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Updated: June 4, 2025
The line of junction between the dark mica-slate and the coarse red granite was most clearly distinguishable from a vast distance: the granite sent many small veins into the mica-slate, and included some angular fragments of it.
This ignorance of localities characterises almost all the guides here, who are chosen from among the most indolent class of the people. We wandered for eight or nine hours among rocks totally bare of vegetation. The mica-slate passes sometimes to clay-slate of a darkish grey. I was again struck by the extreme regularity in the direction and inclination of the strata.
I noticed on the north-west extremity of the basin of the West Indies that the secondary formations dip towards south-east; along the coast of Venezuela rocks of gneiss and primitive mica-slate dip to north-west. The basalts, amygdaloids, and trachytes, which are often surmounted by tertiary limestones, appear only towards the eastern and western banks.
The direction and inclination of the stratum remain the same, and the thonschiefer, which takes the look of a transition-rock, is but a modification of the primitive mica-slate of Maniquarez, containing garnets, cyanite, and rutile titanite.
In the evening I walked down the banks of the river, which flowed in a deep gorge, cumbered with enormous boulders of granite, clay-slate, and mica-slate; the rocks in situ were all of the latter description, highly inclined, and much dislocated.
Wherever the waters evaporate in the air, they form sediments and incrustations of carbonate of lime; possibly they traverse strata of primitive limestone, so common in the mica-slate and gneiss of the coasts of Caracas.
Pyrites dispersed in quartzose veins, crossing the mica-slate, are often auriferous, no doubt; but no analogous fact leads to the supposition that the sulphuretted iron which is found in the schistose marls of the alpine limestone, contains gold. Some direct experiments, made with acids, during my abode at Caracas, showed that the pyrites of Cuchivano are not auriferous.
Near Los Arenales, the mountains on all sides are composed of the mica-slate; and looking backwards from this point up to the bare gigantic peaks above, the view was eminently interesting. The colours of the red granite and the black mica-slate are so distinct, that with a bright light these rocks could be readily distinguished even from the Pampas, at a level of at least 9,000 feet below.
These facts appear to me opposed to the theory, that rock-salt is due to the sinking of water, charged with salt, in mediterranean spaces of the ocean. It is worthy of notice, that the foliation of the gneiss and mica-slate, where such rocks occur, certainly tend to run like the metalliferous veins, though often irregularly, in a direction a little westward of north.
I examined with attention the bed of the torrent of La Guayra; and found it to consist merely of a barren soil, blocks of mica-slate, and gneiss, containing pyrites detached from the Sierra de Avila, but nothing that could have had any effect in deteriorating the purity of the air. Since the years 1797 and 1798, at which periods there prevailed dreadful mortality at Philadelphia, St.
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