MECHANICAL WORK. 673 



most profound significance in the metamorphism of rocks, for it will be 

 fully explained under "Metasomatism" that in so far as mineral particles 

 are strained, and especially as they are under unequal strains, in different 

 directions, solution and redeposition is likely to accomplish deformation. 

 (See pp. 690-692.) The materials are dissolved and mineral particles of 

 different shapes are deposited in an unstrained condition. To accomplish 

 this it is not necessary that the stress difference shall equal or even approach 

 that of the elastic limit of the minerals and rocks. 



We conclude that strain ivithin the elastic limit, either ivith or without 

 a stress difference, but almost universally with a stress difference which for 

 much of the zone of anamorphism often exceeds the crushing strength of the rocks 

 at the surface, prevails throughout the zone, affecting every mineral particle, small 

 and great. 



Where the stress difference surpasses the elastic limit of the rocks 

 under the conditions in which they exist, rupture may take place, and this 

 leads us to the next section. 



STEAIN BEYOND ELASTIC LIMIT. 



Strain beyond the elastic limit in the zone of anamorphism results in 

 disruptive deformation, as in the belt of cementation. But the ruptures 

 are very different in the two. During deformation in the zone of anamor- 

 phism every particle, small or great, takes part in deformation, and it is 

 this fact which gives a fundamental difference between fracturing in the 

 zone of anamorphism and in the belt of cementation, since in the latter belt 

 between the fractures are blocks, small or great, which do not take part in 

 the movements. In this belt but a small number — in many cases an almost 

 infinitesimal fraction — of the mineral particles are actually ruptured, 

 whereas in the zone of anamorphism strain beyond the elastic limit breaks 

 all particles. If a slide be made from rocks deformed in the zone of 

 anamorphism not one square centimeter can be found in which movement 

 has not taken place. Not only so, but no mineral particle has escaped the 

 effect of the deformation. Where strain has extended beyond the elastic 

 limit, under the microscope the deformed mineral particles are seen to give 

 evidence of the above facts by their modulatory extinction and their 

 granulation. (See fig. 17.) 



This contrast in the nature of mechanical deformation between the 

 zone of anamorphism and the belt of cementation is of great significance, 

 mon xlvii — 04 43 



