THE STRENGTH OF THE EARTWS CRUST 225 



is based on the view that Archean and Proterozoic areas have 

 tended to be rising elements of the continent. Erosion instead of 

 sedimentation has been dominant in later geologic time, which is 

 the reason why these rocks are now exposed as surface formations. 

 If there is any deep-seated departure of density from the mean 

 this tendency to rise should correspond, however, to a deficiency 

 of density persisting through the geologic ages, extending through 

 much of the zone of compensation and offsetting the more than 

 average surface density. Such a regional deficiency is opposite 

 in character to the excess which is postulated by Bowie as an 

 explanation of the positive anomalies. 



Assume then as the next step in the argiunent that the density 

 of the zone of compensation beneath the pre-Cambrian areas to a 

 depth of 114 km. is the same as under Cenozoic areas except for the 

 outer 5,000 ft., both having a mean density of 2.75 to 2.80, but 

 taken here as 2.67. The outstanding anomaly in that case is due 

 to a longer mean column for the pre-Cambrian areas and conse- 

 quently greater mass above the level of complete compensation. 

 If the mean radius of these longer pre-Cambrian and shorter Ceno- 

 zoic columns is as great as 166.7 km., then the unit excess or 

 deficiency of mass of 100 ft. at density 2 .67 when spread over these 

 columns will correspond to an anomaly of 0.0024. If the mean 

 eft'ective areas of the pre-Cambrian and Cenozoic formations 

 affecting individual stations are less, the unit mass will give a 

 smaller unit anomaly. If the mean effective areas are greater, the 

 unit anomaly will not, however, rise above 0.0035. Assume then 

 in conclusion a mean radius of 166.7 km., an anomaly of 0.0024 

 dyne as resulting from 100 ft. of added mass of mean density, and 

 the outstanding anomaly not accounted for by the surficial densities 

 but due to an outstanding difference in volume as between 0.008 

 and 0.012. These figures correspond to a differential mean 

 elevation of 330 to 500 feet of the pre-Cambrian above the Cenozoic, 

 due to erosion. To physiographers such a conclusion will seem 

 quite in accord with the geologic evidence testifying to the resistance 

 of pre-Cambrian formations. 



The character of the Archean and Proterozoic anomalies enters 

 into the problem of crustal rigidity in the following way. If there 



