Though we have assumed twelve degrees of longitude as the medium width of this great plain, it is to be remarked, that to many parts of it our examinations have not been extended. In the latitude of 41°, no mountain, and scarce an elevation deserving the name of a hill, occurs between the western range of the Alleghanies and the Rocky Mountains. But at no great distance north of this parallel, low ranges of hills begin to appear in the region south-west of Lake Michigan; and though too inconsiderable in point of elevation to deserve particular notice, still they exhibit peculiar characters, which seem to designate an intimate connection with the Ozark mountains, south of the Missouri. The same succession of strata, the same alternation of crystalline beds, with those of mechanical deposition, and similar depositories of metallic ores, are observed here, as in the regions about the Merameg and St. Francis. A marked difference is also, as we think, to be discovered between the rocks and soils on the different sides of this range. Of this we shall speak more {276} particularly in another place. For our present purpose, it is sufficient to assume as a boundary of the region we propose first to consider, a line running from the confluence of the Arkansa and Canadian rivers on the south-west, to the junction of the Mississippi and Wisconsan, on the north-east. Assuming this as the direction of the range of the Ozark mountains, it will be perceived, by examining the map, that to the north-west of this line spreads an extensive plain, reaching to the base of the Rocky Mountains. This plain has been crossed in three different places by the exploring party, as already detailed in our narrative; once in ascending by the River Platte, between latitude 40° and 41° 30´; again, in descending the Arkansa, in 38°; and, thirdly, by the route of the Canadian, in 34°. To the information collected in these journeys, we have added a little from other sources; but the greater part of this extensive region yet remains unknown. _Of the Great Desert at the base of the Rocky Mountains_