RECORDS LEFT BY RIVERS 



The mineral salts are not evaporated, consequently the 

 lakes become gradually more mineral or, for convenience, 

 let us say, become salter. Among the mineral salts 

 common salt and gypsum are most important; but 

 some bitter lakes contain sodium carbonate or magnesium 

 chloride. The Dead Sea and the Great Salt Lake of 

 Utah show by the deposits round them how they have 

 changed their shape and depth. In the upper terraces 

 of the Great Salt Lake, 1000 feet above the present 

 level of the water, fresh-water shells occur, showing that 

 the basin was at first fresh. The valley bottoms around 

 salt lakes are now crusted with gypsum, salt, or other 

 deposits, and their waters are without sign of life. Such 

 conditions help us to understand how great deposits of 

 salt or gypsum were once laid down in England, Poland, 

 and Germany, and in many other places where now the 

 climate would not permit of the necessary evaporation 

 and condensation of the water. 



