CHAPTER XVII 

 MOUNTAINS; MINING; FORESTRY 



223. Changes in Elevation of Lands. Two kinds of 

 changes in the surface of the earth occur from time to time: 

 elevation or rising, and subsidence or falling. These changes 

 of level are usually due to the contraction and consequent 

 wrinkling of the earth's crust. As a result of the elevation, 

 new land may be lifted above the ocean level, or a portion 

 of a continent may be raised a few feet or thousands of 

 feet, or a great mountain system may be made. The 

 subsidence may reduce the area by bringing portions of a 

 continent below sea level, or it may make valleys between 

 elevations. 



Another change of level called degradation, or lowering, is 

 going on all the time wherever high land is exposed to the 

 physical and chemical changes caused by air and water. 

 The general name of this work is erosion, which means 

 " wearing away." The chief agents of erosion are gravity, 

 running water, moving ice, and wind. In the formation of 

 mountains, erosion plays almost as important a part as 

 elevation. 



224. Mountain Making. Mountains are formed in 

 three ways: by the erosion of high lands; by the breaking 

 and lifting of portions of rock strata as a result of unequal 

 pressures from below; and by the folding of strata over 

 large areas. 



225. Mountains of Erosion. Some elevated lands are 

 composed of horizontal strata which have been lifted so 

 gently from their position beneath the water that the strata 

 have scarcely been disturbed. Slight inequalities of level 



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