410 MOUNTAINS, EARTHQUAKES, VOLCANOES, GEYSERS 



the internal forces. The elevations of the crust are the moun- 

 tains, the depressions are the valleys. 



Not all mountains are due to internal forces ; some are 

 caused by forces at work on the surface of the earth, such as 

 water. As we have seen in Chapter XXXVII, water wears 

 away the weak parts of a region and leaves in bold relief the more 

 resistant parts. The Catskill Mountains in New York owe their 



FIG. 277. The results of an earthquake 



existence to the fact that the weaker rocks of the region were 

 eroded by water, while other rocks withstood erosion. Pikes 

 Peak, Colorado, is another illustration of rocky land which has 

 become a mountain because of the wearing away of weak rocks. 

 Earthquakes. The present-day movements of the earth's 

 crust are usually slight and attract no attention. The deep 

 underground rocks slip and slide such small distances that only 

 faint tremblings or quakes result on the surface. But occasion- 



