308 HAYFORD— THE EARTH. [April 24, 



tiated in part by surface transfers of load, modified by changes of 

 temperature, modified by chemical changes and in the other ways, 

 and on the other hand the facts of the past as to the behavior of the 

 earth recorded in the rocks and read by geologists and others. This 

 comparison should be used to the fullest possible extent to evaluate 

 the relative importance of the various elements in the actions. 



In making this comparison of various hypothetical phenomena 

 with the great accumulated mass of geological facts it should be 

 recognized at once that it is false logic to reason that if a given 

 hypothesis does not account for all the observed facts the hypothesis 

 is necessarily erroneous. On the contrary it is true logic in dealing 

 with such a problem as the earth seen from a physical standpoint 

 to reason that the more facts are accounted for by a given hypothesis 

 the more certain it is that said hypothesis is a statement of a con- 

 trolling element in the complex phenomena and then to study the 

 facts which appear neutral, or conflicting, with reference to the 

 hypothesis, considering them as indicators of other elements of the 

 phenomena which one should attempt to embody in other supple- 

 mentary hypotheses. 



I submit that in studying the earth it is a mistake to think that 

 there is any necessary conflict between the idea that the earth be- 

 haves as an elastic body and the idea that it is yielding in a viscous 

 manner. A body may behave in both ways at once. The earth 

 is probably acting largely as an elastic body under small forces 

 which change rapidly and at the same time is yielding in a viscous 

 manner to forces of larger intensity which are applied in one sense 

 continuously for long periods. 



The object of this address will have been accomplished if it 



serves in time to arouse the imagination and interest of some one 



and to guide him to greater effectiveness in attacking the problems 



presented by the earth as seen from the geophysical standpoint. 



College of Engineering, Northwestern University. 

 EvANSTON, III. 



