Status of the Theory of Isostasy. 291 



Art. XXI. — The Status of the Theory of Isostasy; by 

 Joseph Barrell.* 



Contents. 



Outline of the theory of isostasy. 



Definitions involved in isostasy. 



Interpretations favoring local and nearly perfect isostasy. 



Interpretations favoring isostasy, but regional and imperfect in character. 



Sources of Hayford's errors of interpretation. 



Interpretations adverse to isostasy. 



Influence of datum surface on the theory of isostasy. 



Distribution of isostatic compensation. 



Isostasy in India. 



Outline of the Theory of Isostasy. 



Isostasy embraces the theory of relationships between 

 the relative surface relief of segments of the earth's 

 crust and the densities of those segments. It is a sub- 

 ject closely related to the geological history of continents 

 and ocean basins, and almost as closely to the nature of 

 igneous rocks and the modes of deformation during 

 periods of terrestrial revolution. Although one of the 

 larger fields of geological theory, it is in a concrete 

 form still relatively new and receives either no notice 

 or but bare mention in the geologic texts published dur- 

 ing the past ten to twenty years. The valuable litera- 

 ture on the subject is largely technical and removed 

 from the field of the geologist. There is still need, con- 

 sequently, for a general statement of isostasy before 

 passing into consideration of details. Especially is there 

 need of showing the relationships of isostasy to other 

 fields of geologic theory, in order that the importance 

 and bearings of the subject may become more widely 

 appreciated. 



This article is written in enough detail to be clear to 

 readers who have not closely followed the history of the 

 subject. In the introductory sections the theory of isos- 



* This paper was written for the Journal and was in the hands of the 

 typist at the time of Professor Barrell's death, May 4, 1919. It was his 

 custom to give his papers a final critical reading when in typewritten form. 

 Usually he made only verbal changes but at times entire passages were 

 rewritten. As he has published extensively upon isostasy before, however, 

 and as his reasoning upon this subject had presumably attained to maturity, 

 it is not likely that he would have made extensive changes in this paper. 

 It therefore is thought probable that it appears as he would have presented 

 it. Here and there a few words have been added and minor changes in 

 punctuation made. C. S. 



