WM. H. BKEWEK. 27 



New York, and the Eastern Atlantic states, less than 

 half of the total rainfall evaporates from the surface, 

 the remainder, constituting five-eighths, flows to the sea. 

 If there is a depression in the land it fills up with water 

 in time, forming a lake ; when full, the depression over- 

 flows and a stream runs out of the lake. As rivers are 

 of fresh water, such lakes are always fresh and their 

 outlets are fresh. The many lakes east of the Rocky 

 Mountains are all of this character. 



But a region may be so dry that all the water that falls 

 in it evaporates from the surface. In such a region the 

 streams flow from the higher land into depressions, form- 

 ing lakes, and these lakes become salt. No stream or 

 river has water entirely pure. It dissolves various sub- 

 stances from the soil, which, flowing down into the lakes, 

 is left there by evaporation. Consequently it is the uni- 

 versal rule that lakes without outlets and in dry regions, 

 always become salt. There are but few exceptions on the 

 globe, of which Lake Tschad, in Africa, is the most 

 notable. 



Many of these salt lakes, found in the interior of con- 

 tinents, are of great size. Some are in elevated regions, 

 as is the Great Salt Lake in Utah ; and others are below 

 the level of the sea, as is the Dead Sea and the Caspian 

 Sea. They vary in size from mere salt marshes or salt 

 IDOols to great interior seas, like the Caspian Sea. 



Bear in mind, that the distinguishing feature of all 

 such regions is that they have a very dry climate. If as 

 much rain fell on every square mile between the Rocky 

 Mountains and the Sierra Nevada as falls in this state, 

 the water could not possibly all evaporate, and some, of 

 necessity, would have to flow away ; the hollows would, 

 in time, all fill up with water and overflow, and their 

 outlets would be rivers which would find their way to 

 the sea somewhere, and, moreover, all the lakes would 

 be fresh. As it is now, how^ever, the water that falls alj 



