Chapter 20

Central Chile.—Basal formations of the Cordillera.—Origin of the porphyritic clay-stone conglomerate.—Andesite.—Volcanic rocks.—Section of the Cordillera by the Peuquenes or 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.