296 JOSEPH BARRELL 



Superior region. If such be the case it shows the long endurance of 

 strains borne by this part of the earth. In the almost universal 

 epeirogenic movements which marked the close of the Tertiary and 

 opening of the Pleistocene, the Lake Superior basin showed notable 

 downwarping, its bottom being now beneath the level of the sea. 

 It formed a trough which directed the flow of glacial ice. The 

 latter must have scoured it clean but can hardly be ascribed as the 

 cause of the existence of the basin. The crust movements have 

 doubtless been in the direction of relief of stress, but the relief has 

 been but partial; geodetic investigation reveals that the age-long 

 load is yet borne. 



DEPARTURES FROM ISOSTASY SUSTAINED BY RIGIDITY IN THE ZONE 



OF COMPENSATION 



It was concluded under the last topic that the rigidity over 

 certain parts of the earth probably carries the zone of possible com- 

 pensation as deep as 300 km. even under the assumption of uniform 

 rate, an assumption which tends to minimize the depth; whereas 

 in other regions under that hypothesis it is less than 100 km. in 

 depth. This raises the question whether the regional departures 

 from isostasy are carried as strains within the zone of compensation 

 or are transferred in part to the deeper body of the earth. There 

 are reasons for believing that the former is the case, pointing by 

 inference to a zone of markedly diminished rigidity between the 

 rigid lithosphere and still more rigid centrosphere. 



The geodetic evidence consists in the large values of the squares 

 of the residuals for solution B, the solution which postulates extreme 

 rigidity and compensation at infinite depth. For the whole United 

 States, as shown in the Table XIX, p. 293, the mean value of the 

 squares of the residuals for solution B is 10.7 times the value for 

 solution H. But for group 12, that for which the most probable 

 depth of compensation is 305 km., the distinction is still greater; 

 solution B showing a mean-square residual 28 times greater than 

 for solution E. Dividing in this way the value for solution B by the 

 value for the most probable solution, and taking the mean for all 

 those groups which indicate a depth of compensation greater than 

 the average for the United States, it is found that the ratio is twice 



