»1.] GRAY AND HOOKER ON THE ROCKY MulNTAlX FLORA. 67 



are always expected to su ppor t forests, at least when not cut off from sea winds by 



interposed chains of equal altitude. Trees eaoh mountains will have. Theonl] and 



the real wonder is that the .Surra Nevada ahould rear such Immense trees! 



Moreover, we shall see that this forest la lull ami superb only in our line; that, 

 bi\oudone favorite trihe, it is meagre enough. Such tor situation, and extent, ami 



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

 be compared. 



In order to some to this comparison I must refrain from all account of the Interven- 

 ing 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, ami 

 those mostly in the southern part, ami of the Mexican plateau type; that they are 

 common to the mountain-chains which lie between, Stretched north ami south ni flu- 

 Ion, all through that arid or desert region of Ptah and Nevada, of which the larger 

 part belongs to the Great Basin between the Rooky Mountains ami the Siena Nevada; 

 that most of the Koeky Mountain trees are identical in species with those of the Pacific 

 faesat, except far north, where a few of our eastern ones are tnterminglecL 1 may add 

 that the Koeky Mountainsproper 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 

 ,")00 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 plain should 

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

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

 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 would otherwise share, i-. as w. have seen, 

 arrested on or near the western border, by the coast-ranees 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 obstruct ion. So the diminution of rainfall is gradual instead 

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

 stowed, first and most on the seaboard district, and least on the remote interior. From 

 the Low.i 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 mull 

 ward, and the western holder of the ultra-Mi seissippian pis >SS than ( 



inch' 



Indeed, from ih<- great prevalence of westerly ami southerly winds, what precipita- 

 tion of moisture there i> on our western plains is not from Atlantic BOUTCes, nor much 

 from the Grul£ The rain-chart plainly show.-, that the water raised from the heated 

 <Hilfi> mainly carried northwand and eastward. It i- this which hasgiveo us the 

 Atlantic forest region; audit is the limitation of this which bounds thai forest at the 

 The lim- on the rain-chart indicating twenty-four Inches of annual rain i> not 

 om the line of the western limit of trees, except far north, beyond the Great 

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

 amount of rainfall snfflces foi I rth. 



yfe see, then, why our great plains grow : proceed from the Mississippi 



aid: though we wonder why this should take pi | ,'orupth as 



it do.-. But, as alreadj itated, the general course of the triad-bearing raine from 



tin- Gulf and beyond i- such as to water v. .11 the Mississippi Vallej ami a!' eastwasi| 

 but do! the diet : "t it. 



It doe- no! altOget llel follow • 



