six inches, or it may be six feet. Along the banks of the Mississippi it presents, not infrequently, perpendicular sections of six hundred feet. Its firmness in maintaining its position in such exposed bluffs is due to the infiltration of the cements of lime and iron while in the process of deposition, or subsequently. It is more largely developed along the Missouri than along the Mississippi. There are other places where the surface-soil may be peaty, from the preservation of dead vegetation. Extensive level tracts, that are submerged a large part of the year, may present a peaty soil. Very often also in such peaty places there will be found patches of highly-calcareous soil, resulting from the accumulation of fresh-water shells, or from the precipitation of the carbonate of lime from waters that enter the marsh from limestone districts.
But, whatever the character of the surface-soil, it must be borne in mind that it is accidental, and is always superinduced by causes that have operated since the advent of the drift. Its influence is strictly superficial, rarely exceeding three feet below the natural surface.
No. 2.—We come now to consider that which lies below the surface-soil. If we omit from this enumeration the "bluff-formation," and the alluvium of other streams which sometimes has a considerable thickness, we shall have two different substances, equally pertaining to the drift, and occupying the same relative position in different localities, that claim notice:
1. A clay subsoil.
2. A gravel or sand subsoil.
Now, although these are mentioned as appearing first beneath the surface-soil, it must not be understood that they appear there invariably, nor even usually. It is probably true that throughout the greater portion of the Northwest they are entirely wanting, and that feature of the drift prevails which will next be considered. They are mentioned here because they constitute an essential part of the drift, and must not be overlooked in giving its character and composition.
By the first, here denominated a clay subsoil, is not meant a gravelly clay, or one in which stones are present. It is, rather, a close, plastic, fine clay, with little observable sand. It is impervious to water, and is benefited by artificial drainage. It prevails in much of Southwestern Michigan and Northwestern Ohio. It occupies a large tract in Northeastern Illinois and Northwestern Indiana. It also probably underlies the Red River flats in Minnesota, and perhaps a belt of land rudely conforming to the shore of Lake Superior at its western extremity. When shafts are sunk through this clay subsoil, so as to reveal its composition and arrangement, it is seen to be handsomely laminated horizontally. The individual layers are separated by thinner layers of fine sand. Those of clay are usually about two inches in thickness, but may be no more than one-eighth of an inch; the layers of sand are rarely more than half an inch in thickness, and are apt to
