128 The Geology of the Gold Fields of British Guiana. 



first collected by the Boundary Commissioners it was thought to be 

 gabbro — is due to the development of innumerable, very minute 

 specks and inclusions, and especially exceedingly minute glass bubbles, 

 throughout their mass. 



The district between Paiyuka Cataracts and Quartz Stone — about 

 13 miles along the course of the river — is of granitite-gneiss, in places, 

 as at about 8 miles above Paiyuka, with masses of glassy white quartz, 

 or traversed by pegmatite-veins passing into quartz-veins. 



At the western end of an island, about 1 mile east of the mouth 

 of the Quartz Stone Creek, there is a great mass of white quartz 

 containing numerous nests and needles of black schorl. The rock 

 yielded upon assaying at the rate of one grain of gold per ton. 



The course of the river, about 2 miles north-west of Quartz Stone, 

 leaves the gneissose area. The more or less rounded masses of gneiss 

 hitherto seen in the river are succeeded by angular and pointed masses 

 of schistose rocks. These are metamorphosed porphyrites changed 

 into chlorite and actinolite-schists with an average specific gravity 

 of 2-90. They are well seen in the itabu Ijelow Pap Island, about 

 6 miles in a north-westerly dii'ection from Quartz Stone. Near Pap 

 Island the rocks are sericite-schists. 



Above Pap Island the river crosses a belt of microgranite, the 

 ferro-magnesian minerals of which are more or less completely 

 chloritised. This extends to the Popekai Rapid, the lowest of the 

 Topekai series of rapids. It contains in places, as between three and 

 four miles above Pap Island, large masses of sugary-white quartz. 



The great slab-like masses of rock exj^osed at Popekai Rapid, 

 constantly through the Topekai Rapids, and at Mopay Rapid are 

 hornblende-zoisite-schists. They appear originally to have been porphy- 

 rites and andesites. 



Above Mopay Rapids there are large i^ounded masses of rock in the 

 river. These consist of augite-granitite-gneiss, the rock is usually 

 much chloritised, and in places contains calcite in nests and in veins. 



Between here and the mouths of the Kopang and of the Waiamu 

 Creeks the rocks are fine-grained, more or less schistose, feldspar- 

 porphyrites. In places, as near Waiamu Creek, the rocks are altered 

 and silicified, being changed to masses of quartz with a more or less 

 well-defined schistose structure. 



Similai' rocks are exposed at intervals between the Waiamu Creek 

 and the Waikuri Rapids. Here and there the feldspar-porphyrites 

 pass into felsites and are generally much altered by weathering. 



The rocks exposed at and for a little above Waikuri Rapids ai-e 

 augite-porphy rite . 



Between Waikuri and the Akaiwong or Wakupang Cataracts 

 the rocks are felsites and quartz-porphyries ; they occur in places 

 in rounded masses, but more usually in great slabs and more or less 

 platy masses. The low hills on the right bank of the river consist of 

 more or less decomposed felsite, quartz-porphyry, or feldspar-porphyrite. 

 In many places the silica, which was set free during the decomposition 

 of the rock has separated out as narrow veins of quartz. 



