1901] VEGETATION OF WESTERN TEXAS 207 
plateau area of the Lower Sonoran zone, wich embraces north 
Mexico, southern New Mexico, and Arizona, and while it is only 
surmised that the grass formation would show this floristic rela- 
tion, it is certain that other formations show it in a marked 
degree. 
MOUNTAINS AND SOUTH PLATEAU SLOPE. 
The Rocky mountains proper terminate before reaching the 
Texas region, The mountain axis continues southward, how- 
ever, as an elevated plateau whose surface is much cut up by 
isolated mountain masses, some of which exceed gooo feet in 
altitude. These masses are composed of Carboniferous lime- 
stone and sandstone, of Cretaceous limestone, and of igneous 
material. The high plateaus among them are mostly bolson 
basins filled with the débris of their bordering mountains. The 
soil conditions here and the moisture conditions growing out of 
these furnish the direct causes which operate to determine the 
distribution of plant formation in a region which possesses 
extreme xerophytic conditions independently of edaphic fac- 
tors. 
Viewed from the standpoint ot the grass formation, this is a 
region possessing a grass floor climate in a surprising degree, for 
although the mountain masses are in some instances high and 
rugged, and covered with woody vegetation, the slopes of most 
of these up to 6000 and 7000 feet, the foothills, the wider 
plateau plains (for example, about Marfa and northwestward), 
and even portions of some of the desert bolsons, have a con- 
tinuous grass formation. But in the mountains of the Great 
bend and their intervening bolsons (except in the Rosillos 
mountains, where good and available range grasses are found, 
according to Havard), on bolsons with loose wind-swept débris, 
and on arid gravelly mesas, there is only the most scattering 
bunch-grass formation, or even no permanent vegetation at all. 
The bolson basins of this region, owing to the distribution of 
sedimentary filling, are lowest at the center and undrained. 
Consequently, the drainage water gathers at this place, in many 
cases accumulating in ponds or rather large lakes, and in all 
