(_b._) _The Plateau Country of Utah and Arizona._--This tract, which is drained by the Colorado River and its tributaries, is bounded on the north by the Wahsatch range, and extends eastwards to the base of the Sierra Nevada. Round its margin extensive volcanic tracts are to be found, with numerous peaks and truncated cones--the ancient craters of eruption--of which Mount San Francisco is the culminating eminence. South of the Wahsatch, and occupying the high plateaux of Utah, enormous masses of volcanic products have been spread over an area of 9000 square miles, attaining a thickness of between 3000 and 4000 feet. The earlier of these great lava-floods appear to have been trachytic, but the later basaltic; and in the opinion of Captain Dutton, who has described them, they range in point of time from the Middle Tertiary (Miocene) down to comparatively recent times. (_c._) _The Grand Cañon._--To the south of the high plateaux of Utah are many minor volcanic mountains, now extinct; and as we descend towards the Grand Cañon of Colorado we find numerous cinder-cones scattered about at intervals near the cliffs.[1] Extensive lava-fields, surmounted by cinder-cones, occupy the plateau on the western side of the Grand Cañon; and, according to Dutton, the great sheets of basaltic lava, of very recent age, which occupy many hundred square miles of desert, have had their sources in these cones of eruption.[2] Crossing to the east of the Grand Cañon, we find other lava-floods poured over the country at intervals, surmounted by San Francisco--a volcanic mountain of the first magnitude--which reaches an elevation, according to Wheeler, of 12,562 feet above the ocean. It has long been extinct, and its summit and flanks are covered with snow-fields and glaciers. Other parts of Arizona are overspread by sheets of basaltic lava, through which old "necks" of eruption, formed of more solid lava than the sheets, rise occasionally above the surface, and are prominent features in the landscape. Further to the eastward in New Mexico, and near the margin of the volcanic region, is another volcanic mountain little less lofty than San Francisco, called Mount Taylor, which, according to Dutton, rises to an elevation of 11,390 feet above the ocean, and 8200 feet above the general level of the surrounding plateau of lava. This mountain forms the culminating point of a wide volcanic tract, over which are distributed numberless vents of eruption. Scores of such vents--generally cinder-cones--are visible in every part of the plateau, and always in a more or less dilapidated condition.[3] Mount Taylor is a volcano, with a central pipe terminating in a large crater, the wall of which was broken down on the east side in the later stage of its history. [Illustration: Fig. 25.--Mount Shasta (14,511 feet), a snow-clad volcanic cone in California, with Mount Shastina, a secondary crater, on the right; the valley between is filled with glacier-ice.--(After Dutton).]