The prairies are covered with a dense vegetation of grass and her baceous plants, to the almost entire exclusion of trees, which occur only under peculiar circumstances of moisture and soil, in scattered groups called groves, or along the larger streams, or, occasionally, on low rocky ridges which are sometimes met with. This growth of timber in the river bottoms does not entirely disappear until we reach the border of the " plains," in the longitude of about 98° west of Greenwich. In the swales or depressions, which are comparatively humid, the grass grows very tall and rank; and the higher and drier the prairie the finer its growth, and more dense and closely interwoven the sod. Among the grasses of the prairies is interspersed a great variety of flowering plants, which bloom in constant succession from spring to autumn, and lend a peculiar charm to the landscape, giving beauty and variety to what might otherwise be called a monotonous scene. The characteristic herbs of the prairies would seem to be cora. positx, especially Helienthoid Composites, according to Dr. Asa Gray, who has enumerated the principal species in the 'American Journal of Science,' Series IL, vol xxiii. p. 397.
The upper portion of the material constituting the superficial covering of the prairie is always finely comminuted, and usually has few loose boulders or fragments of rook scattered through it, although they sometimes lie upon the surface in isolated groups, or singly. Upon the great prairies in Central Iowa, one may frequently travel over a large extent of surface without seeing a single stone, not even so much as the smallest pebble. In the swales, and in some of the bottom lands, especially in the southern part of the State, the rich black vegetable mould is very deep, hut ou the prairies it is usually from one to two feet. The subsoil is almost invariably a quite argil laccous loam, and there is a gradual passage downwards into a material in which, though containing sandy portions and occasional pebbles, the argillaceous element greatly predominates.
The material of the superficial covering of the prairie has evidently not beets transported to any great distance. It bears the marks of having had its origin chiefly in the decomposition of the rocks which underlie it, and in fact, the prairies are exclusively confined to regions underlaid by soft sedimentary strata, especially shales and impure limestones. It would be difficult to imagine, Mr. Hall remarks, a prairie soil existing on crystalline rocks.
After describing the prairies, Mr. Hall proceeds to discuss, as follows, the difficult and contested problem of their origin, and of the cause of the absence of trees over so extensive a region :— "The idea is very extensively entertained, throughout the west, that the prairies were once covered with timber ; but that this has been destroyed by the fires which the Indians have been in the habit of starting in the dry grass, and which sweep over a vast extent of surfaco every autumn. A few considerations will show that this theory is
entirely untenable.
" In the first place, the prairies have been in existence at least as far back as we have any knowledge of the country ; since the first explorers of the west described them just as they now.are. There may be limited areas once covered with woods, and now bare ; but, in general, the prairie region occupies the same surface which it did when first visited by the white man.
" But, again, the prairies are limited to a peculiar region, one marked by certain characteristic topographical and geological features, and they are by no means distributed about wherever the Indians have roamed and used fire. Had the frequent occurrence of fires in the woods been the means of removing the timber and covering the soil with a dense growth of grass, there is no reason why prairies should not exist in the eastern and middle states as well as in the western. The whole northern portion of the United States was once inhabited by tribes differing but little from each other in their manner of living.
"Again, were the prairies formerly covered by forest trees we should probably now find some remains of them buried beneath the soil, or other indications of their having formerly existed. Such is not the case, for the occurrence of fragments of wood beneath the prairie sur face is quite rare; and when they are found, it is in such a position as to show that they have been removed to some distance from their place of growth.
" It has been maintained by some, that the want of sufficient moisture in the air or soil was the cause of the absence of forests in the north-west ; and it is indeed true that the prairie region does con tinue westward, and become merged in the arid plains which extend along the base of the Rocky Mountains, where the extreme dryness is undoubtedly the principal obstacle to the growth of anything but a few shrubs, peculiarly adapted to the conditions of the climate and soil which prevail in that region. This, however, cannot be the case in the region east of the Mississippi and near Lake Michigan, wbero the prairies occupy so large a surface ; since the results of meteor°.