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Updated: May 17, 2025
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.
Considering this resemblance, and that the fossils from the Puente del Inca at the base of the gypseous formation, and throughout the greater part of its entire thickness on the Peuquenes range, indicate the Neocomian period, that is, the dawn of the cretaceous system, or, as some have believed, a passage between this latter and the oolitic series I conclude that probably the gypseous and associated beds in all the sections hitherto described, belong to the same great formation, which I have denominated cretaceo-oolitic.
Here the great gypseous formation abruptly terminates, and is succeeded eastward by a pile of more modern strata. Considering how violently these central ranges have been dislocated, and how very numerous dikes are in the exterior and lower parts of the Cordillera, it is remarkable that I did not here notice a single dike.
Knowing that the porphyritic conglomerate formation consists of alternate streams of submarine lavas and of the debris of anciently erupted rocks, and that the strata of the upper gypseous formation sometimes include submarine lavas, and are composed of tuffs, mudstones, and mineral substances, probably due to volcanic exhalations, the richness of these strata is highly remarkable when compared with the erupted beds, often of submarine origin, but NOT METAMORPHOSED, which compose the numerous islands in the Pacific, Indian, and Atlantic Oceans; for in these islands metals are entirely absent, and their nature even unknown to the aborigines.
These effects are the same, whether the caverns form one long and continued range, or several of these ranges lie one over another, as happens almost exclusively in gypseous mountains.
Descending the eastern slope of the Cumbre, the structure becomes very complicated, and generally differs on the two sides of the east and west line of road and section. We now come to the gypseous formation: I will first describe the structure of the several mountains, and then give in one section a detailed account of the nature of the rocks.
The highest peaks of the Cordillera appear to consist of active or more commonly dormant volcanoes, such as Tupungato, Maypu, and Aconcagua, which latter stands 23,000 feet above the level of the sea, and many others. The next highest peaks are formed of the gypseous and porphyritic strata, thrown into vertical or highly inclined positions.
The uppermost beds of the porphyritic conglomerate, on which the gypseous strata conformably rest, are variously coloured, with one very singular and beautiful stratum composed of purple pebbles of various kinds of porphyry, embedded in white calcareous spar, including cavities lined with bright-green crystallised epidote.
"A warrior in armor with a gypseous pipe in his hand leans against a table and blows the smoke far away of himself." "A Dutch landscape along a navigable river which perfuses it till to the background." "Some peasants singing in a cottage. A woman lets drink a child out of a cup." "St. John's head as a boy painted in fresco on a brick."
A specimen also, I may add, of the true T. aenigma, was given me from the neighbourhood of the famous silver mines of Chanuncillo, a little south of the valley of the Copiapo, and these mines, from their position, I have no doubt, lie within the great gypseous formation: the rocks close to one of the silver veins, judging from fragments shown me, resemble those singular metamorphosed deposits from the mining district of Arqueros near Coquimbo.
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