The Yellowstone plateau is a vast lava-deposit. Its material is mostly volcanic, but its landscape--its architecture--is largely glacial. In ages remote, this realm became the scene of volcanic activity. Intermittent outpourings went on through long periods of time. Volcanoes in and near the Park threw forth quantities of ashes, lava, and cinders, which built up a plateau region three or four thousand feet thick. Rhyolite and other forms of lava were last spread over the region. This volcanic activity appears to have ended before the last ice age. No eruption has occurred for centuries. The ice age wrought vast changes in the volcanic landscape. The ice smoothed wide areas, shaped cañons, and rounded mountain-sides, produced and spread soil, and gave the entire region the flowing, attractive lines of glacial landscape. On the rim of the Yellowstone Cañon, about three miles below the falls, an enormous glaciated granite boulder reposes upon lava--rhyolite. It measures about twenty-four by twenty by eighteen feet. It was transported to this resting-place from mountains more than thirty miles away. Here we have a stone foundation laid by volcanic fire, and upon it a stone, shaped, transported, and placed by glacial ice. There are about three thousand geysers, hot springs, and mud-and-water springs in the Park; and as many other vents of steam, acid, and gas. That the geysers have been active in this region for thousands of years is shown in the deep deposits of silica and travertine that overspread extensive area. During the ice age many of these deposits were eroded and others were piled with boulders. It is plain that steam and hot water had been at work long before the last ice age came. During the ice period, a wild conflict probably took place between the deep outspread ice and the insistent eruptions of steam and hot water. The surface of Yellowstone Lake once stood about one hundred and eighty feet higher than it is at present. Its outlet was then through the Snake River to the Pacific Ocean. The Continental Divide then passed over the summit of Mount Washburn. Unwritten as yet is the splendid geological story of this change, which may have been caused by earthquake upheaval or by subsidence. It appears to have occurred about the close of the last glacial epoch. Possibly ice dammed the narrow gorge of Outlet Creek, through which the waters of the lake formerly flowed to the Snake River. Whatever the cause, its outlet waters changed and eroded the now famous and splendidly colored cañon of the Yellowstone.