FOREST GEOGRAPHY AND ARCHZOLOGY. 218 
which the larger part belongs to the great basin between the 
Rocky Mountains and the Sierra Nevada; that most of the 
Rocky Mountain trees are identical in species with those of the 
Pacific forest, except far north, where a few of our eastern 
ones are intermingled. I may add that the Rocky Mountains 
proper get from twelve to twenty inches of rain in the year, 
mostly in winter snow, some in summer showers. 
But the interior mountains get little, and the plains or val- 
leys between them less; the Sierra arresting nearly all the 
moisture coming from the Pacific, the Rocky Mountains all 
coming from the Atlantic side. 
Forests being my subject, I must not tarry on the woodless 
plain — on an average 500 miles wide — which lies between 
what forest there is in the Rocky Mountains and the western 
border of our eastern wooded region. Why this great sloping 
plain should be woodless — except where some Cottonwoods 
and their like mark the course of the traversing rivers — is, 
on the whole, evident enough. Great interior plains in tem- 
perate latitudes are always woodless, even when not very arid. 
This of ours is not arid to the degree that the corresponding 
regions west of the Rocky Mountains are. The moisture from 
the Pacific which those could otherwise share is — as we have 
seen — arrested on or near the western border by the coast- 
ranges and again by the Sierra Nevada; and so the interior 
(except for the mountains) is all but desert. 
On the eastern side of the continent, the moisture supplied 
by the Atlantic and the Gulf of Mexico meets no such obstruc- 
tion. So the diminution of rainfall is gradual instead of 
abrupt. But this moisture is spread over a vast surface, and it 
is naturally bestowed, first and most on the seaboard district, 
and least on the remote interior. From the lower Mississippi 
eastward and northward, including the Ohio River basin, and 
so to the coast, and up to Nova Scotia, there is an average 
of forty-seven inches of rain in the year.. This diminishes 
rather steadily westward, especially northwestward, and the 
western border of the ultra-Mississippian plain gets less than 
twenty inches. 
Indeed, from the great prevalence of westerly and southerly 
