532 



GEOLOGY. 



and expanded to a breadth of 600 or 700 miles in the northern part 

 of the continent. It spread even beyond, occupying the arctic islands 

 and Greenland, where not covered by perpetual ice or snow. The zone 

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

 arctic zone of stunted conifers moved about 12° northward, and expanded 

 into a zone some 400 to 600 miles wide. The cold-temperate belt of 

 deciduous and evergreen trees moved a less distance, but expanded 

 almost equally, while the warm-temperate flora spread itself over the 

 territory abandoned by the last. With each of these vegetal aones 

 went the appropriate fauna. The musk-ox, whose remains have been 

 found skirting the glaciated area in Pennsylvania, West Virginia, 

 Ohio, Kentucky, Indian Territory, Missouri, and Iowa, 1 has since 

 retired to the extreme arctic regions. The reindeer, which had a 

 similar distribution about the ice-edge, made a similar but less extreme 

 migration and still occupies the barrens of the northern border of the 

 continent ; while the fur-clothed animals distributed themselves through 

 the three northerly zones, most notably the sub-arctic zone of the 

 conifers. 2 



The westward spread of these floras and faunas of the southeastern 

 regions seems to have been meager, and individual rather than general. 

 On the whole, the southwestern arid and prairie floras and faunas 

 seem to have had the better of the contest with the forest forms, and 

 to have spread eastward in the mid-latitudes at the expense of the 

 southeastern group; at least arboreous vegetation is found appreci- 

 ably farther west in interglacial deposits than on the present surface. 

 This does not seem to be equally true in the higher latitudes, where 

 the trees of the eastern group are distributed far to the northwest. 

 Intermigration between the floras of the east, the west, and north- 

 eastern Asia, seems to have been less restrained in this northern region, 

 doubtless because the climate was there less differentiated into moist 

 and arid portions. 3 



The arid and semi-arid floras and faunas of the southwest seem 



1 Hay's Catalogue of Fossil Vertebrates in North America, Bull. 179, U. S. Geol 

 Surv., 1902. 



2 Some of these and other features are suggestively discussed by C. C. Adams. 

 The Post-Glacial Dispersal of the North American Biota, Biol. Bull., Vol. IX, 1905 

 pp. 53-71. 



3 Adams, loc. cit. 



