120 6r. G. Hubbard — Air and Water, Temperature and Life. 



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 regions are basins, where the rivers and rainfall either run into 

 salt lakes or are lost in the desert and never reach the ocean. 

 These deserts are caused by the winds which blow either from 

 colder over warm areas and are therefore dry, or over vast plains 

 or mountainous regions upon which they have precipitated their 

 moisture. 



The average rainfall on the great deserts does not exceed ten 

 inches a year, and the evaporation is usually greater than the 

 rainfall. They are situated generally between the twentieth 

 and fortieth degrees of north latitude and between the twen- 

 tieth and thirtieth degrees of south latitude. In the northern 

 belt are the Carson and other basins of Nevada, the Salt Lake 

 of Utah, the desert of Sahara, Arabia, Persia, the Aral-Caspian 

 desert, the Tanin Gobi and Mongolia desert. In the southern 

 belt is the desert of Atacama in South America, Kalahari in 

 South Africa and the Australian deserts. These basins in the 

 northern belt contained formerly, lakes much greater than are 

 now found in either of the continents. 



Salt Lake was formerly much larger and deeper, for its waters 

 once beat upon shores one thousand feet higher up the moun- 

 tain sides than at present; its waters then found their way to 

 the ocean. This was probably in the ice age, when the surround- 

 ing mountains were covered with snow and great glaciers, and 

 the evaporation was much less than the rainfall and the water 

 from the melting glaciers. 



In the desert of Sahara numerous dry water-courses show 

 where great rivers formerly ran into Lake Tchad. 



In Asia the Caspian and Aral seas were connected, covering a 

 territory many times greater than at present, with an outlet to 

 the Bosphorus and Mediterranean. 



We have not sufficient knowledge of x^rabia to know the former 

 condition of that arid country. The process of desiccation is still 

 going on, and how much longer it will continue no one can tell. 



Mountains of America. 



Next we will notice the influence of the mountains on the 

 atmosphere, either in enriching or impoverishing a country, or 

 in intensifying the movements of the currents of air and water. 



The mountains of America rise at the Arctic ocean and form 

 the divide between the Mackenzie and Yukon rivers. A second 

 range runs from northeastern Alaska through Mount Saint Elias. 



