Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/456

346 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 9, sulphuret of silver; that of the Corinna lode is largely mixed with psilomelane, whilst that of the Tigre lode is a jaspery quartz highly charged with auriferous iron-pyrites. The accompanying metalliferous minerals observed are few in number: haematite, red and brown, psilomelane, sulphuret of silver, and iron-pyrites are of frequent occurrence, whilst cinnabar, galena, and carbonate of copper have been found only as isolated specimens.

3. Quartzites, Gneissic and Hornblendic Rocks. — Passing to the north, the felstone is bounded by a band of jaspery quartzite; it is traceable from the west of Callao, along the margin of the Yuruari, to below the Tupuquen ford, a distance of about three miles; the quartzite resembles a jaspery quartz that is met with in several of the auriferous lodes, but it differs in the absence of iron-pyrites, and in its more saccharoid texture; and as its trend conforms to that of the rocks to the north and south, it must be regarded as one of those masses of quartzites which alternate with the gneissic rocks to the north.

From the river Yuruari to the river Orinoco, the strata exhibited are a great breadth of gneiss and gneissoid rocks, and narrower bands of hornblende-slate and quartzite. To the south of the town of Guasipati, blocks of quartzite are scattered about upon an argillaceous surface, undoubtedly resulting from the decomposition of gneiss in place; this quartzite is semicrystalline and micaceous. Again, some fifteen miles to the north of Guasipati, as near the rancho of Platanal, the savannah presents the appearance of a vast cemetery from the varied masses of a semivitreous quartzite; and as similar masses occur to the west, they indicate a band of quartzite two or three miles in breadth coursing about east and west. But in the neighbourhood of Upata, the quartz blocks which strew the undulating ground are largely caverned by casts of iron-pyrites, and are probably of vein-origin.

The gneiss with its interstratified quartzite extends from the river Yuruari, to near the rancho of San Jose, where a narrow strip of amphibole-schist appears, striking east and west, and dipping to the north; a narrow band of gneiss separates it from a similar rock which extends from the rancho of Santa Anna, to the north of that of Candelaria, a distance of about twelve miles; in the bed of the river Carichapo, near Candelaria, this rock strikes east and west with a dip to the south of about 85°; it here encloses non-auriferous quartz-veins.

Before reaching the ford of the Carichapo, north of Candelaria, gneiss appears, and continues with some variation in its constituent minerals to the banks of the Orinoco. At Upata the prevailing variety is a porphyritic felstone, succeeded to the north by a true gneiss, in which the foliation is obscure, but, when viewed on an extensive scale, is generally that of the strike of the beds, which was ascertained to be west 15° north; in some instances the foliation was south-west and north-east. To the north of the rancho of Guacaima and nearer to the Orinoco, the mica of the gneiss is replaced by hornblende; and near Las Tablas, and around Bolivar, the