1995] WHITFORD—FORESTS OF FLATHEAD VALLEY att 
The question arises, what is the cause of this difference in the char- 
acter of the vegetation in so limited an area? So far as could be 
ascertained, trees have been absent from Nigger Prairie for at least 
twenty years, and it is very probable that it has never been forested. 
The character of the soil in both the prairie and the bull pine zone 
surrounding it is decidedly different from that of the country im- 
mediately adjacent. That to the south and west is a residual clay 
from the dolomite shale of Swan Hill; that to the east is a clay rich 
in humus; and that to the northwest, north, and northeast is a pebbly 
clay of glacial origin. On the other hand, the soil of the entire 
prairie and bull pine region is composed of sand mixed in places 
with a considerable amount of humus. The limits of this sandy 
area extend a little beyond the limits of the bull pine forest. It 
seems very probable that the absence of any other trees than bull 
pine is due chiefly to the physical character of the soil. It must be 
remembered that the climate of the area under discussion is such as 
barely to favor a forest, and a slight difference in the amount of 
water which the soil can hold may lead to the exclusion of certain 
trees, and in places of all trees. Since a sandy soil lets water drain 
through it readily, it is very likely that this area is too dry to support 
a mesophytic forest. As shown by wells, the underground water 
level in Nigger Prairie is some distance below the surface, so that 
those trees which have shallow root systems would not be so likely 
to succeed as those whose root systems are extensive and deep. The 
bull pine is one of the latter kind, and it is very likely that this is one 
reason why it has the habit of growing in drier situations than some 
of the other trees. 
If the surface layers of a sandy soil can be made to retain water, 
the objectionable feature of such a soil will be partly overcome, and 
this can be brought about by an accumulation of humus. Lower 
forms of vegetation, even grass, by their decay will add to the humus 
content of the soil. In very dry countries this accretion would be 
exceedingly slow, for the oxidation of the organic débris would be 
great; but even in dry situations by the accumulation of a slight 
amount of humus the water-holding capacity of the soil might be 
increased sufficiently to tide a few trees over the driest months of 
the year. Trees once established in this way would help accumulate 
