98 A TREATISE ON METAMORPHISM. 



to regard them as practically free from stress and strain. However, not 

 infrequently rapid deformation by uplift of an arch or by fracture when a 

 few meters of load is removed, as at the Chicago drainage canal and at 

 the combined lock of Appleton," shows that such rocks are under very con- 

 siderable stress, and therefore must be strained. 



Not only are rocks generally under stress, but because of the com- 

 plexity and variability of rock compositions, structures, and textures, 

 wherever rocks are under stress the amount of stress and therefore of strain 

 continually varies with changing direction and changing position. Variable 

 amount of strain is therefore a universal law. In so far as any mineral 

 particle is strained to a greater degree than an adjacent mineral particle of 

 the same kind similarly strained, the particle under greater strain is more 

 rapidly altered by chemical action. In so far as any portion of a mineral 

 particle is strained to a greater degree than another portion of the same 

 particle similarly strained, the part under greater strain is more rapidly 

 altered by chemical action. Finally, for the same mineral particle or some 

 part of the same the strain varies continually during deformation. 



From the foregoing it follows that the almost universal state of strain, 

 and the not less universal variability in the amount of strain, are of the 

 most profound significance in metamorphism. (See Chapters VI, VII, VIII.) 



strain with rupture — Where deformation produces rupture, another feature 

 enters, also favorable to chemical action. Rupture is favorable to chemical 

 action since thereby the surface exposed to the underground waters is 

 inversely as the average diameter of the mineral particles. Granulation 

 very greatly increases the surface of action. 



Readjustment of panicles — The readjustment of the rock particles with refer- 

 ence to one another can hardly fail to give better opportunities for the 

 chemical action of the ground waters; for during the adjustment the 

 water will necessarily be moving and will come in contact with a succession 

 of mineral particles, and thus promote chemical interchange. Hence I 

 conclude that mechanical action is favorable to metamorphism by chemical 

 action, whether the deformation be strain without rupture, with rupture, or 

 merely readjustment of the rock particles, or, finally, any combination of 

 these. 



<i Cramer, Frank, On the rock fracture at the Combined Locks mill, Appleton, Wis. : Am. Jour. 

 Sci., 3d ser., vol. 41, 1891, pp. 432-134. 



