40 JOSEPH BARRELL 



be an essential factor in measuring the maximum strength of the 

 lithosphere and more especially the asthenosphere. 



Accordance of geologic with geodetic evidence. — ^The United 

 States and its bordering ocean bottoms is a region of moderate 

 reliefs as compared to the great folds of the ocean floor or of the 

 continent of Eurasia. The geologic forces of folding and uplift 

 have not worked here with their greatest intensity and the central 

 and eastern half of the continent has been affected by the world- 

 involving Cenozoic diastrophism to only a moderate degree. It 

 is to be expected then that the greatest strains upon the crust, 

 the maximum departures from isostasy, would not be found here. 

 In accordance with this expectation it has been seen that by 

 far the greatest New Method gravity anomalies are found in 

 other regions and associated in most cases with the greater 

 reliefs of the globe. The geologic evidence is in harmony; the 

 amount of uncompensated relief, parallel to the geodetic evi- 

 dence, is greatest for the lesser wave-lengths; but, throughout, the 

 geologic evidence suggests that the actual burdens which can be 

 borne by the crust, as found in regions of culminating stress, are 

 appreciably greater than those detected by geodetic methods as 

 existing in the region of the United States. 



If, in some past ages, as during the Appalachian or Sierran 

 revolutions, strains were generated in this continent as great as 

 those found now in some other regions, it would appear that the 

 slow changes of geologic time, of erosion and crustal readjustment, 

 have partially eased the crust of its load. We may have, then, a 

 variable crustal strength — a maximum strength exhibited during 

 and following the crises of great diastrophism; another, lesser 

 strength, which measures the loads which the crust without failing 

 can bear through all of geologic time. 



ADJUSTMENT OP LOADS TO THE DISTRIBUTION OP STRENGTH 



It has been seen that the departures from flotational equilibrium 

 may become very notable and are of greatest vertical magnitude 

 for wave-lengths from loo up to 400 km. The strains generated 

 by these loads, if distributed through an elastic crust,consequently 



