Page:1902 Encyclopædia Britannica - Volume 25 - A-AUS.pdf/412

 366

AMERICA

but on the lower part of the Rio Araguay sedimentary beds form low hills, and extend along both the north and south sides of the Amazon valley for hundreds of miles. These beds have generally been regarded as Tertiary, but they have yielded no fossils, and their age is not known. There are fossiliferous Tertiary beds of brackish water origin in the Upper Amazon valley covering a large area, extending westward from Tabatinga, on the Brazilian frontier. Tertiary beds form a narrow belt along the coast of Brazil, probably from the Amazon valley to near Victoria in the state of Espirito Santo. This belt is a hundred miles wide at some places, while at others it is only a fraction of a mile in width, or is entirely wanting. At and for some miles north of Pernambuco the fossiliferous portions of these beds show' them to be of marine origin; south of Pernambuco, on the coast of Alagoas and in the state of Bahia, they are of freshwater origin. The sandstone reefs of the Brazilian coast, the elevated beaches of Bahia and Rio de Janeiro, and the older parts of the coral reefs, may possibly be of late Tertiary age. Beds knowrn to belong to the Tertiary spread over a wide area in the valley of the Rio de la Plata, and over the lowlands of Eastern Argentina and Patagonia. On the west coast Tertiary rocks form much of the narrow plain between the Andes and the ocean, and there are besides isolated Tertiary basins in the high parts of the mountains. Some of the eruptive rocks of the Andes are post-Tertiary, while others are pre-Tertiary. The eruptives of the Parana basin are newer than the Trias, but their age has not as yet been more precisely determined. All through the highlands of Venezuela, the Guianas, and Brazil, the older Pakeozoic rocks are extensively cut by eruptives. Along the north-eastern and south-eastern sides of the continent, most of these eruptives are preTertiary. In only one place—on the Abrolhos Islands —is there a dyke known to be in rock of Tertiary age, while the island of Fernando de Noronha is the only place on the eastern side of the continent at which a volcano of late geologic age is known to have existed. The earliest land areas of the South American continent are approximately represented by the areas of old granites and gneisses. The largest of these is along the eastern

[south

border of the continent, reaching, with few interruptions, from a short distance south of the mouth of the Amazon river to the mouth of the Rio de la Plata. To the west of this land area there were probably many islands over what is now the interior of coatiaent. the continent. North of the present Amazon valley there wus another and smaller land area or islands, where are now the highlands of North-eastern Brazil, South-eastern Venezuela, and Guiana. Another land area, or perhaps more correctly a string of islands, extended from the Falkland Islands the entire length of the west side of the continent. Against these shores the Cambrian or oldest sediments of the continent were laid down. At the close of Cambrian times elevations of the continent added large tracts to the land, but the Silurian seas still covered what is now the Paraguayan basin, and extended from the Serra do Mar on the Brazilian coast to the axis of the Andes, on the Pacific side of the continent. During the Devonian period the land area of the continent was greatly increased in the Brazilian highlands, but the Devonian seas still covered the interior of the continent, extending from the Serra do Mar on the east to the Andes of Bolivia and Peru on the west, and forming a mediterranean sea over twro hundred miles wide and several hundred miles long in the Lower Amazon valley region. In Carboniferous times the Amazonian inland sea was only slightly changed, but in the southern part of the continent the land areas increased enormously. In Southern Brazil there were peat bogs, wdiile farther north and west were shallow brackish water bays or estuaries. But little is known of the Triassic and Jurassic history of the continent. The Cretaceous history of the east coast is preserved in only a few fragments of sedimentary areas. About the mouth of Rio Sao Francisco the land sank, carrying beneath the water large portions of the states of Sergipe and Alagoas. What are now the highlands of the interior of Piauhy, Cearii, Pernambuco, and Parahyba were then under water. The island of Trinidad was two-thirds under water at that time. On the west coast the land was lower than at present, and the ocean washed the base