THE STRENGTH OF THE EARTH'S CRUST 



JOSEPH BARRELL 



New Haven, Connecticut 



PART III. INFLUENCE OF VARIABLE RATE OF ISOSTATIC 

 COMPENSATION 



Introduction and Summary 209 



The Specific Gravity of Rocks 211 



Interpretation of Anomalies in Terms of Mass and Depth 216 



Relations of Anomalies to Exposed Geologic Formations . 221 

 Large Outstanding Anomalies Not Related to Geology or 



Topography 227 



Criteria for Separating Vertically Irregular Compensation 



from Regionally Incomplete Compensation . . . . 228 

 Gravity Anomalies Caused Largely by Regional Departures 



from isostasy 234 



INTRODUCTION AND SUMMARY 



The work of Hayford on the deflections of the vertical, and of 

 Hayford and Bowie on the anomalies of gravity, has supplied the 

 geodetic data from which future work must start. As an initial 

 basis to guide their work, it was desirable to assume the hypothesis 

 that isostatic compensation was complete for each topographic 

 irregularity, giving local compensation, and that it was uniformly 

 distributed to a constant depth. The actual results may then be 

 compared to this ideal of local, uniform, and complete isostasy and 

 the degree of departures noted, as given by residuals and anomalies. 



In Part II the subject of the regional distribution of compensa- 

 tion was examined and the conclusion was reached that the crust 

 was sufficiently rigid to bear such mountains as Pikes Peak without 

 requiring special compensation below. In general it is thought 

 compensation in mountain regions extends to more than 200 km. 

 and in some regions to more than 400 km. In this part are 

 considered the effects of variations in the vertical distribution of 



209 



