FOUNDATIONS OF THE GREAT DEEP 21 



waves run somewhat more slowly across the Atlantic than 

 across the other two oceans. 



Now the velocities of each type of wave depend largely on 

 the nature of the rock constituting the outer fifty miles of the 

 solid earth. The table of values shows, therefore, that the sub- 

 oceanic crust is more elastic than the continental part of the 

 crust. The actual velocities suggest, in fact, that the granitic 

 layer and perhaps also the intermediate layer of the continents 

 are lacking under most of the Pacific and Indian oceans. It 

 looks as if there the crust has the properties of solidified basalt. 

 On the other hand, the slightly smaller velocities of the waves 

 traversing the Atlantic indicate the existence of some lighter 

 and less elastic rock just beneath the Atlantic ooze, though 

 there is no proof that such rock forms a continuous layer all 

 across that oceanic region. 



Recently Mr. J. T. Wilson analyzed the seismograms made 

 at seventeen stations for a world-shaking earthquake that cen- 

 tered at the small circle drawn in the southern part of the map 

 of Figure 9.° From that center of shock a Love wave with a 

 period of 32 seconds ran along great circles around the globe, 

 these paths being indicated by smooth, curved lines that ter- 

 minate at the corresponding named stations. The velocities of 

 the wave, deduced by Wilson, are stated in the first six rows 

 of the last column of Table IV. The first three paths were 

 purely oceanic; the other three paths were "mixed," that is, 

 partly (60 per cent) across ocean basin and partly (40 per cent) 

 across continent. For comparison Gutenberg's velocities of the 

 same type of Love wave, where traversing continent alone, are 

 given in two rows at the bottom of the last column. We see 

 this Surface wave to be faster under the ocean than in the 

 continental traverse. Accordingly we are not surprised to find 

 intermediate values for the velocities along the "mixed" paths. 

 The conclusions to be drawn regarding crustal composition are 



