412 BOTANICAL GAZETTE [ DECEMBER 
vailing about the margin were intermediate between those of 
Greenlandand Alaska at the present time. Inthe former case the 
vegetation is sparse and of tundra type, in the latter the forests 
occur on the stagnant ice margin.’?7. It would appear then that 
the glaciers would not affect the tree distribution at any great dis- 
tance from the ice front. Butthere 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** 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. 
*® BELL, R., The geographical distribution of forest trees in Canada, Scot. Geog- 
Mag. 13: 281. 189 97. 
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