president's address. 839 



Lord Kelvin and Mr. G. H. Darwin, from a study of the long 

 period oceanic tides, conclude that the earth's mass as a whole is 

 more rigid than steel hnt not quite so rigid as glass. Such a 

 degree of rigidity would at first sight appear to preclude any 

 alteration of the levels of the land with respect to the ocean ; 

 we know, however, that certain tracts of the earth's surface are 

 rising and others falling, so that the question arises what such 

 an amount of rigidity implies. 



Mr. R. L. Woodward in a paper entitled '' The Mathematical 

 Theories of the Earth," published in the American Journal- of 

 Science, Vol. 138, p. 34:3, says : — "Whatever may have been the 

 antecedent condition of the earth's mass, the conclusion seems 

 unavoidable that at no great depth the pressure is sufficient to 

 break down the structural characteristics of all known substances 

 and hence to produce viscous flow whenever and wherever the 

 stress difference exceeds a certain limit, which cannot be large in 

 comparison with the pressure." Internal fluidity is therefore not 

 a necessar}" condition to account for movements of the crust. 



Roche considered that geological phenomena were best 

 explained by postulating a solid nucleus with a zone of fusion 

 .separating the crust from the nucleus. 



In a paper entitled " An elementary proof of the earth's 

 rigidity," published in the American Journal of Science, Vol. 139,. 

 p. 336, the author, Mr. George F. Becker, points out that although 

 the earth is a veiy rigid body, it does not necessarily follow that 

 it is solid. The assumption of solidity is objected to by geologists 

 as opposed to the possibility of the occurrence of geological 

 phenomena. There is, however, no conflict betVeen geology andi 

 physics. He says : — " Time enters into the expression of 

 viscosity, and the fact that the earth behaves as a rigid mass to 

 a force which changes its direction by 360° in 24 hours is not 

 inconsistent with great plasticity under the action of small forces 

 which maintain their direction for ages. For a considerable 

 number of years I have constantly had the theory of the earth's 

 solidity in mind while making field observations on upheaval and 



