366 



PHYSICAL GEOLOGY 



of the earth by gravity. The surface of the earth may, therefore, 

 be considered as a mosaic of great polygonal blocks (Fig. 355), which 

 from time to time suffer readjustment, the areas occupied by the 

 continents being the continental segments and those by the oceans 

 being the oceanic segments. Not only is the earth divided into these 



co ntinent 



V 



Fig. 355. — Diagrams representing the conception that the continents were lifted 

 and the ocean basins sunk by movement along definite sliding planes or fault planes. 

 1 he dotted lines may be taken to represent a somewhat uniform original surface, which 

 may be looked upon as the surface before the continents and ocean basins were de- 

 veloped. (After Salisbury.) 



great segments, but these in turn are made up of smaller blocks 

 which by differential movements have produced the high plateaus 

 and low plains of the continents, and the " deeps " of the oceans. 

 'J he theory of isostasy holds that every segment of the earth, 



