The country between the sources of Purgatory creek and the stream on which we were now encamped, is a wide and elevated formation of trap rocks, resting upon horizontal sandstone. It has a loose and scanty soil, in which sand, gravel, and rolled pebbles are rarely seen, except in the vicinity of some parts where the sandstone appears to have been uncovered by the action of currents of water. In traversing it we had collected many new and interesting plants. Among these were a large decumbent mentzelia, an unarmed rubus, with species of astragalus, pentstemon, myosotis, helianthus, &c. Beside the common purslane, one of the most frequent plants about the mountains, we had observed on the Arkansa a smaller species, remarkably pilose about the axils of the leaves, which are also narrower than in P. oleracea. A very small cuscuta also occurs almost exclusively parasitic on the common purslane. July 31st. In attempting to descend the creek from our last encampment, we found the valley so obstructed with fragments of greenstone as to be wholly impassable. We accordingly ascended into the plain; and continuing along on the brink of the precipice, arrived in a few hours at a point where the substratum of sandstone emerges to light at the base of an inconsiderable hill. It is a fine gray sandstone, having an argillaceous cement, and its lamina are so nearly horizontal, that their inclination is not manifest to the eye. It is smooth and fissile, and in every respect remarkably contrasted to the massive and imperfectly columnar greenstone which it supports. The greenstone of this district is not universally characterized by any tinge of green in the colouring, but often, as in the instance of which we are speaking, its colour is some shade of gray, varying from light {275} gray to grayish black. The hornblende and felspar which enter into its composition, are minutely and intimately blended. Its minute structure is rarely, if ever distinctly crystalline; most frequently it is compact, and the fracture nearly even. The hunters were kept constantly forward of the party, and in the course of the morning they killed a small fawn and a heron. At one o'clock we arrived at the confluence of a creek tributary from the east to the stream we were following, and descending into its valley by a precipitous declivity of about four hundred feet, we encamped for the remainder of the day. This valley is bounded by perpendicular cliffs of sandstone, surmounted by extensive beds of greenstone. The fragments of the latter have fallen down into the valley, and being less perishable than the sandstone, they constitute the greater part of the _débris_ accumulated along the base of the cliffs.