_Bad Lands._--In the northern portion of the Great plateaus within the United States the surface rocks over great areas are soft or but irregularly hardened sediments of ancient lakes and streams and have been sculptured by rain, wind, and ephemeral rills into a most marvellous array of monumental and castellated forms. Localities where this minute dissection of the soft horizontal strata is especially well marked over hundreds of square miles occur in South Dakota and Montana, and especially on the borders of the valleys carved by the Loup Fork, Niobrara, White, Yellowstone, and Missouri Rivers. In this region the rainfall is light, the mean annual precipitation being in the neighbourhood of 15 inches, and occurs mostly during the winter months. In the summer season the lands far out on the plateaus are dry and hot, and all but the larger streams disappear. The rocks, consisting mostly of unconsolidated clays and soft sandstones, with occasional hard layers and irregular concretions, have been cut into innumerable channels, leaving steep-sided remnants of the former plain between. The maze of trench-like valleys, the similarity of the sculptured land-forms one to another, and the absence of water, make these desert regions excessively difficult to traverse. The Canadian-French who explored the north-central portion of the Great plateaus in early days of American settlement termed these tracts of country, so difficult to cross, _Mauvaises Terres_, a name now seldom used, but replaced by the English name Bad Lands. Although _bad_ to the hunter and the plainsman, these desert regions are of fascinating interest to men of scientific training. The intense heat, the choking alkaline dust, the absence of water, and the danger of being lost and of perishing of thirst in these wild silent regions, have not checked the ardour of explorers. Not only do the Bad Lands present a most attractive field to the student of erosion and of the origin of earth forms, but their deathlike solitudes have been made to yield the most wonderful procession of strange extinct animals yet unearthed by geologists. They are vast cemeteries in which are interred the skeletons of many genera and hundreds of species of animals which lived in the ancient lakes or wandered through the almost tropical forest that in distant ages clothed the adjacent country. The great lesson to be learned by the geographer in these uninviting regions as they seem to most people relates to the way in which the rocks have been eroded. The prevailing softness of the beds with occasional hard layers, the scarcity of vegetation, the occasional heavy rains, and the considerable height of the country above the master streams combine to favour rapid and deep sculpturing. The precipitous slopes of the small mesas and castle-like rock forms destitute of all vegetation excepting succulent cacti and scattered clumps of bunch-grass, reveal a multitude