62 THEOCEANFLOOR 



possibly explain our failure to obtain echoes from 

 greater sediment depths than a few hundred feet. 

 Experience has shown that lava crusts due to submarine 

 volcanism are not uncommon in the Pacific and Indian 

 Oceans, crusts which occasionally have worked havoc 

 with our long piston-corers. If the sediment is inter- 

 foliated with a number of such layers of sediment- 

 covered lava, it is quite possible that the explosion 

 waves are partly reflected back from their upper sur- 

 faces and so much attenuated in penetrating through 

 the lava crust that the echo returning from a still deeper 

 separation surface is too faint to be recognized on the 

 oscillograms. Here the use of powerful "sediment 

 bombs," exploding below the bottom surface, may 

 increase the range of penetration. 



One disadvantage of Weibull's reflection method is 

 that it fails to give any information regarding the nature 

 of the substratum. For this purpose the so-called refrac- 

 tion method can be used. It is much more complicated 

 and expensive, but it does allow studies of the unknown 

 substratum beneath the sediment carpet. It was first 

 used successfully by Ewing and co-workers off the 

 Atlantic coast of the United States, where a wedge- 

 shaped sediment layer was found, increasing in thickness 

 to about 13,000 feet at the beginning of the continental 

 slope." Similar results were obtained later from the west 

 coast of the British Isles by E. C. Bullard and others.^ 

 In later years Ewing has been able to extend his refrac- 



