and each case appears to have a distinct origin, to be in fact a 

 law unto itself. Each alkali deposit represents generally the 

 resultant from a mixture of salt which has been dissolved and 

 reprecipitated a number of times, and which while dissolved has 

 been seeping through the soil under gravitational forces, or has 

 been moving through the soil as film water under capillary 

 stresses. In either event the salt mixture has been subject to 

 the power for selective absorption peculiar to the particular soil 

 mass through which it has been moving. Re-solution is seldom 

 an instantaneous process, and different rates of solution neces- 

 sarily involve some separation of salts. Finally the alkali de- 

 posit is usually so mixed with other soil material that there 

 cannot be recognized the characteristic solid phases (such, for 

 instance, as the double sulphates of calcium and another base) 

 which serve as guides in laboratory studies and in certain salt 

 mines. Even if the characteristic salts are deposited in surface 

 soils, it is very doubtful, owing to their hygroscopicity, if any 

 but gypsum, halite and Glauber's salt can persist for any length 

 of time. The alternations of temperature from night to day 

 characteristic of arid regions, with precipitation of dews, might 

 easily be expected to make noticeable and rapid changes in the 

 characteristics of any given alkali or salt mixture. 



It is 'not surprising, therefore, that attempts to account for 

 the genesis and present appearance of an alkali deposit by com- 

 parison with artificial depositions of salt mixtures, as worked 

 out in the laboratory, have generally been disappointing. On 

 the other hand, laboratory studies have been quite fruitful in 

 elucidating the phenomena taking place on the leaching of alkali 

 from a soil, or so-called "alkali reclamation." 



Whatever the origin of the alkali, its segregation at or near 

 the surface of the soil is everywhere much the same; that is, 

 there is a translocation and segregation of soluble salts in the 

 below-surface seepage waters, determined mainly by the topo- 

 graphic features, but partly by the texture and structural prop- 

 erties of the soil and subsoil, with a subsequent rise as capillary 

 water consequent upon evaporation at the surface. Precipita- 

 tion of the solutes may take place at the surface ; more commonly 



