190 Part II. Chapter 2. 
the steeper slopes and many of the exposed rock faces then, as now, in the 
Rocky Mountains of the United States and Canada, the Selkirks and in the 
Sierras supported a rich and varied glacial flora which subject to the diverse 
environmental conditions entered a period of mutation with the evolution of 
many new forms. Many glacial plants also exterminated from Europe and 
the eastern United States where the conditions brought on by the increase of 
temperature, decrease of precipitation and increase of the dryness of the air 
were inimic to such alpine species, persisted under the more favorable and 
more diversified physical and climatic environment of the mountains masses of 
western and 'eastern America. The fact that nearly every requirement of a 
glacial plants is met in the physiographic constitution of the New England, 
eastern Canadian and cordilleran system of mountains together with the diverse 
character of the soils is an explanation of the richness of the alpine flora on 
the high mountains of the east and of the west. 
On all mountain ranges which reached above snow line, there might be a 
periodic increase and decrease of snow, and when there were extensive areas 
of plateau at about the same level, the lowering of the snow-line might cause 
such an accumulation of snow as to produce great glaciers and ice fields. 
But along with such depression of the line of perpetual snow, there would be 
a corresponding depression of alpine and sub-alpine belts suitable for. the 
growth of an arctic and temperate vegetation, and what is perhaps more im- 
portant, the depression would necessarily produce a great extension of the 
areas of these belts on all high mountains, because, as we descend the average 
slopes become less abrupt, thus affording a number of new stations for such 
temperate plants as might first reach them. But just above and below the 
snow line is the area of most powerful disintegration and denudation, from 
the alternate action of frost and sun, of ice and water; and thus the more ex- 
tended area would be subject to the constant occurrence of land-slips, berg- 
falls, and floods, with their accompanying accumulation of debris and of allu- 
vial soil, affording innumerable stations in which solitary wind or bird borne 
seeds might germinate and establish themselves. 
In fact, FERNALD ”) has shown that the glacial till which was characteristic 
of the glacial front consisted of an extremely fine, mixed soil derived from 
rocks of very different kinds and it was probably the availability in these soils 
of the potassium, calcium and magnesium which made it possible for arctic- 
‚alpine plants, now confined to soils derived from rocks containing each 0 
these elements specifically on all of our high mountains to grow side by side 
on the bogs, meadows and alluvial plains in front of the great continental ice 
sheet. Consequently the distribution of such plants, as Dryas Drummondi found 
on the Rocky Mountains and the Gaspi Peninsula, 2000 miles apart, and 
Festuca altaica, a common grass on Mt. Albert, also found along the Yukon 
River and the mountains of Asia is explained by this continuity of soil habitat 
1) Soil Preferences of Alpine Plants Rhodora IX: 191, 
