150 



LAKE DEPOSITS 



solubility, the least soluble material being deposited first and the 

 most soluble last. Comparatively little chemical reaction appears 

 to take place in these lakes ; the substances are, for the most part, 

 thrown down merely by the evaporation of saturated solutions and 

 are the same as those carried in very dilute solutions by the tribu- 

 tary springs and streams. If the precipitation of salts is slow 



FIG. 55. — Island of calcareous tufa, Pyramid Lake, Nevada. (U. S. G. S.) 





1 



and occasional, the chemically and mechanically formed deposits 

 are mingled together ; but if such precipitation be rapid, then 

 thick and nearly pure masses of the salts are thrown down 

 in their proper order, as the concentration by evaporation 

 proceeds. 



The first substances to be deposited from solution are the car- 

 bonate of lime and red oxide of iron (CaC0 3 and Fe 2 3 ),and in 

 moderately saline lakes this is about the limit of precipitation. 

 These same materials are thrown down in fresh lakes also, and 



