JVo. 1.1 GRAY AND HOOKER O^ Till': ROCKY MOUNTAIN FLORA. 21 



Spirwa discolor, wliicli in its various forms fiourisbcs under exceed- 

 ingly different altitudes. 



Spirwa Millefolium, which is quite peculiar to the Gr(iat Basin. 



Spiraea cwspitosa should be added, although it spreads in mats over 

 the face of rocks, concealing its trunk, instead of rising into the air. 



Coleogyne ramosissima, a highly ])eculiar geiuis of a single species, 

 found ouly on the southern border of the Great Basin. 



Prunus Andersonii, of the Amygdaleous type, restricted to its south- 

 western rim. 



Hardly elsewhere is such an assemblage to be found. Of otlier shrubs, 

 CeuiiotJms vclutinus and Ribes cereiim are the most widespread and abun- 

 dant. One species of Ephedra extends along the mountains almost to 

 the northern border of the Great Basin, and two or three more are 

 among the characteristic shrubs of the region south of it. 



As to herbs, the genera and the groups mentioned above as predomi- 

 nant at a greater elevation (especially Gilia, Pentsteynon, Phacelia, and 

 EYiogonum) still play a prominent part. Astragali become more numer- 

 ous, as also do white-flowered species of CEnothera, and Helianthoideous, 

 Helenioideous, and Senecionoideous Compositse are conspicuous, yet not 

 more so than in other parts of North America. Few Compositse are 

 peculiar to this zone, and few genera are peculiar to the Eocky Mount- 

 ain region as distiiiguished from the Californian. The more character- 

 istic genera of the whole region may be adverted to in another connec- 

 tion. 



2. The arid or desert interior district, namely, that between the Eocky 

 Mountains and the Sierra Nevada, the central part of which is the 

 Great Basin proper, with no exterior drainage, but which also extends 

 far north between the Eocky Mountains and the Cascades, and is there 

 drained by the Columbia Eiver, and far south over the district through 

 which flow the waters of Eio Colorado and the Gila, with also an exten- 

 sive eastern outlier between the Wahsatch and the Colorado Eocky 

 Mountains, and, as well, north of the Uintas, drained by the Green Eiver, 

 the nuiin and farthest source of the Colorado, where an arid woodless 

 tract, with all the features of the Great Basin, broadly intcrsec^ts the 

 wooded Eocky Mountain ranges. The mountains which traverse and 

 diversify these deserts are thought to occupy about half the area, and al- 

 though many of them appear to be as bare as the intervening valleys, 

 yet their varied surface and exposure and the condensation of moisture 

 which they compel, even from an unwilling air, nourish a dilfereut veg- 

 etation, consisting of a larger number of species. This having already 

 been noted, only the botany of the valleys and plains is uiulei' present 

 consideration. 



The region, in a general botanical view, is one of undershrvbblness ; and 

 the i)revalent growth is composed of Artcmisias, Chenopods, and lig- 

 uescent small-tlowered Composita?. It cannot be better described than 



