

- 416 THE SYLVA OF MONTANA. 
Though we merely skirted the northern limits of the Hes- 
perian region it shows, even there, sufficient distinctness of 
produets to separate it from the "Dacotan" east of Mi 
river. Even its woodless plains differ materially in vegeta- 
tion, having a better growth of grass, and in some very 
sandy tracts, presenting the shrubby forests characterizing 
the whole “Rocky Mountain Province.” It evidently runs 
into the “Saskatchewan” region to the north, which is truly 
a “Campestrian” one. Farther explorations will doubtless 
reveal more spurs of the Rocky Mountains near the one 
hundred and seventh meridian, with the western trees cover- 
ing them, and the fall of the Missouri, with. its lofty cliffs 
throughout this region, plainly shows that even the plains 
form an elevated plateau, or basin, from which the descent 
to the “Dacotah” plains is by a sort of step, often sudden, 
or marked by the protrusion of lower rocks above or near 
to the surface. I have generally found that the base of à 
mountain range formed a stronger limit to the range of spe- 
cies of trees than the summit, and this fact is illustrated in 
the present ease by the change occurring above Milk river 
at the first mountain range. The rule extends also to other 
plants and to animals, as all explorers will testify. 
. At the eastern base of the Rocky Mountains proper _ 
where the Missouri literally cuts through them, the fact is 
repeated, and there I found the following western trees; 
which will probably be found also to reach the more eastern 
ranges: Acer glabrum (tripartitum is a variety), Betula occi- 
dentalis, Alnus viridis or rubra? (viridis is a boreal species ) 
and Populus angustifolia. Amelanchier alnifoliu, although 
a tree on the west side, is but a shrub on the east slope 
of the mountains, from the influence of a drier climate. It 
is said to extend to Lake Superior. Populus tremuloides 18 
also a boreal species, occurring in the mountains everywhere 
above a certain elevation. Some other boreal species have 
.. been found by Dr. Hayden to straggle to the Black Hills, 
.. Such as Pinus Banksiana, Abies nigra (and alba?). Though 


