5o6 JOSEPH BARRELL 



also is more likely to be given by the geologic and geodetic evidence 

 rather than from that yielded by the tides, provided that the present 

 hypothesis of the existence of an asthenosphere is accepted. 



It might seem that if the asthenosphere is strained to its limit 

 by permanent stress and is slowly yielding, that even the small 

 and rhythmic tidal stresses, like the last straw on the camel's 

 back, might reveal a lack of resihence in the region of yielding. 

 The distinction was emphasized in Section A, however, that an 

 elastic limit which is determined for permanent stress by a facility 

 of recrystallization at a high temperature may be a far lower elastic 

 limit than that which would exist for rapid rhythmic stresses. 

 Recrystallization would theoretically go forward a little more 

 rapidly during the additive phase of the tidal stress, but the process 

 is presumably so slow, and the tidal stress so small and rapid, that 

 no appreciable effects would be attained before the following of the 

 negative phase. A high resilience of the earth under tidal stress 

 seems therefore quite compatible with the existence of a slowly 

 yielding asthenosphere. 



THE EVIDENCE OF EARTHQUAKE WAVES ON RIGIDITY AND DENSITY 



The speed of an elastic wave through a solid varies directly 

 with the square root of the modulus of elasticity and inversely 

 with the square root of the density. There are two waves, corre- 

 sponding to the elasticities of volume and form respectively, the 

 one measured by the modulus of compressibility, the other by the 

 modulus of rigidity. The first is the longitudinal or radial wave, 

 the second is the transverse wave. The former outruns the latter 

 and gives rise to the first preliminary tremor by which the earth- 

 quake records itself in distant regions. The transverse vibration 

 is felt as the second preliminary tremor, followed by the much 

 larger oscillations of the principal wave. The first two go through 

 the earth, the latter passes around the surface. The fact that 

 there is a transverse wave shows that the earth is solid throughout. 

 But the vibrations at the point of emergence for waves which have 

 penetrated more than half-way into the earth are so faint because 

 of distance that their beginnings are in doubt, and consequently 

 the speeds of transmission below one-half of the radius are uncer- 



