A few mornings after Glen's experience with the mule, the white tents of Camp Lyle were struck; and at sunrise the long slow-moving trains of wagons had covered the first mile of the many hundreds lying between it and the Pacific. The last railroad had been left behind, and the sound of its whistle was heard no more. Already our young explorer was learning, from his more experienced comrades, to distinguish an Indian pony and lodge-pole trail from that of a buffalo, and a buffalo wallow from an ordinary mud-hole. Already he had seen his first prairie-dog town, and had gazed curiously at several bleached skulls of the mighty bison, some of which were still partially covered with shaggy hair. Already, too, he was filled with that sense of glorious freedom and boundless possibility that can only be breathed with the air of unlimited space. Glen was surprised to find that, instead of being level, as he had always thought them, the Plains rolled, in vast undulations, having a general north and south direction, so that, as the wagons were moving west, they were always ascending some long slope, or descending its farther side. He was almost startled, too, by the intense silence brooding over them, and unbroken at a short distance from the train, save by the plaintive song of meadow-larks.