BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 

 Vol. 33. pp. 375-410 June 30. 1922 



ISOSTASY AND EOCK DENSITY ] 



BY HENRY S. WASHINGTON 



(Presented before the Society December 20, 1921) 



CONTENTS 



Page 



Summary 375 



Introduction 376 



Comagmatic regions 378 



The normative average density 381 



Discussion of the methods 381 



The average rock 382 



Method of calculation 383 



Results 387 



Normative densities and altitudes 391 



The physical average density 399 



The isopiestic level 402 



Comagmatic regions and gravity anomalies 409 



Summary 



In the present paper, which is an expansion of part of a former one, 2 

 there is presented a study of the relations between the average densities 

 of different areas of igneous rocks and their average altitudes, with the 

 object of correlating these relations with the theory of isostasy. The 

 outcome of the discussion is that the general relation holds good : tli e 

 average density of the igneous rocks of a region varies in the opposite 

 sense as the average altitude. The results of the study, therefore, har- 

 monize with and corroborate the theory of isostasy. 



Comagmatic regions, in which the igneous rocks are shown by their 

 similar chemical and mineral characters to be derived from the same 

 magma, are briefly discussed, especially as to their bearing on isostasy. 

 and the distribution of comagmatic regions throughout the United States 

 is described. 



1 Manuscript received by the Secretary of the Society February 11, 1922. 



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

 copies of this article were reprinted for distribution by the Geophysical Laboratory and 

 bear its imprint and the designation *No. 460." 



2 H. S. Washington : The chemistry of the earth's crust. Jour. Franklin Inst., vol. 

 190, 1920, pp. 794-814. 



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