The great central plain of North America, lying between the Rocky and Alleghanny Mountains, and reaching from the Gulf of Mexico to the Arctic Ocean, includes the valleys of the Mississippi, St. Lawrence, Nelson, Churchill, and most of those of the Missouri, Mackenzie's, and Coppermine rivers. It has an area of 3,240,000 square miles, which is 240,000 square miles more than the central plain of South America, and about half the size of the great plain of the Old Continent, which is less fertile; for, although the whole of America is not more than half the size of the Old Continent, it contains at least as much productive soil.

This plain, 5000 miles long, becomes wider towards the north, and has no elevations, except a low table-land which crosses it at the line of the Canadian lakes and the sources of the Mississippi, and is nowhere above 1500 feet high, and rarely more than 700. The character of the plain is that of perfect uniformity, rising by a gentle regular ascent from the Gulf of Mexico to the sources of the Mississippi, which river is the great feature of the North American low lands. The ground rises in the same equable manner from the right bank of the Mississippi to the foot of the Rocky Mountains, but its ascent from the left bank to the Alleghannies is broken into hill and dale, containing the most fertile territory in the United States.