

























THE SYLVA OF MONTANA. 421 
either north or south of these parallels, wherever they find 
the proper amount of rain and heat as combined in these 
mountains. 
Shoshonee region.—The Great Columbia Plains show their 
peculiar features in prairies extending through the valleys 
on the route north to Fort Colville, which are, however, so 
small in extent compared with the forests, as not to be sepa- 
rable from the Kootenay region. Just north of the Spokan 
are the first extensive plains on the uplands, and to the 
south these become rapidly spread to the entire exclusion of 
forest, so that for days together not a tree is seen except 
shrubby willows on the banks of streams. Even the Blue 
Mountains show but a narrow strip of timber just along 
their summits in latitude 469, which is said to disappear 
farther south, though the upper waters of the rivers flowing 
from them are pretty well wooded with deciduous trees. 
The only new ones that occur, and these only as stragglers 
from the south, are Rhus glabra?, Celtis reticulata and, per- 
haps, Crategus sanguinea?, if more than a variety of C. 
rivularis. On the Walla Walla river are also found Populus 
angustifolia, P. monilifera, Alnus rubra? and Betula occi- 
dentalis. Some of the willows are, probably, also distinct 
from those of the mountains, but being undeterminable from 
leaves alone, I have omitted them throughout these remarks. 
(See notes on the trees observed, p. 405.) 
A brief comparison of this with the plains of regions east 
of the Rocky Mountains, will show how little connection 
exists between soils or rocks and the growth of trees, how 
much depends on a proper amount of moisture. 
[ The entire plain is underlaid by basalt, covered thinly 
_ With a fine dusty soil, which I believe to have been also vol- 
canic in origin, having been poured out with lava in the 
form of mud. In parts this has been blown into high 
ridges, while in others it is washed entirely away, leaving 
le bare rock at the surface. This makes no difference 
however in regard to the trees, and little to other vegeta- 
