Status of the Theory of Isostasy. 301 



the topographic feature. As such physiographic divi- 

 sions are often related to differential erosion on strata 

 of varying hardness, the improbability of these extreme 

 views should be evident to geologists. 



Putnam and Gilbert, twenty years before, had reached 

 the conclusion that isostatic compensation was distinctly 

 regional, individual mountain ranges being sustained 

 because of the rigidity of the crust, Hayford's work 

 was so much fuller, however, that it seemed to supersede 

 this older conclusion. It met with a varying reception 

 from geologists which has not yet settled into a general 

 consensus of opinion. 



As illustrations of this diversity, Reid, on the other 

 hand, accepted Hayford's work unreservedly, both the 

 geodetic analysis and Hayford's interpretation. He then 

 proceeded to apply it to the theory of mountain ranges, 

 holding that all other forces and modes of explanation 

 must be subordinated to the vertical forces maintaining 

 an isostatic equilibrium nearly perfect. 3 



Willis, more critical, pointed out, however, that if the 

 mountains and plains of the continents are now in iso- 

 static equilibrium, such regions could not have been in 

 that state before the mountains were upraised by folding. 

 Furthermore, erosion and deposition would ultimately 

 destroy again in considerable measure the present state 

 of isostasy. He therefore held that if Hayford's results 

 were correct in such detail, it meant that during a period 

 of great crust movements, illustrated by the present geo- 

 logic time, isostasy reaches a perfection which- is not 

 usual, and that therefore a high degree of isostatic ad- 

 justment does not imply such a state of weakness as 

 would be inferred if nearly perfect isostasy were a con- 

 stant condition of the crust. Willis was thus the first 

 to test Hayford's interpretation by actually comparing 

 it with the geologic evidence and suggesting a means of 

 explaining the discrepancy. 4 Carrying his argument 

 further, he has made isostatic readjustments following 

 erosion and deposition the basis of an hypothesis of 

 mountain folding. 5 Some serious difficulties in the way 



3 H. F. Beid, Isostasy and mountain ranges, Proc. Am. Philos. Soc, 50, 

 444-452, 1911. 



4 Bailey Willis, What is terra firma? Ann. Kep., Smithsonian Institution, 

 for 1910, pp. 391-406, 1911. 



r ' Bailey Willis, Eesearch in China, Carnegie Institution of Washington, 

 Publ. No. 54, vol. II, 1907. 



