SOUTH AMERICA, with an area of over 7,200,000 square miles, comes fourth in size among the continents, being somewhat smaller than North America. It may, in a preliminary survey, be divided into three great physical regions : the Eastern Highlands of Guiana and Brazil, the Central Lowlands, and the Western Cordillera. The Eastern Highlands, which are cut in two by the valley of the Amazon, constitute the oldest part of the continent. They are the remains of a great mountain system which was worn down and covered, over large areas, with sandstone of different ages. These sandstones lie in horizontal strata, and the land, though much faulted, has the general appearance of a plateau in which the rivers have cut deep valleys. The whole region, therefore, is divided up into a number of tablelands, the steep escarpments of which, when seen from below, present the appearance of mountain ranges ; but the only highlands to which that term can properly be applied lie in the east, where the Serra do Mar, the Serra da Mantiqueira, and the Serra do Espinhaco rise to heights considerably above the level of the massif, which varies in elevation from 1,000 to 4,000 feet, the average height being probably about 3,000 feet. The Guiana massif is also divided into two parts by the Essequibo, the eastern one being an Archaean peneplain, while the western one is covered with sandstone in the more elevated districts.
The Central Lowlands may be divided into two regions, the first lying in the basins of the great rivers and the second being the Pampa-Patagonian area. The land surface of the former is generally flat and low, and is largely composed of sediment deposited by the rivers in the great arms of the sea, which, at one time or another, covered much of the area now occupied by the Central Lowlands. The basin of the Orinoco was occupied by a Tertiary sea which was gradually filled up during Quaternary times by the river and its tributaries ; and the land formed in this way now constitutes the llanos, or great plains, which lie to the north and west of the Orinoco. These llanos, which are cut up into mesas or tablelands, slope down gently towards the river and have nowhere a height of over 800 feet. The Carboniferous sea covered the whole of the lowlands from the mouth of the Amazon to that of La Plata, and during Tertiary times the valley of the Amazon was also under water, Tertiary rocks now being found in different parts of it. Since then, the remainder of the depression has been filled up by more recent deposits, and the slope of the river is now so gradual that, at a distance of 1,250 miles from its mouth, its level is only 260 feet above that of the sea. The lowland in the basin of La Plata was within quite recent geological times occupied by the Pampean Sea. It extends southwards from the Madeira-Paraguay divide in the Llanos de Chiquitos, and includes the Gran Chaco, an immense plain varying in height from 300 to 1,000 feet. Further south, the plains on the right bank of the River Salado and around the lower course of the Parana and Uruguay also fall within the La Plata Lowland.
The Pampa, which reaches to the Colorado, forms a plain sloping down gently towards the east, and consists of rocks of Tertiary age covered with glacial debris, clay, and loess. Only a small part of the drainage of this region reaches the sea, the river basins being generally closed. Further south lies the Patagonian Plateau, which has a height in the west of about 2,000 feet, and slopes gently in terraces towards the Atlantic. Like the Pampa, the land is built up of Tertiary deposits in which, however, there appear, here and there, the remains of an earlier mountain system. Glacial debris, and in places volcanic ash, cover the surface.
The Western Cordillera is the third great physical region into which the continent may be divided, and south of about the thirty second parallel its formation is comparatively simple. Along the coast runs a range with a lower elevation than the main Cor dillera further inland, while between the two there is a great valley. The coastal range is represented south of latitude 42° by a chain of islands, the trend of which is parallel to the mainland, while the central valley, which has been subject to great glaciation, is submerged. North of Aconcagua the main Cordillera consists of two ranges, an eastern and a western, between which lie elevated plateaus. These ranges run more or less in the same direction as the meridian as far north as the nineteenth parallel, where they turn towards the north-west, and finally coalesce in the Cerro de Pasco. The enclosed plateau falls into three divisions : that of the Argentine, which belongs to the Pampa area of inland drainage, that of Bolivia, whose waters make their way to Lake Titicaca, and that of Peru, which is in the basin of the Amazon. The desert of Atacama, between the western and the coastal ranges, continues to the north the central valley of Chile. Beyond the Cerro de Pasco these ranges run in a north-westerly direction almost to the Gulf of Guayaquil, where the western one disappears. The Northern Andes begin at Loja, south-east of the Gulf of Guayaquil, and between Loja and the Knot of Pasto two ranges enclose the elevated plateau of Ecuador, which is much broken up by transverse ridges. From the Knot of Pasto three ranges diverge and traverse Colombia, the most easterly entering Venezuela and finally running eastward along the coast of the Caribbean Sea.
The geological structure of the whole Cordilleran system varies greatly and is still very imperfectly known. South of the fortieth parallel granitic rocks prevail, and the coast ranges are believed to be Archan ; north of the latitude mentioned, the western range of the Cordillera consists chiefly of rocks of Jurassic and Cretaceous age on an Archan base, with eruptive materials interbedded, while the eastern range is built up of Archan and rocks with Cretaceous deposits in places. Volcanic rocks lie between the eastern and western ranges. The Northern Andes consist chiefly of Archan and Cretaceous rocks.