Having given the foregoing brief account of the movements of the expedition, we next proceed to a consideration of the region explored by them, which embraces a very considerable portion of the immense valley situated between the Alleghany and Rocky Mountains. The portion of this valley to which their attention has been more particularly directed, and relative to which intelligence has been collected, is situated between thirty-five and forty-two degrees of north latitude, and eighty and one hundred and six degrees of west longitude, embracing an extent of {198} about five hundred miles in width from north to south, and thirteen hundred miles in length from east to west. As might be expected in a region of this extent, a great diversity of surface is presented to view, exhibiting all the varieties, from the most level and unbroken to the most rugged and mountainous aspect. The most broken parts of this region are those situated along the Ohio, from its source to its confluence with the Mississippi, and on the west of the Mississippi, between Red river and the Arkansa, and between the latter and the Missouri, extending westward about four hundred miles from the Mississippi. The whole region, in a geological point of view, is constituted of three varieties of formations, which characterize the surface throughout; viz. transition, secondary, and alluvial. A tract, however, of considerable extent, including the hot springs of the Washita, and extending northwardly to the lead mines back of St. Genevieve, has, by some, been considered as possessing a primitive character; but it is believed that the rocks discoverable therein are not sufficiently uniform to warrant such a decision. Moreover, an insulated tract of primitive country, surrounded by others exhibiting the most unequivocal marks of their being secondary, and at the same time presenting a similar conformation in their general aspect, is such an anomaly in natural science as requires more than ordinary proof to be admitted. The particular tract under consideration is probably analogous to other tracts within the region above specified, exhibiting a surface characterized by primitive formation superincumbent upon others of a secondary character. In order to give a more distinct conception of the country or region under consideration, it may be regarded as divisible into the following sections: viz. 1st, the country situated between the Ohio river and the Alleghany mountains; 2d, the country situated between the Ohio, Mississippi, and the Lakes; {199} 3d, the country situated between the Mississippi and Missouri rivers; 4th, the country situated between the Red and Missouri rivers, west of the Mississippi and east of the meridian of the Council Bluff; and 5th, the country between the proposed meridian and the Rocky Mountains. _Of the country situated between the Ohio river and the Alleghany mountains_