Andes

ANDES, the name applied to the great mountain system which extends the full length of the western part of South Amer ica. The origin of the name is obscure. It has been suggested that it is derived from anti, the Quechua word for east, a name applied by the Quechua Indians to the range east of Cuzco. Other suggested origins of the name are the Quechua words antasuya, region of metal, or anta, copper. The term "Cordillera of the Andes" is sometimes erroneously used as a name for the whole Andean system, and in South American nomenclature is often applied to the most important range of a given section of the sys tem. From its original form, "Las Cordilleras de los Andes," a term applied by the Spanish conquerors to the series of parallel ranges of which the Andes appeared to be composed, is derived the present widespread use of the word "cordillera," meaning an extensive mountain range or system of ranges.

The Andes are narrowest at the southern end and broadest in the central or Bolivian section and at their northern end, where they divide into f our distinct ranges. In Tierra del Fuego their trend is nearly east and west ; on the mainland the trend is north and south as far north as Lat. 18° S., where, in one of its two broadest sections, the system turns westward to form an almost semi-circular curve convex to the west and follows the configura tion of the west coast as far north as southern Colombia. There it spreads out into three distinct ranges, the easternmost of which again divides into two ranges, one of which forms the northern section of the Venezuela-Colombia boundary while the other crosses the boundary south of Lake Maracaibo and extends north eastward toward the Caribbean sea.

The Patagonian Andes.

Until recently the dominating fea ture of the entire Andean system was pictured as a great mountain barrier, with a well-defined crest line of lofty peaks which coin cided with the water-divide between the Pacific and Atlantic drain age systems, and effectively separated them from each other. As late as 1881, when the Governments of Chile and the Argentine republic signed a treaty in which their common boundary was de fined, they described it as following the highest crest of the Andes, which formed the water-divide between the Atlantic and Pacific oceans. When the demarcation of this boundary was undertaken it was found that throughout the greater part of the cordillera the topography did not correspond to the conception of it on which the treaty was based. South of lat. 38° S. the Andes were found to be, not a simple ridge, but a broad zone, from 25 to 40m. wide, of mountain chains and narrow valleys. The higher summits shift back and forth from one chain to another and do not, except in a general way, mark the water-divide. These sum mits rise above a broad base, represented by high valleys and broad passes, to an average elevation of about 6,500 feet. Few are lower than this average altitude, and only individual peaks exceed feet. Those which have a fairly uniform altitude have been carved by erosion from the older Andean rocks of the plateau like base. The higher peaks are volcanic cones built up on the plateau. The eastern side of this part of the Andes is divided into two sections—a northern from Lake Alumine to Lake Nahuel Huapi, and a southern from Lake Nahuel Huapi southward, which includes the island of Tierra del Fuego. Both sections are characterized by a succession of glacier-fed lakes which stretch back like fiords into the heart of the cordillera, and by valleys cut from west to east across the Patagonian tableland through which, at one time, the lakes drained toward the Atlantic.

In the northern section high transverse ridges separate the lake basins and more or less effectively obstruct passage from one to another. These ridges connect the mountain zone with the Pata gonian tableland, and all but one of the streams of this section rise in the westernmost range and flow eastward to the Colloncura and Limay rivers. Lake Lacar now discharges across the western range to the Chilean lake region, but there is evidence that it too, once drained to the Atlantic.

In the southern section the tableland is separated from the cor dillera by a longitudinal depression which marks the line of con tact between the folded cordillera and the flat tableland and ex tends southward from the low pass between Lake Nahuel Huapi and Lake Bariloche and is continued beyond the mainland by the Canal Ancho of the Straits of Magellan, Useless bay, and White side and Admiralty sounds. The continuity of this depression has, however, been frequently exaggerated. It is really divided into several compartments by masses of different kinds of rocks and by granite ridges. Between Lake Argentina and Lake Buenos Aires it is interrupted by the tableland which rises to a height of 5,000f t. and butts against the cordillera. Similarly between Lake Buenos Aires and Lake General Paz there is no great differ ence in level between the tableland and the depression.

Most of the lake basins here, as well as in the northern section, are continued eastward across the Patagonian tableland as dis tinct valleys. This is the case even in the submerged southern end of the depression, where the east end of the Straits is a submerged valley on the axis of Otway Water and Useless bay and is con tinued eastward across northern Tierra del Fuego by the de pression which ends in the Bay of San Sebastian. The valleys are now, however, with few exceptions, dead valleys, for the drainage is mostly west to the Pacific by way of narrow canyons in the cordillera and the interoceanic water-divide follows the terminal moraines of the old glaciers which confine the lakes on the east.

On the Pacific side of the cordillera a great longitudinal plain extends from the Chacabuco range, by which the basin of the Aconcagua river is enclosed on the south, southward to the Gulf of Ancud. The northern part of this region is narrow and irregu lar and is cut by ridges into small, almost completely separated valleys, and interrupted by isolated hills. At about lat. 36° S. it widens out to a width of 20 to 4o miles. A line of recent vol canoes begins a little south of Santiago and is alined on the cor dillera to about 3 5 ° S. lat., where it gradually departs from the cor dillera until in the south it stands well out in the central plain and forms a less continuous but higher chain west of the cordillera proper. The northern part of the plain is filled with recent geo logical deposits and forms the fertile "Vale of Chile." South of lat. 39° S. it is sown with glacial lakes or depressions formerly occupied by lakes. Not only because of the greater precipitation on the west side of the cordillera in the belt of the westerly winds south of lat. 39° S., but also because of the great condensation of moisture caused by the line of volcanoes in front of the cor dillera, the glaciers of the Glacial period advanced much farther from the water-divide on the western slope than on the eastern, with the result that the glacial lakes on the Chilean side of the cordillera are at a much lower level than those on the Argentine side, and part of them, as, for instance, Ranco and Lanquihue, lie completely separated from the cordillera.

