range are Tuachela, Olca, Mino, and Ollagua, all smoking or emitting lava. South of Ollagua are at least thirty extinct volcanoes, exceeding 16,500 feet in height; among them are Autopalla, 20,920 feet, Socompa, 19,620 feet, and Llullaillaco, 21,670 feet.
In the central Chilean region are Tupungato, San Jose, Maipo, Tinguiririca, all supposed to be extinct. Las Damas and Peteroa are said to have been in eruption in the last century. The volcanoes grouped about Descabezado are quiescent, though appearances indicate recent eruption. Chilean ranges contain several vents, from which lava and ashes have been ejected in recent years. Autuco also has had eruptions within historic times. Farther south, Villarica volcano has frequently been seen in eruption, in all probability, other active volcanoes exist in the fiord region of South Chile, although none has yet been reported.
Hydrography. The Andes system is the source of most of the larger streams of South America. Through nearly its whole extent, wherever the system comprises more than a single main range, the westernmost of these ranges separates the drainage to the Atlantic from that to the Pacific. In Ecuador, however, no fewer than seven of the ten high valleys between the ranges are drained westward, and in southern Chile, as has been seen, glaciers have eroded their sources back across the whole range to the Argentina plains. The western streams are short, and owing to the light rainfall on most of the western slope, have small volume. Hence their cutting power is slight. On the other hand, the streams to the east are long, with great drainage basins, and, except in Argentina, are supplied with abundant precipitation by the trade winds. Hence they are powerful streams of large volume, and have eroded their courses far up into the mountains.
The Andes of Colombia are drained northward to the Caribbean Sea by the Magdalena, Cauca, and Atrato rivers, and eastward to the same body of water by the Orinoco, and to the Atlantic by the Negro and Yapurá great branches of the Amazon. The system in Ecuador, Peru, and most of Bolivia is drained eastward by countless tributaries of the Amazon, among which are the Napo, Marañon, Ucayali, Beni, and Mamore. Of these, the Marañon heads between the ranges far to the south, near the Knot of Cerro de Pasco, flowing northwest within the mountain system for 400 miles before breaking through the eastern range into the Amazon basin. The Huallaga, Mantaro, Apurimac, and Urubamba, tributaries of the Marañon, also head between the ranges, cutting gorges through the eastern range. In Bolivia and northwest Argentina is a great region, 800 miles in length, lying between the ranges, with an average altitude of 13,000 feet, which has no drainage to either ocean. In this region is the great Lake Titicaca, which drains by the Pio Desaguadero to Lake Poopo, where the drainage of this semi-desert region is collected. This lake in earlier times drained to the Amazon, but by shrinkage in volume its outlet has been closed, and now it discharges only by evaporation. The eastern slope of the Andes in southern Bolivia and northern Argentina is drained to the Plata, while farther south shorter streams, the Rio Colorado, the Negro, Chubut, and the Deseado, and the Arroyos Bayo and Salado, and other smaller streams, carry the drainage directly to the Atlantic.
Climate. The climate of the Andes differs widely in different parts, with latitude, altitude, and exposure. The eastern slope of the system from the northern end southward to latitude 25°, comprising the portions drained by the Orinoco and Amazon, and lying almost entirely within the tropics, has a heavy, and, in many localities, a profuse, rainfall. Farther southward in the temperate zone, in the region of prevailing westerly winds, the rainfall on this side of the range diminishes, becoming very light in Argentina, with only eight inches or less in the driest parts. On the west side of the system, the rainfall conditions are very nearly reversed, though in the north, in Colombia, the tropical rainfall passes around the end of the range and extends down the western side for some distance, giving to the valleys of the Magdalena, Cauca, and Atrato abundant moisture, and extending southward along the coast as far as Guayaquil, Ecuador. Thence southward, the western coast is an arid and desert region, as far as latitude 30° S. Below this point the precipitation increases, as the westerly winds bring moisture, and the southern coast is well watered.
From Guayaquil a cooler climate is reached either by going south or by going directly up the mountains. The base of the mountains, within the tropics, has a mean annual temperature of 80° F. or more, while in southern Argentina it is not more than 25°. Within the tropics the temperature ranges from 80° at the base of the mountains to 20° or less at their summits, a range due to altitude alone. Upon the Titicaca plateau Arctic conditions prevail, with frost every month of the year. Where the rainfall is copious, as it is on the eastern side within the tropics, the range of temperature between summer and winter is slight, while upon the west coast, in the same latitudes, where desert conditions prevail, the range is very great. In general, as the mountains are ascended, the contrasts of temperature become greater, owing to the rarefaction of the air. At great altitudes, even, the contrast between day and night is great. South of the latitude of Coquimbo, 30° S., these temperature conditions are reversed, the west slopes having the smallest annual and diurnal range.
Means of Communication. Routes of travel across the Andes are few in number, the passes are very high, and the roads traversing them are, as a rule, very bad. Communication between the peoples on the two sides of the mountains is slight. The high land between the ranges is the best settled part of these sparsely settled countries, and the inhabitants of these elevated regions have some intercourse with the western seaboard, but very little with the low country to the east. But with the development of the mining industry in the mountains and the exploitation of the rubber resources of the upper waters of the Amazon, it may be expected that means of communication across the range will be improved in the near future. In Colombia the main routes of travel follow the valleys of the Cauca and the Magdalena, while the chief route across the Cordillera Central is via Quindio Pass, connecting Cartago, on the Cauca, with the valley of the Magdalena, and ultimately with the capital, Bogota. In Ecuador the main
