

418 THE SYLVA OF MONTANA. 
generally are these prairies limited to the porous strata 
of the later formations that I believe some tracts of high 
prairie on the western slope of the Rocky Mountains indi- 
cate the presence either of tertiary or deep beds of drift, 
which latter cover the prairie summit of Mullan’s Pass. It 
must be remembered, however, that this relation to different 
strata is the only one depending on their porosity, and that 
where rains are more abundant this ceases to prevent the 
growth of trees. Strata resembling the Cretaceous of Ne- 
braska in density are on the west side thickly wooded, so 
that there is no indication of their nature from the absence 
of trees. The impervious rocks and thin soil of the Cœur 
d'Aleüe Range evidently assist the more rainy climate in 
producing a moisture fitted for the peculiar group of trees 
characterizing it, and there is a more marked difference in 
its opposite slopes than in those of the Rocky Mountains, 
more striking, however, on account of the greater number 0 
species of trees found there. The contrast is most impor- 
tant between the west slope of the western rim and the east 
slope of the eastern. . 
Many facts show that the trees are more dependent on à 
certain supply of water than on temperature, as will be -— 
by comparing the profile of the route with the distribution 
of the species. Thus on the Rocky Mountains Pinus con- 
toria grows only between 5000 and 6000 (or more) feet of 
elevation, un altitude just sufficient to catch the moisture 
passing over the general summits of the Cour d'Alene 
Mountains, in which the pass we went through is 5100 feet 
high. It reappears at the east base of the latter range, pe 
cause of the impervious rock there, and the increased mots- 
ture deposited on that rim. The various relations of other 
trees to the influence of moisture are shown briefly by the 
following facts of their distribution and growth : 
. Cerasus Virginiana?, Amelanchier alnifolia, P opulus 
. angustifolia and Pinus ponderosa are distributed entirely 
across, but are most highly developed along the Blackfoot 


