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The whole of this mountainous country, by which the littoral chain of Caracas is linked to the Cordilleras of Cundinamarca, was celebrated in Europe in the middle of the nineteenth century; for that part of the territory formed of gneiss-granite, and lying between the Rio Tocuyo and the Rio Yaracui, contains the auriferous veins of Buria, and the copper-mine of Aroa which is worked at the present day.
The littoral chain of Venezuela, of which the different ranges form the Montana de Paria, the isthmus of Araya, the Silla of Caracas and the gneiss-granite mountains north and south of the lake of Valencia, is joined between Porto Cabello, San Felipe and Tocuyo to the Paramos de las Rosas and Niquitao, which form the north-east extremity of the Sierra de Merida, and the eastern Cordillera of the Andes of New Grenada.
Gardner found in several parts of the province of Ceara, which lies between four and five hundred miles north of Bahia, gneiss with the folia extending E. 45 degrees N.; and in Guyana according to Sir R. Schomburgk, the same rock strikes E. 57 degrees N. Again, Humboldt describes the gneiss-granite over an immense area in Venezuela and even in Colombia, as striking E. 50 degrees N., and dipping to the N.W. at an angle of fifty degrees.
As we advance towards the frontier of the Portuguese province of the Rio Negro the high rocks become more rare and we no longer find the shelves or dykes of gneiss-granite which cause rapids and cataracts in the rivers. The manuscript notes of Don Apollinario also mention the gold of the Rio Uaupes. La Condamine, Voyage a l'Amazone.
They run north 50 degrees east, inclining from 60 to 70 degrees north-west. This is the general direction which I had observed in the gneiss-granite of Caracas and the Orinoco, in the hornblende-slates of Angostura, and even in the greater part of the secondary rocks we had just examined.
In a letter to Mr. Brazil, Bahia, gneiss with disjointed metamorphosed dikes. Strike of foliation. Rio de Janeiro, gneiss-granite, embedded fragment in, decomposition of. La Plata, metamorphic and old volcanic rocks of. S. Ventana. Claystone porphyry formation of Patagonia; singular metamorphic rocks; pseudo-dikes. Falkland Islands, Palaeozoic fossils of.
The folia in the fragment and in the surrounding rock strike in the same N.N.E. and S.S.W. line; but in the fragment they are vertical, whereas in the gneiss-granite they dip at a small angle, as shown by the arrows, to S.S.E. This fragment, considering its great size, its solitary position, and its foliated structure parallel to that of the surrounding rock, is, as far as I know, a unique case: and I will not attempt any explanation of its origin.
It is surrounded by pyroxenic amygdaloid; it would no doubt be seen below, issuing immediately from gneiss-granite, like the phonolite of Biliner Stein, in Bohemia, which contains fragments of gneiss embedded in its mass.
In that part of the Sierra Parime which M. Bonpland and myself visited, gneiss forms a less marked zone, and oscillates more frequently towards granite than mica-slate. I found no garnets in the gneiss of Parime. There is no doubt that the gneiss-granite of the Orinoco is slightly auriferous on some points.
Venezuela is one of the countries in which the parallelism of the strata of gneiss-granite, mica-slate and clay-slate, is most strongly marked. The general direction of these strata is north 50 degrees east, and the general inclination from 60 to 70 degrees north-west.
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