On the west they slope through rolling up !an& to the most peculiar feature of the North American surface, entirely unlike any other part of the globe, the prairies (called savannahs in English books, but never in American speech): a block of undulating plains of enormous ex tent in the centre of the Mississippi basin, com posed mainly of dark, rich loam from a foot to several feet deep over a bottom of clay, and of such composition that natural tree-growth is largely absent even where rainfall is plentiful, though grass and other crops grow abund antly. Often this will be as level as a floor for scores of miles together, and the eye sweeps uninterruptedly over a grassy 'ocean to the horizon. On the west of this extend td the Rockies lands often as flat as the prairies, but lacking their individual trait and called plains instead. The same features are repeated in northwest Canada from Manitoba to the Rockies. Locally they are known as the "Great Plains.' In South America the eastern chain- is simi larly formed of several parallel ranges follow ing the Brazilian coast, on a wide plateau, a reduced copy of it running through the Guianas. The whole centre is an immense plain sloping sharply up to the Andes; but in place of the vast treeless flats of the northern continent there is the most enormous forest of the world, two and a half millions of square miles in ex tent. North of this, however, are considerable plains along the Orinoco called llanos. Below the range the country is a great grassy steppe, rather ill-watered, called pampas, and extending through Argentina and Patagonia.
Geology and Mineral North North American continent may rather easily be divided into a few great geo logical units, the general features of which are simply and briefly stated below. These divi sions may be termed the Canadian Shield, the Coastal Plain, the Eastern Highland, the Cen tral Lowland and the Western Highland.
Canadian The nucleus of the con tinent consists of a great U-shaped area en closing Hudson Bay, and extending in the Lake Superior region southward a short distance into the United States. It also sends a long arm southward into the Piedmont region, along the east border of the Appalachian Highland. This large area consists almost wholly of highly folded Pre-Cambrian (q.v.) (Archean (q.v.) and Algonkian (q.v.)) rocks, granites, schists, quartzites, marbles, etc., of great age; and it was by the erosion of this mass, largely, that the sediments were derived to build up the other portions of our continent. Particularly in Canada, this area contains extremely valuable deposits of gold (Porcupine District), silver and cobalt (Cobalt District), and copper and nickel (Ludbury District). In Michigan it also contains famous mines of metallic copper ; and in Michigan, Wisconsin and Minnesota are great iron ranges, among which the Marquette and Mesaba are the best known. The eastern
part of this large region is known as the Laurentian Highland. The area as a whole is not mountainous, and consists of a rather low, dissected peneplain (q.v.), the aver age elevation of which is about 1,000 to 1,500 feet. It has been glaciated in comparatively recent times, and contains vast areas of swamps and thousands of lakes. Geologically the region is known as the "Canadian,' or "Pre-Cambrian Shield.' Coastal On the eastern and south ern sides of North America, from the vicinity of New York southward, is a low flat plain which sweeps around the southern end of the Appalachian Mountains, and from the Gulf of Mexico merges gradually northward into the great interior plain. It is known as the Coastal Geologically it is extremely young, con sisting wholly of Cretaceous (q.v.) and Tertiary (q.v.) rocks, mostly soft, unconsolidated clays, sands and marls. At a time geologically recent, this area was under the sea, and these beds were laid down as a result of the erosion of older parts of the continent. The region is so new and so low that it has undergone little erosion, and is consequently very flat and monotonous in topography. Portions of it are rich agri culturally, where the soil is not too sandy. Owing to the unconsolidated condition of the rocks, building stone is less abundant than else where. Aside from petroleum products and salt, both of which occur in quantity in the Louisi ana and Texas portions of the area, the Coastal Plain is singularly poor in mineral wealth.
Eastern Highland.— This highland, extend ing from New England southwest to Alabama, has a base of Pre-Cambrian rocks, among which granites gneisses and schists are dominant. Upon these rests an intensely folded complex of shales, limestones and sandstones, embrac ing rocks belonging to every period of the Paleozoic Era. Throughout the Paleozoic, this area was almost continuously beneath the sea, finally emerging at the close of the Era, when it was folded into a marked mountain range. Erosion has one or more times almost com pletely leveled away the range, and still further uplifts without folding have allowed the streams to cut away the soft layers of rock, leaving the upturned edges of the harder layers as long parallel mountains typical of Appalachian topography. To the west this folded region grades into a high plateau of flat-lying rocks chiefly of Carboniferous Age, known as the Allegheny (north) and Cumberland (south) plateaus. The chief mineral wealth of the region lies in its coal, of which the area of Pennsylvania anthracite is the most famous field. Slate, marble and granite are especially important products in the New England portion. The intensely folded area yields little oil and gas, but the flatter plateaus to the west contain the important Appalachian petroleum field.