346 PROCEEDINGS OE THE GEOLOGICAL SOCIETY. [JunC 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 metalli- 

 ferous minerals observed are few in number : hsematite, 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 Hornhlendic 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 argilla- 

 ceous 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 aboat 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 re- 

 placed by hornblende ; and near Las Tablas, and around Bolivar, the 



