THE _Valley of the Mississippi_ is the largest in the world; and differs from any other of very great extent, in the peculiar distinctness of its outline. It is bounded south by the gulf of Mexico, west by the Rocky Mountains, north by the great lakes of British America, and east by the Apalachian Mountains. Its general surface may be classed under three distinct aspects; the thickly timbered, the barren, and the prairie country. This valley extends from the twenty-ninth to the forty-ninth parallel of north latitude, and exhibits every variation of temperature from the climate of Canada to that of Louisiana. It is a wide extent of level country, in which the various rivers, inclosed between two chains of mountains three thousand miles apart, find a common centre, and discharge their waters into the sea by a single channel. Geologically considered, this immense valley presents every where the aspect of what is called secondary formation. Its prevailing rocks are carbonate of lime, disposed in the most regular lamina, masses of limestone, in which seashells or organic remains are imbedded, retaining their distinct and original form. At every step, is presented the aspect of a country once covered by lakes or seas. The soil, stones, and exuviæ of lake or river formation, are, to all appearance, of comparatively recent origin. In the alluvial soils, to the depth of from twenty to an hundred feet, are found pebbles, smoothed by the evident attrition of waters, having the appearances of those masses of smoothed pebbles that are thrown on the seashore by the dashing of the surge. Leaves, branches, and logs are also found at great distances from the points where wood is seen at present, and at great depths below the surface. In the most solid blocks of limestone, split for building, deers’ horns and other animal exuviæ are found incorporated in the solid stone.