1901] VEGETATION OF WESTERN TEXAS 215 
itself when removed, but it is a vigorous agressor upon previ- 
ously open untimbered areas. This persistence may well be 
turned to account in any plans looking toward the maintenance 
of the forest covering on the more arid hills for prevention of 
soil erosion and destructive floods. 
Fic. 13.— Quercus Virginiana (mountain form) formation; Cretaceous hills 
fifteen a west of Au 
XEROPHYTIC FOREST FORMATION OF THE MOUNTAIN SLOPES OF 
TRANS-PEcos TExas.—This formation is the counterpart of that 
just described for the erosion area of the Edwards plateau. It 
is a part of the forest covering of the arid mountain slopes of 
the southwest, including south Colorado, New Mexico, Arizona, 
and northern Mexico. Its area in Texas is practically limited 
to the upper slopes of the mountains in the great bend of the 
Rio Grande (above 6000 feet); to the dry upper cafions in these 
mountains; to the middle slopes of the Guadalupe and Davis 
mountains and their dry cafions. 
The ecological conditions of these mountain slopes are more 
