The rarefaction of the air over this great plain, by the reverberation of the sun's rays during the day, causes an ascending current, which is supplied by the rushing down of the condensed air from the mountains. Though the sun's rays in the middle of the day were scorching and extremely afflictive to our eyes, the temperature of the air, as indicated by the thermometer, had hitherto rarely exceeded 80° Fah. {164} In the forenoon we passed a range of hills more elevated than any we had seen west of the Missouri. These hills cross the Platte from north to south, and though inconsiderable in magnitude, they can be distinguished extending several miles on each side of the river. They consist principally of gravel, intermixed with small water-worn fragments of granite and other primitive rocks, but are based on a stratum of coarse friable sand-stone, of a dark gray colour, which has been uncovered, and cut through by the bed of the Platte. This range may perhaps be a continuation or spur from the black hills mentioned by Lewis and Clarke, as containing the sources of the Shienne, and other tributaries to the Missouri, at no great distance to the north of the place where we now were.[116] [116] The name "Black Hills," which is now associated with the mountains of southwestern South Dakota, has also been applied somewhat indefinitely to hills in Nebraska, Wyoming, and Colorado. Lewis and Clark's map places hills so named along the eastern side of the North Platte, running well into Nebraska. The name is sometimes given to the hills enclosed by the circular course of the North Platte, in southeastern Wyoming; while Frémont (1842) applies it to the hills through which Cache la Poudre River cuts its way to the South Platte.—ED. At evening we arrived at another scattering grove of cotton-wood trees, among which we placed our camp, immediately on the brink of the river. The trees of which these insulated groves are usually composed, from their low and branching figure, and their remoteness from each other, as they stand scattered over the soil they occupy, revived strongly in our minds the appearance and gratifications resulting from an apple orchard; for which from a little distance they might readily be mistaken, if seen in a cultivated region. At a few rods distant on our right hand, was a fortified Indian camp, which appeared to have been recently occupied. It was constructed of such broken half-decayed logs of wood as the place afforded, intermixed with some skeletons of bisons recently killed. It is of a circular form, enclosing space enough for about thirty men to lie down upon. The wall is about five feet high, with an opening towards the east, and the top uncovered.