72 THE OCEAN FLOOR 



The seismic work on the Mid-Pacific and "Capricorn" 

 Expeditions has shown quantitatively how remarkably 

 thin the crust beneath the Pacific really is. Typical 

 thicknesses are five to nine kilometers, compared to a 

 thickness of 30 to 40 kilometers beneath the conti- 

 nents. 



The Nature of the Substratum Underlying 

 the Sediment Carpet 



Investigations from H.M.S. "Challenger" in the 

 Pacific made by T. F. Gaskell and J. C. Swallow are of 

 particular interest. These authors state that they have 

 found three different types of suboceanic crustal struc- 

 ture. The first, typical of the deep ocean floor, consists 

 of less than half a kilometer of sediment lying on a 

 substratum, in which the velocity of sound is only 

 slightly less than in basaltic layers. The second type is 

 similar to that of the deep ocean, except that inter- 

 mediate between the sediments and the basement rock 

 there lies a layer in which the velocity of sound is that 

 which would be expected for volcanic material, having 

 a thickness of U2 to 2^/^ kilometers. This interpretation 

 is supported by the occurrence in the same region of 

 volcanic islands and sea mounts. The third type, 

 characteristic of stations in the western Pacific on the 

 landward side of the "andesite line," shows basement 

 velocities less than those found in the other types and 

 similar to those in the continental surface layers. The 



