Most of the lakes of South America are mountain lakes in the Andes or along its base. Of the Andean lakes, Titicaca and Junin are the largest. (See ANDES.) The glacial lakes along the eastern base of the Andes in southern Argentine and those of the Llanqui hue district of southern Chile are described in the articles on the Andes, Argentina, Chile, and Patagonia. There are many lakes scattered over the flood plains of the great rivers but these are mostly phases of river development. Along the east coast there are occasional lakes, like the Lagoa dos Patos of southern Brazil, of brackish water, produced by depression of the coast and the closing of the mouths of estuaries formed thereby or by barrier beaches thrown up by the sea. Lake Maracaibo is a large narrow necked bay like that of Rio de Janeiro rather than a true lake.
The most extensive old land areas (Pre-Cambrian) of South America are found in the plateaux of Guiana and Brazil where the ancient crystalline granites and gneisses have been laid bare over large areas. The Brasilides, so-called, are a series of Pre Cambrian and Pre-Devonian folds of which traces are found on the western border of the Brazilian plateau and as far south as the Sierras in the Argentine provinces of Cordoba and La Rioia. These old land areas formed the larger units in a chain of islands of which the old rocks of the Falkland islands are a part. On their shores were laid down the sedimentary beds of the Cambrian seas. At the close of the Cambrian period these Pre-Cambrian islands with their Cambrian sediments were uplifted and many of them united to form continental masses of considerable extent. During the Silurian period, however, seas still covered a large area including the present basin of the Paraguay river and a large part of the present basin of the Sao Francisco and extending northward between the axis of the Andes and the Matto Grosso highlands and eastward by way of the region now occupied by the lower Amazon valley.
During the Devonian period further uplift and the consequent disappearance of the Silurian seas from the basin of the Sao Francisco increased the area of the continental mass; but in early Carboniferous times the sea still covered a narrow strip through the lower part of the Amazon valley and part of what is now the Andes south of the Equator. During the Permian the basin of the Paraguay and the south-east coast of Brazil were covered with lagoons and swamps in which here and there coal beds were laid down. Continental Permian deposits have been found at a number of places in the southern part of the Brazilian pla teau, in northern Argentine, and in the Falkland islands. Analo gies between Permian fauna and flora of South America and South Africa are accepted as evidence of a Permian continental mass, known as Gondwana, connecting South America with South Africa and India, the western limits of which are formed by the Argentine pre-cordillera between San Juan and Jachal where the Permian layers are strongly folded. These folded Permian layers are continued south-eastward by the sierras of the Argentine prov ince of Buenos Aires and are known to geologists as Gondwomides. South of them, between the Negro and Chubut rivers, is the Pata gonian massif, which consists, like the Brasilides, of very old rocks and may have been connected with an ancient Antarctic conti nent. Against this massif and between it and the Andes, and ante
dating the latter, is a series of Cretaceous folds known as Pata gonides. At the close of the Devonian there was widespread vol canic activity that covered with lavas large areas in what is now Paraguay and Brazil.
The importance of Jurassic and Cretaceous marine deposits in the Andes is evidence that, during the Mesozoic period, the region now occupied by the Andes was a great geosyncline occupied by the sea between two continental masses, one of which has since subsided to form the Pacific ocean. This geosyncline connected at the north end with a sea that reached from the region of the present West Indies to the present Mediterranean and persisted through the Mesozoic and into the early part of the Tertiary. The South America-South Africa land mass (Gondwana) continued from the Carboniferous tc, the Cretaceous and may have persisted into the Tertiary. There is no evidence on the present coast of Brazil of invasion by the sea between the Devonian and the Creta ceous, while the first invasion of the east coast of Patagonia and, consequently, the formation of the South Atlantic ocean dates from the Upper Cretaceous. The folding of the Andes also began in the Upper Cretaceous and continued into the Tertiary, although how far into the Tertiary is not known.
Great changes took place in the Tertiary period. The continent rose considerably higher than its present elevation and the con tinental land mass was, consequently, much larger than at present, including such coastal islands as the Abrolhos islands on the east coast of Brazil and the Falklands. It is possible, also, that, during this period, South America had land connection with New Zealand and Australia by way of the Antarctic continent or through the south Pacific ocean. The formation of the Isthmus of Panama with the resulting migration of North American faunal species to South America also dates from the Tertiary period; but its for mation could not have taken place until the Pliocene, since the analogies between the marine fauna beds of Navidad (Chile), which date to the end of the Miocene, and European fauna of the same date can only be explained by the existence of free com munication between Europe and the west coast of South America by way of the Tertiary Mediterranean up to the end of the Miocene. The elevation of the Andes during the Tertiary period was due only in part to the elevation of the continent as a whole. It was in large part assisted by the folding of the rocks and the outpouring of lavas and the accumulation of other volcanic mate rials in the neighbourhood of the vents. Nor was this volcanic activity confined to the Andean region. It extended into Venezuela and the islands along the north coast, the Patagonian plains, the highlands of the Parana basin, and as far east as the islands of Fernando de Noronha. Toward the end of the Tertiary subsidence again occurred and the sea entered the Amazon, Orinoco, Cauca, and Magdalena valleys and the Maracaibo basin, and again made an island or groups of islands of the Guiana highlands.