294 JOSEPH BARRELL 



the United States as a whole, a value which may be ascribed to the 

 mountainous relief and the support of individual mountains and 

 ranges by the rigidity of the crust. Nevertheless the residuals for 

 the several solutions fall into a somewhat regular system, and solu- 

 tions E, H, and G are more sharply differentiated from the most 

 probable one than for the whole United States. They may be 

 compared better with the latter if the residuals for the whole 

 country are multiplied by i . 327 as a factor in order to give the same 

 numerical value under solution G. This is done at the bottom of 

 the table. It would appear from these figures as though the argu- 

 ments previously given from geologic analysis receive considerable 

 support from the geodetic results and point to a much shallower 

 depth for isostatic compensation in the Great Basin than over cer- 

 tain other portions of the United States. Furthermore, in the 

 examination of the question of local versus regional compensation, 

 it was only the forty mountain stations classified into two groups 

 according to elevation which gave any suggestion that regional 

 compensation to a radial distance of 166.7 ^m. was not about as 

 probable as more local compensation. In these two lines of geodetic 

 evidence as to limited depth and breadth of compensation there are 

 suggestions therefore which support the geologic inference that the 

 crust of the Cordilleran region may be weaker than over the United 

 States as a whole. On the other hand, the warping or faulting- 

 down of ancient continental areas into marginal sea-bottoms 

 impHes an increasing density of the subcrust and therefore possibly 

 an increasing rigidity and strength under such areas. Such a 

 contrast between the Atlantic Ocean bottom and the Great Basin 

 would correspond to the great strength of crust necessary to sustain 

 the delta of the Niger as compared with the moderate rigidity found 

 by Gilbert for the crust beneath extinct Lake Bonneville, located 

 within the limits of group 8. 



The regions of shallower compensation in the United States are 

 on the whole marked probably by a higher temperature gradient, 

 the regions of deep compensation by a lower. This is illustrated by 

 the very high gradient of the Comstock mine in Nevada and the 

 very low gradient which is found in the Lake Superior copper mines. 

 The temperature gradient may measure the depth to a zone of low 



