348 PROCEEDINGS OP THE GEOLOGICAL SOCIETY. [JUTIO 9, 



gneiss is granitoid. Skirting both, banks of the Orinoco, and lying be- 

 yond 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 Yenezuela, 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^ 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 Yene- 

 zuelan 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 rela- 

 tions of junction of the two formations remain obscure "f, we have 

 no certain data as to their age, except that they are precretaceous. 

 Mr. David PorbesJ 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. 



lY. Intrusive Rocks. — Narrow dykes of basalt were noticed tra- 

 versing the gneiss, between the rancho of Santa Anna and Guasipati ; 

 and blocks of greenstone were observed, but not in situ, at Upata. 



V. Stratitied Detrital Deposits op the Llanos. — The arenaceous 



* El Boletin Comercial : Bolivar, March 1867. 



t Wall, loc. cit. p. 465. 



\ Quart. Journ. Geol. Soc. vol. xvii. p. 53 tt seqq. 



