The prairies coincide, in a general sense, with the middle and upper portion of the Mississippi Valley, embracing the vast region which extends from the Great Lakes to the base of the Rocky Mountains. They are covered in their natural state with a rich herbage, and exhibit a waving sea of grass several feet high. At intervals, toward the banks of the rivers, patches of forest vegetation break the uniformity of the prospect, but the prairie itself is destitute of trees, and (as the name implies) is merely a grassy plain, or meadow. Alternate forest and prairie constitute the great features of natural scenery in the New World. When the rich soil of the prairie-land is broken up by the plow--an operation which is rapidly progressing, year by year, within the Western States of America--it yields abundant crops of corn. There are, however, within the vast extent of the North American continent, immense regions which yet retain the aspect of the wilderness. It was within these regions that the buffalo roamed, in vast herds, and that the native Indian hunter pursued his game ere the advancing footsteps of the white man had driven him from his haunts. The llanos, or savannahs, are vast grassy plains, which occupy nearly the whole basin of the Orinoco River, excepting only toward its highest portion, when they are succeeded by wooded plains. The llanos resemble in general features the prairies of the Mississippi Valley, but have for the most part a lower level, and (owing to the abundant rains of the torrid zone) are annually inundated by the rivers to an immense extent. Whole districts, embracing thousands of square miles, are annually converted, within the interior plains of South America, into lakes, or temporary seas of fresh water, to be rapidly evaporated under the burning rays of a vertical sun. At the close of the rainy season the llanos are covered with grass, and form rich natural pasture-grounds. During the prolonged season of drought which ensues, the verdure is entirely destroyed, and the parched earth opens in wide and deep crevices--again to be laid under water with the recommencement of the rains. The selvas, or forest-plains, belong to the valley of the Amazon, and include an immense area of Brazil, watered by the lower portion of the great stream and its chief tributary, the Madera. Vast regions are here covered by an uninterrupted forest, composed of trees of giant growth, their boughs interlaced by immense creeping plants, and the ground beneath thickly covered with a dense growth of underwood. To the southward of the forest region are vast grassy plains, which stretch in that direction into the valley of the Paraguay.