50 STEPPES AND DESERTS. 



shews itself principally in the district of the Upper Missouri, 

 and still nearer to the eastern declivity of the Eocky Moun 

 tains, where a river bears the native name of Mankizitah- 

 Watpa, or the " river of the smoking earth." Scoriacous 

 pseudo-volcanic products, such as a kind of porcelain jasper, 

 are found in the vicinity of the "smoking hills." Since 

 the expedition of Lewis and Clark an opinion has become 

 prevalent that the Missouri deposits real pumice on its 

 banks. Fine cellular whitish masses have been confounded 

 with pumice. Professor Ducatel was disposed to ascribe 

 this appearance, which was principally observed in the chalk 

 formation, to a " decomposition of water by sulphuric 

 pyrites, and to a reaction on beds of lignite." (Compare 

 Fremont's Keport, p. 164, 184, 187, 193, and 299, with 

 Nicollet's Illustration of the Hydrographical Basin of the 

 Upper Mississipi River, 1843, p. 39-41.) 



If, in concluding these few general considerations on the 

 physical geography of North America, we once more turn 

 our attention to the spaces which separate the two diverging 

 coast chains from the central chain, we find, in striking 

 contrast, on the one hand, the arid uninhabited plateau 

 of above five or six thousand feet elevation, which in 

 the west intervenes between the central chain and the 

 Californian Maritime Alps which skirt the Pacific ; and on 

 the eastern side of the Eocky Mountains, between them and 

 the Alleghanies, (the highest summits of which, Mount 

 Washington and Mount Marcy, are, according to Lyell, 

 6240 and 5066 French, or 6652 and 5400 English feet 

 above the level of the sea,) the vast, well-watered, and fertile 



