With respect to the age at which the several parallel ridges composing the Cordillera were upthrown, I have little evidence.Many of them may have been contemporaneously elevated and injected in the same manner as in volcanic archipelagoes lavas are contemporaneously ejected on the parallel lines of fissure.("Volcanic Islands" etc.) But the pebbles apparently derived from the wear and tear of the porphyritic conglomerate formation, which are occasionally present in the upper parts of this same formation, and are often present in the gypseous formation, together with the pebbles from the basal parts of the latter formation in its upper strata, render it almost certain that portions, we may infer ridges, of these two formations were successively upheaved.In the case of the gigantic Portillo range, we may feel almost certain that a preexisting granitic line was upraised (not by a single blow, as shown by the highly inclined basaltic streams in the valley on its eastern flank) at a period long subsequent to the upheavement of the parallel Peuquenes range.(I have endeavoured to show in my "Journal" 2nd edition page 321, that the singular fact of the river, which drains the valley between these two ranges, passing through the Portillo and higher line, is explained by its slow and subsequent elevation.There are many analogous cases in the drainage of rivers: see "Edinburgh New Philosophical Journal" volume 28 pages 33 and 44.) Again, subsequently to the upheavement of the Cumbre chain, that of Uspallata was formed and elevated; and afterwards, I may add, in the plain of Uspallata, beds of sand and gravel were violently upthrown.The manner in which the various kinds of porphyries and andesites have been injected one into the other, and in which the infinitely numerous dikes of various composition intersect each other, plainly show that the stratified crust has been stretched and yielded many times over the same points.With respect to the age of the axes of elevation between the Pacific and the Cordillera, I know little:
but there are some lines which must--namely, those running north and south in Chiloe, those eight or nine east and west, parallel, far-extended, most symmetrical uniclinal lines at P.Rumena, and the short N.W.-S.E.and N.E.-S.W.lines at Concepcion--have been upheaved long after the formation of the Cordillera.Even during the earthquake of 1835, when the linear north and south islet of St.Mary was uplifted several feet above the surrounding area, we perhaps see one feeble step in the formation of a subordinate mountain-axis.In some cases, moreover, for instance, near the baths of Cauquenes, I was forcibly struck with the small size of the breaches cut through the exterior mountain-ranges, compared with the size of the same valleys higher up where entering the Cordillera; and this circumstance appeared to me scarcely explicable, except on the idea of the exterior lines having been subsequently upthrown, and therefore having been exposed to a less amount of denudation.From the manner in which the fringes of gravel are prolonged in unbroken slopes up the valleys of the Cordillera, Iinfer that most of the greater dislocations took place during the earlier parts of the great elevation in mass: I have, however, elsewhere given a case, and M.de Tschudi has given another, of a ridge thrown up in Peru across the bed of a river, and consequently after the final elevation of the country above the level of the sea.("Reise in Peru" Band 2 s.8:
