686 A TREATISE ON METAMORPHISM. 



circumstances deformation is mainly accomplished, not by mechanical 

 subdivision, but by the chemical action of recrystallization. The funda- 

 mental idea of this process is that as the rock is deformed it takes the new 

 form necessary by means of solution and redeposition of the rock material. 

 But this process is accompanied by strain within and beyond the elastic 

 limit of the minerals. In considering recrystallization these facts also are 

 necessarily taken into account. In order to make this clear the facts of 

 recrystallization will first be considered, and then the theory. 



• RECRYSTALLIZATION. 



Facts of recrystallization. — One would expect, from the section on strain beyond 

 the elastic limit, that the more profound the kneading- the finer would be 

 the granulation of the altered rock, but this is not the case. Many of the 

 most profoundly deformed rocks, instead of being extremely fine-grained, 

 are somewhat coarsely crystalline. 



This anomaly was long a puzzle to me. In examining the deformed 

 rocks, I found that under certain conditions the more profound the defor- 

 mation the finer the granulation; but in tracing the process to the extreme, 

 I found that there was always a limit beyond which the particles did not 

 become more finely granulated. On the contrary, at a certain stage a 

 reverse tendency appeared, and the particles, instead of becoming smaller, 

 gradually became larger. This increase in coarseness of the mineral 

 particles may be followed through all stages to the coarse schists and 

 gneisses. 



In the granulated rocks the mineral particles everywhere show strongly 

 the strains of undulatory extinction, but the mineral particles of many of 

 the coarse schists and gneisses show no more than slight strain shadows. 

 The coarse, perfect schists and gneisses, nearly free from strain shadows, 

 are always found to be those which have been deeply buried and pro- 

 foundly deformed or which are adjacent to great intrusive masses, or both. 

 It is therefore clear that those rocks represent the most advanced stages of 

 metamorphism. 



It is generally agreed that the schists and gneisses of this character 

 have been recrystallized throughout, and therefore strongly contrast with 

 those rocks which have been granulated. However, the granulated and 

 recrystallized rocks are not separated sharply from each other (see pp. 



