FOUNDATIONS OF THE GREAT DEEP 



19 



160 I W Long 



OCEAN 



I DV/ M I DWAY 



PEARL REEF 



C5 



4000 



REEF 

 — 40o c 

 GARDNER 



Depths in meters 



FIGURE 8. MAP OF THE HAWAIIAN ISLANDS. DEPTHS IN METERS. 



concluded, from the travel-times of Push waves, that the upper 

 part of the sub-Pacific crust, surrounding the lofty volcanic 

 pile on which his seismograph rested, has the elastic properties 

 of gabbro or solidified basalt. The granitic layer is thought to 

 be entirely absent, and the reported wave-velocities seem too 

 high to permit us to assume that even the intermediate layer 

 of the continents is represented. 



Much more comprehensive study of the Surface or Long 

 waves has demonstrated strong contrast between the sub- 

 oceanic and continental segments of the earth's crust. In this 

 alternative mode of detection both pseudo-Rayleigh and Love 

 waves are employed. Each of the two kinds is propagated at 

 a velocity depending on the elastic properties of the relatively 

 superficial rock of the earth's body. The result of the double 

 investigation is proof that the sub-oceanic part of the crust is 

 more elastic than the continental part, as if the average rock 

 between the bottom mud and depth of fifty miles is crystallized 



