ATLANTIC COASTAL PLAIN AND ADJACENT OCEAN BASIN 



153 



intermingling of the crustal and mantle rocks, and that this was associated with 

 convection cells in the deep mantle which supply large quantities of basaltic 

 magma and produce extensional forces on the crust and upper mantie ( Heezen 

 et d., 1959). 



In a paper (in press) Heezen and Ewing compare in detail the topography 

 and seismicity of the African rift valleys and the Rift Valley of the Mid-Adantic 

 Ridge. Their conclusion is that the two areas are of basically the same structure, 

 and in fact both form parts of the same continuous structural feature. Since 



the African rift valleys seem clearly to be the result of normal faulting resulting 

 from extension of the crust, Heezen and Ewing conclude that the topo£raphy 

 of the Mid-Adantic Ridge is largely the result of normal faulting. Whether 

 the forces are the result of horizontal extension or vertical uplift remains the 

 most important unsolved problem in connection with the origin of the con- 

 tinental as well as the sub-oceanic rift-valley systems. Hess ( 1954) has proposed 

 a mechanism relating suboceanic uplift to expansion due to serpentization 

 of the upper mande (Heezen et al., 1959). 



