LAKES 205 



exceeds its inflow. Under these conditions concentration may go 

 on to saturation. 



Deposits of salt and other mineral matters are now making in 

 some salt lakes, and formations of the same sort have been made in 

 the past. Buried beneath sediments of other sorts, beds of salt or 

 other precipitates are preserved for ages. Lime carbonate has been 

 precipitated in quantity from some extinct lakes (Fig. 204). 



Lakes which originate by the isolation of portions of the sea 

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

 less and less salty, and may become fresh ultimately; otherwise 

 they remain salt. If evaporation 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 decreasing its range of temperature. They act as reservoirs 

 for surface-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. 



Origin of lake basins. 1 Lake basins arise in many ways, some 

 of which have been pointed out. Most of them arise through 

 processes of gradation. Some are formed by rivers (p. 114), some 

 by waves and shore-currents (p. 186), and some by glacial erosion 

 and deposition. Others are formed by volcanic action, as we shall 

 see, and some by warpings of the earth's surface. A few originate 

 in other ways. 



1 Salisbury's Physiography, Advanced Course, p. 303. 



