ANNOTATIONS AND ADDITIONS. 59 



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 

 Report, pp. 164, 184, 187, 193, and 299, with Nicollet's Illustration 

 of the Hydrographical Basin of the Upper Mississippi River, 1843, 

 pp. 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 Cali- 

 fojrnian Maritime Alps which skirt the Pacific ; and on the eastern 

 side of the Rocky 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 low plain or basin of the Mississippi, the greater 

 part of which is from 400 to 600 French feet above the level of the 

 sea,) or about twice the elevation of the plains of Lombardy. The 

 hypsometric conformation of this eastern region, i. e. the altitude of 

 its several parts above the sea, has been elucidated by the valuable 

 labors of the highly-talented French astronomer, Mcollet, of whom 

 science has been deprived by a too early death. His large and 

 excellent map of the Upper Mississippi, constructed in the years 

 1836-1840, is based, on 240 astronomically determined, latitudes, 

 and 170 barometric measurements of elevation. The plain which 

 contains the basin of the Mississippi is one with the Northern 

 Canadian plain, so that one low region extends from the Gulf of 

 Mexico to the Arctic Sea. (Compare my Relation Historique, t. 

 iii. p. 234, and Nicollet's Report to the Senate ~ of the United 

 States, 1843, pp. 7 and 57.) Where the plain is undulating, and 

 where, between 47 and 48 of latitude, low hills (coteau des prai- 

 ries, and coteau des bois, in the still un-English nomenclature of 

 the natives) occur in connected ranges, these ranges and gentle 

 swellings of the ground divide the waters which flow towards Hud- 



