FOREST GEOGRAPHY AND ARCHAEOLOGY. 213 



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 coining 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 Cotton woods 

 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 Mouu tains 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 

 jis naturally bestowed, first and most on the seaboard district, 

 anof" 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 