The cordillera of Patagonia is divided into a series of massifs separated by longitudinal and transverse depressions. Some of these depressions cut completely through the cordillera and are occupied by rivers which drain lakes on the Argentine side, or by fiords which extend through to the Argentine side. Between 41 ° and, 46° S. lat. the altitude of these massifs is from 6,50o to 8,000ft., only Troncador (1 1,000f t.) attaining a higher altitude. Between 46° and 50° the cordillera reaches altitudes of from to 13,00o feet. It is lower and broken up between Smith channel and the Straits of Magellan, but again rises to more than 6,5ooft. on Tierra del Fuego.

South Chilean Andes.

Northward from lat. 39° S. to lat. 30° S. the character of the Andes is greatly simplified. It con sists of a double line of crests, the westernmost of which carries the Argentine-Chile boundary. The high valleys between the crests are at an altitude of from io,000 to 13,00o feet. The passes to the east and west in the northern part are between 13,00o and 15,000f t. in altitude. South of lat. 31 ° S. they are somewhat lower. Isolated summits rise from 5,000 to 7,000f t. above the high valleys, and between 3 2 ° and 34° S. lat. include three of the highest peaks of the Andes: Mecedario (21,8 7of t.) , Aconcagua (2 2,83of t.) , Tupungato (21,81 oft.). The interior valleys have a direction slightly oblique to the general trend of the cordillera and corresponding to the direction of the folds. The whole section has effective drainage of normal type. The water-divide which the boundary follows shifts back and forth from one ridge to another, and the boundary pillars, which are located, in general, on high passes between headwaters, follow a tortuous line. Erup tions from the chain of recent volcanoes which appears a little south of the latitude of Santiago and extends southward on the Pacific side of the main axis of the cordillera to the Gulf of Ancud have greatly complicated the topography of the cordillera by filling up high valleys or damming up lakes.

East of the cordillera in this section a continuous depression extends northward from the Mendoza river for more than i6om. between the cordillera and the pre-cordillera (a series of short ranges reaching out into the Argentine plain and formed by a north-south alinement of narrow crests of Silurian limestones between sandstones and metamorphosed Devonian schists) . The pre-cordillera continues south of the Mendoza river to about lati tude 36° S. It attains heights of 13,000f t. in the latitude of San Juan and i i,000 on the Paramillo piateau above Mendoza. In spite of their narrow width the Andes in this section offer serious ob stacles to movement from one side to the other because of their elevation and covering of snow. They have no mineral wealth and the few routes across them, which include the Transandine railway between Santiago and Mendoza, are solely for the pur pose of crossing the cordillera and not for exploiting its subsoil. The high valleys furnish some summer pasture and the rivers and lakes afford water for irrigation. Settlements lie not in the mountains but on their border, and the location of the more important of those on the Argentine side has been influenced by the position of the most practicable passes, since it is to cattle and goods trade with Chile that they owe their origin and growth.

The Puna de Atacama.

North of lat. 30° S. the Andes again lose their simple character. The mountain belt widens and long extensions of it reach out south-eastward to the Argentine plain, enclosing in the depressions between them long embayments of the plain. At about the 28th parallel the main range divides into a western and an eastern range, with a belt of very high and extremely cold and desolate country between them (called the Puna de Atacama), made up of mountain chains, isolated peaks, and basins of salt deposits known as salars, arranged along domi nating north-south lines. The average height of the basin floors of the Puna is between i i ,00o and 13,000f t., with the interior peaks and ridges rising only from i,000 to 5,000ft. higher. The range which forms the western border of the Puna is known as the Cor dillera de Los Andes or Maritime cordillera and consists chiefly of a line of high volcanoes with lava flows between them. The passes in the Maritime cordillera average about i 5,000f t. while the peaks reach 20,000 to 23,00o feet. In the range which forms the eastern border of the Puna the passes are from 16,5oo to i 7,000f t. high, but the peaks average only from 18,000 to 20,000 feet. In contrast with the western cordillera in which the volcanic cones are high and have been built up on a lower basement of lava, the cones of the eastern cordillera are low_and have been built up on broad beds of lava. West of the western cordillera is a line of depressions between 3,00o and 4,000f t. lower than the average level of the Puna and enclosed on the west by the Cordillera Domeyko which contains a number of salars, among them the Salar de Atacama and the Salar de Punta Negra.

East of the eastern border of the Puna, the Andes of the Ar gentine provinces of Jujuy, Salta and Tucuman form a broad belt of ridges oriented from north to south and separated by deep depressions. Their altitudes diminish as they extend eastward into the Argentine plains, and their summits, as well as those of the secondary ranges along the main mountains, exhibit an older aspect than their ravined borders. The tops and high upper slopes are covered with grass. The steep headwater slopes and narrow declivities of the ravines, cut into them as the result of recent uplift, are youthful features in contrast to the lawn-like high-level slopes which they are gradually invading.

The Central Andes.

The northern limit of the Puna de Atacama is a transverse range of high snow-capped peaks and ridges at about the 23rd parallel, which separates it from the drainage basin of the Salar de Uyuni. The high plateau continues northward as the or altoplanicie of Bolivia. This alti plano has no outlet to the sea, and is therefore one of the world's great basins of interior drainage. It is about loom. long by loom. wide, and slopes gently from a little over 12,500ft. at Lake Titi caca to slightly more than i 2,000f t. at Lake Poopo, and a little lower at the Salar de Uyuni. It differs from most interior drain age basins in that it contains a large body of fresh water (Lake Titicaca) and an important river drainage (the Desaguadero river). Lake Titicaca is the highest large body of navigable water in the world and has a length of about 125 miles. It owes its ex istence to a deep tectonic depression in close proximity to a high snow-capped mountain range (the Cordillera Real) that keeps it supplied with water. Farther south the altiplano becomes more and more like other interior semi-deserts, for the windward mountains become lower, so that they condense less moisture, and broader, so that the precipitation is more widely distributed. As it flows southward, the Desaguadero river, the outlet of Lake Titi caca, grows smaller and smaller until Lake Poopo, into which it empties, is a shallow pond with only a small overflow to the nearby salt lake or marsh called Coipasa. The altiplano lies be tween two great table-lands with superimposed peaks and ranges. Its eastern edge is particularly well-defined by the sharp rise and straight form of the eastern cordillera. The western edge is less clearly defined because of the volcanic debris that has encroached upon it from the western or Maritime cordillera, forming hilly districts of irregular outline. Between the Salar de Coipasa and the Salar de Uyuni, the altiplano is divided into two interior drainage basins by a line of volcanoes and hills which extend from the western cordillera nearly across to the eastern. North of Lake Titicaca the bordering scarps converge in the Nudo de Vil canota at about 14° S. Lat., and enclose the altiplano on the north.

