In Central Chile, after the deposition of a great thickness of the gypseous strata, and after their upheaval, by which the Cumbre and adjoining ranges were formed, a vast pile of tufaceous matter and submarine lava was accumulated, where the Uspallata chain now stands; also after the deposition and upheaval of the equivalent gypseous strata of the Peuquenes range, the great thick mass of conglomerate in the valley of Tenuyan was accumulated: during the deposition of the Uspallata strata, we know absolutely, from the buried vertical trees, that there was a subsidence of some thousand feet; and we may infer from the nature of the conglomerate in the valley of Tenuyan, that a similar and perhaps contemporaneous movement there took place.We have, then, evidence of a second great period of subsidence; and, as in the case of the subsidence which accompanied the accumulation of the cretaceo-oolitic strata, so this latter subsidence appears to have been complicated by alternate or local elevatory movement--for the vertical trees, buried in the midst of the Uspallata strata, must have grown on dry land, formed by the upheaval of the lower submarine beds.
Presently I shall have to recapitulate the facts, showing that at a still later period, namely, at nearly the commencement of the old tertiary deposits of Patagonia and of Chile, the continent stood at nearly its present level, and then, for the third time, slowly subsided to the amount of several hundred feet, and was afterwards slowly re-uplifted to its present level.
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.Besides the elevation thus gained by angular displacements, I infer, without any hesitation--from the stratified gravel-fringes which gently slope up the valleys of the Cordillera from the gravel-capped plains at their base, which latter are connected with the plains, still covered with recent shells on the coast--that this great range has been upheaved in mass by a slow movement, to an amount of at least 8,000 feet.In the Despoblado Valley, north of Copiapo, the horizontal elevation, judging from the compact, stratified tufaceous deposit, capping the distant mountains at corresponding heights, was about ten thousand feet.It is very possible, or rather probable, that this elevation in mass may not have been strictly horizontal, but more energetic under the Cordillera, than towards the coast on either side; nevertheless, movements of this kind may be conveniently distinguished from those by which strata have been abruptly broken and upturned.When viewing the Cordillera, before having read Mr.Hopkins's profound "Researches on Physical Geology," the conviction was impressed on me, that the angular dislocations, however violent, were quite subordinate in importance to the great upward movement in mass, and that they had been caused by the edges of the wide fissures, which necessarily resulted from the tension of the elevated area, having yielded to the inward rush of fluidified rock, and having thus been upturned.
The ridges formed by the angularly upheaved strata are seldom of great length: in the central parts of the Cordillera they are generally parallel to each other, and run in north and south lines; but towards the flanks they often extend more or less obliquely.The angular displacement has been much more violent in the central than in the exterior MAIN lines; but it has likewise been violent in some of the MINOR lines on the extreme flanks.
The violence has been very unequal on the same short lines; the crust having apparently tended to yield on certain points along the lines of fissures.These points, I have endeavoured to show, were probably first foci of eruption, and afterwards of injected masses of porphyry and andesite.(Sir R.Murchison and his companions state "Geological Proceedings" volume 3 page 747, that no true granite appears in the higher Ural Mountains; but that syenitic greenstone--a rock closely analogous to our andesite--is far the most abundant of the intrusive masses.) The close similarity of the andesitic granites and porphyries, throughout Chile, Tierra del Fuego, and even in Peru, is very remarkable.The prevalence of feldspar cleaving like albite, is common not only to the andesites, but (as I infer from the high authority of Professor G.Rose, as well as from my own measurements) to the various claystone and greenstone porphyries, and to the trachytic lavas of the Cordillera.The andesitic rocks have in most cases been the last injected ones, and they probably form a continuous dome under this great range: they stand in intimate relationship with the modern lavas; and they seem to have been the immediate agent in metamorphosing the porphyritic conglomerate formation, and often likewise the gypseous strata, to the extraordinary extent to which they have suffered.
