502 lUxLF.TiN 84 



About an inch of precipitation is the normal increase for eVery 

 500 feet of added altitude on the border of the plateaus, and for every 

 250 feet within the border of the plateaus. However, the abrupt- 

 ness of the southwestern and western boundaries of the region 

 causes precipitation of moisture from passing winds to such an ex- 

 tent that the lower elevations in plateaus beyond are arid or semi- 

 arid. In general, only the highest plateaus have an annual precipi- 

 tation of more than twenty-two to twenty-four inches. There are 

 distinct summer and winter precipitation maxima and great differ- 

 ences between temperatures at the various elevations. 



ARIZONA HIGHLANDS REGION 



The Arizona Highlands Region, a continuation of the Great 

 Basin, is a mountainous belt from 70 to 150 miles wide, crossing the 

 State from northwest to southeast. It is characterized by short 

 and nearly parallel mountain ranges of monoclinal structure and 

 rarely above 8000 feet elevation. For the most part these moun- 

 tains are abrupt and denuded of soil except in timbered places. 

 Alluvial soils of many of the valleys and basins that occur through- 

 out the area are valuable for agricultural purposes. 



Throughout the Arizona Highlands Region precipitation mostly 

 falls in local torrential showers, and the resultant water erosion is 

 excessive and important. The surface of the region exposes many 

 strata, intrusions, and extrusions, and there are few large uniform 

 areas of interest to dry-farmers, except in the southeastern part 

 where extensive fertile limestone soils are found. The localized pre- 

 cipitation varies greatly from year to year, and over no large area 

 is the average more than sixteen to seventeen inches. Tempera- 

 tures are generally higher and precipitation lower than in the Colo- 

 rado Plateaus Province. The entire Arizona Plighlands Region 

 must be classed as arid and semi-arid. 



