CHAPTER IV. 



RELATION OP LAKES TO CLIMATIC CONDITIONS. 



Lakes may be conveniently divided, into two great classes, fresli and 

 saline, in reference to the chemical composition of their waters. These 

 two classes have no sharply defined boundary between them, but a com- 

 plete graduation may be found between the freshest and most saline 

 examples. 



A convenient test for determining to which class a lake should be 

 referred is to taste its water. If no saline or alkaline taste is perceptible, 

 it evidently falls in the first class ; but if the presence of salts can be 

 determined in this way, it should be referred to the second class. 



It is frequently convenient, however, to recognize an intermediate 

 class, or brackish-water lakes, to include water bodies that are slightly 

 saline or alkaline to the taste, but contain only a small fraction of one 

 per cent of mineral matter in solution. 



The more pronounced differences in chemical composition, shown by 

 lakes, depend mainly on climatic conditions. Fresh A^-ater lakes overflow 

 or else their surplus water escapes by percolation, while saline lakes are 

 without outlets. Exceptions to this rule may occur, but they are accom- 

 panied by unstable conditions, and the presence of an outlet to a saline 

 lake or its absence in the case of a fresh lake, are temporary phases that 

 have not continued long enough to bring about the changes toward 

 which they tend. 



Fresh lakes occur principally in humid regions, while saline lakes, 

 with the exception of those formed by the isolation of bodies of sea 

 water, are confined to regions of small rainfall. Whether a lake shall 

 overflow or not, depends ordinarily on the relation of the rainfall over its 

 hydrographic basin to evaporation from the lake surface. As lakes fre- 

 quently receive the water of fissure springs, the sources of which may be 

 far distant, it will he more exact to say that whether a lake held in an 

 impervious basin shall overflow or not, depends on the ratio of the amount 

 of water contributed to it to the amount evaporated from its surface. If 

 the inflow is in excess of evaporation, the water will rise and its area 

 •increase until an equilibrium is established or until an outlet is found. 



