ISOSTASY AS A WORKING HYPOTHESIS 269 



can not be hydrostatic, as if the solid "were a portion of a perfect fluid/' 



That this conclusion is correct is proved by the distinction between 

 granular and schistose rocks. The former, having cooled from a molten 

 condition, crystallized under hydrostatic stress. Their components are 

 not definitely oriented. The latter, which, according to theory, observa- 

 tion, and experiment, are known to have recrystallized in a solid state 

 under non-uniform stress, exhibit definite orientation of the component 

 crystals. 



There is, furthermore, a distinction between liquid and solid flow in 

 regard to the intensity of stress in various directions. Hydrostatic press- 

 ure is uniform in all directions, and can develop only in a perfect fluid — 

 that is, in a medium whose internal resistance is practically zero. When 

 the internal resistance or viscosity becomes a significant factor, it neu- 

 tralizes a portion of any stress tending to produce flow. All resultant 

 stresses must, therefore, be less intense. 



Darwin, Hayford, Love, and Barrell have all recognized this fact in 

 calculating the stresses within the lithosphere. The term "hydrostatic'^ 

 as used in this connection, therefore, needs qualification. 



In spite of the fact that the lithosphere is a rigid solid, there must be 

 mobility to permit adjustments of equilibrium to take place in such a 

 manner as to produce that approach to isostasy which exists. N"ow, 

 mobility may be conceived to be either a permanent or a temporary con- 

 dition. If it be permanent, the mobility of the rock may be compared 

 with that of pitch, for instance ; but if temporary the mobility is that of 

 a crystalline substance dissolving and recrystallizing momentarily and 

 little by little, just above and below the critical temperature and pressure. 



The concept of permanent mobility underlies Hayford's statement that, 

 for reasons detailed by him, "it appears probable that under the stress 

 differences within the earth due to the weight of continents and moun- 

 tains, the material must slowly yield, the continents slowdy sink down- 

 ward and the ocean bottoms rise." ^* Time is the factor appealed to in 

 this argument to give competency to relatively small stresses to overcome 

 relatively great strength of rock-masses and to induce movement. 



Permanent mobility, notwithstanding high rigidity, was postulated by 

 Willis in 1907, and the concept of temporarily increased mobility was 

 introduced to account for the periodicity of diastrophic effects. ^^ The 

 very great strength of the lithosphere since demonstrated by Adams gives 



2* J. F. Hayford : The earth a failing structui'e. Phil, Soc. of Wash., Bull. XV, 1907, 

 p. 63. 



22 B. Willis : Research in Chink. Carnegie Institution of Washington, Publication 54, 

 vol. 2, 1907, p. 130. 



