Mapping Soils and Alkali. 19 



Imperial series. — Delta soils of the Colorado Desert. 



Salem series. — Derived from interbedded red sandstone, shale, 

 and highly ferruginous basalt. Pacific coast. 



San Luis series. — Lacustrine deposits of volcanic rock material, 

 underlain by gravel within 3 or 4 feet. Inter-mountain valleys. 



Scheme of soil classification. — While it is not possible to make 

 a rigid classification of soils from the mechanical analyses, much 

 may be done to systematize and bring into uniformity the classi- 

 fication of soils by the different field parties. The following 

 scheme seems to fit very closely the best field classification, and 

 has been made up from the examination of several thousand sam- 

 ples which have been described by the field men and analyzed 

 in the physical laboratory of the Bureau. 



Coarse sand contains more than 20 per cent of coarse sand and 

 more than 50 per cent of fine gravel, coarse sand, and medium sand, 

 less than 10 per cent of very fine sand, less than 15 per cent of silt, 

 less than 10 per cent of clay, and less than 20 per cent of silt and 

 clay. 



Medium sand contains less than 10 per cent of fine gravel, more 

 than 50 per cent of coarse, medium, and fine sand, less than 10 

 per cent of very fine sand, less than 15 per cent of silt, less than 

 10 per cent of clay, and less than 20 per cent of silt and clay. 



Fine sand contains less than 10 percent of fine gravel and coarse 

 sand, more than 50 per cent of fine and very fine sand, less than 

 15 per cent of silt, less than 10 per cent of clay, and less than 20 

 per cent of silt and clay. 



Sandy loam contains more than 20 per cent of fine gravel, coarse 

 sand, and medium sand, more than 20 per cent and less than 35 

 per cent of silt, less than 15 per cent of clay, and less than 50 per 

 cent of silt and clay. 



Fine sandy loam contains more than 40 per cent of fine and very 

 fine sand and more than 20 per cent and less than 50 per cent of 

 silt and clay, usually containing 10 to 35 per cent of silt and from 

 5 to 15 per cent of clay. 



Silt loam contains more than 55 per cent of silt and less than 25 

 per cent of clay. 



Loam contains less than 55 per cent of silt, and more than 50 

 per cent of silt and clay, usually containing from 15 to 25 per cent 

 of ciav. 