The western cordillera descends by relatively smooth slopes to the coastal deserts of Tarapaca and Atacama, famous for their nitrate deposits, and the desert region of southern Peru. In the rare cases in which its streams permanently reach the piedmont, they furnish water to small but important fields of vegetables and alfalfa. Viewed from the west, the cordillera presents, for long distances a strikingly even skyline, generally the line along which the uplifted peneplane has been warped up to form the highlands, broken only by the summits of the highest volcanoes which stand back toward the altiplano or, in places, by volcanoes like El Misti and Tacora, which rise on the western edge of the cordillera. The most recent of these volcanoes, such as El Misti, Tacora and Sajama, are symmetrical cones surmounted by craters; but the greater number are much older than these, and have more or less lost their original form by erosion.

The eastern cordillera, where it borders the altiplano, falls into two sections of quite different physiographic aspect. In the northern section the character of the uplifted peneplane has been almost completely destroyed because the concentration of the rain fall in a narrow belt in this region and the relatively steep gradi ent of the rivers have combined to enable the streams to cut the old surface to pieces, leaving only the lofty snow-clad Cordillera Real standing on a narrow base. The Cordillera Real is a central core of resistant rocks whose superior hardness and greater initial elevation have preserved it from the effects of great denudation recognizable all around it. The Cordillera Real is cut between the Nevado de Illimani and the Nevado de Quimza Cruz by the chasm of the La Paz river, a tributary of the Madeira river of the Am azon system, which, by virtue of the enormous condensation of moisture on the eastward slope of the eastern cordillera, has been able to break through this great barrier to the altiplano; but the axis of the Cordillera Real extends southward across the La Pas valley through the Nevados de Araca, Quimza Cruz and Vera Cruz, forming a line of heights as definite in trend and structure as the Cordillera Real and continuing its features southward nearly 5o miles.

Northward from Lake Titicaca the western range turns north westward, while the eastern range, which has already in the Cor dillera Real taken a north-westerly trend, turns still further to the west and cuts off the altiplano by merging with the western range in a rough mountain mass known as the Nudo de Vilcanota. The Eastern Cordillera, here called the Cordillera de Carabaya, is a lofty snow-capped range separated from the Cordillera Real by a narrow and relatively low divide between the headwaters of the Mapiri river (a tributary of the Beni) and the Lake Titicaca drainage. On the west flank of the Cordillera de Carabaya is a group of glacial lakes in narrow depressions between the spurs of the range. There is no record of any soundings having been made in them, but from descriptions of their deep colouring and the character of the depressions in which they lie, they appear to be among the deepest of the high glacial-fed lakes of the Andes. The Cordillera of Carabaya lowers very rapidly to the north-east to the Amazon plain, and larger tributaries of the Beni cut directly across the lines of low ranges which front it on the east. Farther north, however, the valleys of the tributaries of the Madre de Dios have a longitudinal trend and have cut deep troughs oriented from south-east to north-west like the ridges which separate them.

The Peruvian Plateau.

The Andes of Peru, north from the Nudo de Vilcanota, are usually described as three distinct ranges which merge in the Nudo de Pasco and then continue north again as separate ranges. Recent studies show that they are a single broad plateau from 13,000 to 15,000 ft. high, on which narrow cordilleras rise to 18,000 and 21,000 feet; and that the nudos or knots are only gigantic erosion remnants. The Amazon tributaries have cut deep gorges almost to the western edge of the plateau, while the numerous streams of the arid Pacific slope are short and, with the exception of the Rimac and Santa rivers, carry little water as far as the ocean.

The Andes are divided into three natural regions—the mon tana, the eastern slopes of the Amazon forest ; the sierra, the temperate regions of the slopes and secondary valleys of the plateau; and the puna, cold, monotonous regions in the cordilleras, of vast extent at altitudes of from 10,000 to 15,00o feet. In iso lated sections above the puna are areas known as jalcas which resemble the paramos of Colombia and Ecuador. They are cov ered with a dense, steppe-like vegetation that varies but little from season to season. North-west of the Nudo de Vilcanota is a narrow trough sunk below the level of the plateau to an eleva tion of 11,000ft., known as the Cuzco valley, from. the City of Cuzco which lies at its upper end, on the site of the ancient Inca capital. The trough is 19m. long and is divided into three basins of flat-lying land arranged like the links of a chain. Its bordering rims have an average altitude of 13,500f t., with peaks exceeding 14,5ooft., and are cut through in many places by streams leading to the Urubamba and Apurimac rivers. The valley itself is drained by the Huantaney river which flows through the chain of basins and joins the Urubamba through a broad pass in the mountain wall. The most broken section of the Peruvian Andes is between Abancay and Huancayo where the Apurimac, Pachachaca, Pam pas and Mantaro rivers have cut deep gorges 6,000ft. and more below the surface of the puna. The zone of the interoceanic water divide is here a gently undulating plateau, with lakes of glacial origin which the recently eroded valleys have not yet reached. The many secondary valleys, such as those of Ayacucho and Lircay, have long been centres of grain cultivation.

