Atlantic coast, is the mighty Amazon itself, whose system drains the great valley included between the plateau of Brazil on the southeast, the plateau of Guiana on the north, the Andes on the west, and the highlands of the Cordillera Geral and Matto Grosso on the south, thus embracing about one-third of South America. The Amazon pours a vastly greater quantity of water into the ocean than any other river on the globe. The plateau of Brazil is drained chiefly by the Tocantins, which flows to the north and empties into the Pará estuary; a number of smaller streams which flow northeast and enter the Atlantic between the mouth of the Pará and Cape St. Roque; the São Francisco, which has a generally northeastern direction, and a few smaller streams which drain the short eastern slopes along the whole extent of coast between the mouth of the São Francisco, lat. 10° S., and the estuary of the Plata, lat. 35° S. The Plata, which receives the waters of the Paraná, Paraguay, and Uruguay, drains the whole of the south central part of South America, from the Amazon watershed in lat. 15° S. to lat. 35° S., and embraced between the coast sierra on the east and the Andes on the west. This great river system has been compared with the Mississippi River system, with which it has certain features in common. South of the Plata are a number of rivers, including the Colorado, Negro, and Chubut. On the Pacific coast the drainage is effected by short, torrential streams scarcely worthy the name of river. See Amazon; Orinoco, etc.
Geology. The geological history of North America, considered in a broad way, is not complex. The oldest part of the continent, the first to be elevated above the sea, is the northeastern section, including the Adirondacks of New York and the Laurentian Highlands of Canada, and a region about the Great Lakes, together with a southward projection just east of the Blue Ridge in the Southern States. This is the Archæan area. From this, as a nucleus, the continent grew westward, as is indicated by the surface formations, which become successively more recent. The eastern portions of the Appalachians are in great part composed of Silurian beds. The plateau forming the western part of the system is Carboniferous, which formation also underlies much of the Mississippi Valley. The great plains which form the eastward slope of the Cordilleran plateau are floored, in westward succession, by Triassic, Cretaceous, and Tertiary beds.
The mountains of the Cordilleran system are mainly of recent formation, and show strata of all ages, as they have been much disturbed by uplift, and the beds exposed by subsequent erosion. Upon the mountains granitic rocks largely predominate, as the stratified beds which formerly covered them have been eroded away, while in very many cases these stratified beds still remain on the flanks of the ranges, as hog-back ridges. The valleys are often partially filled with detritus from the mountains. In this region many great areas have been covered by outflows of lava, some of them in very recent times. The regions bordering the coasts of the Atlantic and the Gulf of Mexico are floored with Cretaceous and Tertiary deposits, indicating their comparatively recent uplift. There are no active volcanoes in the United States proper or in Canada. Within historical times eruptions have been reported on the coast of Alaska, and several peaks on that coast are still smoking. In Mexico, Central America, and the West Indies there are many active volcanoes. See Geology under United States; Canada, etc.
South America.—The eastern highlands are of Archæan and Paleozoic formations, with a superimposed layer of sandstone. No subsequent submergence has occurred, and no folding has taken place since Paleozoic times, so that no recent marine deposits have been made, and the deep valleys are due to erosion rather than to irregular faulting, the rock layers lying horizontally. These eastern highlands are but the remains of a great mountain system which has been worn away to the existing condition in the filling up of the plains below, to which they have contributed their material. The western highlands (see Andes), while of more recent origin than the eastern, are made up of ranges differing in geologic age. Most of the great peaks of the Andes are of volcanic origin, and many of them are still active, or have been eruptive in recent and historic times. The lowlands east of the Andes are, so far as known, floored with Tertiary deposits, with broad bands of alluvium bordering the larger streams. See Geology under Brazil; Argentina, etc.
Glaciation. In recent geologic times nearly all of Canada and much of the United States was covered by a great sheet of ice, the Laurentian glacier. In the United States it covered New England and New York, extended southward to the Ohio River, and westward to the Missouri. Throughout this area the surface has been modified by erosion and deposition by ice. Stream courses have been changed, countless lake basins have been formed, and the surface covered with drumlins, kames, and other morainal deposits. In the northern part of the Cordillera, evidences of former glaciation are everywhere abundant, and in the higher ranges many glaciers still exist. Indeed, in the mountains on the Alaska coast, where the precipitation is profuse, there are many glaciers of great magnitude, some of which reach the sea. The Muir Glacier covers fully a thousand square miles, and there are others of equal size. Even these great glaciers, however, are but the much reduced relics of far larger ones, which covered the coast and eroded the fiords which intersect it.
In South America the glacial history, so far as known, is confined to the Andes. Most of the higher peaks, even those under the equator, have glaciers upon their upper slopes, while in the southern portion of the system glaciers are extremely abundant, and the configuration of the land shows that in past time they covered it, lying in every gorge and fiord, which are evidently products of ice erosion.
Climate. Stretching from the south temperate zone through the tropics to the north polar zone, America has many climates, dependent upon latitude, prevailing winds, and the distribution of the relief features. The main body of North America is principally within the region of the anti-trades or prevailing westerlies. These winds give to the western coast of the United States and Canada, and to southeastern Alaska, an insular climate, with great uniformity of temperature and a heavy rainfall. Their influence extends inland but a short distance, owing to the mountain ranges which border this coast, and the rest of the United States and Canada have a
