408 MOUNTAINS, EARTHQUAKES, VOLCANOES, GEYSERS 



Basin west of the Mississippi, are higher than some mountains. 

 Rivers flow through plateau regions just as they flow through 

 mountainous regions and plains; in fact, a plateau region is 

 usually dissected or crossed by streams, and broken up into 

 hills and valleys and smaller plateaus. 



How mountains are made. Mountains are formed in 

 different ways, but most of them are caused by the warping 

 and folding of the earth's crust, and by upheaval of rock from 

 the earth's interior. Earthquakes give evidence that strong 



FIG. 274. Hard rock is always found below the soil. 



forces are at work in the interior of the earth. The forces which 

 cause warping, folding, or upheaval of the earth's crust do not 

 always work in a conspL uous way ; more often they work slowly 

 and gradually and almost unnoticed. Forces within the earth 

 are slowly raising the ground around Stockholm, Sweden, I foot 

 in 50 years. There is evidence that Mt. St. Elias of Alaska is 

 being raised higher and higher by these internal forces, and there 

 is equally good evidence to show that some coasts are slowly 

 sinking. The forces which cause changes in the earth's crust 

 are not entirely understood, but there is no doubt that they 

 were more powerful in the past than they are to-day. 



