GENESIS OF THE SOIL AND ITS POSSIBILITIES 197 



when ordinary irrigation only is practised. It may 

 thus become possible to reclaim large extents of alka- 

 line soil little by little by treating them with an excess 

 of irrigation water in connection with thorough under- 

 drainage. 



ADOBE SOILS. 



In many parts of the arid regions of this country 

 which can be recovered for agricultural purposes by ir- 

 rigation the soil has peculiar characteristics. 



The name adobe as commonly used applies to both 

 the sun-dried bricks of the arid regions of the West and 

 Southwest, and to the materials of which they are com- 

 posed. The material is described by Russell as a fine 

 grained porous earth, varying in color through many 

 shades of gray and yellow, which crumbles between the 

 fingers, and separates most readily in a vertical direc- 

 tion. The coherency of the material is so great that 

 vertical scarps will stand for many years without form- 

 ing a noticeable talus. 



The area over which adobe forms a large part of the 

 surface has not been accurately mapped, but enough 

 is known to indicate that it is essentially co-extensive 

 with the more arid portions of this country. In a very 

 general way it may be considered as being limited to 

 the region in which the mean annual rain-fall is less 

 than twenty inches. It forms the surface over large 

 portions of Colorado, New Mexico, western Texas, 

 Arizona, southern California, Nevada, Utah, southern 

 Oregon, southern Idaho, and Wyoming. Adobe occurs 

 also in Mexico and may there reach a greater develop- 

 ment than in the United States, but observations con- 

 cerning it south of the Rio Grande are wanting. 



In the United States it occurs from near the sea- 



