the river, which are of quartz-porphyrite. Near the south-west 

 end of this island thei'e are several exposures of more or less schistose 

 fine-grained hornblende-porphyrite. At the end of the Marabisi 

 Channel, where the course of the river turns from the west, a belt of 

 porphyrite extends across the river in an east-north-east and west-south- 

 west direction, about thirty feet in breadth being exjjosed above the 

 water, and here the rock has a well-marked rectangular jointing. On 

 the north point of the river bank at this bend there are numerous 

 great masses of concretionary ironstone, evidently derived from a 

 hill of diabase at the point. From about half a mile from this point 

 exposures of feldspar-porphyrite extend at intervals for about two miles. 

 At about a mile and a half from the first exposure of feldspar-porphyrite, 

 near the left bank of the river, large rounded masses of a coarse-grained 



