1^71.] .SAWKINS BRITISH GUIANA. 423 



After passing decomposed granite, syenite, and quartz, about a 

 mile below the Caruma creek, on the north, and the Wairyarra, on 

 the south, the rocks become decidedly schistose. There is also a 

 small dyke of quartz of a red and white colour in the course of the 

 river ; but by microscopic examination no gold could be seen in it. 



At the base of one of the hills in the vicinity, gold in very small 

 quantities was soon obtained. 



On passing over the rapids of Olupikai a variety of greenstone and 

 talcose-schist occurs, very slaty in structure, particularly at the 

 rapids of Atete, whore it is of a greenish colom\ 



At the cataract of Waicuri there is an extensive development of 

 the red brecciated rock lying on a phonolite, with small crystals of 

 pyrites, and a blackish mineral that, like the pyrites, dissolves out 

 of the rock, leaving square cavities more or less filled with oxide of 

 iron. 



At the falls of Wakupany, the rocks are priacipallj- greenstone, 

 or coarse schist, on which there is a considerable deposit of white 

 sand. A little up the river the rocks become more slaty and foli- 

 aceous by the presence of talcose-schist with quartz veins 2 inches 

 in diameter, and a good deal of iron pyrites. 



The granite rocks in the river from this point became more 

 gneissose, with occasional quartz veins ; therefore the space between 

 the Copang and the Maruparu creeks may be regarded as the 

 schistose district on the Cuyuui river, mthin the boundary of 

 British Guiana. 



From Maruparu talcose schist occupies a considerable portion 

 of the north bank. On approaching the falls of Duquari, the granite 

 rises in large bosses alternating with syenite, porphyry, greenstone-, 

 and hornblendic and talcose schist, which lie on the granite at different 

 angles. Above the Iroma river, these rocks are more decomposed, and 

 assume a red colour, but still preserve their laminated structure. 



At Duquari falls there is a better opportunity of observing these 

 various rocks and determining their relative age. It appeared to 

 me evident that the greenstones have displaced the granite, and in- 

 truded through lines of weakness, or fissure, without materially 

 altering the granite, by which it also appears that the greenstone 

 was at a low temperature during its ejection, i. e. below a white heat, 

 like the lava of the present day at Yesuvius, and at Mouna Loa, 

 in Hawaii. 



From the Duquari falls to the Otomong river, granite is general, 

 except where the cataracts of Aroroyamo, Callo, Darra, Corrocoro- 

 roopung, Icarima, Poinchaima, Tacarric, and Aberiwaika traverse 

 the river by dykes of greenstone, gneiss, &c. 



After passing the Paccu falls there is a small creek called Cartuni, 

 along the banks of which there is an Indian path that leads to the 

 Puruni, a tributary of the Mazuruni river, over which I made the 

 transit on foot. I estimated the distance to be twenty-one miles. 



The rocks observed on the way were granite, granular quartz, 

 syenite, and trappe. In the creeks black sand occurred very gene- 

 rally ; and I consider this part of the Cuyuni valley at least as likely 



