2fo.l.] GRAY AND HOOKER ON THE ROCKY MOUNTAIN FLORA. It 



Platanus Wrightii, &c., Quercus Emoryi and Q. hypoleuca, &c. Along, 

 with these, as equally foreign to the timber region of the Eocky Mount- 

 ains and the accessory ranges, wo should eliminate and i)lace by them- 

 selves those trees which are characteristic of the southern arid plains, 

 rather than of the mountains. A few of these come into Utah and 

 Nevada, but they mostly belong to Arizona, and to a district, which, 

 with all its aridity, receives a portion of the subtropical summer rainfall. 

 To this category belong — 



Olneya Tesota, a peculiar genus of papilionaceous Leguminoso}. 



ParJcinsonia Torreyana, the Palo Verde {Cercidium of authors). 



Frosopisjtil'ijlora, the true Mesquite, and P. puhescens, the Screo^ Bean 

 or Screw-pod Mesquite, the pods and seeds of which furnish food and 

 forage, the bark a kind of gum-arabic, and the wood good fuel. 



Acacia Greggii, the only one which in this district becomes abores- 

 cent. 



ChiIo2)sis saligna, the Desert Willow, fringing water-courses in the 

 arid district. 



Morus microphyUa, a Texas Mulberry which extends along the south- 

 ern part of New Mexico and Arizona. 



It might be expected that a fair number of trees represented iu the 

 moister and cooler district of the IsTorthern Rocky Mountains would dis- 

 appear from the scantier, interrupted or scattered or restricted woods of 

 the southern mountains ; but we miss from them only one of the north- 

 ern trees above enumerated, namely, the Larch of the region, Larix 

 occidentalism while we miss from the northern mountains no small number 

 of those iu the southern. 



This is not the place to institute a comijarison between the Eocky 

 Mountain forest and the eastern ; but it may be remarked that, while 

 angiospermous, round-headed, and deciduous-leaved trees prevail in the 

 latter, largely in the number of species and genera and conspicuously 

 in the extent of surface occupied, the Eocky Mountain sylva, in its char- 

 acteristic features, is gymnospermous, spiry, and evergreen. In the 

 importance of its useful products, such as lumber, the difference between 

 the two sorts, as a whole, in the Atlantic forest cannot be great. But 

 with perhaps only one exception, that of the so-called ]\[ountain Ma- 

 hogany, Cercocarpns ledi/olius (a small tree or more commonly a shrub), 

 the economical value of the Eocky Mountain forest is almost wholly iu 

 its coniferous trees, and iu the mountains these alone strike the eye. 



Disregarding unessential and inconspicuous features, and eliminating 

 those outlying small trees of the Mexican border, we may say that the 

 Eocky Mountain forest is composed of the fcjllowing species, whieli are 

 arranged somewhat in the order of their conspicuousness and impor- 

 tance : 



Pinus pondcrosa, called Yellow Pine, and sometimes Long-leaved Pine, 

 which distinguishes it well from the next. It is a composite species, and 

 the form of it which we are concerned with, and to wbicli Engelmann 



