16 FIELD OPERATIONS OF THE BUREAU OF SOILS, 1917. 



number by tenants, and a very few by managers. This hio^h per- 

 centage of farms operated by owners is due mainly to the large 

 Indian population. 



The average assessed value of land in 1910 was $27.47 an acre. It 

 is very difficult to estimate the average value of farm lands at the 

 present time, as no sales of the Indian holdings take place, and 

 transfers of other lands have been few in recent years. Values prob- 

 ably range from a few dollars to $75 an acre, depending upon the 

 means of irrigation and other local conditions. 



SOILS. 



The soils of the Middle Gila Valley area are derived from a variety 

 of rocks, all the soils except those identified with the several small 

 buttes and parts of the adjacent mountains having been transported 

 and laid down by water. The predominating rocks in the mountains 

 and buttes are granite, mica schist, and other crystallines. Igneous 

 rocks of low quartz content and of basic character occur on some of 

 the buttes, and have entered into the formation of the soils to a 

 small extent. In the western and southern parts of the area small 

 fragments and bowlders of igneous rocks have been washed down 

 over the desert plains by torrents. 



The soils may be classed in three main groups — namely, residual 

 soils, or those derived in place through the weathering and distin- 

 tegration of consolidated rocks; old valley-filling or desert soils, 

 Avhich are derived from the weathering and other modification of 

 old unconsolidated, water-laid material; and recent alluvial or 

 stream bottom soils, consisting of comparativel}^ recent alluvial prod- 

 ucts which have undergone no important changes by weathering or 

 internal modification subsequent to. deposition. Another main 

 group — namely, wind-laid soils — is represented, but is not of suffi- 

 cient extent in this survey to warrant separate grouping, and it is 

 included with the old valley-filling soils. 



The main groups of soils are divided into series in which the 

 soils are similar in color, origin, mode of formation, subsoil condi- 

 tions, and other essential features except texture. The series are 

 divided into types, the latter being determined by the proportions 

 of sand, silt, and clay present in any particular case. The soil type 

 is the unit of mapping. 



Following this system of classification, five series of soils, each 

 represented by one or more types, are recognized in this survey. 

 In addition, two miscellaneous, nonagricultural soils are shown on 

 the map — viz. Rough stony land and Riverwash. 



Residual soils. — The residual soils in the Middle (rila Valley area 

 are of little importance. They are represented only by Rough stony 



