Upham.] 
144 
[May 21, 
often attributed to the effect of fires. Through many centuries 
fires have almost annually swept over these areas, generally de- 
stroying all seedling trees and shrubs, and sometimes extending the 
border of the prairie by adding tracts from which the forest had 
been burned. Late in autumn and again in the spring the dead 
grass of the prairie burns very rapidly, so that a fire within a few 
days sometimes spreads fifty or a hundred miles. The groves that 
remain in the prairie region are usually in a more or less sheltered 
position, being on the border of lakes and streams and sometimes 
nearly surrounded by them ; while areas that cannot be reached by 
fires, as islands, are almost always wooded. If fires should fail to 
overran the prairies in the future, it can hardly be doubted that 
much of that area would gradually and slowly be changed to for- 
est. Yet it does not appear that fires in the western portion of 
our great forest region are more frequent or destructive than east- 
ward ; and our inquiry must go back a step further to ask why fires 
east of the Appalachian mountains had nowhere exterminated the 
forest, while so extensive areas of prairie have been guarded and 
maintained, though not apparently produced, by prairie fires here. 
Among the conditions which have led to this difference, we must 
undoubtedly place first the smaller amount, and somewhat less equa- 
ble distribution throughout the year, of rain in the prairie region. 
The comparatively shaded and moist southern bluffs of valleys, as 
of the Minnesota and Assiniboine rivers, are generally wooded, 
while the opposite northern bluffs, exposed to the drying sunshine, 
are prairie or have only bushes and scattered small trees. On the 
western and southwestern plains, a still more arid climate sets a 
limit to the grassy prairies, and in their stead dreary wastes of sage- 
bush and cacti cover the parched soil, which yet in many portions 
needs only irrigation to give it fertility and cause it to yield boun- 
tiful harvests. 
Trees and shrubs . — Many species of trees, which together con- 
stitute a large part of the eastern forests, extend to the Red river 
basin, reaching there the western or northwestern boundary of their 
range. Among these are the basswood, sugar maple, river maple, 
and red maple, the three species of white, red, and black ash, the 
red or slippery elm, and the rock or cork elm, the butternut, the 
white, bur, and black oaks, iron- wood (Ostrya Virginica, Willd.), 
the American hornbeam (Carpinus Caroliniana, Walt.), the yellow 
