Rocky Mountains. 3jS 



superincumbent upon horizontal strata of secondary sand 

 stone. But, the volcanoes whence they originated, have 

 left no vestiges by which their exact locality can be deter- 

 mined. In all probability they were extinguished, previously 

 to the recession of the waters that once inundated the vast re- 

 gion between the Alleghany and Rocky Mountains. 



Of the riv, rs 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 iMountains, and after an east- 

 erly course of about eight hundred miles, falls into the Mis- 

 siouri, 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 Moun- 

 tains. 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 the 

 Platte has two tributaries of a respectable size, 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 vallies of the Platte and its several tributa- 

 ries are extremely broad and in many places, possessed of a 

 good soil. 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 ompo- 

 nent part of the soil, a quality invariably derogatory to the 

 fertility of any soil. The valley of the Platte, from its mouth 

 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. 



The river in several places, expands to the width of many 

 miles, embosoming numerous islands, some of which are 



VOL. II. 45 



