The great "backbone" of the American Continent is the Rocky Mountains, and the summits of its main range make the parting of the waters, the "Continental Divide." Its name of the Rockies is appropriate, for on these mountains and their intervening plateaus, naked rocks are developed to an extent rarely equalled elsewhere in the world. The leading causes of this are the great elevation and extreme aridity, the scanty moisture preventing growth of vegetation, and the high altitudes promoting denudation of the rock-material disintegrated at the surface. Enormous crags and bold peaks of bare rocks, mostly compose the mountains, while the streams flow at the bases of towering precipices in deep chasms and canyons filled with broken rocks. Being unprotected by vegetation, the winds sweep the hills clean of soil and sand, the steep slopes of the valleys are strewn with fragments of the enclosing cliffs, and the rivers are usually without flood-plains or intervales, where soils may gather. In the extensive and highly-elevated plateaus, the streams usually run in the bottoms of deep canyons, their channels choked with _débris_. Added to this the whole Rocky Mountain region has in the past been a scene of great volcanic activity, many extinct volcanoes appear, broad plains are covered with lava, and scoria and ashes are liberally deposited all about. The aridity is not a feature of the Pacific coast ranges, however, for the moisture from that ocean abundantly supplies water; there are good soils, and in the northern parts usually dense forests. The Rocky Mountain system extends from Mexico up to Alaska and the Arctic Ocean, its greatest development being between 38° and 42° north latitude, where the various ranges cover a breadth of a thousand miles. The highest peak of the Rockies is Mount Logan, in British America, on the edge of Alaska, rising nineteen thousand five hundred and thirty-nine feet. In the United States these mountains rise from a general plateau extending across the country, and reaching its maximum elevation of about ten thousand feet in Colorado, whilst towards the north the surface descends, entering Canada at an elevation of four thousand feet. The plateau descends westward into the basin of the Colorado River, then the surface rises in Nevada to six thousand feet, and thence farther westward it gradually descends to the base of the Sierra Nevada in California. To the eastward the plateau throughout steadily descends in the long, undulating and generally treeless slope of the Great Plains to the Mississippi, the many tributaries of the Father of Waters carving their valleys down through its surface. There are numerous mountain ranges, plateaus and parks, under different names in this extensive mountain region, and the higher peaks in the United States generally rise to thirteen to fifteen thousand feet elevation. These mountains and the plains to the eastward compose the