294 Status of the Theory of Isostasy. 



of the land is more dependent npon erosion than upon 

 hidden density, tends to magnify the differences between 

 nature and the mathematical picture. 



The subject of isostasy has grown through the past 

 half century, but has been brought into the field of exact 

 analysis in the past decade by the comprehensive and 

 very valuable geodetic researches of Hayford and Bowie. 

 The writer, in common with some other geologists, was 

 unable, however, to accept the geologic interpretation 

 which Hayford gave to his work with the implication 

 that whether geologists liked it or not they must accept 

 it, and some years ago, in order to test the validity of 

 the geodetic evidence and its geologic implications, made 

 an extensive investigation of the geodetic data. 1 * Since 

 that time a number of articles of various values, some 

 supporting the theory of isostasy, some adverse to it, 

 have appeared. As giving source material especially, 

 should be mentioned Bowie's most important mono- 

 graph, 2 which has greatly extended the data previously 

 available and has treated them from some new stand- 

 points. The time seems appropriate therefore for a 

 further review and analysis of this subject. 



As the following analysis must deal largely with the 

 simple mathematical pictures, the impression might be 

 given that a writer mistakes them for the realities of 

 nature. It is desirable, therefore, before taking up the 

 details of the subject, to state briefly the general con- 

 clusions entertained regarding the limits of isostasy. 

 The writer believes the evidence demonstrates beyond 

 controversy that the larger relief of the earth is in greater 

 part balanced by corresponding variations of density 

 within the outer fiftieth of the earth's radius, that the 

 distortional stresses at a depth of a twentieth of the 

 radius are only a small fraction of what they would be 

 if the density of the outer crust were uniform. On the 

 other hand, it is believed that the variations of density 

 are irregular, imperfect, and mostly concentrated in the 

 outer hundredth of the radius, with a tendency to pro- 

 gressively disappear with depth. The outer crust is very 

 strong, capable of supporting individual mountains, 

 limited mountain ranges, and erosion features of cor- 



1 Joseph Barrell, The strength of the earth 's crust, Jour. Geology, vols. 

 22, 23, 1914, 1915. 



2 William Bowie, Investigations of gravity and isostasy, Special publica- 

 tion No. 40, U. S. Coast and Geodetic Survey, 1917. 



