PHYSICAL CHANGES OF THE OUTER SHELL 



63 



degree, save for such exceptional and sudden disturbances as 

 landslides and earthquakes. But an examination of the 

 rocks of almost any region discloses evidence of movements 

 which have taken place, and there is even proof that such 



FIG. 44. Chief topographic divisions of North America. Compare this 

 with the photograph of the model on the opposite page. 



movements are continually recurring. Indeed, the study of 

 geology can hardly fail to emphasize the fact that the earth is 

 forever undergoing changes of many kinds, which after long 

 lapses of time produce great results. These changes include 

 crustal movements of one kind or another. Some of the 



