Igor] VEGETATION OF WESTERN TEXAS 281 
plain below the Balcones escarpment, and at many places on 
stony slopes as far east as the Colorado. The great tufts of this 
so-called bunch grass form the most conspicuous vegetation on 
the finer débris below the yucca belt in the Rio Grande plain. 
THE AGAVE VEGETATION. 
In addition to species of the Agaveae which associate in the 
mixed cactus-yucca-agave formation to be next described, two 
Species require notice. Agave Wislizeni, or ‘Texas mescal,” is 
a mountain species (apparently upper Sonoran to Transition) 
occurring (according to Dr. Havard) in all the mountains of 
western Texas, and formerly much used for food, beverage, and 
medicine. Agave Americana, or ‘‘ Mexican maguey,” enters the 
Texas region in the lower Rio Grande plain, and two or three 
more insignificant species are also present. 
MIXED CACTUS, YUCCA, AND AGAVE FORMATION OF ARID STONY SLOPES. 
While the trans-Pecos region possesses more or less of each 
of the three types included in the above named formation, in 
most of its physiographic provinces a little investigation shows 
that in the main the three are found upon one type of physio- 
graphic structure, either more or less equally represented, or one 
type predominating. In traversing the high grass plains of the 
Stockton plateau and westward it is noticed that in approaching 
the crest of one of the broad undulations, or any relief feature 
notably above the plains level, a unique yucca vegetation sets in, 
to be followed again presently by other stretches of grass plain. 
Where the margin of the plain rises up into the bordering foot- 
hills, there again is the yucca vegetation. Again, on the rim of 
the enclosed bolson basins this formation is present, and finally 
on many of the gentler slopes of the mountains (especially those 
with timberless arid slopes) the yucca vegetation is present as 
the chief vegetation feature. This is the apparent condition 
seen at a distance, and sometimes the Yucceae are alone in the 
formation. Buta nearer look commonly reveals the less con- 
Spicuous cactus and agave members of the association (jig. 22). 
In all the physiographic features cited, practically the same 
