THE EARTH AS IT IS. 87 



165. Rivers. Rivers come from mountains and pla- 

 teaus, and, having gathered the waters from these, run 

 through the lowlands into the sea. There is system 

 here as well as in the arrangement of mountains. For 

 example, the great basin included between the Rocky 

 Mountains and the Apalachian range has a grand river 

 system, the Mississippi being its principal river. Then 

 there are two other great systems in North America, in 

 which the St. Lawrence and the Mackenzie are the chief 

 rivers ; but these are not as extensive as that of the 

 grand basin of the continent. In South America there 

 are also three principal systems, that of the La Plata, 

 corresponding with that of the Mississippi in North 

 America, that of the Amazon in the east, and that of the 

 Orinoco in the north. The systematic arrangement, 

 which is so obvious in the case of the large rivers of the 

 globe, exists as really with the smaller rivers, though, 

 from the influence of local circumstances, it is not always 

 as manifest. 



166. Lakes. Lakes are formed by rivers and streams 

 of various sizes, which pour their waters into depressions 

 on the land from which they can not escape readily, in 

 some cases not at all. .These depressions are of various 

 shapes and sizes. Those from which the water has no 

 outlet are salt lakes, as, for example, the Dead Sea in 

 Asia, and the Great Salt Lake of this country. 



167. Relation of Mountains to Fertility. The position 

 of mountains has a great influence upon the fertility of a 

 country by detaining the winds and condensing their 

 moisture. Observe how the winds are produced. There 

 are two principal causes of them, heat and the rotation 

 of the earth, and consequently the winds that blow from 

 the tropics, or trades, as they are called, are eastern 

 winds, while the winds coming from the colder regions 

 are western. The winds from the east, being warm, are 

 therefore loaded with moisture, while the cold western 

 winds have comparatively little moisture, and continual- 



