78 



Biology in America 



has both inlet and outlet, its water is being continually 

 changed, and consequently it contains nearly the same amount 

 of salt from year to year. But if a change of climate occurs, 

 sg that evaporation exceeds precipitation, the lake begins to 

 shrink, its outlet is lost and the salts which are carried into 

 the lake by its tributary rivers accumulate, and a little inland 

 sea is formed. Thus have arisen the various salt lakes and 

 inland seas such as the Caspian, the Dead Sea and Great Salt 

 Lake. If now we measure the amount of w^ater carried by 

 the tributary rivers of a closed lake, and determine the amount 

 of salts cai'i-i(^d by them, wo cnii estimate the number of j-ears 



Shore op Salton Sea, Showing Old Lake Level ix Background 

 The character of the beaches of extiuct lakes gives a ckie to the 

 weather of the past. 



After MacDougal. 



required for the lake to acquire its present degree of saltiness 

 since the time when it had an outlet. Such measurements are 

 at best approximate, due largely to the fact that when rain- 

 fall was greater the rivers carried a greater amount of salt 

 than they do at present, but Avhen compared with the testi- 

 mony of the old shore lines, they furnish a means for 

 determining probably to within 50 or 100 years the periods 

 of heavy and light rainfall in the past. 



These ancient lake beaches can often be traced for miles 

 with the greatest ease. When a lake maintains the same level 

 for a number of years it leaves an unmistakable record "on 

 the sands" or gravels ''of time" in a clear-cut beach or 

 terrace, where the waves have undermined and cut away the 



