816 A TREATISE ON METAMORPHISM. 



pressed grains, and without the intervention of water or any other solvent."" 

 If this last sentence were written with the word may before "flow" no 

 exception to the conclusion would be taken. While the process of flatten- 

 ing by movement along gliding planes undoubtedly occurs in marbles, it 

 by no means follows, even in the 21 rocks in which the residual strain 

 effects are described, that this was the chief process in their metamorphism. 

 Indeed, in the production of the marbles from the original limestones, 

 in order that the rocks could change from the very fine-grained condition 

 of limestone to that of marble, recrystallization must have occurred, as 

 explained on pages 808-809. Therefore to explain the metamorphism of 

 the 21 rocks showing residual strain effects, we must place recrystallization 

 as the chief process. During- or after this, mechanical deformation supple- 

 mented recrystallization and gave the various strain effects described. 

 Indeed, the history of these 21 rocks is very nearly analogous to the com- 

 plete history of the Carrara marble experimentally deformed by Adams 

 and Nicolson. Nature, in producing the Carrara marble free from strain 

 effects, utilized recrystallization as the chief process. Later Adams sup- 

 plemented this process by mechanical deformation. This later artificial 

 deformation was an unimportant episode as compared with the recrystalli- 

 zation by nature. 



Therefore, while I have no doubt that during the deformation of marble 

 all the mechanical movements which Adams and Nicolson have described 

 occurred, I hold that these, in the case of the production of marble from 

 limestone, are entirely subordinate to, although they assist, the dominant 

 process of solution and deposition, or recrystallization. And if this be true 

 for marbles it can not be doubted that for those rocks composed of minerals 

 which are less mobile than calcite and dolomite, and which do not have 

 ready gliding planes in various directions, recrystallization was the process 

 of paramount importance. It is certainly yet to be proved that, in such 

 rocks as the slates, schists, and gneisses, movement along gliding planes is a 

 process of consequence in their metamorphism. 



CHERTY LIMESTONES, CHERTY DOLOMITES, AND CHERTY MARBLES. 



Chert is a term used to include all forms of finely crystalline nonfrag- 

 mental silica, including- opaline, semicrystalline, and completely crystalline 

 varieties. It differs from quartzite and novaculite in being nonfragmental. 



« Adams and Nicolson, cit., p. 398. 



