The mountains forming the Cordillera between Magellan Strait and 41° S. lat. are higher than those previously mentioned in Tierra del Fuego. Generally composed of granite, gneiss and Palaeozoic rocks, covered in many parts by rugged masses of volcanic origin, their general height is not less than 6500 ft., while Mount Geikie is 7500 ft. and Mount Stokes 7100 ft. To the north are Mounts Mayo (7600 ft.), Agassiz (10,600 ft.), and Fitzroy, in 49° S. lat. (11,120 ft.). The section from 52° to 48° S. lat. is a continuous ice-capped mountain range, and some of the glaciers extend from the eastern lakes to the western channels, where they reach the sea-level. The level of the lakes begins at 130 ft. at Lake Maravilla and gradually ascends to nearly 700 ft. at Lake San Martin. Passing the breach through which Lake San Martin empties itself into Calen Inlet, in 48° S. lat., is found a wide oblique opening in the range, through which flows the river Las Heras, fed by Lake Pueyrredon, which is only 410 ft. above the sea-level to the east of the Andes, while Lake Buenos Aires, immediately to the north, is 710 ft. The Andes continue to be to the west an enormous rugged mass of ice and snow of an average height of 9000 ft., sending glaciers to all the eastern fjords.
Mount San Lorenzo, detached from the main chain in the pre-Cordillera, is 11,800 ft. high. Mount San Valentin (12,700 ft.) is the culminating point of the Andes in the region extending from 49° to 46° S. lat., a little north of which is the river Huemules which is followed by the breach of the river Aisen. These two rivers have emptied a large system of lakes, which in pre-Glacial times occupied the eastern zone, thus forming a region suitable for colonization in the broad valleys and hollows, where the rivers, as in the case with those in the north, cut through the Andes by narrow gaps, forming cataracts and rapids between the snowy peaks. Volcanic action is still going on in these latitudes, as the glaciers are at times covered by ashes, but the predominant rocks to the east are the Tertiary granite, while to the west gneiss, older granite and Palaeozoic rocks prevail. The highest peaks, however, seem to be of volcanic origin. Farther north, up to 41° S. lat., the water gaps are situated at a lesser distance one from the other, owing mainly to more continuous erosion, this section of the continent being the region of the maximum rainfall on the western coast to the south of the equator. Between the gaps of the river Aisen and river Cisnes or Frias, which also pierces the chain, is found a huge mountain mass, in which is situated Mount la Torre (7150 ft.). These form the continental watershed, but in this region erosion is taking place so rapidly that the day is not far distant when Lakes La Plata and Fontana, situated to the east at a height of 3000 ft. and now tributaries of the Atlantic, may become tributaries of the Pacific. Already filtrations from the former go to feed western affluents through the granitic masses. To the north of Mount la Torre flows in the river Cisnes, 44° 48' S. lat., across another water gap, continuing the range to the north with high peaks, as Alto Nevado (7350 ft.) and Cacique (7000 ft.). The glaciers reach almost the western channels, as is the case at the river Quelal. The northern glaciers, descending nearly to sea-level, are situated at 43° 40' S. lat. To the north 45° S. lat. a well-defined western longitudinal valley, at some recent time occupied by lakes and rivers, divides the Cordillera into two chains, the eastern being the main chain, to which belong Mounts Alto Nevado, Cacique, Dentista, Maldonado, Serrano, each over 7000 ft. high; and Torrecillas (7400 ft.), Ventisquero (7500 ft.), and Tronador (11,180 ft.); while the western chain, broken into imposing blocks, contains several high volcanic peaks such as Mounts Tanteles, Corcovado, Minchimahuida, Hornopiren and Yates. The rivers Palena, with its two branches, Pico and Carrenleufu, Fetaleufu, Puelo and Manso cut the two chains, while the rivers Reñihue, Bodadahue and Cochamo have their sources in the main eastern ridge. Mention has been made of active volcanoes in 51°, 49° and 47° S. lat., but these have not been properly located. The active volcanoes south of 41°, concerning which no doubt exists, are the Huequen, in 43° lat., and the Calbuco, both of which have been in eruption in modern times.
