SECT. 2] 



THE MID-OCEANIC RIDGE 



393 



oceans are remarkably free of seismicity except along the epicenter belt. The 

 continuity of the epicenter belt over its entire length of 40,000 miles has been 

 established beyond doubt. Thus, the co-relation of the seismic belt and topo- 

 graphy, in the moderately well surveyed j)ortions of the North Atlantic, may 

 be extended to areas less well sounded. 



60^ 



30°- 



60° 



Fig. 5. World map of the mid-oceanic rift. This plot shows essentially the mid-oceanic 

 epicenter belt. In every area in which the bathymetry is known, however crudely, 

 the epicenter belt follows the crest of the mid-oceanic ridge. In every area where 

 detailed submarine topographic studies have been carried ovit in the region of the 

 epicenter belt a median rift valley has been discovered. Wherever the rift extends 

 across the continental margins it joins either rift grabens, as in East Africa and Iceland, 

 or great strike-slip faults, as in California and New Zealand. 



3. South Atlantic 



The major physiographic provinces, described above for the North Atlantic, 

 have been recognized in 40 trans-Atlantic sounding lines across the South 

 Atlantic (Fig. 4) (Heezen and Tharp, 1961). The pioneer sounding profiles 

 of the Meteor Expedition across the South Atlantic (Stocks and Wiist, 1935) 

 have been shown to be extremely dependable when compared with the more 

 recent precision soundings of Vema, Atlantis, and Crawford. Epicenters, al- 

 though few in number, at least j^artially owing to unfavorable location of 



seismographic stations, fall near the location of the rift valley. 

 14 — s. Ill 



