206 BOTANICAL GAZETTE [MARCH 
The giant arborvitae is another tree that needs brief mention here. 
In the plot under discussion, along the base of the Swan Range 
there are three isolated places where the arborvitae was noted. It is 
usually found in moist places and probably ought to be considered 
as a meso-hydrophytic rather than a mesophytic tree (fig. 13). West 
of the Cascade Mountains, where it reaches its greatest development, 
it is distinctly a tree of the moister regions. 
Another tree found in Swan valley is alpine fir, mentioned before 
as associated with Engelmann spruce in the moist cafions of the 
mountains. From here it spreads to the subalpine regions where 
it occupies a prominent place in the basins where snow lies all the 
year. In places it also finds its way into the lower altitudes in rather 
moist situations, as in Swan valley. The birch maintains a place 
throughout the mesophytic area; especially is it frequent in the 
moister situations and mixed with the lodgepole forests. 
The soil in the mesophytic area is variable in nature, the character 
of the vegetation changing to a certain extent with the variations. 
It is probably not the chemical composition that determines the 
kind of forest, but rather the physical composition, the capacity of 
the soil for holding water being the controlling factor. If the under- 
ground water level is not far below the surface, it makes little differ- 
ence whether the soil is sand, clay, or gravel, so long as there is 
sufficient humus to furnish the needed nitrates. However, back 
from the low-lying lands, where the water level is too far below the 
surface to be reached by the roots of the trees, the physical character 
of the soil plays an important role. As a rule the soil of the valley 
is made up of clay, probably derived from the decay of the dolomitic 
shale, the principal rock of the surrounding mountains. This has 
been washed in and deposited in the bed of the former more extended 
Swan Lake, and is mixed more or less with silt. Humus has acct 
mulated in places, and this is more abundant where fires have nom 
been so prevalent; for not only do fires destroy the humus, but 10 
the open places left after the destruction of the forest it dries out 
rapidly. In contrast with the rich beech and maple woods of the 
eastern United States, the humus content of the soil is considerably 
less in these coniferous forests. This is of course due to the fact that 
the needle leaf of the conifers is not so good a humus producer as the 
