Precise information on the extent and peculiar characters of the Prairies, obtained in an elaborate survey, has recently been given by gr. James Hall, state geologist of Iowa, a man of science advantageously known to English geologists. This is introduced in an account of the physical geography of that region, forming Chapter I. of Mr. Hall's ' Report on the Geological Survey of the State of Iowa,' published by authority of the legislature of that state, 1858. It involves many par ticulars having an important bearing on the structure of the plains of North America in general, and with some slight omissions we shall now transfer it to our columns. Fortunately fur the investigation of this subject, the state of Iowa consists, mainly, of prairie country.
The prairie region of the west occupies a vast extent of country, extending over the eastern part of the Ohio, Indiana, the southern portion of Michigan, the southern part of Wisconsin, nearly the whole of the states of Illinois and Iowa, and the northern portion of Missouri, and gradually passing, in the territories of Kansas and Nebraska, into the arid and desert region which lies at the base of the Rocky Moun tains, called—in that local technicality which is so characteristic of America, and which, in every part of the world appears to have pre ceded scientific geographical description—die Plains. This passage takes place in the region between the parallels of 97' and 100°, west of which belt the country becomes too barren to be inhabited and worth less for cultivation. The passage from the heavily wooded region of the north and east into the treeless plains of the west is a gradual one, and the disappearance of the waderwood and the predominance of " oak openings," or groves of oak and other forest trees, not crowded together, but scattered over the surface at a considerable distance from one another, without any low shrubs or underbrush between them, is the characteristic of the border of the prairie region. To one coming from the dense thickets of low and tangled shrubs which choke the forests of the north, and render travelling through them difficult, even to the pedestrian, the open woods of the north-west offer a striking contrast to that which has been left behind, and form a natural intro duction to the grassy plains or prairies.
The elevation of the prairie region above the level of the sea gra dually increases from the east to the west. The lower part of Illinois varies from 100 to 250 feet above the river at Cairo, or 400 to 550 feet above the sea-level. In the central portions of the state the levellings
along the line of the Illinois central railroad, indicate an average eleva tion of from 650 to 750 feet, which increases towards the northern boundary to 800 or 900 feet above the sea. Some of the highest swells of the prairie attain an elevation of nearly 1000 feet. The more elevated portions of Southern Wisconsin are about 1100 feet above tide-water. The region from the Mississippi westward is a gradually ascending plain as far as the very base of the Rocky Mountains. Within the limits of Iowa the table-land along the water-parting between the streams flowing into the Mississippi and the Missouri, the " Plateau du Coteau des Prairies " of Nieollet, attains the altitude of from 1400 to 1500 feet above the sea; and to the west of the Missouri the ascent still continues, the soil being more and more arid, until at the 105th meridian, an elevation of 5000 feet is attained.
The prairie is designated as " flat " or " rolling," its surface in the one case being nearly level, and in the other gently undulating. The flat prairie is found chiefly south and west of Lake Michigan, on the head-waters of the Illinois and Wabash rivers. In other districts the rolling prairie greatly predominates over the flat, especially in Iowa, where there are but few tracts of any extent which are not more or less undulating. Even in the rolling prairie the irregularities of the surface are but trifling in amount, compared with the vast extent which can be taken in at one view : so that it often happens that a region which, seen from a distance, appears to be almost a dead level, is, in reality, furrowed by broad depressions, which give a wave-like character to its surface. Thus, the traveller crossing the prairie in any direction except along its water-parting, will be'surprised to find himself constantly ascending and descending, although only hills of moderate elevation. The depth of these depressions below the mean level of the prairie may frequently be as much as 50 feet; but it does not often greatly exceed that amount, without the drainage becoming suffi cient to convert the water, which collects in them, into a running stream. The "swales," or " sloughs," of the prairie, as these depressions are called, are wet and marshy except in the dry seasons, and, in the spring and autumn make the roads almost impassable.