286 B. WILLIS DISCOIDAL STRUCTURE OF THE LITHOSPHERE 



horizontal;, but they will approach those directions and may be discussed 

 accordingly. 



These stresses, in the writer's judgment, will produce no effect of 

 movement in the masses other than a very slow elastic recovery, unless 

 external forces occasion recrystallization. The most competent external 

 force is heat, which increases the speed of chemical reaction as the powers 

 of 2 with each rise of 10 degrees centigrade in temperature, within the 

 range of temperatures commonly employed. 



A rise of temperature may conceivably result from the gathering of 

 heat energy, conducted from within the earth, as postulated by Cham- 

 berlin, more rapidly through the denser interior than through the outer 

 shell, and therefore accumulating in the latter ;^° or it may be due to the 

 intrusion of rocks melted at a deeper level. It is to be noted that heat 

 may also be developed by movements, but we are now concerned with 

 masses which, as a w^hole, are at rest, although subject to non-uniform 

 pressure. 



The effect of non-uniform pressure on melting or solution is far greater 

 than that of uniform pressure and is itself greatly augmented by a rise 

 of temperature. Johnston and Adams, by their investigation of the 

 physical and chemical behavior of solids under high pressures, demon- 

 strated that non-uniform pressure "always lowers the melting point and 

 raises the solubility, and by amounts which are many times greater than 

 • the corresponding changes with uniform pressure." They emphasize 

 further the effects of high temperature, which they cite as an agency 

 which is coordinate with non-uniform pressure, saying: 



"Even with unequal pressure, the effect of change of temperature is so very 

 important that the two factors must be considered simultaneously." " 



In the geologic case, typified by the underbodies here considered, any 

 rise of temperature will first affect the deeper-seated levels in which the 

 direction of maximum stress is approximately vertical and that of least 

 stress is approximately horizontal. In the underbody of a depressed mass, 

 which is under added load, and therefore under elastic compression, the 

 direction may be truly horizontal. In the direction from the loaded to 

 the unloaded the influence of the elastic relief due to unloading will form 

 a resultant, which will trend upward and will approach or reach verti- 

 cality in the unloaded mass. 



*o Chamberlin and Salisbury : Manual of Geology, vol. i, p. 629. Hypothesis of 1909. 



*i John Johnston and L. H. Adams : On the effect of high pressures on the physical 

 and chemical behavior of solids. Am. Jour, of Sci., 4th series, vol. xxxv, 1913, pp. 206- 

 253. Conclusions, p. 251. 



