SECT. 1] GRAVITY AT SEA 173 



mentary accumulations in the region near the ocean bottom. Probably, in the 

 regions of continental margins, this view will prove to be substantially correct. 

 In regions such as the mid-ocean ridges and volcanic-island arcs, it may be 

 necessary to consider some or all of the deficiency of mass represented by the 

 down-bulge of the M-discontinuity to occur within the crustal layers. Only 

 more definitive data can decide. Since the low-density matter of these layers 

 is probably of lower velocity than the overlying material, it will be masked to re- 

 fraction seismology. It may be necessary to detect the layers from surface- wave 

 studies or from studies of shadow zones, both of which look difficult at present. 

 It now appears that the crust beneath the trenches is only slightly different 

 in thickness from an oceanic crust and that, in some trenches, there is a large 

 accumulation of sedimentary material while in others there is only a small 

 amount. 



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