308 Status of the Theory of Isostasy. 



reduction, which is equivalent to depth of compensation 

 infinite, gives 0-111. Thus, unless a deficiency of gravity 

 near the surface should be balanced in some wholly 

 improbable manner by an excess at greater depth, there 

 is here a real load, negative in sign but of great magni- 

 tude, borne by the strength of the crust. Recent unpub- 

 lished work by the U. S. Coast and Geodetic Survey 

 shows this belt of deficiency of gravity to extend north- 

 ward along the east side of Puget Sound, the anomaly 

 at Everett being — 0-073 and at Bellingham — 0-043 

 dyne. 16 The deficiencies of density in this region are thus 

 seen to give strains in the crust at least comparable to 

 the excavation of the Grand Canon of the Colorado. 



In Part V the methods for interpreting the geodetic 

 data in terms of mass are further developed and applied 

 in illustration to a region of excess of mass in central 

 Texas, lat. 30°-31°, long. 99°-100°. The data are not 

 adapted to more than a first approximation. The de- 

 flection residuals and gravity anomalies indicate, how- 

 ever, an excess of mass whose center of attraction is 

 about 12 miles deep and which is roughly equivalent in 

 load to a cylindrical mass at the surface, density 2-7, 

 height 2,500 feet, diameter 125 miles. 



Having developed the evidence of the regionally im- 

 perfect nature of isostasy in the previous parts, the point 

 of view is shifted and in Part VI the contrary side of 

 the problem is taken up. The hypothesis of no isostasy 

 introduces large systematic errors into the computations 

 of the deflections' of the vertical and the intensity of 

 gravity. The comparison of the maps showing the 

 Bouguer, Free Air, and New Method anomalies gives 

 convincing evidence that not only are continental and 

 oceanic segments fairly closely balanced with respect to 

 density, but such features as the Cordilleran plateaus 

 also. This existence of a high degree of isostasy for 

 areas of broad span throws light on the nature of the 

 crust and subcrust. There must be capable of operation 

 a mechanism of readjustment, necessary when surface 

 agencies destroy to a sufficient degree the isostatic equi- 

 librium. It is shown in this paper, from the distribu- 

 tion of stresses, that the transference of mass must take 

 place in a zone below the depth of isostatic compensation. 

 Furthermore, this zone must be a zone of marked weak- 



16 William Bowie, personal communication. 



