NORTH AMERICA, which may be considered to extend as far south as the Isthmus of Tehuantepec, has an area, exclusive of Greenland and the West Indies, of a little more than 8,000,000 square miles. It therefore contains about one-seventh of the land surface of the globe, and comes third in the list of continents according to size ; it is about twice as large as Europe, and has an extent about sixty-six times that of the British Isles.
Taking into consideration the geological and physical features of the whole continent, several main physiographical regions may be recognised. The first of these is the Laurentian Plateau. If a line be drawn from the Arctic Ocean, east of the mouth of the Mackenzie River, through the Great Bear and the Great Slave Lakes to the western extremity of Lake Athabasca, and from there in a gentle southward curve to the northern end of Lake Winnipeg, along the eastern shore of that lake, and then southwards to the Lake of the Woods, eastwards from there to Lake Superior, along the northern shores of the Great Lakes as far as the north eastern extremity of Georgian Bay, and from there at a varying distance from Lake Ontario and the St. Lawrence River to a point on that river a few miles below Quebec, it will mark off the old Archaean nucleus about which the rest of the continent has grown up. This Laurentian region consists of an old mountain mass, whose height has been reduced by weathering and glacial action so that it now presents the appearance of a roughened plateau with an average elevation in the east of 1,500 to 1,600 feet, though in the west it is somewhat lower.
From the St. Lawrence, there runs in a south-westerly direction an elevated region known as the Appalachian Highland, which may be divided longitudinally into four belts parallel to one another. The first of these belts, the Piedmont Plateau, stretches from central Alabama to New York with a width varying from 60 miles in the north and south to 150 miles in the centre, and it also reappears in the east of New England. On the south-east, where
it adjoins the coastal plain, it has an elevation of 250 to 300 feet, but on the north-west, along its junction with the second belt, it rises to 1,000 feet above sea-level. This second belt is a mountain range, highest in New Hampshire and in North Carolina, formed of the same hard and resistant rock as the Piedmont Plateau. In Virginia it is known as the Blue Ridge, while in New England it forms the Berkshire Hills of Massachusetts and the Green Hills of Vermont. The third belt, the Great Appalachian Valley, stretch ing from the St. Lawrence to Alabama, with an average breadth of about seventy-five miles, is a region of relative depression carved out of softer Palaeozoic rocks, though within it are many short ranges running parallel to one another. In the north it is occupied by Lake Champlain and the Hudson, and further south by the Shenandoah and the upper waters of the James and the Tennessee. The western boundary of this belt is formed by the escarpments of the Allegheny and Cumberland plateaus, of more or less horizontal Palaeozoic strata, which slope away towards Lake Erie, the lower Ohio, and the lower Tennessee. These upland regions vary in height ; in Pennsylvania they do not exceed 2,000 feet, but in Kentucky and West Virginia they have an elevation of 3,000 to 4,000 feet.
Between the Piedmont Plateau and the Atlantic, there is a low coastal plain which sweeps round the southern end of the Appala chian system, and, skirting the Allegheny Plateau, extends north ward as far as the mouth of the Ohio. On the west its boundary touches the Ozark uplift, and then runs in a south-westerly direc tion to the Rio Grande Del Norte. This region, which may be called the Atlantic and Gulf Coastal Plain, slopes upwards from the shore, but seldom exceeds a height of 300 feet on its inland margin. It is of recent formation, and consists of weak and unconsolidated rocks, usually covered with deep and fertile soil.