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Note that the geology in this article currently reflects views from the first decade of the 20th century. It is a good example of the descriptive geology of that time, but because it is pre-plate tectonics cannot be relied on when explaining the existence of these ranges and some of the structural commentary. If you update this article with more current information, please remove this disclaimer.
The Andes is a vast mountain system forming a continuous chain of highland along the western coast of South America. It is roughly 7000 km (4400 miles) long, 500 km (300 miles) wide in some parts (widest between 18° to 20° S latitude), and of an average height of about 4000 m (13,000 feet).
The connection of this system with that of the Rocky Mountains, which has been pointed out by many writers, has received much support from the discovery of the extensive eruptions of granite during Tertiary times, extending from the southern extremity of South America to Alaska.
The Andean range is composed of two great principal chains with a deep intermediate depression , in which, and at the sides of the great chains, arise other chains of minor importance, the chief of which is that called the Cordillera de la Costa of Chile. This starts from the southern extremity of the continent and runs in a northerly direction, parallel with the coast, being broken up at its beginning into a number of islands and afterwards forming the western boundary of the great central valley of Chile. To the north this coastal chain continues in small ridges or isolated hills along the Pacific Ocean as far as Colombia, always leaving the same valley more or less visible to the west of the western great chain.
Tierra del Fuego
Of the two principal chains the eastern is generally called Los Andes, and the western La Cordillera, in Colombia, Peru and Bolivia, where the eastern is likewise known as Cordillera Real de los Andes, while to the south of parallel 23°S in Chile and Argentina, the western is called Cordillera de los Andes. The eastern disappears in the centre of Argentina, and it is therefore only the Cordillera de los Andes that is prolonged as far as the south-eastern extremity of the continent.
The Cordillera de la Costa begins near Cape Horn, which is composed principally of crystalline rocks, and its heights are inconsiderable when compared with those of the true Cordillera of the Andes. The latter, as regards its main chain, is on the northern coast of the Beagle Channel in Tierra del Fuego, bounded on the north by the deep depression of Lake Fagnano and of Admiralty Sound . Staten Island appears to be the termination to the east. The Cordillera of the Andes in Tierra del Fuego is formed of crystalline schists, and culminates in the snow-capped peaks of Mount Darwin and Mount Sarmiento 2200 m(7200 feet), which contains glaciers of greater extent than those of Mont Blanc. The extent of the glaciers is considerable in this region, which, geographically, is more complex than was formerly supposed. Although in the explored portion of the Fuegian chain the volcanoes which have been mentioned from time to time have not been met with, there seem to have existed to the south, on the islands, many neo-volcanic rocks, some of which appear to be contemporaneous with the basaltic sheet that covers a part of eastern Patagonia. The insular region between Mount Sarmiento and the Cordillera de los Andes, properly so called, i.e. that which extends from Magellan Strait northwards, is not fully explored, and all that is known of it is that it is principally composed of the same rocks as the Fuegian section, and that the greater part of its upper valleys is occupied by glaciers that reach down to the sea amid dense forest.
