FOUNDATIONS OF THE GREAT DEEP 17 



angles to the wave-path or direction of propagation. Rhythmi- 

 cal motions of the kind have been demonstrated in the seis- 

 mograms, so that a class of Love waves or Surface-shear waves 

 is recognized. 



For a given earthquake the Push wave, Shake wave, pseudo- 

 Rayleigh wave, and Love wave have each its own particular 

 velocity, and each of the corresponding travel-times can tell us 

 something about the nature of the sub-oceanic crust of the 

 earth. 



Study of the travel-times for Push and Shake waves has 

 already shown that the first fifty miles of the continental part 

 of the crust is in a general way made up of three layers, as 

 shown in the cross-section of Figure 7. The top layer, about 10 



Depth 



(Km.) 







VENEER 



' BASEMENT 'COMPLEX 



15 





GRANITIC' 



'INTERMEDIATE" 



"GABBROIC" 



"SUBSTRATUM" 



5/al 



Crystallized 

 Sima " 



Vitreous 

 "Sima" 



FIGURE 7. DIAGRAMMATIC CROSS-SF.CTION OF THE CONTINENTAL PART OF 

 THE EARTH'S CRUST, AS INFERRED FROM THE FACTS OF SEISMOLOGY AND 



GEOLOGY. 



miles in thickness, carries the waves at speeds indicating an 

 average composition like that of common granite, and seismol- 

 ogists have adopted the name "granitic layer." Below it a 

 layer, about 15 miles thick, has wave-velocities and elastic 



