672 A TREATISE ON METAMORPH1SM. 



of the pressure in the different directions varies from zero to the elastic 

 limit of a mineral particle under the conditions in which it exists. The 

 difference between the stresses in the different directions where they are 

 great in all directions has been called by Darwin the "stress-difference."" It 

 has been assumed by Darwin that if a mineral particle or a rock be sub- 

 jected to stress in all directions, when the stress-difference between the 

 maximum and minimum pressure is as great as the strength of the particle 

 or rock at the surface, deformation by rupture will take place. But this 

 by no means necessarily follows. It is highly probable that a greater 

 stress-difference is required for deformation when the rocks are under 

 pressure in all directions than when the pressure in one direction is zero, as 

 at the surface. Hallock found'' that when a solid of a certain composition, 

 readily plastic under ordinary conditions, was subjected to a pressure of 

 6,000 atmospheres in cylinders on one side of which were placed coins and 

 tacks, the plastic substance, instead of flowing around the coins, pressed 

 them against the surface of the cylinder so as to fit it perfectly. Moreover, 

 the coins and the steel tacks were forced against the cylinder so strongly 

 that their impressions were left on the steel holder so as to be seen and felt. 

 From this experiment Hallock concludes that "in general for one and the 

 same substance, over considerable ranges of condition, the rigidity dimin- 

 ishes as the intermolecular distances increase." c Or, reversing this gener- 

 alization to accord with his experiment, the rigidity increases as the 

 intermolecular distance decreases. Now in the zone of anamorphism, 

 where great pressure obtains in all directions, the molecules are brought 

 closer together than at the surface, and probably therefore a much greater 

 stress-difference is required to reach the elastic limit of mineral particles 

 in rocks in the zone of anamorphism than at the surface. How much 

 greater the stress-difference must be it is impossible to conjecture. 



This statement applies only to mechanical deformation. In so far as 

 change in form takes place through solution and deposition, or by recrys- 

 tallization, the above rule does not apply, and this is probably a fact of 



« Thomson, W., and Tait, P. G., Treatise on natural philosophy, Cambridge Univ. Press, London, 

 edition of 1890, pt. 2, p. 423. 



('Hallock, William, The flow of solids; or, liquefaction by pressure: Am. Jour. Sci., 3d ser., vol. 

 34, 1887, pp. 277-281. 



c Hallock, cit., p. 278. 