As Admiralty Sound and Lake Fagnano bound the Cordillera to the north in Tierra del Fuego, so at the eastern side of the Cordillera in the southernmost part of the continent there is a longitudinal depression which separates the Andes from some independent ridges pertaining to a secondary parallel broken chain called the pre-Cordillera. This depression is occupied in great part by a series of lakes, some of these filling transversal breaches in the range, whilst others are remains of glacial reservoirs, bordered by morainic dams, extending as far as the eastern tableland and corresponding in these cases with transversal depressions which reach the Atlantic Ocean. Between the larger lakes, fed by the Andine glaciers of the eastern slope of the Southern Andes, are Lakes Maravilla , 260 km² (100 mile²), and Sarmiento , 67 km² (26 mile²), 51°S, which overflow into Last Hope Inlet ; Argentino ,1,476 km² (570 mile), 50°S; and Viedma , (450 mile²) 1,165 km², 49°30'S, which empty into the river Santa Cruz ; the fjordian Lake San Martin/O'Higgins , 49°S, and Lakes Nansen , 47 km² (18 mile²) ; Azara ,21 km² (8 mile²); and Belgrano , 47 km² (18 mile²) (), which are dependents of Lake San Martín/O'Higgins 984 km²(380 mile²)), and Lakes Pueyrredon/Cochrane 254 km² (98 mile²)) and Buenos Aires/General Carrera 1,813 km² (700 mile²)), which now overflow into the Pacific, through one of the remarkable inlets that are found throughout the Cordillera, the Calen Inlet , which is the largest western fjord of Patagonia. To the north of Lake Buenos Aires there is Lake Elizalde , which, while situated on the eastern slope, sends its waters to the Pacific Ocean, and Lakes Fontana 78 km² (30 mile²)) and La Plata 88 km² (34 mile²)), 45°S, which feed the river Senguerr , which flows to the Atlantic. Lake General Paz 171 km² (66 mile²)) on the eastern slope of the Andes, at 44°S, is the principal source of the Palena River , which cuts all the Cordillera, while Lakes Futalaufquen 52 km² (20 mile²)) Menendez 73 km² (28 mile²)), Rivadavia 26 km²(10 mile²)), and other smaller lakes, also situated between 43°30', and 42°30'S on the eastern slope send their waters to the Pacific by the river Futaleufu which cuts through the Andes by a narrow gorge. The waters of Lake Puelo 47 km² (18 mile)) likewise flow into the same ocean through the river of that name, which also cuts the Cordillera, and of which the principal affluent (the Manso River) likewise drains the waters of a system of small lakes, the largest of which, Lake Mascardi , measures 44 km² (17 mile²), which in comparatively recent times formed part of the basin of Lake Nahuel Huapi 536 km²(207 mile²)), 41°S. An extensive area of glacial deposits shows that a sheet of ice formerly covered the whole eastern slope to a great distance from the mountains. To the west another sheet reached at the same time the Pacific Ocean.
From the Strait of Magellan up to 52°S, the western slope of the Cordillera does not, properly speaking, exist. Abrupt walls overlook the Pacific, and great longitudinal and transversal channels and fjords run right through the heart of the range, cutting it generally in a direction more or less oblique to its axis, the result of movements of Earth's crust.
The mountains forming the Cordillera between Magellan Strait and 41°S 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 2000 m (6500 feet), while Mount Geikie is 2300 m (7500 feet) and Mount Stokes 2200 m (7100 feet). To the north are Mounts Mayo 2300 m(7600 feet), Agassiz 3200 m (10,600 feet), and Fitzroy (also called Chalten ), in 49°S (11,120 feet). The section from 52°to 48°S 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 at40 m (130 feet) at Lake Maravilla and gradually ascends to nearly 200 m (700 feet) at Lake San Martin/O'Higgins . Passing the breach through which Lake San Martin empties itself into Calen Inlet , in 48°S, is found a wide oblique opening in the range, through which flows the river Las Heras, fed by Lake Pueyrredon/Cochrane, which is only 125 m (410 feet) above the sea-level to the east of the Andes, while Lake Buenos Aires/General Carrera , immediately to the north, is 220 m (710 feet). The Andes continue to be to the west an enormous rugged mass of ice and snow of an average height of 2700 m (9000 feet), sending glaciers to all the eastern fjords.
