120 HARVESTING THE FOREST CROP 
Owing to the fact that the mountain chains in North 
America run north and south, many tree species escaped 
extinction during the glacial period when the northern 
portion of the United States was covered with a sheet 
of ice. 
In Europe, where there are but one hundred tree 
species against the five hundred upon this continent, the 
mountain barriers run east and west, and so when the 
tree communities were blotted out by the enormous ice 
sheets, certain species were not able to surmount the 
mountain ranges and thus regain the ground they for- 
merly occupied. 
Forest Regions.—The study of the original forests of 
this continent is most interesting, and from forest re- 
mains found in rocks far beneath the surface, it is 
known that certain species now quite limited in their 
range once were distributed over a wide stretch of 
country. The redwood now occupying a thin belt along 
the coast range of California was once found far in 
toward the Rocky Mountains and also flourished in 
Europe and Western Asia. 
The Pacific Northwest then, as now, was covered with 
dense forests in which hemlock, cedar, and the firs 
predominated, massive timber above and dense under- 
growth, holding the world’s record for uniform den- 
sity. Passing to the east, the Rocky Mountain forest, 
containing lodgepole and yellow pine, or Douglas and 
white fir, or Englemann spruce, depending upon lati- 
tude or elevation, soon gives way to the treeless plains. 
Here mile after mile of open grassy plains are found 
and the early emigrant bound for California stopped 
his prairie schooner and made camp along the water 
courses, since there only could firewood be secured from 
the cottonwoods, box elders and willows. Still farther 
