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Updated: May 5, 2025
The stratified beds, which we have hitherto followed across the range, a little further down are seen to lie, I believe unconformably, on a broad mountainous band of clay-slate and grauwacke.
In a thick mass of laminated, feldspathic and altered clay-slate, interposed between two great strata of porphyritic conglomerate in Central Chile, and where there could be but little doubt about the bedding, I observed similar slight differences in composition, and likewise some distinct thin layers of epidote, parallel to the highly inclined cleavage of the mass.
The term contemporary formation is here taken in the sense attached to it by geologists, in speaking of beds of quartz in clay-slate, granular limestone in mica-slate or feldspar in gneiss.
Between Coquimbo and Guasco there are two considerable formations of mica- slate, in one of which the rock passed sometimes into common clay-slate and sometimes into a glossy black variety, very like that in the Chonos Archipelago.
Nearer we look down on the small desert archipelago of the four Morros del Tunal, the Caribbee and the Lobos Islands. The mica-slate changed suddenly into carburetted and shining clay-slate.
Vast blocks lie superficially on the ridges; and the tops of all the outer mountains, as of Khersiong spur, of Tonglo, Sinchul, and Dorjiling, appear a pile of such masses. Injected veins of quartz are rare in the lower beds of schist and clay-slate, whilst the gneiss is often full of them; and on the inner and loftier ranges, these quartz veins are replaced by granite with tourmaline.
From these several facts, and more especially from the case of the clay-slate in Tierra del Fuego, it must, I think, be concluded, that the same power which has impressed on the slate its fissile structure or cleavage has tended to modify its mineralogical character in parallel planes. Let us now turn to the foliation of the metamorphic schists, a subject which has been much less attended to.
In the Eastern Cordillera, in the Portillo Pass, there is a grand mass of mica- slate, foliated in a north and south line, and with a high westerly dip: in the Uspallata range, clay-slate and grauwacke have a highly inclined, nearly north and south cleavage, though in some parts the strike is irregular: in the main or Cumbre range, the direction of the cleavage in the feldspathic clay-slate is N.W. and S.E.
At Port Famine much of the clay-slate is calcareous, and passes either into a mudstone or into grauwacke, including odd-shaped concretions of dark argillaceous limestone. Ancyloceras simplex, d'Orbigny "Pal Franc" Mount Tarn. 2. Natica, d'Orbigny "Pal Franc" Mount Tarn. 4. Pentacrimus, d'Orbigny "Pal Franc" Mount Tarn. 5. Lucina excentrica, G.B. Sowerby, Port Famine. 6.
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.
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