Mount San Lorenzo , detached from the main chain in the Pre-Cordillera, is 3600 m (11,800 feet) high. Mount San Valentin 3900 m (12,700 feet) is the culminating point of the Andes in the region extending from 49° to 46°S, 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, 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 2200 m (7150 feet). 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 900 m (3000 feet) 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, across another water gap, continuing the range to the north with high peaks, as Mount Alto Nevado 2240 m (7350 feet) and Mount Cacique 2100 m (7000 feet). 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. To the north, 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 2100 m 7000 feet high; and Torrecillas 2250 m (7400 feet), Ventisquero 2300 m (7500 feet), and Tronador 3400 m (11,180 feet); 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 , Futaleufu, Puelo and Manso cut the two chains, while the rivers Renihue , Bodadahue and Cochamo have their sources in the main eastern ridge. Mention has been made of active volcanoes in 51°, 49°, and 47°S, but these have not been properly located. The active volcanoes south of 41°, concerning which no doubt exists, are the Huequen , in 43°, 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 130 km² (50 mile²)), to which the access is easy and of which the scenery is of surpassing beauty. Between 41°and 38°S, among other smaller lakes, are Lakes Traful 117 km²(45 mile²)), Lacar 83 km²(32 mile)), 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 40 km²(15 mile²)), Huechulafquen 117 km² (45 mile²)), and Lake Alumine 54 km² (21 mile²)). The volcanoes of Lanin 3700 m (12,140 feet), Quetrupillan 2800 m (9180 feet), Villarrica 3200 m(10,400 feet), Llaima 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 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 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. Outflows 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.
Chile-Argentina from 38°S northward
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 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 4700 m (15,317 feet) belongs to a lateral eastern ridge. The principal peaks between this and Mount Tupungato at 33°S are: Mount Cochico 2100 m (8255 feet), Campanario, 4000 m (13,140 feet), Peteroa 4050 m (13,297 feet), Tinguiririca, Castillo 5000 m (16,535 feet), Volcano Maipu 5350 m (17,576 feet), Alvarado 4450 m (14,600 feet), Amarillo 4670 m (15,321 feet), Volcano San Jose 6050 m(19,849 feet), Piuquenes 5430 m (17,815 feet), and Volcano Bravard 5980 m (19,619 feet).
North of Maipu volcano, ascended by R. P. Gussfeldt 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 2700 m (9000 feet) high; and in that part of the Cordillera are situated the highest passes south of 33°S, one of which, the Piuquenes Pass, reaches 4060 m (13,333 feet), whilst the easiest of transit and almost the lowest is that of Pichachen 1980 m (6505 feet), which is the most frequented during winter. Mount Tupungato reaches 6800 m (22,329 feet), 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 5680 m (18,645 feet) and Mount Polleras 6180 m (20,266 feet). The Pircas Pass is situated at a height of 5170 m (16,962 feet). The gaps of Bermejo and Iglesia, in the Uspallata road, the best known of all the passes between Argentina and Chile, are at 3970 m (13,025 feet) and 4100 m (13,412 feet) altitude respectively, while the nearest peaks, those of Juncal and Tolorsa, are 5900 m and 6100 m(19,358 and 20,140 feet) high.
Mounts Tupungato, Aconcagua 7130 m (23,393 feet) and Mercedario 6700 m (21,982 feet) 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 4500 m(14,803 feet) and Los Patos or Valle Hermoso 3580 m (11,736 feet), 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 3000 m (10,000 feet), but most of them between 4000 m and 4300 m (13,000 and 14,000 feet). The pass of Quebrada Grande is 3800 m (12,468 feet) in altitude; Cencerro, 3950 m (12,944 feet); Mercedario, 4030 m (13,206 feet); Ojota, 4360 m (14,304 feet); Pachon, 4400 m (14,485 feet); while Gordito is 3150 m (10,318 feet). Farther north the passes are higher. Barahona Pass is 4600 m (15,092 feet); Ternera,4850 m (15,912 feet); San Lorenzo, 5000 m (16,420 feet), while the peak of the volcano reaches 5530 m (18,143 feet); Mount Olivares, 6240 m (20,472 feet); Porongos, 5940 m (19,488 feet; Tortolas, 6130 m (20,121 feet); and Potro, 5900 m (19,357 feet).
