In the vicinity of the Rocky Mountains, southwardly of the Arkansa river, the surface of the country, in many places, is profusely covered with loose fragments of volcanic rocks. On some occasions, stones of this description are so numerous as almost to exclude vegetation. A multiplicity of ridges and knobs of various sizes, containing rocks of this character, also make their appearance. All these formations seem to be superincumbent upon horizontal strata of secondary sandstone. But the volcanoes whence they originated have left no vestiges by which their exact locality can be determined. In all probability, they were extinguished previously to the recession of the waters that once inundated the vast region between the Alleghany and Rocky Mountains. Of the rivers that have their courses through this section, those of most note are the Platte, the Konzas and its forks, the Arkansa, and the Canadian tributary to the Arkansa. The Platte rises in the Rocky Mountains, and after an easterly course of about eight hundred miles, falls into the Missouri, at the distance of about seven hundred miles from the Mississippi. It derives its name from the circumstance of its being broad and shoal; its average width being about twelve hundred yards, exclusive of the islands it embosoms; and its depth, in a moderate stage of water, so inconsiderable, that the river is fordable in almost every place. The main Platte is formed of two confluent tributaries of nearly equal size, called the North and South forks, both of which have their sources considerably within the range of the Rocky Mountains. They unite about four hundred miles westward from the mouth of the Platte, having meandered about the same distance eastwardly from the mountains. Besides these, {227} the Platte has two considerable tributaries, the one called the Elk Horn, entering a few miles above its mouth, and the other the Loup Fork, entering about ninety miles above the same place. The valleys of the Platte and its several tributaries are extremely broad, and in many places considerably fertile. They gradually become less fertile on ascending from the mouths of the rivers on which they are situated, till at length they exhibit an arid and sterile appearance. The alluvion of which the bottoms are composed contains a large proportion of sand, which, added to the nitrous and saline matter blended with it, occasions frequent appearances of complete barrenness. Magnesia also appears to be a component part of the soil, a quality invariably derogatory to the fertility of any soil. The valley of the Platte, from its mouths to its constituent forks, spreads to the width of ten or twelve miles, and forms a most beautiful expanse of level country. It is bounded on both sides by high lands, elevated twenty-five or thirty feet above the valley, and connected therewith by gentle slopes.