392 GEOLOGY. 



deposits often underlie those of salt. Deposits of salt and other mineral 

 matters once in solution are making in some salt lakes at the present 

 time, and considerable formations of the same sort have been so made 

 in the past. Buried beneath sediments of other sorts, beds of common 

 salt or of other precipitates are preserved for ages. Lime carbonate 

 has been precipitated in quantity from some extinct lakes (Fig. 333). 



The lakes which originate by the isolation of portions of the sea are 

 salt at the outset. If inflow exceeds evaporation, they become fresher 

 andmay ultimately become fresh; otherwise they remain salt. If evap- 

 oration exceeds inflow they diminish in size and their waters become 

 more and more salt or bitter. 



Indirect effects of lakes. — Lakes tend to modify the climate of the 

 region where they occur, both by increasing its humidity and by decreas- 

 ing its range of temperature. They act as reservoirs for surf ace- waters, 

 and so tend to restrain floods and to promote regularity of stream flow. 

 They purify the waters which enter them by allowing their sediments 

 to settle, and so influence the work and the life of the waters below. 



Composition of lake-waters. — The accompanying table^ shows the com- 

 position of various inclosed lake-waters, and gives some idea of the wide 

 range, both in kind and quantity, of the mineral matter held in solution 

 by them. It is to be noted that the table shows the composition of the 

 waters of exceptional, rather than common, lakes. The waters of fresh 

 lakes do not depart widely from those of rivers (p. 107). 



1 Copied from Russell's Lake Lahontan, Mono. XI, U. S. Geol. Surv. 



