426 JOSEPH BARRELL 



existence of a subcrustal zone with but little strength, readily 

 yielding under vertical loads when these are of such breadth that 

 the strains resulting from them are not confined and absorbed 

 within the strong outer crust. 



There has thus been developed a paradox, an apparent conflict 

 of evidence which becomes more insistent of explanation with con- 

 tinued accumulation of proofs of high rigidity from the domain of 

 geophysics and of proofs of regional isostasy from the equally 

 precise field of geodesy. 



In the consideration of such broad problems, the hope of an 

 ultimate definitive solution rests upon the use of the method of 

 multiple working hypotheses. The surety, significance, and 

 breadth of appHcation of the facts must be established. By these 

 the various hypotheses must be tested and molded. All hypotheses 

 must be kept for further consideration provided they are not posi- 

 tively excluded. In the complexity of relationships there is com- 

 monly a complexity of cause, and hypotheses which seem at first 

 to be mutually exclusive may be found to co-operate in giving a 

 completer explanation. Those which originally appeared antago- 

 nistic may thus come to be seen as participating and dividing the 

 field of cause between them. A paradox often points to this kind 

 of a conclusion. 



To pass to the particular problem of the relations of isostasy to 

 the physical conditions of the earth's interior; the hypothesis devel- 

 oped in this study — of the existence of a zone of weakness under- 

 lying a zone of strength — must not be regarded at present as the 

 only available hypothesis. Searching investigation must be 

 carried forward to see if other and possibly antagonistic hypotheses 

 cannot be developed which will equally well co-ordinate and explain 

 the facts. Even if true in the main, it is likely, as has been the case 

 with other hypotheses, that time will show that in certain directions 

 it has been carried too far. Such testing, however, can best be 

 done by others, and after the implications of this hypothesis are 

 seen. In this part, the concluding article of this series, a discussion 

 had best be given of the lines of adjustment by which the hypothesis 

 here favored may be brought into harmony with other fields of 

 geophysical evidence. With this understanding of the relation of 



