SHAPE AND STRUCTURE OF OCEAN BASINS 



13 



Table I. Wave Velocity and Layer Thickness for Typical Continental and 

 Oceanic Crustal and Upper Mantle Sections" 



° The section on "Mantle Structure of Ocean Basins," page 18, contains a fuller 

 discussion of the data given here for the mantle. 



The oceanic layer does not show systematic differences in 

 thickness or composition from one ocean to another; but in all 

 oceans for which we have data there are exceptional areas in which 

 this layer deviates considerably from the normal, e.g., the mid- 

 oceanic ridges, the island arcs and deep-sea trenches, and the 

 continental borderlands. 



A considerable quantity of data (largely unpublished) on seismic 

 reflections in the oceans, for both vertical and wide angle reflection 

 arrangements, has been gathered by the Lamont Geological 

 Observatory staff. In a few areas there is found a good reflection 

 whose depth agrees with that found for the oceanic layer by 

 refraction measurements. There are usually one or more strong 

 reflections from within the sedimentary layers. At the majority of 

 stations the reflection from the oceanic layer is weak to unde- 

 tectable (by the rather primitive methods used) in comparison 

 with the reflection originating within the sedimentary column. 

 There is need for continuing investigation of the structure of the 

 layers above the oceanic layer and of the nature of their contact 

 with it. For example, the investigation of these layers by drilling 

 and coring is many-fold easier to accomplish and offers scientific 

 rewards many-fold greater than the proposed investigation of the 

 upper mantle by drilling — the so-called Mohole project. 



Conversions of part of the P wave energy into 5, at some point 

 on the downward path of refracted rays, and recon\-ersion at the 



