May, 1922 dynamics of the lithosphere 203 



kgm. per cm.-^ while the pressure at the center of the earth is 

 estimated at approximately 3,380,000 kgm. per sq. cm.^^ 

 There can be no question but that rocks under these conditions 

 will readily adjust to stress or strain. Yet Oldham-^ calculates 

 that waves emerging at a point 90 degrees distant from their 

 origin have passed through material having 12 times the 

 resistance of granite and 15 times its rigidity. He adds: "Prob- 

 ably this is only what can be traced to increased pressure." 



THE CRUST. 



The term "crust" is used to describe that portion of the 

 earth which lies above the level of compensation, and conse- 

 quently includes all fractures, cavities, and most of the adjust- 

 ment of the earth's solid substance. Crust and centrosphere 

 constitute the lithosphere. 



Depth. — -Isostatic investigations have placed the compensa- 

 tion surface — by which is meant the surface at which compen- 

 sation is complete — at 122 kilometers, which seems by results 

 obtained to be near the truth; from other evidence the allow- 

 ance seems ample. Van Hise, assuming the crushing strength 

 of rocks to be 1,700 kilograms per square centimeter, surface 

 density to be 2.7, and cavities to be supported by hydrostatic 

 pressure, found that no cavities or crevasses w^ould be possible 

 at a depth of 10,000-12,000 meters. Fracture, however, might 

 occur at this depth. Seismographic evidence would bear out 

 this conclusion. Most seismic foci lie above 17 kilometers of 

 depth. In the case of the Neapolitan earthquake, the deepest 

 estimate fell near 39 kilometers, although the mean depth fell 

 at 10}/2 kilometers. It is unlikely that this estimate is correct, 

 because of the imperfect methods of calculation and the 

 unknown density of the rocks through which the waves passed. 

 Assuming it to be correct, it is still a very shallow phenomenon 

 and well within the limits of 122 kilometers. Oldham^* believes 

 the more or less heterogeneous material may not be more than 

 a score of miles thick. 



"Special Rpt. No. 10, U. S. C. & G. Survey, p. 111. 

 28Fisher, O.: Physics of the Earth's Crust. 



2'Oldham: Some Conclusions as to the Interior of the Earth. Q. J. Geol. 

 Soc. Lond. (1906), V. 62, p. 456-75; Vol. 63, pp. 344-350. 

 2801dham, R. D., loc. cit. 



