422 FIRST YEAR SCIENCE 



condition. Their uplift has been no simple process, each 

 age has added its peculiar impulse to their growth. As 

 the forces of elevation have been lifting them up, those of 

 degradation have been cutting them down. Their broad 

 brows have been carved into peaks and pinnacles, and 

 gorges and caverns have been cut into their flanks. 



The different rock masses which enter into their struc- 

 ture have each assumed its own peculiar lineaments under 

 the carving of the wind, rain, streams, avalanches and 

 glaciers, and thus the variegated beauty of the whole mass 

 has been produced. The central part of massive moun- 

 tains is composed of igneous rocks, but on the sides over- 



Fig. 120. 



lying these, sedimentary rocks are found. The Rockies, 

 the Alps and the Himalaya Mountains are of this kind. 



199. Mountains that no longer Exist. The mountains 

 which are now such prominent features of the earth's sur- 

 face are neither all the same age nor are they the only 

 representatives of this kind of land forms that have ever 

 existed. All the kinds of mountains thus far considered 

 are young in geological age, although some are older than 

 others. 



All parts of the earth's surface are being gradually worn 

 down by the action of water, but the higher portions are 

 worn more rapidly than those lower, as here the forces of 

 denudation act more intensely. Thus if mountains stop 

 growing, they decrease in height until finally they are too 

 small to be called mountains. Their rocks will be crumpled 



