936 GEOLOGY 



In the west, the northern life was driven by ice behind, hemmed in 

 by mountain and other barriers at the sides, and resisted by arid 

 tracts in front. The arid tracts were themselves shifted in some 

 measure, but the restraint of migration to the east and west became 

 increasingly formidable as glaciers gathered on the mountain 

 heights and occupied the passes. As the trend of the mountains 

 was mainly north and south, they defined a series of meridional 

 tracts which directed the life migrations. Even east of the moun- 

 tains, climatic differences seem to have appreciably restrained east- 

 west migration. 



In the eastern half of the continent, the forests and forest-life 

 were driven southward in a more unrestrained way, but for the 

 greater part they kept within the eastern humid tract. 



Following the last ice-retreat, the life of each of these sections 

 moved northward, each biotic zone, arctic, subarctic, cold-temper- 

 ate, and temperate, expanding as it went. It was as though the 

 life-zones were elastic bodies which had been compressed to narrow 

 limits about the edge of the advancing ice, and then recovered their 

 normal breadth by expansion northward as the ice withdrew. The 

 arctic or tundra flora and fauna that had probably been crowded 

 into a narrow zone fringing the ice-sheet, moved northward through 

 about 20 of latitude, and expanded to a breadth of 600 or 700 

 miles in the northern part of the continent, and occupied the arctic 

 islands w T here not covered by perennial ice and snow. The zone of 

 this arctic flora and fauna now lies mostly north of 60. The sub- 

 arctic zone of stunted conifers moved northward about 12, and 

 expanded into a zone some 400 to 600 miles wide. The cold- 

 temperate belt of deciduous and evergreen trees moved a less dis- 

 tance, but expanded almost equally, while the warm-temperate 

 flora spread itself over the territory abandoned by the last. 



With each of these vegetal zones went the appropriate fauna. 

 The musk-ox, whose remains have been found skirting the. glacial ed 

 area in Pennsylvania, West Virginia, Ohio, Kentucky, Oklahoma, 

 Missouri, and Iowa, 1 lias since retired to the extreme arctic region-:. 

 The reindeer, which had a similar distribution about the edire of 



1 Hay's Catalogue of Fossil Vertebrates in North America. Hull. 17" I . 

 S. Geol. Surv., 1902. 



