240 CALIFORNIA ACADEMY OF SCIENCES. 



While the drift mentioned may have destroyed previously 

 •existing land shells in the Atlantic States, as it brought the 

 marine arctic mollusk-fauna as far as the St. Lawrence ba- 

 sin at least, and may have helped to make the vast collec- 

 tion of land shells in the loess of the Mississippi valley, it 

 did not exterminate any spacies as far as known, and there 

 must have been a great extension of the range of many of 

 them toward the north since then. 



On the west slope no such general influence seems to have 

 existed, although Prof. Whitney has shown that the glaciers 

 of the Sierras once had a vastly greater extension downward 

 than now, which he attributes to greater precipitation of 

 moisture, accompanied by greater heat. It seems, however, 

 unlikely that such was the case, but rather that the glaciers 

 co-existed with the eastern drift, a heated epoch being follow- 

 ed by one of increased cold throughout the North Atlantic 

 slopes of both continents, and in some degree increasing the 

 glacial deposits of the western mountains. This glaciation 

 seems to have caused the final extinction or modification of 

 the pliocene creation in the temperate zone. 



The fossil tertiary flora of California, plainly showing a 

 sub-tropical and moist climate, is not contemporaneous with 

 the glacial moraines, but seems to have preceded them. 

 Whether the glacial epoch was an effect of temporary cold- 

 ness of the sun's rays is not material to this question, but 

 the fact is evident that the ice has since then decreased all 

 over the continent, and we may conclude that w r armth has 

 increased again. The pre-glacial hot climate was probably 

 connected with a less elevation, much of the sea borders 

 being submerged, the interior valley of California a sea, and 

 the Great Basin of Utah and Nevada a chain of lakes. As 

 marine pliocene shell beds were raised several hundred feet 

 at the end of that epoch, the Sierras probably rose also. 



Unfortunately the land shells of the tertiary strata of Cal- 

 ifornia are rare and have not been much investigated, but from 

 the fact that the trees show a gradual succession of changes> 



