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Updated: June 4, 2025
On the other hand the immediate superposition of the limestone of Cumanacoa on mica-slate and transition clay-slate the rarity of the petrifactions which have not yet been sufficiently examined the strata of silex passing to Lydian stone, may lead to the belief that the soils of Cumanacoa and Caripe are of much more ancient formation than the secondary rocks.
This phenomenon is general from the Quebrada of Tocume, between Petares and Caracas, as far as Porto Cabello. It would seem as if the impulsion had everywhere come from the south-east; and this fact is the more striking, as the strata of gneiss and mica-slate in the Cordillera of the coast are generally directed from the south-west to the north-east.
Where in one district the rocks are fissile, and in another adjoining district they are foliated, the planes of cleavage and foliation are likewise generally parallel: this is the case with the feldspathic homogeneous slates in the southern part of the Chonos group, compared with the fine foliated mica-schists of the northern part; so again the clay- slate of the whole eastern side of Tierra del Fuego cleaves in exactly the same line with the foliated gneiss and mica-slate of the western coast; other analogous instances might have been adduced.
The beds, over a vast extent of land, make the same angle with the meridian of the place; they present a parallelism, which may be considered as one of the great geologic laws capable of being verified by precise measures. Advancing toward Cape Chuparuparu, the veins of quartz that cross the mica-slate increase in size.
If, in the granitic series, where a very complex formation, we would distinguish mineralogically between the rocks of granite, gneiss, and mica-slate, it must be borne in mind that coarse-grained granite, not passing to gneiss, is very rare in this country.
In this case and at Rat Island, the passage of the gneiss into imperfect hornblendic or into chloritic slate, seemed to be connected with the segregation of the veins of quartz. Greenough page 78 "Critical Examination" etc., observes that quartz in mica-slate sometimes appears in beds and sometimes in veins.
We were teased by sand-flies this evening, although the thermometer did not rise above 45°. The country through which we had travelled for some days consists principally of granite, intermixed in some spots with mica-slate, often passing into clay-slate.
By STRATIFICATION, as applied to these formations, I mean those alternate, parallel, large masses of different composition, which are themselves frequently either foliated or fissile, such as the alternating so-called strata of mica-slate, gneiss, glossy clay-slate, and marble. I may add that Mr.
Near the Downs of Bathurst I passed over much pale-brown, glossy clay-slate, with the shattered laminae running north and south; I mention this fact, because Captain King informs me that, in the country a hundred miles southward, near Lake George, the mica-slate ranges so invariably north and south that the inhabitants take advantage of it in finding their way through the forests.
Five miles west of the bifurcation of the Beagle Channel, the slate- formation, instead of becoming, as in the more southern parts of Tierra del Fuego, feldspathic, and associated with trappean or old volcanic rocks, passes by alternations into a great underlying mass of fine gneiss and glossy clay-slate, which at no great distance is succeeded by a grand formation of mica-slate containing garnets.
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