_Sand-Hills._--The sands winnowed by the winds from the bare plains and steep bluffs are in certain places on the Great plateaus gathered into dunes which cover great areas with a succession of low dome-shaped hills. On the borders of the Niobrara River there is a detached area of about 20,000 square miles, which has been covered in this manner with loose sands. This region, as described by F. V. Hayden, presents a succession of round-topped hills, some of them scooped out by the swirling winds so as to resemble volcanic craters. These sand-hills were formerly a favourite resort of the bison, which fed upon the scanty but very nutritious grasses in the little valleys and intervals among the mounds and ridges. There is, for the most part, an abundant supply of water in the lakelets scattered through the region, and fed by the seepage from the porous sands, which drink in all the water that falls upon them and allow it to percolate slowly into the adjacent depressions. Some of the lakelets and ponds are highly alkaline, while others are fresh; the former can be easily distinguished from the latter by the absence of vegetation about their borders. The hills, although seemingly utterly desolate, on a nearer view sometimes reveal considerable vegetation, including yuccas or "Spanish needles," which shelter the sands from the winds. Many other regions on the western border of the Great plateaus, in the valleys of the Rocky Mountains and on the desert plains of Utah, Nevada, and California, are buried beneath drifting sands, which have the characteristic features of a dune-covered seacoast. The sources of the sands in these interior plains are usually to be found in the disintegration of the rocks under the action of the dry air with its many and sudden changes of temperature, but occasionally they come from desiccated lake beds not yet clothed with vegetation.