Page:EB1911 - Volume 01.djvu/1017

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964
ANDESINE—ANDESITE

In the western chain, which is remarkable for its regularity, the highest peak is 11,150 ft., and the lowest pass 6725 ft. Colombia. 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 (13,360 ft.) and Chumbul (15,720 ft.). The volcanoes Campainero (12,470 ft.) and Pasto (14,000 ft.) are also in that zone. Farther north is the volcano Purace, which presents a height of 16,000 ft.; then come Huila (18,000), Santa Catalina (16,170), and Tolima (18,400), Santa Isabel (16,760), Ruiz (17,390) and Hervas (18,340). The eastern chain begins north of the equator at 6000 ft., gradually rises to the height of Nevado (14,146 ft.), Pan de Azucar (12,140 ft.), and in the Sierra Nevada de Cochi attains to peaks of 16,700 ft.

The snow-line of the Andes is highest in parts of Peru where it lies at about 16,500 ft. Its general range from the extreme north to Patagonia is 14,000 to 15,500 ft., but along the Patagonian frontier it sinks rapidly, until in Tierra del Fuego it lies at about 4900 ft.

Structure.—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.

Authorities.—John B. Minchin, “Journey in the Andean Tableland of Bolivia,” Proceedings of Geographical Society (1882); Paul Güssfeldt, Reise in den centralen chileno-argentinischen Andes (Berlin, 1884); John Ball, Notes of a Naturalist in South America (London, 1887); Alfred Hettner, Reisen in den colombianischen Andeen (Leipzig, 1888); “Die Kordillere von Bogotá,” Peterm. Mitteilungen, civ. (1892); Edward Whymper, Travels amongst the Great Andes of the Equator (London, 1892); Teodoro Wolff, Geografia y Geologia del Ecuador (Leipzig, 1892); E. A. Fitzgerald, The Highest Andes (London, 1899); Sir Martin Conway, “Explorations in the Bolivian Andes,” Geogr. Journ. xiv. (London, 1899); The Bolivian Andes (London and New York, 1901); Carl Burckhardt, Expédition géologique dans la région Andine, 38°—39° S. lat.; Leo Wehrli, “Cordillère argentino-chilienne, 4o° et 41° S. lat.,” Revista del Museo de La Plata (1899); F. P. Moreno, “Explorations in Patagonia,” Geogr. Journ. xvi. (1900); Hans Steffen, “The Patagonian Cordillera and its Main Rivers, between 41° and 48° S. lat.,” Geogr. Journ. (London, 1900); Paul Kruger, Die chilenische Reñihue Expedition (Berlin, 1900); Carl Burckhardt, “Profils géologiques transversaux de la Cordillera argentino-chilienne,” Anales del Museo de La Plata (1900); Argentine-Chilian Boundaries in the Cordillera de los Andes, Argentine Evidence (London, 1900); “South America; Outline of its Physical Geography,” Geogr. Journ. xvii. (1901); Maps of Cordillera de los Andes, Surveys of Argentine Boundary Commission; L. R. Patron, Cordillera de los Andes (República de Chile, Oficina des Limites) Santiago (Chile), 1903 et seq.); Sir T. H. Holdich, “The Patagonian Andes,” Geogr. Journ. xxiii. (1904).

ANDESINE, a member of the group of minerals known as plagioclase felspars, occupying a position in the isomorphous series about midway between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8); its chemical composition and physical characters are therefore intermediate between those of the two extremes of the series. Distinctly developed crystals or crystallized specimens are rarely met with, the mineral usually occurring as embedded crystals and grains in the igneous and gneissic rocks, of which it forms a component part. It occurs, for example, in the andesite of the Andes, from whence it derives its name.

ANDESITE, a name first applied by C. L. von Buch to a series of lavas investigated by him from the Andes, which has passed into general acceptance as the designation of a great family of rocks playing an important part in the geology of most of the volcanic areas of the globe. Not only the Andes but most of the Cordillera of Central and North America consist very largely of andesites; they occur also in great numbers in Japan, the Philippines, Java and New Zealand. They belong to all geological epochs, and are frequent among the Silurian and Devonian rocks of Britain, forming the ranges of the Cheviots, Ochils, Breidden Hills, and part of the Lake district. The well-known volcanoes, Montagne Pelée, the Soufrière of St Vincent, Krakatoa, Tarawera and Bandaisan have within recent years emitted great quantities of andesitic rocks with disastrous violence. No group of lavas is more widespread and more important from a geographical standpoint than the andesites.

They are typical intermediate rocks, containing on an average about 60 % of silica, but showing a considerable range of composition. Most of them correspond to the plutonic diorites, but others more nearly represent the gabbros. Their essential distinguishing features are mineralogical and consist in the presence of much soda-lime felspar (ranging from oligoclase to bytownite and even anorthite), along with one or more of the ferro-magnesian minerals, biotite, hornblende, augite and hypersthene. Both olivine and quartz are typically absent, though in some varieties they occur in small quantity. Orthoclase is more common than these two, but is never very abundant. The andesites have mostly a porphyritic structure, and the larger felspars and ferro-magnesian minerals are often visible to the naked eye, lying in a finer groundmass, usually crystalline, but sometimes to a large extent vitreous. When very fresh they are dark-coloured if they contain much glass, but paler in colour, red, grey or pinkish when more thoroughly crystallized. They weather to various shades of dark brown, reddish-brown, green, grey and yellow. Many of them are highly vesicular or amygdaloidal.

The older (pre-Tertiary) andesites are grouped together by many German, and formerly by British petrologists, under the term porphyrites, but are distinguished only by being, as a rule, in a less fresh condition. Apart from this there are three great subdivisions of this family of rocks, the quartz-andesites or dacites, the hornblende- and biotite-andesites, and the augite and hypersthene-andesites (or pyroxene-andesites). The dacites, a term first applied by Karl Heinrich Hektor Guido Stache (b. 1833) to quartz-bearing andesite of Transylvania or Dacia, contain