1905] WHITFORD—FORESTS OF FLATHEAD VALLEY 209 
indeed everywhere that the mesophytic forest grades into the Douglas 
spruce-bull pine combination the forest floor has prairie plants rather 
than forest forms. 
It has been shown that the western larch-Douglas spruce associa- 
tion is found in those areas where the amount of water in the soil 
is less than in the areas that support the spruce formation. Again, 
attention must be called to the fact that in the former extension of the 
lake many parts of the valley now having mesophytic forest were 
moist enough to support spruce forests, and as the water of the lake 
receded the amount of water in the soil became less and less until 
the encroaching mesophytic forests crowded out the spruce forests. 
As there is a relation in origin, therefore, between the meadows and 
the spruce forests, so there is also a genetic relation between the 
spruce forests and the western larch-Douglas spruce forests. In 
places where the hills end precipitously near the lake shore, as on 
the west side of Swan Lake for instance (map), this relation is not 
So apparent, for the mesophytic forests reach in many cases to the 
water’s edge. Yet even here narrow, flat areas at the foot of the 
Mountains on the borders of the lake have small meadows with scat- 
tered Engelmann spruce and cottonwood in them. These will give 
Way, of course, to the encroaching mesophytic element when the 
waters of the lake have further withdrawn. 
In conclusion it may be pointed out that the climax forest of the 
plot under consideration is the western larch-Douglas spruce for- 
mation. This association occupies a greater portion of the com- 
paratively level stretches of Swan River valley (fig. 8). It also is 
found on the protected slopes and foothills of the low Swan Range. 
No attempt was made to determine its range outside of the region 
mapped. The trees that are characteristic of this forest are western 
larch, Douglas spruce, silver pine, lowland fir, Engelmann spruce, 
and lodgepole pine. 
THE DOUGLAS SPRUCE-BULL PINE (MESO-XEROPHYTIC) AND THE 
PRAIRIE (XEROPHYTIC) FORMATIONS. 
Since the Douglas spruce-bull pine associations and the prairie 
are more or less connected, they will be treated together. A refer- 
ence to the map will show that a rectangular plot of ground known 
