546 A TREATISE ON METAMORPHISM. 



especially the former. Indeed, the surface may thus become saturated 

 with these salts. 



The process of migration of alkalies to the surface is well illustrated 

 in various irrigated districts. Before irrigation the soil may have contained 

 only a sufficient amount of alkalies to be very fertile. In many districts, 

 by irrigation for some years, so large a quantity of alkalies has been brought 

 to or near the surface as to injure vegetation or even to make crop growing 

 impossible. Where the water used is sweet, these effects follow from over- 

 irrigation. Where more than a sufficient amount of water is used a part-of 

 it passes to some depth below the surface. Between the times of flooding 

 a part of this is brought to the surface by capillarity, carrying with it 

 various dissolved salts. Also by continual seepage from canals at higher 

 levels water passes below the surface, and by capillarity is brougbt to the 

 surface in the cultivated areas, carrying with it dissolved salts. As the 

 water evaporates these salts are precipitated in the soil. This process is 

 cumulative, and after a number of years, as already stated, the quantity of 

 alkalies may become so great as to lead to the abandonment of formerly 

 productive tracts. This process is well illustrated in San Joaquin Valley, 

 California, and in Salt Lake Valley, Utah. At the latter place alone 

 Whitney states that of the 50 square miles once irrigated about 10 square 

 miles, or one-fifth, has become unfit for agriculture." 



Where the waters used for irrigation are themselves very rich in 

 alkalies, as, for instance, in Pecos Valley, New Mexico, the accumulation 

 of alkalies in the soils is prevented in a different way, namely, by very 

 abundant use of water with considerable intervals between and by thorough 

 underdrainage. By this method the salts which are precipitated between 

 the periods of irrigation by evaporation are dissolved by the abundant 

 waters at the time of irrigation, and thus accumulation of the alkalies in the 

 soil is prevented. 6 



While the more readily soluble constituents are of far more importance as 

 precipitates at or near the surface than the less soluble salts, the latter are pre- 

 cipitated in the soils. Of these difficultly soluble compounds silica and ferric 

 oxide are perhaps the only ones of sufficient importance to require mention. 



Many sandstones which have long been exposed to the weather have a 



« Whitney, Milton, Field operations of the Division of Soils: Rept, Div. of Soils, U. S. Dept. 

 Agric, No. 64, 1899, p. 25. 



& Whitney, Milton, tit:, pp. 22-24. 



