1st. _Red sandstone._--This rock, which is the lowest of the horizontal or fletz rocks met with in this part of the country, is very abundant in all the region immediately subjacent to the Rocky Mountains. We have never met with a similar rock in the eastern part of the valley of the Mississippi. It occurs at intervals along the base of the mountain, reposing against the primitive rocks, in an erect or highly inclined position. It varies in colour from bright brick red, to dark brown; and is sometimes found exhibiting various shades of yellow and gray. It is, however, almost invariably ferruginous; and the predominance of red in the colouring certainly entitles it to the distinctive appellation of red sandstone. The lowest part of the stratum has frequently least colour, and is also the most compact and hard. This is not, however, invariably the case; for in the neighbourhood of the Platte, that part of it which lies immediately upon the granite is white, and contains beds of coarse conglomerate or puddingstone. At the lowest points we have been able to examine, are found embodied large oval or irregular masses of hornstone, usually of a yellowish-white, or bluish colour; and near the surface of these masses are found the few well-marked organic relics the stratum can be said to contain. Higher {286} up the rock becomes much softer, and usually of a browner colour. It is disposed in immense horizontal laminæ or strata, which, when broken transversely, exhibit some tendency to separate into fragments of a rhombic form. Near the upper part of the stratum are frequently seen broad belts of a lighter colour, conspicuously marked with reticulating yellowish veins. The cross fracture of the stone is even and earthy, except in the conservatories. When divided in a direction parallel to that of the strata, small scales of mica are seen; but this is usual only in those parts of the stone where natural seams or fissures existed. Small specimens from many parts of this stratum could not be distinguished from the red sandstone quarried at Nyae in New Jersey, and used in great quantities in the cities of New York, Albany, &c. for building. The character which most particularly distinguishes this rock from "the old red sandstone of Werner," pointed out by Maclure in New York and New Jersey, appears to be the constant accompaniment of gypsum, and muriate of soda; the colour of the stratum is also in general of a brighter red, approaching vermilion, and is more copiously imparted to such streams of water as traverse it.