As far as 28°S 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 4580 m (15,026 ft), Penas Negras 4400 m (14,435 ft), Pircas Negras 4150 m (13,615 ft), La Gallina 4950 m (16,240 ft), Tres Quebradas 4735 m (15,535 ft), and Aguita 4720 m(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 he considered as amongst the highest of the whole range. Mount Aguita is 6000 m (20,600 ft), and the culminating peak of those of Tres Cruces reaches 6900 m (22,658 ft). To the east of the eastern longitudinal valley, at 27°S, 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 6400 m (20,998 feet), Mount Bonete 6700 m (21,980), Mount Reclus 6300 m (20,670), Mount Pissis 6750 m (22,146), Mount Ojo del Salado 6600 m (21,653), and Incahuasi 6620 m (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:
- Juncal, Panteon de Aliste, Azufre or Listarria 5680 m (18,636 feet)
- Llullaillaco 6620 m (21,720)
- Miniques 5950 m (19,357)
- Socompa 6080 m (19,948)
- Licancaur 6000 m (19,685)
- Viscachuelas 6280 m (20,605)
- Tapaquilcha 5950 m (19,520)
- Oyahua 5860 m (19,242)
- Ancaquilcha 6200 m (20,275)
- Olca 5840 m (19,150)
- Mino 6130 m (20,112)
- Sillilica 6430 m (21,100)
- Perinacota 6380 m (20,918)
- Sajama 6542 m (21,463)
- Tacona 6020 m (19,740)
- Misti 5800 m (19,029)
To the east closes in the intermediary high plateau which begins at 28°S in Argentina. The principal peaks of the Bolivian Andes and its prolongation from south to north, are Famatina , in the centre of Argentina, 6200 m (20,340 feet), Languna Blanca 5600 m (18,307), Diamante 5500 m (18,045), (Cachi 6000 m( 20,000), Granadas, Lipez 6000 m (19,680), Guadalupe 5760 m(18,910), Chorolque 5630 m (18,480), Cuzco 5465 m (17,930), Enriaca 5700 m (18,716), Junari 4940 m (16,200), Michiga 5300 m (17,410), Quimza-Cruz 5570 m (18,280), Illimani 6460 m (21,190) and Sorata 6550 m (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 , (4700 m (15,400 feet) 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 5760 m (18,898 feet), San Pedro 5700 m (18,701), Antuco 5800 m(19,029), Antofalla 6100 m (20,014), Rincon 5450 m (17,881), Pastos Grandes 5350 m(17,553), Zapalegui 5350 m (17,553), Suniguira 5870 m(19,258), Tahue 5320 m(17,458); volcanoes which have been elevated from a lncustrine 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 are continued in Peru. The Cordillera Blanca rises abruptly from the Pacific coast escorted by the Western range known as Cordillera Negra and the Eastern one called Cordillera Oriental . The western, which reaches an altitude of about 3000 m (10,000 feet), 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.
Peaks reaching over 5000 metres in the Peruvian Andes include:
Ecuador has one of the world´s greatest concentration of volcanoes. There are more than thirty, of which at least eight are considered to be active. The Eastern Range, sometimes called the Cordillera Real (Royal Range) is older and higher than the Western Range. Several smaller peaks, not of volcanic origin, are found on the eastern slopes of this range towards the Amazon basin. Further to the east there are other isolated mountains of volcanic origin completely isolated from the Eastern Range by jungle, being the most important Reventador (3485 m) and Sumaco (3900 m).
Cotopaxi 5897 m (19,348), Antisana 5704 m (18,715), Cayambe 5790 m (18,997) are in the eastern range, with many other peaks over 5000 m (16,000 feet) which still contain glaciers. Sangay 5230 m (17,160 feet), under the equator, according to Wolf, appears to be the most active volcano in the world. Pichincha 4800 m (15,804 feet) and Cotocachi 4970 m (16,297 feet) 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.
