Note 53 — Chapter V · PATAGONIA AND SHEEP-REARING

The great mass of the Patagonian Andes differs considerably in geological structure from the Argentinian Andes. The Paleozoic sedimentary rocks and the lofty chains of the pre-Cordillera cease at 36° S. lat. The Mesozoic beds—variegated breccie and porphyritic conglomerates, sandstones, limestones, and marls—which form the western slope of the Andes in central Chile, pass to the eastern slope at 35° S. lat., where they develop in regular folds, in the direction south-south-east, obliquely to the general line of the range. These folds account for the orientation of the interior valleys, which is remarkably uniform from the Rio Negro to the Collon Cura. They pass in the south-west under the sandstones of the tableland. West of this sedimentary zone, the zone of the sub-Andean granites and diorites, which have not been exposed further north except at the base of the western slope, opens out in the Patagonian Andes, of which it is the main body between Lake Lacar and the Gulf of Ultima Esperanza. In fine, the Patagonian Andes are characterised by volcanic formations. They are seen on the eastern slope about 36° S. lat., in the lava-flows and ashes of Payen and Tromen. Further south volcanoes with acid lava and characteristic cones are restricted to the central zone (Lanin, etc.) and the Chilean flank, but flows of fluid basic lava cover enormous stretches at the eastern fringe of the Andes, and they have spread over a good deal of the Patagonian tableland outside the Andean region.