

Groundwork for Study of Megadiastrophism. 267 



asthenozones, the loci and methods of magmatie gene- 

 ration, etc. 



The rigidity and elasticity disclosed by the body 

 tides. — The body tides are simply the yielding and res- 

 toration of the earth in response to differential stresses 

 from the moon and snn. The quick restoration to form 

 implies a high state of elastic rigidity. The evidence in 

 this case differs from that of the seismic waves in that 

 the tidal oscillation shows that the earth's action as a 

 whole is that of a highly elastic rigid body under existing 

 conditions. The periods of action are also longer than 

 those of the earthquake vibrations but still relatively 

 short. 



Now, as before, the body tides show the existence of the 

 properties of rigidity and elasticity in the earth acting 

 as a whole in spite of all effects of the earth's present 

 temperatures, pressures, differential stresses and inher- 

 ited effects. 



The rigidity and elasticity disclosed by the nutation of 

 the earth. — Nutation may be regarded as a third line of 

 testing or sampling the rigidity and elasticity of the earth. 

 Like the tides, it discloses the response of the earth as a 

 whole to differential stresses, but it supplements the testi- 

 mony of both tides and seismic vibrations in an important 

 way because its changes of stress are slow and the oscil- 

 lations long, the period of the Chandlerian cycle being 

 about 14 months ; besides which there is an annual cycle, 

 and some deviations that imply other slowly changing 

 stresses. Now the significance of the properties of 

 rigidity and elasticity under these slower stress actions 

 is to be interpreted, as before, in the light of all existing 

 temperatures, pressures, and stress differences notv 

 affecting the earth. 



AVe have thus three lines of independent testimony, 

 differing in kind and in period, and yet none of them has 

 revealed the limits of the elastic and rigid properties. It 

 falls to anyone who assumes any specific limits to give the 

 evidence of such limits, or frankly acknowledge that his 

 assumptions are simply speculative. 



By correlating these three lines of evidence with other 

 data, as has been done by Schweydar, rather definite 

 quantitative values may be reached. Schweydar con- 

 cludes that the constant of rigidity of the earth is 3 X 10 11 



