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 Frémont’s Report, 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 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 Mississipi, 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 labours 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 Mississipi, 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 Mississipi is one with the Northern Canadian plain, so that one low region extends from the Gulf of Mexico to the Arctic Sea. (Compare my Rélation Historique T. iii. p. 234, and Nicollet’s Report to the Senate of the United States, 1843, p. 7 and 57.) Where the plain is undulating, and where, between 47° and 48° of latitude, low hills (côteaux des prairies, and côteaux 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 Hudson’s Bay from those which seek the Gulf of Mexico. Such a dividing line is formed north of Lake Superior by the Missabay Heights, and more to the west by the “Hauteurs des Terres,” in which were first discovered, in 1832, the true sources of the Mississipi, one of the largest rivers in the world. The highest of these ranges of hills hardly attains an elevation of 1400 to 1500 (1492 to 1599 English) feet. From St.