272 RELATIONS OF AIR AND WATER TO TEMPERATURE AND LIFE. 



lu the desert of Sahara iiuiueroas dry water-courses show where great 

 rivers formerly ran iuto Lake Tchad. 



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

 tory many times greater than at present, with an outlet to the Bos- 

 phorus and Mediterranean. 



We have not sufficient knowledge of Arabia to know the former con- 

 dition 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 t he Arctic Ocean and from the 

 divide between the Mackenzie and Yukon rivers. A second range 

 runs from northeastern Alaska through Mount St. Elias. Then these 

 two bands extend through British Columbia, gradually widening as 

 new ranges arise until they obtain a width of 500 miles at the boundary 

 line between British Columbia and the United States, and a width of 

 1,000 miles on theline of the Union Pacific Eailroad. These two ranges, 

 the Sierra Nevada and the Eocky mountains, come together in southern 

 Mexico and extend as a single range through Central America and the 

 Isthmus of Panama. On entering South America this range again 

 divides, forming the Cordilleran and the Andes systems, and thence 

 they extend southward, with a varying width between them of from 

 40 to 200 miles. They are connected from east to west by several cross- 

 ranges or spurs. From southern Chile the Andes continues as one 

 chain through Patagonia and Terra del Fuego to Cape Horn. This is 

 the longest and most persistent chain of mountains in the world. The 

 peaks gradually rise in height from north to south until in Chile, 

 Aconcaqua, 22,427 feet in height, is the culminating point; thence 

 southerly the range gradually lowers to an elevation of a few hundred 

 feet only at the Straits of Magellan and Cape Horn. Several volcanoes 

 in this long range rise to a greater elevation than any of the non- 

 volcanic peaks. 



In North America the currents of air from the Pacific Ocean, in 

 passing over the Coast, Sierras, and other ranges, deposit a large por- 

 tion of their moisture on the mountains. Between these ranges are 

 warm valleys, and the winds chilled in crossing the mountains evapo- 

 rate the little moisture in these valleys, and they are left dry and arid 

 unless irrigated by mountain streams. Thus we have a succession of 

 arid valleys and green mountain ranges moistened with rain and snow, 

 and rich in forests and vegetation. A number of these valleys are 

 inclosed basins, from which the mountain streams have no outlet to 

 the ocean and in some of which saline lakes are fouiul. 



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