Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/455

1869.] TATE—GUYANA, IN VENEZUELA. 345 the thickness is very considerable, and the area undoubtedly great. The dip and strike of the rocks are given with reference to the magnetic meridian. The sequence of the various rocks can be made out with tolerable precision; on the whole the felspar predominates in the lower strata, whilst hornblende and mica increase in quantity as we rise in the series*.

1. Felstone and Talcose Schists.—To the extreme south of the line of section, and beyond the river Yuruari, is situated the Caratal gold-field, which until the last ten years remained hidden in the solitude of the virgin forest. The auriferous veins are included chiefly in a felstone, "piedra azul" of the miners, and also in a talcose schist. From the usual mode of association of auriferous veins with diorites in Central America and Bolivia, I was led at first to regard the chief auriferous rock of the Caratal gold-field as a diorite in which the hornblendic constituent was wanting; but its immediate association with the talc-schist, and its conformity with the metamorphic series of the country, have finally induced me to view it as constituting a part thereof. Unfortunately, the absence of sections has prevented my satisfying myself on this point; but one section on the Rio Mucupio exhibits the talc-schists overlying the felstone, dipping together to the north at from 60°-70°, and striking west 30° north: there are no evidences of alteration along the line of junction; and elsewhere the talcose rock appears incorporated with the felstone.

The felstone is homogeneous, of close texture, and usually semi-columnar; but where it constitutes the vein-stone of the Tigre and Santa-Maria lodes, it contains isolated quartz-crystals and chlorite. To the south-west of Nueva Providencia and high up on the river Yguana the strike approaches north and south, and the dip is 85°-90° westerly.

2. Caratal Gold-field.—The auriferous area at present known is about twelve square miles, the whole of which is traversed by gold-bearing quartz-veins, averaging in thickness from 2-3 feet, most of which have yielded visible gold in abundance. The richness of some of the lodes, and of the "placer" diggings of this auriferous tract, fully justifies the appellation of "El Dorado" which has been given to it; and it may be the "El Dorado" that incited Raleigh to penetrate Venezuela, but which he failed to discover.

The veins present little conformity as regards their strike; but many apparently trend about N.E. and S.W.; the dip varies from 45°-90°. I have obtained gold in the felstone and talc-slates, in the vicinity of the auriferous lodes; but it has not occurred to me disseminated in the main masses of these rocks. Great diversity in the character of the quartz prevails, so much so that specimens from the various lodes can in the majority of cases be readily identified, as, for instance, that of the Callao lode is highly vitreous, with the gold more or less lamelliform and arborescent; that of the Potosi lode is saccharoid, with finely disseminated gold, and granular gold localized in the parallel strings of haematite, which also are rich in

presented by the Author to the Society.—Edit.]
 * [Specimens of rocks and minerals, in illustration of this paper, have been