Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/573

 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 colour.

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 principally 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 foliaceous 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 Cuyuni river, within 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 intruded 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 Vesuvius, 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