DISTRIBUTION OF DISSOLVED MATERIAL. 545 



The upward transfer of material, while largely due to the water which 

 does not pass beyond the belt of weathering, is also partly dne to water which 

 under the influence of molecular attraction passes upward from the sea of 

 ground water. The amount of such water increases in proportion as the sur- 

 face of ground water is near the surface. It has already been pointed out 

 (pp. 419-422) that when the ground water is comparatively near the surface 

 a very large amount may thus migrate upward and be evaporated. This 

 material carries with it all the salts which it had in solution in the belt of 

 cementation and such as it may take into solution during its upward move- 

 ment above the level of ground water. Locally, as a result of over-irrigation, 

 seepage from irrigation ditches or unusual precipitation for several years, 

 the level of ground water may be raised. A layer of the belt of weathering 

 may be thus encroached upon. This belt is likely to contain a large amount 

 of soluble material. To illustrate, Hilgard states that "The investigations 

 of the California station have shown that in the arid region few uplands 

 normally contain less than from 2,000 to 2,500 pounds [900 to 1,300 kilo- 

 grams] of soluble salts per acre in 4 feet [1.22 meters] depth, and much 

 more has been found in the silty substrata of the Salton Basin in southern 

 California even to 22 feet [6.7 meters] depth." When the level of ground 

 water is raised this material is abundantly dissolved by the water. If the 

 rise continues so that the level of ground water is within 3 meters or less of 

 the surface, a very large amount of salts may be carried upward to the 

 surface from the belt of cementation by molecular attraction." 



The solutions cany more of the readily soluble compounds than of 

 the difficultly soluble ones, and therefore the substances transferred in 

 greater amounts are the alkalies and the alkaline earths. In humid regions 

 the material thus transferred to the surface layer is largely taken into solu- 

 tion at times of abundant precipitation and carried downward again. There- 

 fore, if there be no change in the composition of the compounds, there is little 

 accumulation of such material near the surface. But in the semi-arid and 

 arid regions, where the belt of weathering is comparatively rich in alkalies 

 and alkaline earths, the material carried upward during the long periods 

 of drought may not be compensated by downward transfer at the rare 

 occasions of abundant precipitation. Consecpiently the upper part of the 

 soil may become exceedingly rich in salts of the alkalies and alkaline earths, 



"Hilgard, E. W., The rise of alkali salts to the soil surface: Science, new series, vol. 15, 1902, pp. 

 314-315. 



MON XLVII — 01 35 



