CHAPTER XXIX 

 A STUDY OF LAKE BASINS 



Freshwater and saline lakes. Lakes require for their exist- 

 ence a basin within which water may be impounded, and a supply 

 of water more than sufficient to meet the losses from seepage and 

 evaporation. If there is a surplus beyond what is needed to meet 

 these losses, lakes have outlets and remain fresh; their content 

 of mineral matter is then too slight to be detected by the palate. 

 If, on the other hand, supply is insufficient for overflow, continued 

 evaporation results in a concentration of the mineral content of 

 the water, subject as it is to continual augmentation from the in- 

 flowing streams. 



As we have seen, there are in areas of small rainfall special 

 weathering processes which tend to bring out the salts from the 

 interior of rock masses, these concentrated salts generally first 

 appearing as a surface efflorescence which is ultimately transferred 

 through the agency of wind and cloudburst to the characteristi- 

 cally saline desert lakes. 



Lake basins may be formed in many ways. Depressions of 

 the land surface may result from tectonic movements of the crust ; 

 they may be formed by excavating processes; but in by far the 

 greater number of instances they result from the obstruction in 

 some manner of valleys which were before characterized by uni- 

 formly forward grades. In relatively few cases loose materials 

 are heaped up in such a manner as to produce fairly symmetrical 

 basins. 



Newland lakes. On land recently elevated from the sea, 

 basins of lakes may be merely the inherited slight irregularities 

 of the earlier sea floor, in which case they may be assumed to be 

 largely the result of an irregular distribution of deposits derived 

 from the land. Lakes of this type are especially well exhibited 

 in Florida, and are known as newland lakes (Fig. 430). Such 

 lakes are exceptionally shallow, and are apt to have irregular out- 

 2D 401 



