THE STRENGTH OF THE EARTH'S CRUST 295 



rigidity, determined by a certain relation of temperature and 

 pressure. 



Within an overlying zone of high rigidity, even where it is of 

 uniform depth, the geodetic measurements of the depth of com- 

 pensation may not, however, show uniformity. If the density is 

 unequally distributed, the compensation of a region may be nearly 

 completed at some depth above the base of the rigid zone, the lower 

 part consisting of rock of mean density and therefore not possessing 

 influence. A region of deep and marked rigidity, if characterized 

 by notable irregularities in the distribution of either density or 

 relief, would show large residuals. A region characterized by more 

 uniform distribution of density and gentle reHef would show lower 

 residuals even with the same rigidity. A region with deep com- 

 pensation would show within the limits of the group lower residuals 

 for the same degree of uniform compensation, than where compen- 

 sation was at lesser depths, since the attracting masses are spread 

 over a greater distance. 



As applications and tests of these principles, it is to be noted 

 that group 5, embracing the Lake Superior region with its low- 

 temperature gradients, has the highest residuals of any group in the 

 United States. Further, the mean values of the least squares for 

 the different solutions show less differentiation than in any other 

 group. These facts suggest irregular distribution of density, high 

 rigidity, and the zone of rigidity may extend below the most 

 probable depth, 152 km., indicated for the limits of compensation. 

 The topographic deflections are only 58 per cent compensated. 

 The contiguous group to the southwest, No. 12, shows the lowest 

 residuals of any group, the separate solutions are sharply differen- 

 tiated and the depth is the greatest in the United States. On the 

 side of this area, the gravity anomaly at St. Paul, 0.059 dyne per 

 gram, is, next to Seattle, the largest found thus far in the United 

 States. It may be concluded, then, that in this part of the con- 

 tinent, undisturbed by igneous activity or mountain-building since 

 the pre-Cambrian, the depth of the zone of rigidity appears to be 

 very great. The irregularities in residuals in group 5 may date 

 from the Keweenawan period, when enormous masses of basic and 

 therefore heavy magmas were intruded and extruded in the Lake 



