1901 ] VEGETATION OF WESTERN TEXAS 203 
rainfall. The soil water level is too far beneath the surface to be 
drawn upon even: by deep-rooted trees. The vegetation is 
dependent, therefore, upon the immediate surface for its mois- 
ture, and alternates quickly between active growth and the dor- 
mant conditions according as the surface is moist or dry. But 
the porosity of the soil enables it to retain a larger percentage 
of precipitated moisture, so that the xerophytic conditions are 
not so extreme as the low rainfall (15 to 18 inches), the high 
FIG. 9.— Characteristic occurrence of Acacia amentacea or black chaparral; on 
silty clay aauke plain near Eagle 
altitude (3500 to 4000 feet), the high evaporation capacity of 
the air (60 to 70 inches), and its high velocity would lead us to 
expect. The Staked plains do not terminate at the south in an 
escarpment, but pass into the plains portion of the Edwards 
plateau by a broken series of outlying buttes and sand hills and 
plains. These sand plains not only form the transition from the 
Staked plains, but extend northward néarer their western side 
beyond the center of this province. 
In its temperature relations the Staked plains is mainly Upper 
Sonoran, The transition from Lower Sonoran is made about 
the 3500 foot contour. Beginning at the southern limit of the 
plains, the Lower Sonoran vegetation, notably the chaparral 
