BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 

 VOL 33, PP. 317-326 JUNE 30, 1922 



ISOSTASY AND EARTH MOVEMENTS l 



BY HARRY FIELDING HEJ1) 



(Read before the Society December 29, 1921)- 



( UNTENTS 



Page 



Introduction 317 



Evidences of isostatic equilibrium 318 



Objections to former isostatic equilibrium 319 



Vertical uplift of mountain ranges 319 



Relation of isostasy to folding and uplift 323 



Isostasy and peneplanation 324 



Isostasy and the rigidity of rock 324 



Conclusion 326 



Introduction 



Geologists often ask too much of the principle of isostasy. When they 

 find that isostasy will not explain all earth movements, they think it is 

 not a true principle. Many of the most important earth movements are 

 not due to an attempt to reestablish a disturbed isostatic equilibrium. 

 James Hall made a splendid contribution to our knowledge of the geo- 

 logic history of mountain ranges in 1859. 3 He showed that the sediments 

 of the Appalachian Mountains were laid down in a sinking syncline, and 

 he inferred that the sinking was due to the weight of the sediments. 

 While this weight must have contributed to the sinking, it is clear that 

 great depressions have taken place without the addition of material ; for 

 many of the great deeps of the ocean, and as a particular example, the 

 Tonga Deep, are so situated that they could never have received any large 

 amount of sediments; and the elevation of mountain ranges after the ac- 

 cumulation of sediments could not be due to a reestablishment of isostatic 



1 Manuscript received by the Secretary of the Society December 22, 1922. 

 This paper is one of a series composing a symposium on isostasy. 



2 The main part of the evidence for the uplift of mountains by vertical forces, given 

 in this article, was presented to the Geological Society at the Chicago meeting, in 1920. 

 See this Bulletin, vol. 32, p. 33. 



3 Natural History of New York, part vi, Paleontology, vol. iii, pp. 50-S5. 



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