_The Great Plateaus._--The boundary between the prairie plains of the central portion of the interior Continental basin and the Great plateaus (Great plains) bordering them on the west is usually indefinite. The prairies pass into the more elevated and drier plateaus by insensible gradations. The plateaus rise gradually from east to west, and along their western margin, adjacent to the east base of the Pacific mountains, attain a general elevation of from 5,000 to 6,000 feet. Over vast areas these monotonous plateaus, with their even sky-lines, are higher above the sea than the crests of the Appalachians, and along their western margin in many localities even surpass in elevation the most prominent peaks in the eastern portion of the United States. Accompanying this increase in elevation from east to west there is a decrease in precipitation, and in consequence a marked change in the vegetation. The plateaus, like the prairies, are treeless in their most characteristic portions, but the larger rivers winding across them are margined in many instances by giant cottonwoods. The mental picture that a traveller over the broad plateaus retains in after-years is of a vast treeless level tract of country, boundless as the ocean, which is bright green and decked with lowly flowers in the early spring, but becomes yellowish brown as the heat and dryness of summer increase and the grasses lose their freshness. Various portions of the plateaus, however, have their own individuality and present characteristics which make them conspicuously different from other portions of the same great series of steppes. At the south, in the region of the Rio Grande and of the Pecos and Canadian Rivers, the plateau is dissected by stream-cut valleys 1,000 feet or more deep, and from one to two score miles across, which divide it into a number of individual table-lands. The plateau margins for many miles on each side of the larger river-valleys have been carved by a complex system of secondary and usually ephemeral streams into a great variety of rock forms with deep trenches between. These conspicuously sculptured areas constitute what are commonly termed Bad Lands. In certain regions also the surfaces of the plateaus, more especially in Nebraska and South Dakota, are broadly undulating or reveal a seemingly endless succession of ridges and hills separated by shallow depressions, due to the presence of large tracts of drifting sand. In spite of these several variations, however, the leading characteristics of by far the larger portion of the plateau country are the generally level grass-covered surfaces extending away in all directions far beyond the reach of vision. On the rolling prairie one can frequently see the undulating surface about him for a distance of 15 or 20 miles, but the curvature of the earth usually draws still narrower limits to the region within the view of the plainsman. In riding over the plains the scene changes but