for the Study of MegadiastropJiism. 



395 



earth which passed through a stage of fusion resulting 

 in a coarse stratification by density and which, since 

 crustification, has possessed a yield zone beneath a com- 

 paratively thin outer shell. This latter viewpoint is not, 

 as Daly has pointed out, 6 incompatible with the basic 

 postulates of the planetesimal hypothesis of geogenesis. 

 The purpose of the present paper is to bring together 

 in summary form from the various sources the evidence 

 leading to the conclusions that the earth passed through 

 a molten stage and that there exists at no great depth a 

 zone which yields to diastrophic differential stresses. 

 The writer believes that this evidence, much of it direct 

 evidence of seismic transmission and close field studies, 

 cannot be lightly set aside. 



The Rigidity and Elasticity of the Earth. 



General Considerations. — The term "rigidity" has been 

 generally loosely interpreted. Rigidity is dependent not 

 only on the stress applied but upon the time the stress 

 is applied. A material may be rigid for one set of stress 

 conditions and non-rigid for another set. The earth is 

 spoken of as having a higher rigidity than steel. Another 

 equally good way of stating the same fact is to say that 

 the earth yields under the same conditions that steel 

 will yield and, as a matter of fact, steel does yield quite 

 readily. Even Adams' 7 and Bridgman's s experiments 

 besides demonstrating the high strength of rocks under 

 cubic compression also show that these same rocks 

 eventually yield if the pressure is great enough and if 

 the time the pressure is applied is long enough. Rigidity, 

 then, is merely a relative term. Nothing that we know 

 of has absolute rigidity. And plasticity, or yielding, is 

 not necessarily dependent on the physical state of a 

 material. Crystals become plastic at sufficiently high 



6 E. A. Daly: The Planetesimal Hypothesis in Eelation to the Earth, Sci. 

 Monthly, May, 1920, p. 495. 



7 F. D. Adams : An Experimental Contribution of the Question of Depth 

 of the Zone of Flow in the Earth's Crust, Jour. Geology, vol. 20, pp. 97-118, 

 1912. 



On. the Amount of Internal Friction developed in Eocks during Deforma- 

 tion and on the Relative Plasticity of Different Tvpes of Rocks, Jour. Geol- 

 ogy, vol. 25, pp. 597-637, 1917. 



8 P. W. Bridgman: The Failure of Cavities in Crystals and Rocks under 

 Pressure, this Journal (4) vol. 45, pp. 243-268, 1918. 