The surroundings of Mount Tronador, consisting of Tertiary granite and basalt, form one of the most interesting regions in the Patagonian Andes for the mountaineers of the future. To the east extends the large and picturesque lake of Nahuel-Huapi, to the west is Lake Todos Los Santos (50 sq. m.), to which the access is easy and of which the scenery is of surpassing beauty. Between 41° and 38° S. lat., among other smaller lakes, are Lakes Traful (45 sq. m.), Lacar (32 sq. m.), which, properly belonging to the system of Atlantic lakes, empties itself by the only water gap that occurs in this zone of the Cordillera into the river Valdivia, a tributary of the Pacific, Lake Lolog (15 sq. m.), Huechu-lafquen (45 sq. m.), and Lake Alumine (21 sq. m.). The volcanoes of Lanin (12,140 ft.), Quetropillan (9180 ft.), Villarica (10,400 ft.), Yaimas and Tolhuaca are all more or less active; the first is in the main chain, while the others are on the western slope. The scenery in the neighbourhood is magnificent, the snowy cones rising from amidst woods of araucaria, and being surrounded by blue lakes. While the scenery of the western slope of the Andes is exceedingly grand, with its deep fjords, glaciers and woods, yet the severity of its climate detracts considerably from its charm. The climate of the eastern slope, however, is milder, the landscapes are magnificent, with wooded valleys and beautiful lakes. The valleys are already partly settled by colonists. Between 52° and 40° S. lat. erosion has carried the watershed of the continent from the summit of the Cordillera to the eastern plains of Patagonia.
From 40° S. southward the Chile-Argentine Boundary Commission under Sir T. H. Holdich carried out important investigations in 1902; and between 38° and 33° S. lat. the Andes were somewhat extensively explored about the close of the 19th century by Argentine and Chilean Commissions. The highest peaks in the latter section are volcanic and their eruptions have sensibly modified the character of the primitive ridges. Out-flows of lava and tufa cover the mountain sides and fill up the valleys. The Jurassic and Cretaceous formations, which in the Southern Cordillera are situated outside of the range to the east, form to a considerable extent the mass of the great range, together with quartz porphyry, the Tertiary, granite and other eruptive rocks, which have been observed along all the chain in South America up to Alaska in the north. Gneiss is seldom met with, but there are crystalline rocks, belonging chiefly to the pre-Cordillera of the eastern and to the Cordillera de la Costa on the western side.
About 38° S. the Andes take a great transversal extension; there are no wide intermediate valleys between the different ridges but the main ridge is perfectly defined. Volcanic Chile-Argentina from 38° S. northward. cones continue to predominate, the old crystalline rocks almost disappear, while the Mesozoic rocks are most common. The higher peaks are in the main chain, while the Domuyo (15,317 ft.) belongs to a lateral eastern ridge. The principal peaks between this and Mount Tupungato at 33° S. lat. are: Mount Cochico (8255 ft.), Campanario, (13,140 ft.), Peteroa (13,297 ft.), Tinguiririca, Castillo (16,535 ft.), Volcano Maipu (17,576 ft.), Alvarado (14,600 ft.), Amarillo (15,321 ft.), Volcano San Jose (19,849 ft.), Piuquenes (17,815 ft.), and Volcano Bravard (19,619 ft.).
North of Maipu volcano, ascended by R. P. Güssfeldt in 1883, the Cordillera is composed of two huge principal ridges which unite and terminate in the neighbourhood of Mount Tupungato. The valley between them is 9000 ft. high; and in that part of the Cordillera are situated the highest passes south of 33° S. lat., one of which, the Piuquenes Pass, reaches 13,333 ft., whilst the easiest of transit and almost the lowest is that of Pichachen (6505 ft.), which is the most frequented during winter. Mount Tupungato reaches 22,329 ft., according to Argentine measurement. To the north of this mountain, situated at the watershed of the Andes, extends a lofty region comprising peaks such as Chimbote (18,645 ft.) and Mount Polleras (20,266 ft.). The Pircas Pass is situated at a height of 16,962 ft. The gaps of Bermejo and Iglesia, in the Uspallata road, the best known of all the passes between Argentina and Chile, are at 13,025 ft. and 13,412 ft. altitude respectively, while the nearest peaks, those of Juncal and Tolorsa, are 19,358 and 20,140 ft. high.
Mounts Tupungato, Aconcagua (23,393 ft.) and Mercedario