East of the huge central mountain system there rolled away from the base of the range to the Mississippi endless plains resembling a petrified ocean; the prairies, treeless, sublime in their immensity. For about half the distance from the mountains to the river, approximately as far as the 100th meridian, this enormous territory was well-nigh rainless, thus presenting an additional barrier to investigation from the eastward. The remaining half was invitingly fertile. Across these wide prairies meandered eastwardly several branches of the Missouri and the Mississippi, chief among them the Platte, the Arkansas, and Red River. West of the Backbone lay a maze of mountains, "parks," deep gorges, now called canyons, cliffs, plateaus, and valleys, limited on the far western side by a second mother-range rivalling in height and extent and impenetrability the central system itself. This was the Sierra Nevada and its upper continuation, the Cascade Range. About midway between the two master ranges another, the Wasatch, extended northerly and southerly, forming the eastern limit of the dry bed of an ancient sea of which a small remnant remained concentrated in a salt lake some fifty miles in length. This vanished sea is now known as Lake Bonneville, its old bed as the "Great Basin." Its southern rim breaks down from an altitude of about ten thousand feet in a series of mighty cliffs, like cyclopean steps, to the canyons of the Colorado, and near the summit of this rim a river starts north down into the basin, sweeping along for many miles to turn suddenly to the westward and end in a lake without visible outlet, in the middle of a stretch of desert. This is now the Sevier. West of the salt lake another stream, the Humboldt, took its rise and, darting boldly toward the Sierra as if to cut it in twain, meekly collapsed in a small lake at the foot. Between the Wasatch and the Rocky Mountains lay the valley of the Colorado, already mentioned, a marvellous labyrinth of canyons and cliffs, of dead volcanoes, lava beds, plateaus, and mountain peaks of rare beauty. [Illustration: The Yosemite Valley. Photograph by C. C. Pierce & Co.]