RELATION OF LAKES TO CLI:MATIC CONDITIONS. 55 



When evaporation counter! )alances the inflow for a long- period, the 

 waters are concentrated and become charged with mineral matter, for the 

 reason that all streams and springs contain foreign substances in solution 

 wliicli are left when evaporation takes place. 



It has been found by observation that in regions where the topographic 

 conditions are favorable, a rainfall of about 20 inches per year, and an 

 evaporation from lake surfaces in excess of 50 inches per year, is fre- 

 quently accompanied by the formation of lakes that do not rise sufficiently 

 to find an outlet. When the difference in the direction indicated between 

 precipitation and evaporation is still greater, or when the area from which 

 a lake receives the drainage is small in reference to the area where a lake 

 would naturally form, desiccation may be complete and permanent lakes 

 rendered impossible. 



Whether a lake shall be fresh or saline depends, therefore, on climatic 

 conditions and on the configuration of its hydrographic basin. 



Fresh Lakes. 



3Iatorial in Solution. — As all fresh lakes may be considered as 

 the expansions of streams, their chemical composition is indicated where 

 the actual lake waters have not been analyzed, by the composition of the 

 streams flowing to or from them. It follows, therefore, that the average 

 composition of the waters of fresh lakes would be shown with consider- 

 able accuracy, by the average composition of the princi})al rivers in the 

 region where they occur. 



Analyses of the waters of 20 of the principal rivers of the United 

 States have shown that they contain on an average 0.15044 part per 

 thousand of total soliclj^ in solution, of which 0.056416 part per thousand 

 is calcium carljonate. This may be taken as the average composition of 

 tlie fresh lakes of this country, but more particularly of those in the 

 humid regions. 



In a table of 48 analyses of European river waters given in Bischof's 

 Chemical Geology, the average of total solids in solution is 0.2127 and 

 the average of calcium carbonate 0.1130 i)art per thousand. From tlie 

 analyses of the waters of 36 Furo})can rivers given in Koth's ('lu-mie-al 

 (ieology, including some of those menti(uied by IJisi-hof, the average of 

 total solids is 0,2033 and of calcium carbonate O.OOoOS part per thousand. 



In both American and European rivers, as determined from the above 

 data, the average of total solids in solution is 0.1888 and of calcium car- 



