The country of the third order, agreeably to the subdivision above given, viz. the valley country, is situated upon the rivers, and is included within the hilly and plain country above described. The tracts belonging to this order, usually denominated bottoms, are altogether alluvial, being composed of alternate layers of sand and soil deposited from the water of the rivers upon which they are respectively situated. The alluvion thus deposited, having once constituted a part of the surface of the countries drained by the watercourses tributary to the rivers along which the deposit has been made, it will readily be inferred that the fecundity of the valleys will in some measure correspond with that of the countries whence their alluvion was derived. Accordingly we find the bottoms more or less productive in proportion to the fertility of the regions in which the rivers take their rise and through which they flow. In the valley of the Ohio the quality of the soil appears to improve from its source downwards. The alluvion, of which it is composed is supplied by the Alleghany and Monongahela rivers, which have their origin and courses in a hilly and mountainous country, possessed in general of a sandy surface. The alluvion, supplied by other tributaries entering the Ohio at various points between its source and its mouth, is of a better quality, being composed principally of argillaceous and calcareous earth, which are prevailing ingredients in the soil of the country drained by those tributaries. It should be remarked, however, in relation to all the varieties of alluvia, that they are partially composed of the fine particles of decayed vegetable {206} matter with which the water drained from the surface of the ground is invariably charged. This property in alluvial deposits often prevails to such a degree as to render soils, apparently sandy and sterile, remarkably productive. The alluvial bottoms throughout the United States afford innumerable examples of this fact. The fertilizing matter often exhibits itself in the slimy deposits left upon the surface of the ground after an inundation.