The frontier army post of Fort Lincoln on the bluff alongside the river testifies to the time not yet remote when the Sioux and Crow Indians of the Dakotas needed a good deal of military control. The deer, buffalo and antelope then roamed these boundless prairies, but they have all disappeared. Beyond the Missouri River is the region of the Dakota "Bad Lands." The surface rises into sharp conical elevations known as "buttes," and soon this curious district of pyramidal hills known as Pyramid Park is entered, fire and water having had a remarkable effect upon them. Their red sides are furrowed by the rains, and smoke issues from some of the crevices. The lignite and coal deposits underlying this country have produced subterranean fires that burnt the clays above until they became brittle and red. There are ashes and scoriæ in patches, and cinders looking much like the outcast of an iron furnace. The buttes are at times isolated and sometimes in rows, many being of large size. Their sides are often terraced regularly, and frequently into fantastic shapes, occasionally appearing as the sloping ramparts of a fort. There are frequent pot-like holes among them, filled with reddish, brackish water, and sometimes excavated in the ground with regularly square-cut edges. When the railway route cuts into a butte, its interior is disclosed as a pile of red-burnt clay fragments mixed with ashes and sand. Little prairie dogs dodge in and out of their holes, but there is not much else of life. The boundary is crossed into Montana, and the "Bad Lands" gradually give place to a grazing section. Here stands up the great Sentinel Butte, with its reddish-yellow sides, near the Montana border, and the railway route then descends from the higher region to the valley of the Yellowstone.