62 



PHYSICAL GEOLOGY 



and lowlands (Figs. 43 and 44). Some of the basins contain 

 lakes or seas, while others do not. In the oceans likewise 

 there are irregularities, such as projecting islands, and the 

 hollows known as " deeps " ; but, on the whole, the ocean 



FIG. 43. Photograph of a relief model of North America. 



floors are far less rugged than the lands. This is due partly 

 to the fact that streams, glaciers, and other agencies which 

 roughen the land surface do not operate in the oceans, and 

 partly to the fact that the deposition of sediment upon the 

 sea bottom tends constantly to smooth out such irregulari- 

 ties as may exist. 



Movements within the crust. As a matter of human 

 experience the earth seems to be firm and stable to the last 



