TIk> Mnzaruni River. 145 



On the hill on the left bank of the Tiboku Cataracts, for about 

 fifty feet above the level of the river, the rocks consist of quartz- 

 porphyrite and of feldspar-porphyrite, generally of a dark-grey tinge, 

 but speckled with yellowish patches near their junction with diabasf . 

 From about fiftj'- feet to the summit of the hill, which is about three 

 hundred feet above the level of the river, the rock consists of coarse- 

 textured diabase, masses of which, in some cases from thirty to forty 

 feet high, are scattered through the dense forest. 



For some distance above Tiboku Cataracts the rocks in the channel 

 and on the banks of the river consist of quartz-porphyrite and of 

 feldspar-porphyrite, but at about six miles above Tiboku, near the left 

 bank of the river close to the mouth of a small creek, is a large 

 exposure of a fine-grained diorite, intersected by an intrusive vein of 

 compact hoi'nblende-porphyrite. 



The general structure of the district from above Tamanua channel 

 to the turn from the west, about six miles above Tiboku Cataracts, is 

 that the course of the river and the low-lying country which extends for 

 some distance from its bank, consists of gneiss and of porphyrites 

 intersected by occasional dykes of diabase. At a little distance back 

 from the banks of the river on both sides of it are low ranges of rounded 

 hills, their contours being those usual among the diabase-bosses so com- 

 mon in many parts of the interior of the colony. The ridge extending 

 along the left bank of the river stretches southward for about seven miles, 

 deflecting the river from its general course of west and east to a north 

 and south one curving round the range and cutting through it at a 

 little above the Tiboku Cataracts, where broad dykes of diabase cross 

 the river channel. The general structure of this part of the Mazaruni 

 lliver valley resembles that of the Potaro-Konawaruk goldfields,especialIy 

 of the upper j^arts of the valleys of the Mahdia and Minnehaha. 



Above the exposures of diorite the country traversed for about 

 eleven miles is quartz-porphyrite and porphyrite. About two miles 

 above the diorite the river is crossed by a dyke of diabase, and about 

 two miles and a half higher quartz is exposed in the river, samples of 

 which yielded upon assay gold at the rate of seven grains per ton of 

 the i-ock. In the neighbourhood of these exposures of quartz are 

 several exposures of moi'e or less schistose porphyrites, in places 

 containing much calcite. 



At about ten miles from Tiboku a dyke of diabase, having about sixty 

 feet in width exposed, strikes north 20° east and south 20° west across 

 the river, while about one and a quarter miles higher another of 

 similar apparent width trends in the same direction. About four 

 miles higher up the river a belt of hornblende-schist about two hundred 

 yards across occurs, the strike of its foliation being approximately 

 east and west. In this portion of the course of the river the land is 

 generally low-lying but with occasional low hills, as near Warima Creek 

 in the neighbourhood of the hornblende-schist. 



Above the exposures of hornblende-schist the river's course is very 

 tortuous, the general direction being from the north-north-west — the 

 country for a distance, as the crow flies, of about nineteen miles being 



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