A conspicuous character in the sandstones about the central and western portions of the region under consideration, is the great proportion of mica, in large scales, which enters into their composition. Fragments of the sand-rock, about the mouth of the Poteau, might be mistaken for mica slate. This mica is rarely if ever of that dark coloured variety which prevails in the Rocky Mountains; and in the other materials of these aggregates, there is a manifest want of resemblance to those mountains. A very slight comparison of the secondary formations at the base of the Rocky Mountains, with the similar aggregates in the Ozark range, will be sufficient to convince any one that they have resulted from the wearing down of primitive mountains, very dissimilar in character to each other. We might have remarked, when speaking of the Rocky Mountains, the absence of any formation of talcose rocks, and indeed of magnesian fossils of any kind, and a corresponding deficiency of talcose and chloritic sandstones among the secondary rocks. We no sooner arrive at the western margin of the secondary belonging to the Ozark mountains, than we meet with extensive beds of sandstone, in which the prevalence of magnesia forms a conspicuous character. The beautiful argillaceous chlorite sandstone at the rapids of the Canadian, has been already described, and similar beds are not uncommon in many places in the vicinity of extensive depositions of coal. Another peculiar variety of sandstone occurs, in connection with the sulphuret of lead, at the old mines of St. Michael, and at many places thereabouts. This bears apparently the same relation to the common sandstones, as the crystalline limestone above {306} mentioned does to the earthy varieties, and it alternates with and passes into the common rock in a similar manner. Its particles are crystalline, and appear to remain undisturbed in the position in which they were originally deposited from solution in water. Nevertheless the aggregate is manifestly secondary, and embraces the relics of many organized beings, as is common in the other secondary rocks. There is also about the lead mines a sandstone composed of small glimmering grains of transparent quartz, and so loosely cemented as to fall rapidly to pieces, forming a light gray sand. In this variety we have sometimes observed the lead ore either disseminated, or forming horizontal veins between the laminæ of sandstone. An examination of some spots might lead to the conclusion that the soil in which most of the lead has hitherto been found, has resulted from the disintegration of a sandstone of this kind.