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Considering that in the Uspallata range the lowest strata on the western flank lie unconformably on the clay- slate, as probably is the case with those on the eastern flank, whereas in the Cordillera all the overlying strata lie conformably on this formation: -considering that in the Uspallata range some of the beds, both low down and high up in the series, are marked with vegetable impressions, showing the continued existence of neighbouring land; considering the close general resemblance between the deposits of this range and those of tertiary origin in several parts of the continent; and lastly, even considering the lesser height and outlying position of the Uspallata range, I conclude that the strata composing it are in all probability of subsequent origin, and that they were accumulated at a period when a deep sea studded with submarine volcanoes washed the eastern base of the already partially elevated Cordillera.
The lower beds of the gypseous formation resemble the corresponding and probably contemporaneous strata of the main Cordillera; whilst the upper beds in several respects resemble those of the Uspallata chain, and possibly may be contemporaneous with them; for I have endeavoured to show that the Uspallata beds were accumulated subsequently to the gypseous or Neocomian formations of the Cordillera.
From this correspondence between the Uspallata and Portillo ranges, perhaps in age and certainly in geographical position, one is tempted to consider the one range as the prolongation of the other; but their axes are formed of totally different intrusive rocks; and we have traced the apparent continuation of the red granite of the Portillo in the red porphyries diverging into the main Cordillera.
Bed 8, a greenish mudstone, with several layers of gypsum. Bed 9, a highly indurated, easily fusible, white tuff, thickly mottled with ferruginous matter, and including some white semi-porcellanic layers, which are interlaced with ferruginous veins. This stone closely resembles some of the commonest varieties in the Uspallata chain.
Strata of this nature alternate with much black and brown siliceo-calcareous slate, remarkable from the wonderful number of huge embedded logs of silicified wood. This wood, according to Mr. Some of the layers of the black siliceous slate contained irregular angular fragments of imperfect pitchstone, which I believe, as in the Uspallata range, has originated in a metamorphic process.
At length, on January 17, 1817, he began the passage of the Andes with about 5,000 men, 1,600 horses, and 9,000 mules, the latter carrying the field artillery, ammunition, and provisions. The summit of the Uspallata Pass is 12,700 feet above the sea-level, 5,000 feet higher than the Great St. Bernard, by which Napoleon led his army over the Alps.
Concluding remarks on the denudation and elevation of the Portillo line. Section by the Cumbre, or Uspallata Pass. Porphyries. Gypseous strata. Section near the Puente del Inca; fossils of. Great subsidence. Intrusive porphyries. Plain of Uspallata. Section of the Uspallata chain. Structure and nature of the strata. Silicified vertical trees. Great subsidence. Granitic rocks of axis.
Some other little hillocks of similar composition, with their strata highly inclined, range N.E. and S.W., obliquely to the main Uspallata line.
Disregarding local disturbances, the beds are either horizontal or inclined at a small angle eastwards: hence, when standing on the plain of Uspallata and looking to the west or backwards, the Cordillera appear composed of huge, square, nearly horizontal, tabular masses: so wide a space, with such lofty mountains so equably elevated, is rarely met with within the Cordillera.
The Uspallata range is separated from the main Cordillera by a long narrow plain or basin, like those so often mentioned in Chile, but higher, being six thousand feet above the sea.
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