SECT. 2] 



THK MID-OCEANIC KIIXJK 



389 



7cy 



Continental 

 Margin 



Ocean Basin Floor 



Mid Oceanic Ridge 



Ocean Basin Floor Continental 

 Margin 



Fig. 1. Major physiographic divisions of the North Atlantic and earthquake epicenters 

 1905-56. This threefold pattern of continental margin, ocean-basin floor, and mid- 

 oceanic ridge in the North Atlantic has been found in the South Atlantic, Indian. 

 South Pacific and Arctic Oceans. Epicenters from Gutenberg and Richter (1954) 

 and the epicenter cards of the United States Coast and Geodetic Survey. Accuracy 

 of epicenter location varies from ± |° to ± 1 J°. The major morphological divisions 

 and the Mid-Atlantic rift valley are shown on the profile, which extends from North 

 America to Africa. (Vertical exaggeration of profile is 40:1.) 