From the Huancayo to Cerro de Pasco the plateau is chiefly the basin of the upper Mantaro river. In the Cordillera de Huaro chiri through which the Central railway from Lima to Oroya cuts, at an altitude of nearly i 6,000f t., on red porphyritic sandstones between crests of dark andesites, the divide rises to elevations of over 17,000 feet. On the western slopes of the Cordillera de Huarochiri are many glacial cirques, and immediately north of the route of the Central railway as it enters the cordillera is a great cluster of lakes at different levels, still fed by glaciers, which drain into the Rimac river close to the edge of the plateau. East of the crest a network of glaciated valleys with many lakes descends in steps to the Mantaro river. East of the Mantaro a little known range, whose snow-capped summits approach 16,500f t., cuts off the horizon north-east of the plain of Jauja. Its altitude dimin ishes to the north-west, and north of Tarma no longer carries permanent snow.

The Mantaro river rises at an altitude of about i 5,000f t., near the mining town of Cerro de Pasco, in a marshy plain with many shallow lakes. Most of these are only small ponds, but Lake Junin, the largest of them, is about tom. long. A group of par tially glacier-fed lakes close to the main divide, of which Lake Punriun (15m. long) is the largest, also drains to the Mantaro river near the outlet of Lake Junin. The Mantaro flows south eastward in a long limestone gorge to the plains of Jauja and Huancayo. These plains stand at 12,500f t. and i 1,600f t. respec tively, and together form a depression, 25m. long by 6 to 8m. wide, bordered on both sides by rows of bare hills behind which rise snow-capped ranges. The Mantaro leaves the depression in a deep gorge and swings round in a great bend to the Amazon plain about 8om. below Huancayo. The Cerro de Pasco region, north of the plain of Junin, has been given great importance in descriptions of the Peruvian Andes as a mountain knot in which are merged three distinct cordilleras. In reality it does not exceed the altitude of the puna and is only a fragment of the continuous plateau of sandstones and limestones, with occasional crests of andesite, which extends north of the plain of Junin and south between the plains of Junin and Jauja.

From Cerro de Pasco to the Pongo de Manseriche the plateau forms the drainage basin of the Huallaga and the upper Maranon rivers. North-west of Cerro de Pasco the Cordillera de Huay huash forms the interoceanic divide. The Maranon river flows from a chain of glacier-fed lakes on its north-east flank, while on the same slope, a short distance to the south, are the sources of the Huallaga river. The Huallaga valley is fairly wide as far as the gorge east of Huanuco in which it turns northward toward the Amazon and the valley floors of both the main streams and its tributaries in the plateau are well-irrigated and have a con siderable population. Below the Huanuco the Huallaga is a swif t flowing stream, with many rapids and gorges, and flows in a northerly direction through the foothills and low ranges for 18om. before it finally breaks through to the Amazon plain in the gorge or pongo of Aguirre, at lat. 6° 30' S. The Maranon flows in a deep trench and has only a sparse population of Indian com munities, which occupy the upper basins of its affluents and occasional sugar plantations at the mouth of the larger tributaries or at rare points where the valley widens.

The Cordillera Blanca, north of the Cordillera de Huayhuash has the same south-east to north-west trend as the latter but stands slightly to the west of its axis. These are both lofty snow-capped ranges with elevations that may be classed among the highest of the whole Andean highland, Cerro Carnicero in the Cordillera de Huayhuash being 21,76oft. high, and Cerro Huascaron in the Cordillera Blanca 22,18o feet. The valley of the Santa river (known as the Callejon de Huaylas) which separates the Cordillera Blanca from the Cordillera Negra (so-called because its crests do not exceed 16,5oof t. and, therefore, bear no permanent snow) is the most densely populated district of this section of the Andes. The river has its source in Lake Conococha in the high puna; flows, with a descent of 6,5ooft. in 6om., through a string of lacus trine basins of rich alluvial soil; and carries a large permanent stream through a gorge across the Cordillera Negra to the Pacific ocean.

Ecuador.

North of the Peru-Ecuador boundary the plateau is higher, rising to altitudes of over io,000ft., but the interoceanic divide continues narrow as far north as lat. 3° S. From there northward to the Nudo de Pasto, in southern Colombia, the Andes appear to be a single broad plateau, the top of which is in places 40m. wide, surmounted by irregularly distributed volcanic masses which rise to elevations from 11,000 to 12,000ft. above the plateau, and divide it into a number of separate basins (lioyas) at an average elevation of from 7,500 to 9,00o feet. It has beer the custom to describe this section of the Andes as consisting of two parallel cordilleras (a western cordillera and an eastern cor dillera, or Cordillera Real) with a high plateau between them which is divided into separate basins by cross ranges from the main cordilleras. Our present knowledge does not support this description, and it seems to be only the rather general north and south alinement of some of the outstanding volcanoes on the east and west borders of the basins and, particularly, of the Quito and Latacunga-Riobamba basins themselves, that has given rise to such a characterization. In the interior basins is concentrated the population of the plateau. Their rich volcanic soil and temperate climate had permitted their In dian inhabitants to attain, long before the conquest, a state of civilization far above the savage. They are in the rain-shadow of the mountain masses which enclose them on the east and west, and have, at the most, only moderate rainfall and, in some cases, require irrigation. By contrast, both outer slopes have very heavy rainfall at about the level of the basins and are covered with luxuriant vegetation, while above 10,00o to ii,000ft. they have very heavy rainfall on both sides. These high regions of heavy rainfall are the regions of the pdramos, cold, desolate areas on both the slopes of the cordilleras and on the cross ranges, with a covering of moss and other bog growths and enshrouded in almost perpetual mist.

