1907] Fernald,— Soil Preferences of Alpine Plants 193 
state of the finest particles, as shown above, the surface-area presented 
by the ton would be enormous — in fact, it would be equal to 256,000,- 
000 square feet, 5,877 acres, or more than 9 square miles." ! 
It is quite conceivable, then, that the polar plants, which had been 
forced to find temporary homes on the potassic, calcareous, or mag- 
nesian areas or the mixed soils of our more southern states and in 
Mexico, found in the extensive alluvial deposits, which, after the 
receding of the ice, covered much of the northern half of our continent, 
a mixed soil in which they were able to spread both eastward and 
westward as well as poleward; and then, as the particular soil- 
element upon which they most depended gradually became exhausted 
from the mixed soil and as the climate at low altitudes became increas- 
ingly warmer, these plants found upon bogs, cold méadows or sheltered 
alluvial shores, or upon cold cliffs and exposed mountain summits, 
the only habitats in which they have been able to persist within our 
temperate regions. 
In areas of conglomerate rocks, it should be added, as for instance 
on some of the conglomerate cliffs along the lower St. Lawrence, 
where the component pebbles are of varied origin (limestones, ser- 
pentine, feldspathic rocks, etc.), there is sometimes a mixture of floras 
similar to that found on the bogs or meadows, and such plants as 
Empetrum and Hedysarum or Zygadenus will be found occasionally 
on the same cliffs. In such mixed areas it is, of course, difficult with- 
out actual examination of the soil and analyses of the ash of the 
plants, such as are now being made by Mr. H. H. Bartlett, to deter- 
mine the controlling soil-elements. 
Similarly, in the mixed soils which cover the ordinarily tillable 
districts of our northern states and Canada the problem is complicated, 
and in these temperate areas the preference of certain plants for one 
soil or another is best seen in rock- and cliff-habitats. There is need 
of close observations along these lines, and, although, in the alpine 
and subalpine areas here chiefly considered, the distribution of plants 
is largely controlled by the preponderance in the soil of potassium, 
calcium and magnesium, it will be found that in various areas, sodium, 
iron, and other elements are of equal importance in determining the 
ranges of our plants. 
1 Cushman, 1. c. 27, 28. 
