70 LAKES OF NORTH AMERICA. 



tain, on an average, 34.4 parts per thousand, or, in round numbers, 3.5 

 per cent. 



One of the most instructive features connected with the Caspian is 

 the manner in which it loses its saline constituents by discharging into 

 a secondary basin, where the waters are still more highly concentrated. 

 On its eastern shore there is a deep bay or gulf known as Karabogaza, 

 which is nearly shut off from the main water-body by intervening sand 

 bars, and receives its only inilux through an opening in the bar about 

 140 yards broad and 5 feet deep. The water escapes from Karabogaza 

 solely by evaporation, and is replaced by a current from the Caspian 

 which has been estimated by Von Baer to carry 350,000 tons of saline 

 matter daily from the sea to the gulf. The waters of the gulf have 

 reached the point of saturation for common salt, and precipitation is tak- 

 ing place. These peculiar conditions are* of great interest, not only in 

 showing how deposits of salt may accumulate, but also in illustrating 

 the manner in which an enclosed lake may deposit a large part of its 

 foreign matter without the entire Avater-body becoming highly concen- 

 trated. 



Saline lakes of terrestrial origrin. — The existence of lakes of this 

 class depends upon a combination of topographic and climatic conditions. 

 The basins they occupy may originate in almost any of the various ways 

 enumerated in Chapter I. As a rule the lakes of this class in North 

 America occupy depressions formed by movements in the earth's crust 

 which have cut off large areas from free di-ainage to the sea. Such en- 

 closed basins, however, can only continue in regions where the rainfall 

 is small, for the reason that if precipitation were in excess of evaporation, 

 they would become filled to overflowing. The most favorable conditions 

 for the formation of inland saline lakes are found where high mountains 

 discharge their drainage into basins where the climate is arid. A region 

 of condensation of atmospheric vapors and a region of concentration by 

 evaporation are thus supplied, which supplement each other. 



The saline lakes of arid regions are peculiarly sensitive to climatic 

 changes, and undergo many fluctuations. When the mean annual influx 

 and the mean annual loss by evaporation are nearly evenly balanced, lakes 

 frequently exist only during the rainy season, and disappear entirely dur- 

 ing the hotter portions of the year, leaving broad, smooth mud plains. 

 Plains of this character are a characteristic feature of tlie arid region of 

 North America, and are known in Mexico and in the southwestern part 