The Cuenca basin, in the south, and the Latacunga-Ambato and Riobamba basins, in the central plateau, drain to the Amazon ; while the Alausi basin, north of the Cuenca basin, and the Quito and Ibarra basins in the north, drain to the Pacific. The Andes in this section slope steeply to both the Pacific and the Amazon plains. The streams from the basins have, therefore, steep gradi ents. Because of this 'and the steady supply of water from the adjoining mountaintop pdramos, the streams have eroded deep beds and few lakes remain, of which San Pablo, in the Ibarra basin, is the only one of considerable area. Erosion is slight at the parts of the basins farthest from the outlet through the mountains, but deepens rapidly as the outlet is approached. In the case of the Guaillabamba river, which drains the Quito basin, the total depth is over 3,000ft., of which 1,200 15 a steep-sided canyon.

An essential characteristic of the northern half of the Andes of Ecuador is the volcanoes which not only border the western side of the interior basins, from Chimborazo to the Nudo de Pasto in southern Colombia, and the eastern side from nearly lat. 2° S. to the Equator, but are grouped in many places between the basins and, in places, stand completely in them. They really owe more than half their height to the base of ancient rocks on which they stand, but so impressive are they that even conscientious ob servers have frequently exaggerated the steepness of their slopes. The principal volcanoes of the more westerly line of volcanoes are Chimborazo (2o,7ooft.), Canhuairazo (16,784), Quilatos Iliniza (17,4o5), Pichincha (15,718), Catacachi (16,292), Cimbal (15,715), Chiles (15,682) ; of the eastern, Altar (17,729), Tun guragua (16,689), Cotopaxi (19,498), Antisana (18,851), Cayambe (19,062). Sangay (17,47o) lies still farther to the east, and Sumaco (12,70o), which is believed to be a volcanic cone, lies far out on the eastern slope east of the Cordillera Guacamayo.

The eastern slopes of the Andes of Ecuador are little-known. Recent explorers have described the Amazon lowland as joining the base of the plateau at elevations averaging between 3,60o and 4,000ft., and sloping' gently eastward with a descent of i,000ft. in the first 7o miles. There has been no exploration of the southern part of the lowland. North of the Napo river recent explorations have revealed a number of short ranges and mountain masses of moderate elevation rising above the general level. Among these are the Cordillera Guacamayo and the Cordillera Galeras, with elevations reaching 8,000ft. and 6,000ft. respectively, and the cone-shaped Sumaco (i 2, 7oof t.) . Still farther north a short range on the`Equator called the Cordillera Lumbaki and two high tain masses north-west of Sumaco have recently been discovered.

The Cordilleras of Colombia.

In southern Colombia the Andes divide into three separate ranges: the western cordillera, or Cordillera de Choco ; the central cordillera, or Cordillera del Quindio; and the eastern cordillera, or Cordillera de Bogota. The valley of the Magdalena river which separates the eastern cordil lera from the central and part, at least, of the Patia and Cauca val ley between the central and western cordilleras are not narrow, eroded trenches, but broad tectonic depressions with a deep allu vial cover. The western cordillera forms the coast range of Colombia in the south and, to the north, extends on the east side of the valleys of the San Juan and Atrato rivers to the Caribbean coast. It is the lowest of the cordilleras, with elevations varying, in the southern section, from a little over 5,000f t. on the Pacific railway between Buenaventura and Cali, to 7,2ooft. west of Car tago and io,000ft. west of Popayan, and rising to elevations of i r ,000 and i 3,000f t. in the west and northwest of the province of Antioquia. The southern 2 5om. of the depression between the western and central cordilleras is occupied by a series of three broad longitudinal plains. The southernmost is drained by the Patia river to the Pacific ocean, while the northernmost, with a length equal to the combined length of the other two, is the famous Cauca valley and drains to the Magdalena by the Cauca river. The central plain, in which is the city of Popayan, contains the low divide between these two drainage basins. Its elevation (6,000ft.) is 3,000ft. higher than those of the Cauca and Patia plains, and its northern and southern borders are, therefore, deeply trenched. The Cauca plain has many aspects of an old lake basin filled with detritus. The city of Cali stands slightly above the level of the plain on a gently sloping alluvial fan. North of the Cauca plain the Cauca river flows by a series of gorges through a region of hills and valleys between the western and central cordilleras. Its lower plain is bordered by gradually disappearing spurs until it finally merges in the plains of the lower Magdalena river.

The central cordillera is the highest of the cordilleras of Colom bia and the shortest, its last spurs disappearing south of the junc tion of the Cauca river with the Magdalena, i 7om. from the Caribbean. It contains the only recent volcanoes in the Colombian Andes. Of these, Tolima (18,438f t.) and Ruiz (r 8,400f t.) are the most important. At lat. 5 ° 3o' N. the cordillera widens to form the plateau of Antioquia, whose last spurs extend as far as lat. 8° N., where they are lost in the plains of the Magdalena. This plateau appears to be a block of ancient crystalline rocks with in trusions of diorites and diabases divided into two parts by the deep trench of the Porce and Nechi rivers. The groups of high lands on both sides of the Porce river preserve, in their gently broken surfaces, traces of a very ancient cycle of erosion. The larger of these covers an area of about 1,200 sq.m. N. of Medellin and has an average elevation of 8,5ooft., with diminishing heights toward the north-east. The other extends south-east of Medellin, and contains the upper valley of the Nare or Negro river. Medel lin, the second city of Colombia, occupies an advantageous position in the Porce valley, where it widens out for a distance of about 8m. into a fertile alluvial plain.

The eastern cordillera is somewhat analogous to the Andes of Ecuador in that its most marked characteristic is a series of high intermontane plains or savannas, at a nearly uniform elevation of 8,000 to 9,000ft., of which the most important are at the head waters of the Bogota, Suarez and Sogamoso rivers on the eastern border of the Magdalena basin. Their subsoil is formed of beds of fine gravels and clays, with some beds of peat, and the majority contain shallow lagoons and bogs which are partly submerged dur ing the rainy season. Most of them have areas in their centres too wet for cultivation, so that the population is grouped about their edges. These high savannas were the centre of the pre colonial Chibcha civilization and now contain a third of the entire population of Colombia. The Savanna de Bogota is the largest and most densely populated of them, and has only a narrow strip of useless bog land in its centre. Bogota is built on its eastern edge.

