The mountain system rises abruptly on both sides throughout its course. Everywhere it presents a steep wall to the Pacific, and on the east it drops abruptly to the Amazon Valley; farther south, in Argentina, the land rises somewhat to meet it, and there are outlying ridges, but the main ascent is everywhere steep. The southern part of the system consists of a single range, with here and there outliers of comparatively little height, but from northern Chile and Argentina to Colombia, it consists of a high, broad plateau, capped by two or three ranges, with hundreds of high volcanic peaks, some active, others dormant or extinct.
Geologically, the system is of recent origin, although its age has not been closely determined. The material of which it is composed is in the main granites, with schists, slates, and other metamorphic rocks and the oldest of stratified rocks; here and there upturned beds of more recent formation, up to the Jurassic, lie upon the flanks of the ranges, while in Peru, the eastern range is composed largely of Silurian beds. Over all, in the neighborhood of the volcanic peaks, which are very numerous in all parts of the range, are spread lava, pumice, scoria, and ashes, in many places burying deeply the metamorphic rocks. Near the northern end, in Colombia and Venezuela, the eastern branches are composed of recent stratified rocks. For detailed description it will be convenient to refer to the countries traversed by this mountain system, and this order will be followed, commencing at the north.
Colombia and Venezuela. The Andes originate on the north in Colombia and Venezuela in several distinct ranges, which, trending south to southwest, meet and coalesce in the Pasto Knot in southwest Colombia. The westernmost range of magnitude is the Cordillera Occidental, which rises just east of the mouth of the Atrato, and trends southward, parallel to the coast, throughout Colombia. In this are summits 10,000 and 11,000 feet in height, the highest peaks being in the southern part. Near the boundary line with Ecuador it is cut through by the Rio Patia, which flows south and west into the Pacific. East of the Cordillera Occidental, and separated from it by the narrow valley of the Cauca, a branch of the Magdalena, is the Cordillera Central. This range rises from the lowlands between the Cauca and the Magdalena, and attains a great height, with Cumbal, 15,715 feet; Guacau, 16,683 feet; Guican, 15,748 feet; Santa Marta, 19,029 feet; Santa Isabel, 16,732 feet; Herveo, 18,045 feet; Ruiz, 17,388 feet; Sugarloaf, 16,000 feet; Tolima, 18,425 feet, and many others of equal height. The range is composed mainly of crystalline schists, while the higher peaks are volcanoes, which have spread lava and ashes over many parts of the range. East of the Cordillera Occidental and across the valley of the Magdalena, is the Cordillera de Bogota, originating in several ranges in the north of Venezuela, which trend in a general southwest direction and come together at various points; the principal ones are the Parija and Merida ranges, which unite near Bogota, beyond which point the range is single. Its highest peak is Cocui, 16,680 feet high. The range is in the main composed of strongly folded Cretaceous and Tertiary beds, and contains no volcanoes.
Ecuador. The Andes of Ecuador form two ranges, the Cordillera Occidental, the continua- tion of the range of the same name in Colombia, and the Cordillera Oriental, or Real; the two are separated by a high plateau, from 70 to 100 miles wide, with an average elevation of 8000 feet; connecting cross ranges divide this plateau into ten basins or high mountain valleys. The western range is composed of porphyries, diorites, and greenstone, and the eastern and higher range is composed of gneiss, schist, and granite. Above them tower many high volcanic cones, which have spread lava and ashes over great areas. The great peak of the western range is Chimborazo, 20,498 feet; with Cotocachi, 16,300 feet; and Pichincha, 15,918 feet, the last named near the city of Quito, while in the eastern range are Cotopaxi, 19,613 feet; Antisana, 19,335 feet; Cayambe, 19,186 feet; Altar, 17,736 feet; Illiniza, 17,023 feet; and Carahuairazo, 16,515 feet, with the active volcanoes Tunguragua, 16,690 feet, and Sangai, 17,464 feet. The cross ranges also contain many volcanic peaks, indicating that the whole region must once have been the centre of tremendous volcanic activity.
Peru. In Peru, Bolivia, and the northern part of Chile, the system is much broader and more complex. The Andes of Peru consist of three ranges, the two westernmost being the Maritime or Black, and the Central Cordillera, trending parallel to one another and to the coast, and in the north separated only by a narrow, high plateau, known as the Puna, with an average height of 12,500 feet, and in the south by the narrow valley of the Rio Huay. The Eastern Cordillera, though otherwise continuous, is cut through by no less than six of the head tributaries of the Amazon. The broad, elevated region lying between this and the Cordillera Central, known as the Sierra, is broken by mountain spurs, with broad valleys and plateaus. East of the Eastern Cordillera, or the Andes, as it is locally known, are several lower ranges, trending parallel with the system, and separating tributaries of the Amazon. The Maritime and Central Cordillera are composed of crystalline and volcanic rocks, with stratified beds of Jurassic age resting upon their outer flanks. The Eastern Cordillera is composed mainly of stratified beds of Silurian age, with some intrusions of granite. These ranges are connected at the mountain knot of Cerro de Pasco, 14,293 feet high, and again further to the southeast, at the Knot of Vilcanota, 17,390 feet. South of this latter peak the Central and Eastern Cordillera enclose the lofty plateau on which is Lake Titicaca, situated partly in Peru and partly in Bolivia, and 12,545 feet above the sea. North of the Cerro de Pasco, the Sierra comprises the upper valley of the Marañon, the largest and longest of the head branches of the Amazon, which cuts through the Eastern Cordillera just south of the Ecuador frontier. Between the Cerro de Pasco and the Knot of Vilcanota, the Sierra is drained by the head streams of the Ucayali, a large tributary to the Amazon. These streams also cut gorges through the eastern range. This region was the site of the ancient Inca civilization, and is still thickly settled. Among the high peaks of this part of the Andes are Huascan, 22,051 feet; Huandoy, 21,089 feet; Misti, 20,013; Chacani, 19,820 feet; and Tutupaca, 18,960 feet.
Bolivia. In Bolivia the system comprises two main ranges, one of which is formed by the coalescing of the two westernmost of the Ecuador
