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Igor ] VEGETATION OF WESTERN TEXAS 107 
Indian Territory to the southern extremity of the Rio Grande 
plain. 
MOISTURE, WIND, AND LIGHT. ‘ 
MOISTURE, 
RAINFALL.— Beginning with an average fall of about thirty 
inches at the g8th meridian, the annual rainfall of the west 
c.3-— Precipitation in the Texas region: /, over 50 in.; //, over 45 in.; Z//, 
over 40 in.; /V, over 35 in.; V, over 30 in.; V/, over 25 in.; VJ, over 20 in.; VIII, 
over 15 in.; ZX, over 10 in.— From Hill’s Phys. Geog. aes 
Texas region decreases to nine inches at El Paso. This decrease 
is in general so uniform that the state may be divided into north 
and south zones of precipitation of five inches difference in the 
annual amount. The organization of vegetation with respect to 
moisture emphasizes these zones. The greatest interruption in 
their north-south equality occurs in the Guadalupe and Davis 
mountains, where the rainfall on the summits is fully double that 
of the bolson plains between the ranges (fig. 3). 
