The Esspquibo River. 151 



grey tinge : and above Aharu a coarse-grained variety, capped by a 

 fine-grained one, is noticeable, the latter closely resembling that at 

 Bethany Island and Kumaka Serima. The difference in their modes of 

 weathering is very marked, the former producing great rounded masses, 

 the latter angular boulders. Near Moneri Island the coarse granite is 

 traversed by frequent' veins of pegmatite, and by one of a hornblende- 

 granitite or quartz-diorite. 



The rock exposed at Rockstone is a granitite-gneiss with faintly 

 marked foliation. Similar rocks are exposed in the channel near Gluck 

 Island when the water is very low. At about one and a half miles 

 south of Rockstone a remarkable ridge of rocks running north and 

 south for a distance of one hundred and fifty yards occurs in the 

 eastern channel. The rocks are quartzite, made up of rounded pebbles 

 and grains of clear quartz cemented together by secondary quartz, 

 which shows as minute distinct crystals in small hollows in the rock. 

 The quartzite did not yield to assay any trace of gold. Possibly this 

 rock is an outlier of the great sandstone formation, or else is an 

 altered portion of a still earlier sedimentary deposit. From this 

 place to Arriwini Island, at the east of the south end of Gluck Island, 

 no rocks are seen but the decomposition-products exposed on the banks 

 of Gluck Island, and on those of the mainland are all of a granitic or 

 gneissose nature. 



Arriwini Island consists of diabise, apparently an intrusive boss 

 having its long diameter lying north and south. South of the end 

 of Gluck Island, on the western bank of the river, gneissose rocks are 

 exposed in places through the sandbanks. On the same bank to the 

 south of Tikuru Inlet are large exposures of fine-grained, light-grey 

 gneiss with highly contorted foliation, containing many basic inclusions 

 of hornblende schist, their dark glistening surfaces showing strongly in 

 the surrounding lighter rock. The gneiss is traversed by elvans of 

 aplite and of pegmatite, and by thin veins of quartz. The general 

 strike of the foliation of the gneiss is south-east and north-west. 



ISTo rocks are visible between Tikuru and Moco-Moco Point, a 

 distance of about nine miles, and the decomposition-products noticed on 

 the banks of the river are all of granitic origin, their structure in many 

 places showing them to have been derived from gneissose rocks. This 

 stretch of river contains many large sandbanks, which hide any rocks 

 which otherwise might be exposed near the frequently occurring islands. 



About three-quarters of a mile north of Moco-Moco Creek, on the 

 west bank, two parallel dykes of diabase strike north-east across the river 

 through hornblende-granitite-gneiss, the more northerly one being about 

 twenty feet across, the other about seventy, and both send off narrow 

 tongues through the gneiss. The larger dyke is of coarse texture, with a 

 M-ell-defined chilled edge of very fine texture. The smaller dyke is fine- 

 grained, while the narrow tongues are very compact and closely grained. 

 The gneiss is traversed by many thin veins of quartz, and by broader 

 elvans of aplite. It is noticeable that the gneiss, where in contact 

 with the diabase intrusion, loses its foliated structure, and, in hand- 

 specimens, strongly resembles a massive granite. 



