86 Expedition to the 



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 

 points, 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 spe- 

 cies of astragalus, pentstemon, myosotis, helianthus, &c. Be- 

 side the common purslane, which is one of the most fre- 

 quent 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. 



31st. In attempting to descend the creek from our last 

 encampment, we found the valley so obstructed with frag- 

 ments of greenstone, as to be wholly impassable. We ac- 

 cordingly ascended into the plain, and continuing along 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 ah argillaceous cement, and its laminae are so nearly 

 horizontal, that their inclination is not manifest to the eye. 

 It is smooth and fissile, and in every respect remarkably con- 

 trasted to the massive and imperfectly columnar greenstone, 

 which it supports. 



The greenstone of this district is not universally marked 

 by any tinge of green in the colouring, but often, as in 

 the instance of which we are now speaking, its colour 

 is some shade of gray, varying from light gray to grayish 

 black. The hornblend and feldspar which enter into its 

 composition, are minutely and intimately blended. Its mi- 

 nute structure is rarely, if ever, distinctly crystalline; most 

 frequently it is compact, and the fracture nearly even. 



The hunters were kept constantly in advance of the party, 



