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Show A NOT..l.TIONS AND ADDITIONS. 59 l\Iissouri deposits real pumice on its banks. Fine, ceUular, whitish rna e have been confounded with pumice. Professor Ducatel was di posed to a cribe 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'slllustration 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 Californian 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, wellwatered, 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, Nicollet, 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 (ooteau des prairies, 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- |