Notes on American Geology. 241 



rect opposition to another theory of these same geologists, that a 

 higher mean temperature prevailed over the northern regions at 

 that period, than now reigns in temperate climes. This would 

 not have been the case, all other things being equal, if the north- 

 ern half of the continent had been nearly all formed by the ocean, 

 notwithstanding the mean temperature is greatly modified in the 

 same parallel of latitude, by the presence or absence of large bod- 

 ies of water, rising with the former and falling with the latter 

 physical condition of the globe. Whence then this immense 

 body of ice, which has scattered boulders over so vast a tract of 

 country, appearing too at an epoch subsequent to the extinction 

 of the mastodon and other mammalia, which evidently lived in 

 this region and enjoyed an equatorial climate anterior to the icy 

 period ? Nothing can reconcile this apparent contradiction, but 

 the admission of a fall of temperature far below that which pre- 

 vails in our day, freezing the enormous lakes of that period, and 

 converting them into immense glaciers, which probably con- 

 tinued undiminished during a long series of years. At the same 

 time, elevations and depressions of the earth's surface were in 

 progress, giving various degrees of inclination to the frozen sur- 

 faces of the lakes, down which boulders, sand and gravel would 

 be impelled to great distances from the points of their origin. 

 This in some cases might result from gravity alone ; but in oth- 

 ers, during the close of the epoch, when the temperature had 

 risen, and avalanches began to descend from the mountain tops, 

 and from numerous less elevated places, there occurred, on a vast 

 scale, the same phenomena which now are familiar to the trav- 

 eller among the Alps. Land slides, like that of one of the hills 

 bordering the Saco river in New Hampshire, and avalanches of 

 mud, filled with detritus of all sizes, some angular, as torn from 

 the surface of the rocks, others having been rolled in the beds of 

 torrents, would be propelled for many- miles over the frozen lakes ; 

 and when the ice disappeared, sand, gravel, pebbles and boulders 

 would lie promiscuously together. That a considerable elevation 

 of land has occurred in some regions subsequent to one of the 

 newest tertiary depositions, is certain, from the occurrence of 

 shells of recent species two hundred feet above the level of the 

 sea. 



M. Agassiz attributes the polished surfaces of the rocks in Swit- 

 zerland to the agency of ice, and the " diluvial scratches," as they 



Vol. XXXV.— No. 2. 31 



