[53] 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 deg. 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 deg. 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 deg. 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. It is on the Alumine, about 39 deg. S. lat., that we find traces of glacial erosion, as they spread over the landscape. At present there is no ice on the mountain except on the peaks of Lanin and Tronador, but from the Rio Puelo onward (42 deg. S. lat.), glaciers clothe all the summits which rise above 6,500 feet. North of the Aisen they form a narrow, but almost continuous, line. From the Aisen to the Calen fiord, and beyond the gap of the fiord as far as 52 deg. S. lat., the ice spreads in a considerable sheet which in some places attains a breadth of eighty miles. The tongues of the glaciers reach the Pacific below 46 deg. S. lat., and Lake San Martin on the Argentine slope below 49 deg. S. lat. In Tierra del Fuego the snow-line is at 2,300 feet, and the glaciers which the snows feed, reach as far as the fiords and Lake Fagnano.