Il6 THE SOIL, SOLUTION 



The third general origin of alkali supposes that wind-borne 

 sea-spray carries into the air salts which are left in very fine 

 particles on the evaporation of the water, or are deposited on 

 the ordinary atmospheric dust and carried over the land; and 

 that this dust is precipitated here and there as may be determined 

 by the various meteorological conditions which it encounters. 

 All the land surface is supposed to be receiving more or less of 

 it from time to time, but in arid regions the rainfall and drainage 

 is not sufficient to return to the sea as much as is received there- 

 from. 1 



It is very probable that wind-borne salts from the sea are 

 being carried over and to some extent being deposited on all the 

 land surfaces of the earth. To what extent this process is tak- 

 ing place, and whether it is sufficient to account for the alkali 

 of any particular region, available data fail to answer satisfac- 

 torily. Probably it is always associated with one of the origins 

 of alkali already discussed and is in itself generally of second- 

 ary importance. 



An argument frequently advanced against the validity of the 

 hypothesis that wind-borne sea-spray is the origin of alkali is 

 that the relative proportions of the several constituents in "alkali" 

 are seldom if ever those obtaining in sea water. This argu- 

 ment does not take into consideration, however, that the several 

 salts in the spray probably separate into crystals of widely dif- 

 ferent size and specific gravities, and there may well be taking 

 place a selective or sorting action by the wind. More important, 

 undoubtedly, is the selective action taking place in the soil itself ; 

 it can only be an accidental coincidence that the constituents of 

 alkali in any particular occurrence should have the same quanti- 

 tative relations as in the material from which it originated, no 

 matter what may have been the nature of its origin. 



In the field, alkali is found in a bewildering array of forms 

 and types. Quite different combinations of constituents may be 

 found in the same field within a few rods or even a few feet, 

 1 For a recent interesting and valuable discussion of this subject 

 with reference to a particular area, see: The origin of the salt deposits 

 of Rajputana, by Sir Thomas H. Holland and W. A. K. Christie, 

 Records of the Geological Survey of India, 38, 154-186 (1909). 



