THE DOCTRINE OF ISOSTATIC COMPENSATION 697 



by Bridgman were made upon hollow metal cylinders, while Adams 

 has interpreted his results to show that the crushing strength of 

 granite is at least seven fold as great as has been supposed. Test 

 blocks of Westerly (Rhode Island) granite at ordinary tempera- 

 tures first began to flow with pressures of 200,000 pounds to the 

 square inch. In a discussion of these results King says : 



No state of shearing stress in the crust of the earth due to the weights of 

 continents and mountains can cause the collapse of the rock in the neighborhood 



of a small cavity At a temperature of 550 C. supposed to exist eleven 



miles below the earth's surface cavities will remain open when submitted to 

 considerably greater pressures than are found at this depth. 1 



Though applying to forces whose continuance of application is 

 short (six hours), many lines of evidence confirm the assumption of 

 the great rigidity of the earth's crust — much the most exact as well 

 as the most recent being the determination by Michelson that in 

 this respect it exceeds. solid steel. 2 



Recent unpublished experiments by Bridgman have an impor- 

 tant bearing upon this point. I am permitted to quote the following 

 from a personal letter : 



1 have recently made a few experiments which show that at least for some 

 substances the viscosity increases enormously with increasing pressure. The 

 effect may certainly be as great as two hundred times for an increase of 1,000 

 atmospheres, and increases at least as rapidly as the square of the pressure. 



Hayford's negative argument in favor of isostasy has thus upon 

 the basis of later work been shown to be fallacious. In his official 

 monographs he has supplied what he considers conclusive positive 

 evidence in support of his contention that isostatic compensation 

 is nearly perfect at a depth of 76 miles below sea-level — in other 

 words, that elevations above the surface persist only by virtue of 

 a deficiency of mass, and that basins are situated above a basement 

 of exceptional density. Upon his hypothesis the quantity of matter 

 is the same in all vertical columns of the same cross-section and 

 limited below at the assumed depth below sea-level of 76 miles. 

 This "positive" argument rests, however, upon Pratt's hypothesis, 

 and implies a weak and failing earth shell. 



z Op. cit.,p. 137. 



2 A. A. Michelson, "Preliminary Results of Measurements of the Rigidity of the 

 Earth," Jour. Geol., XXII (1914), 97-130. 



