1901} VEGETATION OF WESTERN TEXAS 311 
But it is of varying degrees of xerophytism according to its 
relation to the zones of decreasing moisture from east to west, 
that in the eastern provinces possessing some of the elements 
of the vast mesophytic forest of east Texas, while the extreme 
of xerophytism is reached in the desert bolsons of extreme west 
Texas. Recalling the common conception of the west Texas 
region as a grass country, it becomes a matter of surprise to 
learn how great is the proportion covered by woody formations, 
that is arborescent or shrubby vegetation in sufficient amount 
to form the chief covering. 
This includes the following areas: the erosion area of the 
Edwards plateau; the hills, bluffs, and escarpment of the Grand 
prairie; the granite, Carboniferous, and upper cross timber 
areas; the higher mountain slopes and summits of the trans- 
Pecos mountains; the escarpment of the Staked plains and the 
Callahan divide; the chaparral of the Rio Grande plain, of 
trans-Pecos Texas, and of the lower Staked plains and adjacent 
Edwards plateau; the open streamways and the cafions; and 
finally many thousands of miles of mesquite-covered prairie. The 
energy and rate of encroachment of woody vegetation during 
the past half century lead one to believe that there is scarcely 
an area of consequence in the state that woody vegetation of 
some type will not occupy and cover more or less completely, 
granting of course that no artificial means are employed to 
check it. 
The woody vegetation is here discussed under several types 
of formation, based largely upon moisture relations. 
ARBORESCENT VEGETATION. 
XEROPHYTIC FOREST FORMATIONS OF THE EROSION MOUNTAIN- 
OUS AREA OF THE EpwarbDs PLATEAU (including the hill, bluff, 
and escarpment timber of the Grand prairie). —That portion of 
the Edwards plateau embraced between the Colorado and Devils 
rivers, and extending back 100 to 125 miles from the Balcones 
escarpment (southern downthrow of the Great plains) is very 
deeply cut by erosion channels, until the appearance is that of 
