30 JOSEPH BARRELL 



therefore to a relation of flotational equilibrium within the solid 

 earth. 



Most geologists in former years have utilized but little the prin- 

 ciples of isostasy, as may be seen by reference to the standard 

 manuals. On the one hand, the weight of sediments may be spoken 

 of as the cause of downsinking with such equal pace that the condi- 

 tion of a shallow sea prevails for a geologic period, though perhaps 

 accompanied by the deposition of thousands of feet of sediment. 

 On the other hand, and without argumentation to explain the 

 apparent inconsistency, the same geologist may state that tangen- 

 tial forces have built folded mountains miles in height which may 

 be subsequently largely removed by erosion before marked vertical 

 warping of the crust occurs. 



In contrast to the geologists, certain geodesists have argued 

 in recent years for a high degree of isostatic adjustment; isostasy 

 being regarded by Hayford, for example, as largely complete in 

 areas probably between one square mile and one square degree in 

 size, the mean departure of these unit areas from the level of com- 

 plete compensation being stated by him as ranging from 250 to 

 570 ft. These figures he does not regard, however, as of a high 

 order of accuracy, the latter being probably the more reliable of 

 the two. He states that their significance is mainly in showing 

 that isostatic compensation is nearly perfect. It has even been 

 argued by Button, Wilhs, and Hayford, as an outflow of geodetic 

 studies, that those vertical movements of the outer crust which 

 tend to give isostatic equilibrium are the ultimate causes of the 

 periodic great compressive movements. 



There is here between geologists and geodesists a tendency to 

 a fundamental difference of opinion, resulting from the emphasis 

 upon one or the other of those opposing forces which work in the 

 outer crust. The truth must lie within the broad zone between 

 these two extremes of theory. To try to bring them together in 

 harmony is the problem before us. 



The first part of the paper, on the geologic tests of the limits of 

 strength, opens with a brief review of the lines of geologic evidence 

 which may be used as tests of the degree of resistance or response 

 by the crust to vertical stresses, having regard to both area and 



