DEFINITION OF METAMORPIIISM 391 



Opinion is divided concerning the place of mere volatilization without 

 the formation of new cr3'Stalline matter. The change from lignite, 

 through bituminous coal, to anthracite has been sometimes described as 

 "metamoriDhic.-' Yet geological manuals and special works on coal, in 

 describing this change, ver}- seldom use the word. "Transformation" is 

 there commonly preferred to "metamorphism," which is thus unnecessar}^ 

 in dealing with the coals. A mud is altered in composition by the expul- 

 sion of some of its water, and a bituminous sediment is altered by the 

 expulsion of natural gas or oil; but few geologists are impelled to call 

 either an instance of metamorphism. For reasons to be stated in a fol- 

 lowing section, it seems better to exclude all such cases of pure volatiliza- 

 tion from the domain of metamorphism. 



PROPOSED DEFINITION OF METAMORPHISM 



If pure volatilization were included, metamorphism might be defined 

 as the sum of the processes which, working below the shell of Aveathering, 

 lead to the alteration of rocks through the activity of solutions — gaseous, 

 liquid, or solid — the change in each case not being accompanied by gen- 

 eral melting of the rock or by general simultaneous solution of its con- 

 stituents. 



If volatilization be excluded (as here advocated), metamorphism may 

 be defined as the sum of the processes which, working below the shell of 

 weathering, lead to the alteration of rocks through the constructive ac- 

 ti\'ity of solutions — gaseous, liquid, or solid — the change in each case not 

 being accompanied by general melting of the rock or by general simulta- 

 neous solution of its constituents. More concretel}^, the definition may 

 be phrased thus: Metamorpliism is the sum of the processes toliicli, luorlc- 

 ing hdowtlie shell of ■weatliering, cause the re crystallization of the origi- 

 nal crystalline materials in rods (with or without chemical reactions) or 

 the crystallization of original amorphous materials in rods, the change 

 in each case not being accompanied by general melting of tlie rode or by 

 general simultaneous solulion of its constituents. 



New crystallization in non-magmatic rock substance is the one basic 

 principle that seems best to express the essential idea shared by Lyell and 

 most other geologists since 1S:J;3. That petrographical criterion has its 

 counterpart in the physico-chemical criterion of the alternative defini- 

 tion, namely, the proof of the constructive activity of solutions. The 

 definition covers the unequivocal changes, such as those from granite to 

 gneiss, from argillite or volcanic ash to schist, from limestone to marble, 

 from argillite to hoi-nfels. It co\ers also the change from coal to graph- 

 ite, the change from anhydrite to gypsum, the change from calcai'cous 



