THE STRENGTH OF THE EARTH'S CRUST 669 



that narrow columns should be able to rise or sink. This may be 

 illustrated by the following example: 



Suppose a region 50 km. in radius possesses a mean departure 

 from isostatic equilibrium equal to 76 m. of rock (250 ft.) and that 

 the surrounding regions are out of adjustment by the same amount 

 but in the reverse direction. This is the maximum area for regional 

 isostasy which in Hayford's opinion is to be expected, and 250 ft. 

 is the mean departure from isostasy as given by him in his Minne- 

 apoHs address. But in this example the adjacent regions are each 

 assumed to be out of adjustment in opposite directions by this 

 amount and, therefore, the differential load is twice this or 500 ft. 

 of rock. The case is one which he would regard consequently as 

 rather extreme. Now a cylinder 100 km. in diameter and 122 km. 

 deep could not fail through its bending moment, as in the flexing 

 of a beam. It would have to fail as in punching a rivet hole 

 through a metal plate, in other words, by circumferential shear. 

 The shearing stress per unit area is obtained by dividing the total 

 load by the total shearing surface. With the data taken as above 

 this gives ^=8.4 kg. per sq. cm. or 120 lbs. per sq. in. But strong 

 rock at the surface can readily carry a shearing stress of from 700 

 to 1,000 kg. per sq. cm. (10,000 to 14,000 lbs. per sq. in.). Isostatic 

 perfection to this degree would therefore require the zone of com- 

 pensation as a whole to be only about one-hundredth as strong under 

 permanent stress as is solid rock at the surface. This calculation 

 alone would tend to show that the loads and areas by which the 

 crust departs from isostatic equilibrium have been much under- 

 estimated by the advocates of extreme isostasy. 



It should be noted, however, that, following the Hnes of his 

 rejoinder to Lewis, Hayford would answer that he regarded the 

 landward isostatic flow as taking place within the zone of isostatic 

 compensation and the vertical shear as operating, consequently, 

 through a depth far less than the thickness of the entire zone of 

 compensation. There are, however, a number of inconsistencies 

 in this argument, some of which have already been made evident. 

 Others will appear as a result of the later discussion of this chapter. 

 But it may be noted that even granting this contention — that only 

 the outer third of the zone of compensation was involved — the 



