470 Central Chile. paet n. 



CHAPTER XIV. 



CENTRAL CHILE; STRUCTURE OF THE CORDILLERA. 



Central Chile — Basal formations of the Cordillera — Origin of the 

 porphyrittc clay-stone conglomerate — Andesite — Volcanic rocks — 

 Section of the Cordillera by the Peuqueneg or Portillo Pass — Great 

 gypseous formation — Peuquenes line; thickness of strata, fossils of 

 — Portillo line, conglomerate, orthitic granite, mica- schist, and 

 volcanic rocks of — Concluding remarks on the denudation and eleva- 

 tion of the Portillo line — Section by the_ Cuvibre or TJspallata Pass 

 — Porphyries — Gypseous strata — Section near Puente del Inca ; 

 fossils of — Great subsidence — Intrusive porphyries — Plain of Us- 

 pallata — Section of the Cspallata chain — Structure and nature of 

 the strata — Silieijied vertical trees — Great subsidence- Granitic 

 rock* of axis — Concluding remarks on the TJspallata range ; origin 

 subsequent to that of the main Cordillera ; tno periods of subsid- 

 ence ; comparison nith 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 moun- 

 tains, 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, or of an allied euritic porphyry ; in 

 the low country, besides these granitic rocks and green- 

 stone, 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 



