387] H, EH. Pulling 189 
formed from rock flour in the bed of ancient lake Aggasiz. 
The upper limit of frozen soil encountered during the sum- 
mer varied from a depth of a few inches, near the water’s 
edge on a shore with a north exposure, to about six feet on 
a slope well above the water line and with a southeast ex- 
posure. It is highly probable that one of the most effective 
agencies conditioning local distribution of plant species is 
the depth at which frozen soil is encountered. Large trees 
and other deeply rooted plants could not exist in soils made 
too shallow by the presence of perpetually frozen soil near 
the surface. . : 
The soil of the spruce forest, which is the characteristic 
type of this region, is covered chiefly by sphagnum, often to 
a depth of several feet. Large amounts of water are held 
by the moss so that these forests resemble those of temperate 
regions at the borders of swamps and marshes. If the forest 
is situated on a hillside, however, the soil underneath the 
moss is usually dry and if exposed in windy weather will 
blow as dust. This may perhaps be explained in the light 
of knowledge of the conditions above and below the dry 
layer. This dry stratum rests upon frozen soil, which be- 
cause of its lower vapor pressure and of other probably less 
effective properties, should continually absorb water from 
the adjacent, unfrozen soil. Thus, making the easily justified 
assumption that the soil was originally wet, the conditions 
exist for almost completely drying it, provided it should not 
regain the water lost. The light snowfall in this region 
would be unlikely to produce large amounts of water in the 
spring, especially on these slopes where drainage in the spring 
is rapid over the frozen surface of the soil, the relatively 
small amount remaining being conceivably retained by the 
highly absorbent sphagnum covering. ‘The summer rains, 
which, although frequent, bring comparatively little water, 
are apparently no more than sufficient to supply the trans- 
piration loss of the plants exposed to almost continuous winds 
and often to bright sunshine for many hours a day. 
Roots penetrate this dry layer only to a slight extent, 
