Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/458

348 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [JUNE 9, gneiss is granitoid. Skirting both, banks of the Orinoco, and lying beyond the granitoid gneiss are hummocky protuberances of hornblende rock, highly crystalline, and containing garnets; the rock is jointed by partition planes, having a north and south bearing, and is in part further broken up into rhomboidal masses.

There is great presumptive evidence that these gneissic rocks, as well as those of northern Venezuela, contain disseminated gold, as the sands of most of the streams flowing from the Itacama mountains have yielded gold in a fine state of division; the stratified debris of these rocks, at San Isidore, show on assay as much as 4-1/2 ounces of gold per ton*. The region occupied by the gneiss is comparatively easy of exploration, and has been well prospected for gold-veins, but up to the present time without success.

The gneiss, three miles to the north of Upata, contains a rich deposit of red haematite; this was observed on the slopes of two hills, about two miles apart, and it is highly probable that the ferruginous mass is interstratified with the gneiss. Mr. Siegert, a local surveyor, has discovered in this neighbourhood evidences of ancient metallurgical operations for the reduction of silver.

4. Conjectures as to the Age of the Auriferous Series.—From the known distribution of gold in the rocks of the American continent, speculations as to the relative age of the auriferous rocks of Venezuelan Guyana will be necessarily directed to the Silurian and Oolitic strata. But as the Oolitic rocks are only auriferous by the intrusion of certain diorites, and as such do not appear to have played any part in the development of gold in the rocks of the district under consideration, there remains but the inquiry into the evidences of their being of Silurian age. The metamorphic series of Northern Venezuela, which I regard as contemporaneous with that of Guyana, is overlain by unmetamorphosed Neocomian strata; but as "the relations of junction of the two formations remain obscure"†, we have no certain data as to their age, except that they are precretaceous. Mr. David Forbes‡ has determined that the eminently auriferous rocks of Bolivia belong to the Silurian system; and though consisting chiefly of clay-slate, shales, and grauwackes, yet this author refers the gneissic and metamorphic schistose rocks in the desert of Atacama to the same series. The mineral contents of the auriferous rocks of Guyana have not been sufficiently ascertained to afford data for the determination of the age of the rocks by analogy of the mineralogical features. And though the contemporaneity of these auriferous metamorphic rocks with those of Bolivia has not been proven, yet the balance of evidence is in favour of such association, rather than with the Oolitic series of the same country.

IV. Intrusive Rocks.—Narrow dykes of basalt were noticed traversing the gneiss, between the rancho of Santa Anna and Guasipati; and blocks of greenstone were observed, but not in situ, at Upata.

V. Stratified Detrital Deposits of the Llanos.—The arenaceous


 * El Boletin Comercial: Bolivar, March 1867.

† Wall, ''loc. cit''. p. 465.

‡ Quart. Journ. Geol. Soc. vol. xvii. p. 53 et seqq.