254 



The Realm of Nature 



CHAP. 



barrier of which gave way in successive steps separated 

 by long intervals of time. A landslip, the melting of a 

 glacier, or the flow of a lava stream, sometimes obstructs 

 a valley by forming a barrier of earth, moraine stuff, or 

 solid rock, through which the issuing stream cuts very 

 slowly, and the lake so formed is permanent as far as 

 the observations of a lifetime can discover. Hollows 

 produced by the irregular deposit of boulder -clay left 

 by the melting of an ice-sheet form lakes in regions where 

 rainfall exceeds evaporation. Rock-basins ( 339) con- 

 tain the typical Alpine lakes of mountainous regions. Slow 

 upheaval of the end of a valley, subsidence of a plain, or the 

 collapse of caverns, are also processes of lake formation, and 

 the craters of extinct volcanoes often collect a large quantity 

 of rain, forming lakelets with neither inflow nor outflow. 



333. Great Lakes of the World. The Caspian Sea is 

 the largest lake, and a typical example of a hollow isolated 

 by upheaval of surrounding land. Lake Superior comes 

 next in size and is the largest fresh lake. Lake Baikal in 

 Asia, at an elevation of 1360 feet above the sea, is the 

 deepest known lake, the maximum sounding obtained in it 

 being almost 800 fathoms. The highest lake yet measured 

 is Askal Chin in Tibet, 16,600 feet, and the lowest is the 

 Dead Sea, 1290 feet below sea-level. 



THE LARGEST LAKES 



334. Function of Lakes. When water begins to flow 

 over a new land surface, either freshly upheaved from the 



