18 THE OCEAN FLOOR 



blocks through a highly resistant substratum, an energy 

 for which no adequate source was suggested, only 

 subcrustal convection currents set up in the substratum 

 at the escape of radioactive heat from the continental 

 blocks through the ocean floor. On the other hand, 

 movements of continents or parts of continents in a 

 vertical direction are generally admitted to have oc- 

 curred. They have been invoked in order to explain the 

 so-called regressions and transgressions of the shore 

 line, and have been attributed to a varying load of inland 

 ice on accumulated sedimentary layers. But horizontal 

 displacements of great magnitude are in general dis- 

 credited. 



Thus there still remains considerable uncertainty 

 regarding the early history of the ocean basins, and 

 many problems concerning their origin and that of the 

 water masses filling them remain unsolved. Highly 

 interesting contributions to the discussion of these 

 problems have recently been offered by American 

 authorities like Harold Urey and William Rubey. Al- 

 though the theme is too vast to be further dealt with here, 

 certain aspects of it will be considered later. 



