RELATION OF LAKES TO CLIMATIC CONDITIONS. 71 



of the United States as |)/rt_yrt.«. It is convenient to adopt this name, 

 and eall the tenipoiarv Avatei-bodies to which phiyas owe their orio'in. 

 jyJaija lakes. These lakes may be formed by a single shoAver and disap- 

 pear in a few hours, or they may endure for a series of years and oidy l)e 

 evaporated to dr3'ness during seasons of exceptionally low rainfall or un- 

 usually active evaporation. 



When enclosed lakes of arid regions are more permanent, they fluctu- 

 ate in volume, and consequently in extent and in density, from season to 

 season, and are so sensitive to climatic changes that they sho\\' marked 

 variations when ordinary weather observations, taken at a limited number 

 of localities in their neighborhood, fail to indicate analogous changes in 

 atmospheric conditions. 



The terrestrial saline lakes of North America are confined to the arid 

 region, of Mexico and the United States, although small pools of alkaline 

 water do occur on the great plains in the sul)-humid region east of the 

 Rocky mountains both in the United States and Canada. Tlie saline 

 lakes of the United States are confined almost entirely to Utah and 

 Nevada and adjacent portions of the Great Basin. The distribution of 

 some of the more important lakes here referred to, is indicated on the 

 accompanying ma}) forming Plate 14. The chemical composition of their 

 waters is slioAvn in tlie taljle on page 72. 



Cliemical precipitates. — The deposition of mechanical sediments, as 

 clay and sand, in lake basins has alread}^ been referred to. This takes 

 ]>laee in all lakes Avithout special reference to their chemical composition. 

 When lake Avaters become concentrated by evaporation, however, the 

 inatcrial coiitril)uted to them in solution may l)e precipitated, and 

 either mingle with the mechanical sediments or form deposits of 

 purely chemical origin. Chemical, precipitates, like mechanical sedi- 

 ments, may furnish evidence of important changes in a lake's history, 

 and are also frequently of great interest on account of their connncicial 

 value. 



As already seen, enclosed lakes are constantly ivceiving conti'ibutions 

 from streams, springs, and rain, l)Ut do not overflow, the iiitlnx l)cing 

 c(uuiterbalanced by evaporation, 'i'liis assures us that in the earlier stages 

 (»f their history, at least, the amount of saline matter held in solution nuist 

 increase from year to year and tVoni cciitury to century. This ])ro(H'ss 

 continuing, a time is cvcntnally rcaclicd wlicn the waters will be salnratc<l 

 with one or more ot its saline eonstitnents and precipitation begin. Waiei's 



