12 THE OCEAN FLOOR 



the viewpoint of both the present and the remote past, 

 that is, paleontology). Specific hypotheses have been 

 advanced to account for the distribution of plants and 

 animals and their past migrations from continent to 

 continent. It will be necessary to consider these 

 hypotheses briefly, since they have played an important 

 part in discussions of the problem before us, the ocean 

 floor. 



The evidence is overwhelming that large parts of the 

 present continental surface have in earlier phases of 

 the evolution of our planet been submerged and formed 

 the bottom of relatively shallow epicontinental seas. 

 Thus, the material from which sedimentary rocks such 

 as chalk, shists, and sandstones were built is ancient 

 sediment deposited on the bottom of vanished seas 

 which were laid bare at the regressions of the shore line 

 and gradually hardened into rocks. Our loftiest moun- 

 tains, the Rockies, the Andes, the European Alps, and 

 the Himalayas, are built up largely from such ancient 

 sea beds, which by enormous horizontal forces have 

 been folded and crinkled into mountain chains. On the 

 other hand, very few if any deposits from great ocean 

 depths are found in the continents. This fact, among 

 others, has led to the theory of the "permanence of the 

 ocean basins," propounded 90 years ago by the great 

 American seolodst James Dwight Dana and his school. 

 The strongest opposition to this view came from the 

 biologists and paleontologists, who claimed to find 

 evidence for "land bridges," now out of sight below the 



