Central Chile.
Basal formations of the Cordillera.
Origin of the porphyritic clay-stone conglomerate.
Andesite.
Volcanic rocks.
Section of the Cordillera by the Peuquenes are Portillo Pass.
Great gypseous formation.
Peuquenes line; thickness of strata, fossils of.
Portillo line.
Conglomerate, orthitic granite, mica-schist, volcanic rocks of.
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.
Concluding remarks on the Uspallata range; origin subsequent to that of the main Cordillera; two periods of subsidence; comparison with the Portillo chain.
The district between the Cordillera and the Pacific, on a rude average, is from about eighty to one hundred miles in width.It is crossed by many chains of mountains, of which the principal ones, in the latitude of Valparaiso and southward of it, range nearly north and south; but in the more northern parts of the province, they run in almost every possible direction.Near the Pacific, the mountain-ranges are generally formed of syenite or granite, and or of an allied euritic porphyry; in the low country, besides these granitic rocks and greenstone, and much gneiss, there are, especially northward of Valparaiso, some considerable districts of true clay-slate with quartz veins, passing into a feldspathic and porphyritic slate; there is also some grauwacke and quartzose and jaspery rocks, the latter occasionally assuming the character of the basis of claystone porphyry: trap-dikes are numerous.Nearer the Cordillera the ranges (such as those of S.Fernando, the Prado (Meyen "Reise um Erde" th.
1 s.235.), and Aconcagua) are formed partly of granitic rocks, and partly of purple porphyritic conglomerates, claystone porphyry, greenstone porphyry, and other rocks, such as we shall immediately see, form the basal strata of the main Cordillera.In the more northern parts of Chile, this porphyritic series extends over large tracts of country far from the Cordillera; and even in Central Chile such occasionally occur in outlying positions.
I will describe the Campana of Quillota, which stands only fifteen miles from the Pacific, as an instance of one of these outlying masses.This hill is conspicuous from rising to the height of 6,400 feet: its summit shows a nucleus, uncovered for a height of 800 feet, of fine greenstone, including epidote and octahedral magnetic iron ore; its flanks are formed of great strata of porphyritic claystone conglomerate associated with various true porphyries and amygdaloids, alternating with thick masses of a highly feldspathic, sometimes porphyritic, pale-coloured slaty rock, with its cleavage-laminae dipping inwards at a high angle.At the base of the hill there are syenites, a granular mixture of quartz and feldspar, and harsh quartzose rocks, all belonging to the basal metamorphic series.I may observe that at the foot of several hills of this class, where the porphyries are first seen (as near S.Fernando, the Prado, Las Vacas, etc.), similar harsh quartzose rocks and granular mixtures of quartz and feldspar occur, as if the more fusible constituent parts of the granitic series had been drawn off to form the overlying porphyries.
In Central Chile, the flanks of the main Cordillera, into which Ipenetrated by four different valleys, generally consist of distinctly stratified rocks.The strata are inclined at angles varying from sometimes even under ten, to twenty degrees, very rarely exceeding forty degrees: in some, however, of the quite small, exterior, spur-like ridges, the inclination was not unfrequently greater.The dip of the strata in the main outer lines was usually outwards or from the Cordillera, but in Northern Chile frequently inwards,--that is, their basset-edges fronted the Pacific.
Dikes occur in extraordinary numbers.In the great, central, loftiest ridges, the strata, as we shall presently see, are almost always highly inclined and often vertical.Before giving a detailed account of my two sections across the Cordillera, it will, I think, be convenient to describe the basal strata as seen, often to a thickness of four or five thousand feet, on the flanks of the outer lines.
BASAL STRATA OF THE CORDILLERA.