The peaks reaching over 5000 meters in the Ecuadorian Andes, in order of altitude are:
- Mount Chimborazo 6310 m
- Cotopaxi 5897 m
- Cayambe 5790 m
- Antisana 5704 m
- El Altar 5320 m
- South Iliniza 5263 m
- Sangay 5230 m
- North Iliniza 5126 m
- Carihuairazo 5020 m
- Tungurahua 5016 m
In the western chain, which is remarkable for its regularity, the highest peak is 11,150 feet, and the lowest pass 2050 m (6725 feet). The central chain, separated from the western chain by the valley of the Cauca and from the eastern by the valley of the Magdalena, is unbroken; it is the more important owing to its greater altitudes and is of volcanic character. To the south, near the equator, are Mounts Arapul 4070 m (13,360 feet) and Chumbul 4790 m (15,720 feet). The volcanoes Campainero 3800 m (12,470 feet) and Pasto 4270 m (14,000 feet) are also in that zone. Farther north is the volcano Purace, which presents a height of 5000 m (16,000 feet); then come Huila 5500 m (18,000), Santa Catalina 4930 m (16,170), and Tolima 5600 m (18,400), Santa Isabel 5100 m (16,760), Ruiz 5300 m (17,390) and Hervas 5590 m (18,340). The eastern chain begins north of the equator at 1800 m (6000 feet), gradually rises to the height of Nevado 4300 m(14,146 feet), Pan de Azucar 3700 m (12,140 feet), and in the Sierra Nevada de Cochi attains to peaks of 5100 m 16,700 feet.
The snow-line of the Andes is highest in parts of Peru where it lies at about 5000 m (16,500 feet). Its general range from the extreme north to Patagonia is 4300 m to 4700 m (14,000 to 15,500 feet), but along the Patagonian frontier it sinks rapidly, until in Tierra del Fuego it lies at about 1500 m (4900 feet).
Warning: this section should be treated with caution, it is based on pre-plate tectonics, please remove this message if you update this article to modern geological terminology
The structure of the Andes is least complex in the southern portion of the range. Between 33° and 36°S the chain consists broadly of a series of simple folds of Jurassic and Cretaceous beds. It is probably separated on the east from the recent deposits of the pampas by a great fault, which, however, is always concealed by an enormous mass of scree material. The Cretaceous beds lie in a broad synclinal upon the eastern flank, but the greater part of the chain is formed of Jurassic beds, through which, on the western margin, rise the numerous andesitic volcanic centres. There is no continuous band of ancient gneiss, nor indeed of any beds older than the Jurassic. There is very little over-folding or faulting, and the structure is that of the Jura mountains rather than of the Alps. The inner or eastern ridge farther north of Argentina consists of crystalline rocks with infolded Ordovician and Cambrian beds, often overlaid unconformably by a sandstone with plant-remains (chiefly Rhaetic). In Bolivia this eastern ridge, separated from the western Cordillera by the longitudinal valley in which Lake Titicaca lies, is formed chiefly of Archaean and Palaeozoic rocks. All the geological systems, from the Cambrian to the Carboniferous, are represented and they are all strongly folded, the folds leaning over towards the west. West of the great valley the range is composed of Mesozoic beds, together with Tertiary volcanic rocks. (The Cordillera of Argentina and Chile is clearly the continuation of the western chain alone.) In Ecuador there is still an inner chain of ancient gneisses and schists and an outer chain composed of Mesozoic beds. The longitudinal valley which separates them is occupied mainly by volcanic deposits. North of Ecuador the structure becomes more complex. Of the three main chains into which the mountains are now divided, the western branch is formed mostly of Cretaceous beds; but the inner chains no longer consist exclusively of the older rocks, and Cretaceous beds take a considerable share in their formation.
The great volcanoes, active and extinct, are not confined to any one zone. Sometimes they rise from the Mesozoic zone of the western Cordillera, sometimes from the ancient rocks of the eastern zone. But they all lie within the range itself and do not, as in the Carpathians and the Apennines, form a fringe upon the inner border of the chain.
The curvature of the range around the Brazilian massif, and the position of the zone of older rocks upon the eastern flank, led Suess to the conclusion that the Andes owe their origin to an overthrust from east to west, and that the Vorland lies beneath the Pacific. In the south Wehrli and Burckhardt maintain that the thrust came from the west, and they look upon the ancient rocks of Argentina as the Vorland. In this part of the chain, however, there is but little evidence of overthrusting of any kind.
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