The shape of the plateau in Washington Territory is an irregular square with a diameter each way of about 150 miles. Followed into Idaho and Oregon, the diameters would reach 200 miles. Its surface is generally smooth, but there are frequent patches of rock, and sometimes large areas are roughened by rocky outcrops. The plateau is elevated and rolling, rising from 1,000 to 3,000 feet above the surface of Columbia River. Its elevations usually are mere swells, except along the precipitous edges of coulées. I know of only one mountain upon it, and that is quite a small one; but it served as a refuge for Lieutenant Steptoe and his handful of soldiers when attacked by the Indians; and hence is called Steptoe Butte. The surface of the plain is scarred in a number of places with coulées, or dry river-beds, which are cut down twenty to one hundred feet, and sometimes more, and their sides are usually marked by bluffs, often of rock. These coulées are an advantage, or a disadvantage, in road-making, according to whether the road goes with, or across, the coulée. The Northern Pacific Railroad found it convenient to use one of them for a long distance. The deepest cuts in the plateau are made by its rivers. Of these the Columbia is chief. This river, as already intimated, has cut a channel for itself along the north and west edge of the plain from 1,000 to 2,000 feet below the general level. The Snake River, which is the largest affluent of the Columbia, has numerous branches, all cut deep into the basalt which underlies the plain. [Sidenote: Columbia and Snake Rivers.] The Columbia and Snake are both steamboat rivers, but navigation is interrupted by rocky rapids, which prevent through lines of steamers. The Columbia is one of the largest rivers in the world, and has abundant water for steamboats from its mouth to a point in Canada, north of Farwell, where it is crossed by the Canadian Pacific Railway, and steamboats run at intervals to the most northerly point; and there are navigable stretches not yet used for boats which will have steamers in connection with future railroads. The steamers on Snake River are very useful, and run to Lewiston, in Idaho, and perhaps further. This plateau, or Great Plain of the Columbia, as it is called sometimes, is a most interesting and important region, concerning which I shall have much to say under subsequent heads. ALTITUDES IN WASHINGTON TERRITORY.