412 BOTANICAL GAZETTE [DECEMBER 



vailing about the margin were intermediate between those of 

 Greenland and Alaska at the present time. In the former case the 

 vegetation is sparse and of tundra type, in the latter the forests 

 occur on the stagnant ice margin. 17 It would appear then that 

 the glaciers would not affect the tree distribution at any great dis- 

 tance from the ice front. But there are other factors which would 

 affect the breadth of the zone of conifer dominance. As we may 

 learn from their present distribution, a dry climate, a youthful 

 topography in which erosion is active, high elevation and sterile 

 soil, all of which imply great variations in temperature and rela- 

 tive humidity, are more favorable to conifers than to broad- 

 leaved deciduous trees. 



It should also be noted in connection with the development 

 of the continental glacier that, as the ice sheets spread from the 

 two great centers of accumulation, they unite in the region north 

 of lakes Superior and Huron. With their near approach to the 

 lakes, the area of conifers is divided into an eastern and western 

 section. As the development proceeds toward the Wisconsin 

 terminal moraine, the western section would be forced toward 

 the Great Plains, while the eastern division would spread south to 

 the Appalachian highlands and coastal plain. 



But in the interior the Ohio basin was occupied by the oaks, 

 ash, hickories; elms and maples. Judging by the present 

 northern limits 18 of some of these species it is doubtful if the 

 conifers could compete with them at any great distance from 

 the ice front, so that the belt of tundra and conifers may have 

 extended as far south as the Ohio, but it seems probable that 

 even north of this river species of oak, ash elm, and maple per- 

 sisted. 



DISTRIBUTION DURING MAXIMUM GLACIATION. 



To be more definite, let us briefly note the conditions that 

 would prevail during the time of maximum extension, from the 

 Atlantic to the Rockies. In New Jersey, with its extensive area 

 of sand and slow-flowing streams, conditions must have been 



*7 RUSSELL, I. C., Glaciers of North America. Ginn & Co. 1901. 

 18 BELL, R., The geographical distribution of forest trees in Canada. Scot. Geog. 

 Mag. 13:281. 1897. 



