I90r] VEGETATION OF WESTERN TEXAS 275 
end of the Stockton plateau, the lower slopes and foothills, the 
arid mountain slopes (especially in the Great bend), the dry 
cafions of the Great bend region, and the bolson deserts. On 
the whole it is more of a rock vegetation than the Rio Grande 
chaparral, but it also occupies the finer débris-covered slopes 
and bolson plains. 
Compared with the Rio Grande chaparral it is in large meas- 
ure floristically different. Where it contains the same species, 
as for example Prosopis juliflora, the habit is very different, being 
in this case dwarfed and shrubby bushes, while in the Rio 
Grande plain it is habitually low arborescent. The Mimoseae 
and Caesalpineae form less than 10 per cent. of the species, indi- 
viduals occur less abundantly, and, excepting Prosopis julifiora, 
are different species from those of the Rio Grande plain. A 
more striking individuality as ecological types is attained in the 
Species of this formation. This is seen in the greater abundance 
of the Ephedra type, the felt-covered Eurotia and Croton type, 
those impregnated with volatile resins, like Larrea and Flouren- 
sia, and the wand-like, fluted, thorny stems of Fouquiera (fig. 
20). 
About thirty species are recognized as more or less promi- 
nent constituents of chaparral formation of the trans-Pecos 
Texas. Apparently not so great a percentage of these species 
associate on any one area to constitute the formation, though 
the number of species occurring, for example, on the sides and 
bluff of the Pecos cafion would be little less than half the total. 
Considering types of the trans-Pecos chaparral formation in 
detail, it must suffice to point out the following. 
Larrea Mexicana or Mexican greasewood formation —This is by 
far the most notable formation of any single species. It is espe- 
cially characteristic of high gravelly mesas (as about Fort Bliss, 
El Paso), and of the bolson deserts, extending even to highly 
charged alkaline soils at the center of such basins (fig. 27). 
This greasewood formation is composed of such regular open 
growth as to appear like plantations. The plant is a shrub with 
Spreading top, averaging less than three feet tall. Its special 