East of Girardot and Honda the cordillera is a series of parallel ranges, running from south-south-west to north-north-east, of which only the easternmost, the Sierra de Cocuy (i7,5ooft.), reaches the level of permanent snow. North of the Sogamoso river the ranges have a north-south direction and in the latitude of Bucaramanga are united in a broad block. Farther north the central ranges stop and the western and eastern continue on, diverging toward the north and north-east as the Cordillera de Ocana, of which the Sierra de Perija forms the boundary between Colombia and Venezuela, west of Lake Maracaibo. North-east of the delta of the Magdalena river, the Sierra Nevada de Santa Marta, a triangular massif with steep slopes to the north and west, rises abruptly from the Caribbean coast to snowy summits i 7,000f t. high. It is separated from the Sierra de Perija by the valleys of the Cesar and Rancheria rivers.

A great many lakes exist in the cordillera east of Bogota and on both sides of the upper Sogamoso, but the pdramos have usually gentle forms and show traces of a long erosion period. It is, how ever, only on the high plains and the pdramos that the streams of the interior now meander. They are rapidly cutting the greater part of their courses and upon leaving the savannas they flow through deep canyons. The falls of Tequendama south-west of Bogota are the most striking example of the manner in which these streams have dissected the borders of the high plains, but the Suarez and Sogamoso rivers exhibit the same characteristics. Tributaries of the Orinoco river have cut through the eastern range to the neighbourhood of the savannas at points south of the Sogamoso and Tunja rivers. South of Bogota the eastern cor dillera is narrow. The Paramo de Sumapaz reaches elevations of i 4,000f t., and snow remains there throughout most of the year, but farther south the range is much lower. In the Sierra de Perija, which forms its northern extremity, the cordillera narrows to about 16m., and its highest crests do not exceed 8,000 feet.

Venezuela.

The Andes of Venezuela are separated from the eastern cordillera of Colombia by a depression of about 4,600f t. altitude, between the Tachira and Torbes rivers, and extend north east and east for about 5oom., across the northern part of the country. Transverse valleys cut them into three sections, the area and altitude of which decrease from west to east. The western most section, from the sources of the Torbes river to the sources of the Tocuyo river is known as the Cordillera de Merida. It is about i 6om. long and 3o to 5om. wide. Its snow-capped summits are all above i o,000f t., and in the Sierra de Santo Domingo and the Sierra Nevada de Merida exceed 14, 50o feet. In the Andes of Venezuela the coastal sierras have a west to east direction, and are separated from a second line of much lower mountains by a de pression in which are the basins of Lake Valencia and the Tuy river. Eastward from Puerto Cabello the range rises steeply from the coast. Its western end is only a little over 5,000ft. high near Nirgua, but it reaches 9,000ft. on the Silla de Caracas north-east of Caracas, and falls to 7,5ooft. west of Cape Codera. The Caracas valley, in which is the city of Caracas at an altitude of 2,600ft., is only 6m. from the sea, and is not a part of the central depres sion. It appears to be, rather, a subsided local block covered with alluvial soil.

East of Cape Codera the Gulf of Barcelona interrupts the moun tain zone for a distance of 8o miles. Only a narrow line of sand stones and limestones continues eastward from the interior range along the flat coast. Its altitude lowers rapidly from 4,000ft. to r,600ft., and it is completely interrupted by the Unari and Aragua rivers, which drain a part of the llanos to the Gulf of Barcelona. The Sierra de Cumana extends for 8om. between the Aragua river and the Gulf of Paria. Its interior is a massif of sandstones and folded limestones which rises to 7,5ooft. in the Cerro Turumiquire. The double peninsula of Araya and Paria is a narrow belt of crystalline schists, with an altitude of i,3ooft. west of the town of Carupano and a little over 3.000ft. at its eastern end near the Boca del Dragon, beyond which it is continued as the northern range of Trinidad island.

Geology.—The beds of the Primary series are found in the Andes in a longitudinal zone on the eastern side of the cordillera. They form the eastern part of the plateaus of Bolivia and northern Argentina, where the high anticlinal ranges are of Cambrian quartzites, while Silurian schists, surrounding the recent granite crests of the Cordillera Real, cut by deep valleys, and overlapped in the synclines by Devonian and Secondary red sandstones, form the base of the series. They are found, also as Silurian sandstones, schists and Devonian sandstones on the eastern side of the Chile Argentine cordillera as far south as lat. 3 5 ° S. and in the pre cordillera.

The zone of the Secondary beds is of much greater length. Breccias and porphyritic conglomerates are the most common f or mation, forming almost the whole of the western part of the An dean system. In Peru they are succeeded to the east by gray lime stones, which cover the greater part of the plateau. Still farther east, on the Maranon and upper Huallaga rivers, the ridges are of Secondary sandstones above Silurian schists. In Ecuador the Sec ondary conglomerates of the western side of the plateau rest directly upon the Andean gneisses and mica-schists, of which the eastern part is composed. In Colombia the central cordillera is formed of mica-schists, with almost no trace of sedimentary cov ering; while, in the eastern cordillera, even on the border of the plains of the Orinoco, the conditions which prevail farther south are completely reversed and the Secondary beds have their greatest development. The importance of the Secondary marine deposits in the Andes, especially in the Jurassic and Cretaceous series, seems to indicate that, during the Secondary period the region of the Andes was a geosyncline inundated by the sea be tween two continental masses, one of which occupied the place of the present Pacific ocean. The relations between the Andes and the vorland to the east is obscured by an enormous development of alluvial forms which partly cover the zone of contact between the lowlands and the cordilleras. It has been suggested that it is a rigid zone against which were exerted the forces to which the folding of the geosynclinal Andes was due.

