^0.1.] GRAY AND HOOKER ON TIFE ROCKY MOUNTAIN FLORA. 67 



are always expected to support forests, at least when not cut off frfnu sea wiiuls hy 

 interposed chains of equal altitude. Trees such mountains will have. The only and 

 the real wonder is that the Sierra Nevada should rear such iniincnse trees! 



Moreover, we shall see that this forest is rich and snitcrb only in one lino; that, 

 beyond one favorite trihe, it is meagre enough. Such for situation, an<l extent, and 

 surrounding conditions, are the two forests— the Atlantic and Pacific — wliich are to 

 be compared. 



In order to come to this comparison I must refrain from all account of the interven- 

 iug forest of the Rocky Mountains, only saying, that it is comparatively poor in the 

 size of its trees and the number of species; that few of its species are peculiar, and 

 those mostly in the southern part, and of the Mexican jdateau type ; that they are 

 common to the mountain-chains which lie between, stretched north aiul south en eche- 

 lon, all through that arid or desert region of Utah and Nevada, of 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 valleys between them less ; 

 the Sierra arresting nearly all the moisture coming from the Pacific, the Rocky Moun- 

 tains 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 oureastern wooded region. Why this great sloping i)lain should 

 be woodless — except where some Cotton woods and their like mark the course of the trav- 

 ersing rivjers — is, on the whole evident enough. Great interior plains in temperate 

 latitudes are always wuodless, 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 would 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 obstruction. So the diminution of rainfall is gradual instead 

 of abrupt. But this moisture is spread over a vast surface, and it is naturally be- 

 stowed, 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 iu)rthwe8t- 

 ward, and the western border of the ultra-Mississi^jpian plains gets less than twenty 

 inches. 



Indee<l, from the great prevalence of westerly and southerly winds, what precipita- 

 tion of moisture there is on our vrestern plains is not from Atlantic sources, nor much 

 from the Gulf. The rain-chart plainly shows that the water raised from the heated 

 Gulf is mainly carried northward and eastward. It is this which has given us the 

 Atlantic forest region; and it is the limitation of this which bounds that forest at the 

 west. The line on the rain-chart indicating twenty-four inches of annual rain is not 

 far from the line of the western limit of trees, except far north, bcyoiul the Great 

 Lakes, where in the coolness of high latitudes, as in the coolness of mountains, a less 

 amount of rainfall suffices for forest growth. 



We see, then, why our great plains grow bare as we proceed from the Mississippi 

 westward ; though we woQder why this should take place so soon aud so abruptly aa 

 it does. But, as already stated, the general course of the wind-bearing rains from 

 the Gulf and beyond is such as to water well the Mississippi Valley aud all eastward, 

 but not the district west of it. 



It does not altogether follow that, because rain or its equivalent ia needed for forest, 



