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 rnean temperature is greatly. modified in the 
same parallel af: 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 
f 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. 
© 'Phis 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 Score 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 
- hewest tertiary _ depositions, is certain, from the occurrence of. 
shells of recent species two hundred feet above the level of the 
S€a., 
. M. Agassiz attributes the polished surfaces of the rocks in Swit- 
zerland to the agency of i ice, a a “ diluvial scratches,” as they 
Vou. XXXV.—No. 2 