The beginning of the folding of the Andes dates from the upper Cretaceous and continued during a part, at least, of the Tertiary. Tertiary marine forms are, therefore, lacking in the interior of the cordillera and are known only in eastern Patagonia, on the extreme edge of the cordillera. The Tertiary is represented in the greater part of the Andes by continental deposits without fossils, which furnish no precise information on the age of the folding. North of the Patagonian Andes as far as lat. 4° S. the western ranges contain Jurassic rocks and porphyritic rocks of similar age folded together. Both are of interest, the Jurassic because they are the only marine sediments of that age south of the Equator, the por phyritics because they are the most important evidence we have of volcanic activity in Mesozoic times. North of lat. 4° S. the Jurassic and porphyritic rocks are comparatively rare and the Primary rocks are absent. The majority of geological cross sections which have been made in the Andes indicate foldings of the simple Jura type rather than the complex type of the Alps. The cross-sections are, however, still comparatively few, and greater complexity may be revealed by further studies.

Evidence of the uplift of the Andes is found not only in the existence of Tertiary and Quaternary marine deposits on the Pacific coast and many examples of uplifted shore-lines, but also in the frequent occurrence throughout the cordillera of surfaces which could only have been formed at an altitude near sea-level. These areas are found even in the more humid parts, where the contrast between them and the deep ravines of the headwater . streams which are now invading them is most striking. The pene plane has been most completely conserved, however, in the des ertic Maritime cordillera of northern Chile and southern Bolivia. In the terracing of the valleys, particularly on the eastern side of the cordillera, there is evidence of a succession of vertical movements, interrupted by periods of rest ; while in the coastal terraces all the way from Paita to Antofagasta we find evidence not only of periods of rest but also of at least one subsidence fol lowed by an uplift, which is still in progress at the present time. In fact, along the entire western seaboard the region has suffered enormous disturbances in the past, while the frequent earthquakes that have occurred in recent years in many parts of the coastal region and the cordillera are evidence that these disturbances are still going on. The abrupt transition from high tableland to abys mal ocean depths that is characteristic of the entire coast indi cates the remarkably unstable condition of the region. In addition to the general or regional movements of uplift and subsidence, the existence of drop-faulting has been noted throughout the whole length of the cordillera. The upper Magdalena valley, the Cuzco basin, the Iglesia and Calingasta valleys between the cordillera and the pre-cordilleras of the Argentine republic, and the central valleys of Chile have all been thus explained.

The volcanoes of the Andes occur in three notable groups, southern Colombia and northern Ecuador, southern Peru and northern Chile, and the group of central Chile, Neuquen and Patagonia. All types are present, from ancient volcanoes almost completely destroyed by erosion and extinct volcanoes, with or without craters, but still fresh in form, to active volcanoes. They have played an important part in the levelling of the interior basins. Fluid lavas exposed appear only on the Patagonian pla teau. Elsewhere the lavas are chiefly of the viscous acidic type and, although accumulated to great depths in many places (as much as i zm. deep in the Cordillera de Vilca pampa in southern Peru), have not been removed far from the craters. The volcanic ash, however, has been carried away by erosion in such quantities that they have overcharged the streams and filled depressions and valleys to a great depth.

Climate.

It is not on the Equator but between lat. i 5 ° and 20° S. that the highest temperatures of the Andes are found, and we must go south of the Tropics to find mean annual temperatures comparable to those on the Equator. The abnormal depression of temperatures in the equatorial Andes is due to the heavy rainfall, and it is the lack of rainfall that raises the temperatures to the south. The effect of the higher temperatures of the drier Andes is to elevate the upper limit of agriculture and human occupation. In Colombia the highest agricultural zone is between 6,50o and i,000ft., while on the moister portions of the Bolivia altiplano agriculture is carried on over 3,000f t. higher. The rich agricul tural basins of Cuzco and Jauja, if they lay at the same elevation in the humid equatorial Andes, would not belong to the agricul tural regions at all. The lower limit of the colder agricultural zone is not, however, parallel to the upper limit, and as one leaves the Equator frosts occur at lower and lower levels. As a result, sugar cane, which grows at a level of 6,5ooft. and more in Ecuador and Colombia, does not reach above 3,000f t. in northern Argentina.

The lower limit of perpetual snow in the tropical Andes is between 15,00o and i 6,000 feet. On account of the increasing aridity, it rises rapidly south of the equatorial region to i 7,000f t. on the western ranges of the Peruvian Andes, above Lima, and to i 8,000 and 2o,000f t. in the summits which border the Puna de Atacama, to the north-east. South of the Puna de Atacama it lowers again to i 8,000f t. in the Famatina ranges, and to between 14,5oo and i6,5ooft. in the ranges of Juan and Mendoza. From these it falls rapidly to 6,5ooft. at lat. 3 7 ° S., 5,000f t. at lat. 40° S., and 2,3ooft. on Tierra del Fuego. In the tropical and sub tropical sections of the Andes the present glaciers rarely extend beyond the snow line. South of lat. 40° S., however, they have more extensive fields of supply, and come well down into the valleys. South of lat. 46° S. the ice-fields, though only a shrunken remnant of the ice-fields of the Glacial period, still form a con tinuous cap over the entire central zone of the cordillera. Through out the Andes there have been recognized, in front of the present glaciers and in regions to-day free of ice, glacial moraines, cirques, outwash plains and lakes confined behind glacial dams, that are proofs of several periods of extensive Quaternary glaciation.

