:;0() n. A. I)AI>Y METAiMOrxrniSlM AKV rTS PHASKS 



pag-e 215), like Kemp (1908, page 144), are logically compelled to do this, 

 but the result only goes to show the difficulties raised by their definition, 

 for all oeolo2-ists are vitally interested in the classification of rocks as well 

 as in that of metamorphic processes. 



Befoi'e attempting a definition that may meet the approval of the ma- 

 jority, a list of the alterations affecting rocks below the shell of weather- 

 ing should be scrutinized. It covers : 



1. Simple crushing. 



2. Consolidation by pure coliesion. 



o. Consolidation by cements, amorphous and crystalline. 



4. Consolidation by both cementation and recrystallization. 



5. Concretionary action. 



6. Pseudomorphic changes in constituent minerals. 



7. Polymorphic changes in constituent minerals. 



8. Devitrification. 



9. Eecrystallization in general, 



10. Volatilization; for example, dehydration, carbonization of organic 

 matter. 



11. Complete fusion or simultaneous solution of most of the rock con- 

 stituents. 



The simple crushing of a rock or its consolidation by pure cohesion, in 

 neither case accompanied by new crystallizations, is not usually regarded 

 as an independent metamorphic process. Both are very rare phenomena. 

 Some mylonites and other breccias may represent the one, as a few strati- 

 fied rocks may represent the other. 



The problem of rock cementation is much more difficult. Most writers 

 have, apparently, voted to regard it as a phase of metamorphism. Since 

 cementation merges into load metamorphism with utter gradualness, and 

 since the conditions of cementation are largely of the same quality with 

 those controlling both load and dynamic metamorphism, this view of the 

 majority seems well taken. For practical reasons, however, the writer 

 believes it best to admit in true metamorphism only those kinds of cc- 

 iiientation that are accompanied by new crystallizations in the rock body 

 concerned. 



By common agreement concretionary action, pseudomorphism, poly- 

 morphism, and devitrification are regarded as phases of metamorphism 

 as well as of rock weathering. 



Nearly or quite complete melting or solution of a rock, even though 

 followed by crystallization, is usually treated as a magmatic, rather than 

 a metamorphic, phenomenon. AVith that understanding the following 

 definitions have been framed. 



