STRUCTURE OF OCEAN BASINS 



43 



been found for longitudinal waves, in basalt 5-5f 

 km. /sec. It is very probable that the values found 

 for the upper layer beneath the continental areas 

 correspond to granite under somewhat higher pres- 

 sure. No waves through more basic rocks have been 

 investigated by means of explosions. It seems to be 

 very probable that the continental layers consist of 

 granite at the top, and rocks with increasing basicity 

 at greater depths, that the bottom of the Atlantic 

 Ocean is formed by the same types of rocks, the 

 layers being noticeably thinner, and that the 

 entire bottom of the Pacific Ocean and all regions 

 of the earth at dei;)ths of more than 50 kilometers 

 consist of a very much more basic material than is 

 characteristic of the uppermost part of the con- 

 tinents. 



There are other observations confirming these 

 results. Surface waves undergo a certain amount of 

 extinction when propagated. For very long waves, 

 this seems to be the same everywhere. Indeed, the 

 energy of these waves is propagated almost com- 

 pletely at considerable depths, the wave-length 

 being a few hundreds of kilometers, and the structure 

 at that depth apparently is the same in every region 

 of the earth. But if we use short waves we find a 

 very definite dependence of extinction upon the 

 region. The least values are to be found at the 

 bottom of the Atlantic Ocean and on the continents. 

 The values for the bottom of the Pacific Ocean are 

 somewhat scanty, as in this case epicenter and sta- 

 tion must be situated in the ocean (Honolulu, Apia). 

 They do not differ much from those just mentioned; 

 however, noticeably larger loss of energy is found 

 for waves which have traversed the boundary of the 

 Pacific Ocean, even if the station is situated very 

 close to the ocean. In particular, the values found 

 from paths along the coast (Japan-Manila, Japan- 

 Batavia) are very high, indicating that it is not a 

 high absorption of the energy at the bottom of the 

 Pacific Ocean that is the cause of the large values 

 there, but the fact of the crossing of the coasts. 

 No corresponding effects have been found from 

 waves passing the coasts of the Atlantic Ocean. 

 In this case, no surfaces between layers of different 

 material must be crossed, but as we found before, 

 and as is stated by the investigation of the extinction 

 of surface waves, the physical coast of the Pacific 

 Ocean (Japan-Philippines-New Guinea) is the 

 boundary, between two completely different kinds 

 of material. The large losses of energy of the 

 surface waves crossing this vertical surface between 

 the material at the bottom of the Pacific Ocean 



