SOIL CHANGES DUE TO IRRIGATION 



83 



tration of dissolved substances. This is well shown by 

 any of the analyses made of river water taken at different 

 distances from the river head. For instance, in the fol- 

 lowing rivers the salinity or the parts of soluble matter in 

 1,000,000 parts of water was as follows: 



The Chalis River, Algeria, is an exception to the rule 

 because tributaries, carrying relatively pure water, enter 

 and dilute the main river near its lower end. 



The quantity of dissolved substances in natural water, 

 that is, the salinity, varies from exceedingly small quan- 

 tities, as in rain-water, to almost saturated solutions, as in 

 the waters of the Dead Sea and the Great Salt Lake. The 

 following table, based upon the classical work on "The 

 Data of Geochemistry," by F. W. Clarke, shows the pro- 

 portions of dissolved substances found hi some of the 

 river waters of the world. No such table, however elabo- 

 rately constructed, can be wholly accurate. At best, only 

 a few of the rivers of the world have been subjected to 

 chemical analysis, and even the rivers that have been 

 most thoroughly studied, have not been analysed at all 

 seasons of the year for a sufficient number of years to 

 make the averages absolute in their values. 



