268 B. WILLIS DISCOIDAL STRUCTURE OF THE LITHOSPHERE 



mathematical physics and restored to the conditions of its natural en- 

 vironment. They impose certain limitations, among which is the require- 

 ment that the solidity of the earth be recognized as the controlling con- 

 dition. 



The second erroneous assumption of Hayford's hypothesis is that "at 

 and below the depth of compensation the condition as to stress of any 

 element of mass is isostatic^ — ^that is, any element of mass is subject to 

 equal pressures fro7n all directions as if it were a portion, of a perfect 

 fluid." ^^ This interpretation of the term "isostatic" is restated in all the 

 later writings of Hayford and Bowie, and is accepted by Barrell, although 

 the latter recognized the artificial character^^ of the assumption that 

 isostasy is equivalent to hydrostatic balance. 



There is a difference between solid flow and liquid flow, to which atten- 

 tion is called on a preceding page (265). In the case of liquid flow 

 there is no orientation of stress, for under hydrostatic pressure stress is 

 equal and uniform in all directions. The movement of the fluid occurs 

 in threads or currents which are incapable of transmitting compressive 

 stress in a definite direction. They bend aside. On the other hand, they 

 respond directly to a tensile stress or lessened resistance, flowing toward 

 the point or area of least resistance. This is readily seen in colored 

 liquids. 



In the case of solid flow there is definite orientation of stress, which 

 obeys the law of shear. I was struck with this fact when making experi- 

 ments on folding and thrusting in solid plastic substances, in 1886-1887. 

 The more homogeneous the solid and the lower the ratio of its viscosity 

 or internal friction to the applied pressure, the more perfect the shear- 

 ing. Applying this observation to solid flow in the lithosphere, shearing 

 should, and no doubt does, become more perfect as the ratio of strength 

 to load, the factor of safety, decreases — that is, as depth increases. 



Movements which take place in obedience to the law of shear are defl- 

 nitely oriented and persistent in direction. The shearing surface origi- 

 nates as a plane. There may be one or an infinite number of parallel 

 planes. The movement results from a compressive stress, which is the 

 resultant of all the stresses acting on the element of mass, less the inter- 

 nal friction. 



At and below the depth of compensation, shearing stresses in solid 

 rock may be equilateral^ if the resistances on all sides be equal, but they 



22 J. F. Hayford : The geodetic evidence of isostasy. Proc. Wash. Acad, of Sci., vol. 

 viii, 1906, p. 28. 



23 J. Barrel! : The strength of the earth's crust. Chicago Jour, of Geol., vol. xxii, 

 1914, pp. 662-663. 