Mining is an important industry in all of the Andean countries. In Chile and Bolivia it far exceeds all other industries in the value of its products. The most important mineral deposits are those of the younger igneous rocks which include the gold quartz lodes of Colombia, the silver-bearing copper deposits of Peru and Chile, and the tin-silver-bismuth deposits of Bolivia. The copper deposits, which are found in many sections of the Andes, but are particularly abundant in Chile and Peru and, to a lesser gree, in Bolivia, differ greatly in their geological occurrence and char acteristics. They occur most frequently as replacements of rock near intrusions of igneous material. At Portrerillos, Chile, they have replaced the easily soluble limestone. At the Braden mines, near Rancagua, Chile, and the Cuquicamata mines, the copper minerals have filled cracks and openings in less soluble rocks with little or no replacement. In many places, as at Cerro de Pasco, Peru, rich silver ores at the surface give place to copper ores al lower levels. The tin deposits of Bolivia are true fissure veins or disseminations filled by products of igneous intrusives. The only gems occurring in the younger igneous rocks are the emeralds of Colombia. Of the ores which occur mainly in associa tion with the ancient crystalline rocks, gold-bearing and silver bearing veins in the pre-Cambrian schists, gneisses and granites, in many places in the cordilleras, and iron ore, chiefly magnetite, in Chile, are the most important. Mineral deposits of economic value in the sedimentary rocks are not extensive. Chile, Peru and Colombia have extensive coal deposits, but they are of inferior quality as compared with foreign coals, and except in Chile, are so inaccessible that they have been worked but little, except for local use. Petroleum resources have been studied intensively, and indications of oil-bearing strata have been reported on the eastern border of the Andes all the way from Venezuela to Tierra del Fuego. The areas which are at present yielding the most valuable flows are the Maracaibo basin, in Venezuela, the basin of the Sogamoso river on the western border of the eastern cor dillera of Colombia, and the coastal region of northern Peru at Talara. Of the placer minerals, gold is the most important, and the extensive placer operations have of recent years been mainly confined to Colombia, particularly in the Choco district, where they yield considerable platinum.

From the standpoint of human occupation, the Andes are di vided into three sections—an unpopulated section extending north to the Puna de Atacama ; an arid central section from the Puna de Atacama to northern Peru, where mining is the chief industry; and a humid northern section, of which the chief industry is agriculture. The southernmost section has little mineral or agri cultural resources, and the population is, therefore, not in the mountain zone but on its borders; concentrated in irrigated dis tricts on the eastern side and more uniformly distributed in the central valleys of Chile on the western side. La the Puna de Atacama there is a sparse population of shepherds, and a still smaller element engaged in collecting salt from the salars for sale in the settlements on the mountain border. From there north to the Caribbean sea, however, the greater part of the population of the west coast and Caribbean republics, and, with few excep tions, the chief cities, are on the plateau. This populated section of the Andes contains a third of the whole population of South America. It consists of two sections of distinctly different char acter. The southern and more arid section is a region in which mining is the dominant industry, and practically the only source of export. All other industries are maintained for the purpose of supplying it with food, clothing and means of transportation. Railways which penetrate into the cordillera from the Pacific coast are constructed solely to transport the products of the mines to the nearest ports. The influence of the mining industry, since the mining districts are mainly on the high pdramos, has com bined with climatic conditions to elevate the altitudinal limits of human habitation to levels which are among the highest in the world. Since before the conquest the characteristic of this section of the Andes has been the establishment of small agricultural communities on the alluvial floors of high, sheltered valleys in close proximity to the mining centres which form the sole market for their produce. In the more humid Andes of Ecuador, Colom bia and Venezuela mining is of secondary importance, the chief industries are agricultural and the chief exports the products of the tropical levels—cacao in Ecuador; coffee, in Colombia and Venezuela. Here the internal trade is not the supplying of agri cultural products to a dominant industry but an exchange of products between zones at different levels and, therefore, of dif ferent climates.

Throughout the populated sections of the Andes, transportation between different parts of the plateau, between the plateau and the eastern lowlands, and, to a considerable extent, between the plateau and the Pacific coast, is still chiefly by pack-trail. The mule has come to be the principal pack-animal, largely supplant ing the llama of pre-conquest times, although the latter is still used locally in large numbers. In parts of Colombia the ox is still used for long trips with bulky commodities such as coffee and tobacco, while in the montanas of the eastern slopes and in the Choco region of Colombia, transportation is still dependent upon Indian bearers. Everywhere the arrieros, or muleteers, are an important element of the population. Often they are grouped in villages or particular districts where their work is the chief source of income. In Colombia it is estimated that they form a third of the population. Whereas the railway has supplanted the pack-train on most of the long routes from the plateau to the Pacific coast, the number of arrieros in the Andes as a whole has probably not been decreased, since the penetration of the railways into the plateau has increased the demand for foreign goods and, consequently, for means of distributing them from the railheads.

Except in the broad central plateau of Bolivia, the Andes offer such obstacles to road and railway construction that the west coast republics between Chile and Colombia are effectively divided into two lowland regions separated by the cordilleras. From the point of view of Government administration and national soli darity, railways across the Andes to connect the two lowland regions are of much importance; but so great would be the cost of construction and maintenance that, unless the economic interdependence of the two regions develops to a point where the cost is justified, it is probable that few of the many lines pro posed will be constructed. Such rail connections as the mountain zone now has are short lines built to connect important mining or agricultural districts with the nearest seaports. Only two railway routes cross the Andes. The most famous of these is the railway opened in 1911 between Los Andes, Chile and Mendoza, Argentine republic, which crosses the Chile-Argentine boundary by a tunnel 1 o,000f t. long and I o, 5oof t. above sea-level, and affords direct rail connection between Buenos Aires and Val paraiso. The recently completed railway from the Bolivia-Argen tine boundary to Uyuni connects on the Bolivian altiplano with three railways to the Pacific coast, and gives through routes from Buenos Aires to Antofagasta and Arica by rail, and by rail and steamer on Lake Titicaca, to Mollendo. (For railway lines east ward from the altiplano of Bolivia see BOLIVIA. For railways which penetrate the Andes in Peru, Ecuador, Colombia and Venezuela, see articles on these countries.) (R. R. P.)