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 (21,982 ft.) are the highest peaks of the central Argentine-Chilean Andes. These three peaks are formed of eruptive rocks, surrounded by Jurassic beds which have undergone a thorough metamorphosis. While in the west of the Andes, from the latitude of Aconcagua, the central valley of Chile runs without any notable interruption to the south end of the continent, a valley which almost disappears to the north, leaving only some rare inflexions which are considered by Chilean geographers and geologists to be a continuation of the same valley; to the east in Argentina a longitudinal valley, perfectly characterized, runs along the eastern foot of the Cordillera, separating this from the pre-Cordillera, which is parallel to the Cordillera de la Costa of Chile. Between Aconcagua and Mercedario are the passes of Espinacito (14,803 ft.) and Los Patos or Valle Hermoso (11,736 ft.), chosen by the Argentine General San Martin, when he made his memorable passage across the chain during the War of Independence. North of Valle Hermoso the Andean ridges, while very high, are not abrupt, and the passes are more numerous than in the south; some of them descending 10,000 ft., but most of them between 13,000 and 14,000 ft. The pass of Quebrada Grande is 12,468 ft. in altitude; Cencerro, 12,944 ft.; Mercedario, 13,206 ft.; Ojota, 14,304 ft.; Pachon, 14,485 ft.; while Gordito is 10,318 ft. Farther north the passes are higher. Barahona Pass is 15,092 ft.; Ternera, 15,912 ft.; San Lorenzo, 16,420 ft., while the peak of the volcano reaches 18,143 ft.; Mount Olivares, 20,472 ft.; Porongos, 19,488 ft.; Tortolas, 20,121 ft.; and Potro, 19,357 ft. As far as 28° S. lat. the Cordillera de los Andes has been principally formed by two well-defined ridges, but to the north, recent volcanic action has greatly modified its orography. Only a single line of passes characterizes the main ridge, and amongst them are the passes of Ollita (15,026 ft.), Peñas Negras (14,435 ft.), Pircas Negras (13,615 ft.), La Gallina (16,240 ft.), Tres Quebradas (15,535 ft.), and Aguita (15,485 ft.). To the north of Mount Potro the peaks in the Cordillera are not very prominent as far as the great mass of Tres Quebradas, but here are to be met with some that may be considered as amongst the highest of the whole range. Mount Aguita is 20,600 ft., and the culminating peak of those of Tres Cruces reaches 22,658 ft. To the east of the eastern longitudinal valley, at 27° S. lat., begins a high volcanic plateau between the Cordillera and the southern prolongation of the Bolivian Cordillera Real, which contains lofty summits, such as Mount Veladero (20,998 ft.), Mount Bonete (21,980), Mount Reclus (20,670), Mount Pissis (22,146), Mount Ojo del Salado (21,653), and Incahuasi (21,719). To the north of Tres Cruces is a transversal depression in the Cordillera, which is considered to be the southern termination of the high plateau of the Puna de Atacama. The Cordillera of the Andes borders the Puna to the west, while the Bolivian Cordillera Real bounds it to the east. In that region the Cordillera of the Andes is of comparatively recent origin, being principally constituted by a line of high volcanoes, the chief summits being those of Juncal, Panteon de Aliste, Azufre or Listarrìa (18,636 ft.), Llullaillaco (21,720), Miñiques (19,357), Socompa (19,948), Licancaur (19,685), Viscachuelas (20,605), Tapaquilcha (19,520), Oyahua (19,242), Ancaquilcha (20,275), Olca (19,159), Miño (20,112), Sillilica (21,100), Perinacota (20,918), Sagama (22,339), Tacona (19,740), Misti (19,029); to the east closes in the intermediary high plateau which begins at 28° S. lat. in Argentina. The principal peaks of the Bolivian Andes and its prolongation from south to north, are Famatina, in the centre of Argentina, (20,340 ft.), Languna Blanca (18,307), Diamante (18,045), Cachi (20,000), Granadas, Lipez (19,680), Guadalupe (18,910), Chorolque (18,480), Cuzco (17,930), Enriaca (18,716), Junari (16,200), Michiga (17,410), Quimza-Cruz (18,280), Illimani (21,190) and Sorata (21,490).

While the western range of the Cordillera is principally formed by volcanic rocks, the eastern (to the east of the range is Cerro Potosi, 15,400 ft.) Andes of Bolivia are chiefly composed of old crystalline rocks. Between the ranges in the high plateau north to 27° are numerous isolated volcanoes which have been in activity in recent times, such as Peinado (18,898 ft.), San Pedro (18,701), Antuco (19,029), Antofalla (20,014), Rincon (17,881), Pastos Grandes (17,553), Zapalegui (17,553), Suniguira (19,258), Tahue (17,458); volcanoes which have been elevated from a lacustrine basin, which very recently occupied the whole extension, and the remains of which are, in the south, the Laguna Verde, at 28°, and in the north Lake Titicaca. The discovery of great Pampean mammals in the Pleistocene beds of that region shows that this upheaval of the latter is very recent, for in the heart of the Cordillera, as well as on the west coast of Bolivia and Peru, there have been discovered, in very recent deposits, the remains of some mammals which cannot have crossed the high range as it now exists.

The two Cordilleras that formed the Andes to the north of 28° S. lat. are continued in Peru. The western, which reaches an altitude of about 10,000 ft., then ceases to exist as a continuous chain, there remaining only a short, high ridge, called by Edward Whymper the “Pacific range of the equator,” and between this ridge and the crystalline Andean axis, the “avenue of volcanoes,” to use his words, arises amidst majestic scenery. Chimborazo, which is not in the main chain, reaches 20,517 ft.; Cotopaxi (19,580), Antisana (19,260), Coyambo (19,200) are in the eastern range, with many other peaks over 16,000 ft. which still contain glaciers. Sangay (17,380 ft.), under the equator, according to Wolff, appears to be the most active volcano in the world. Pichincha (15,804 ft.) and Cotocachi (16,297 ft.) are the loftiest volcanoes of the western range. In Colombia the three principal chains are continuations of those under the equator, and show very slight traces of volcanic action,