410 F. A. DALY METAMORPHISM AND ITS PHASES 



B. Local metamorphism (caused by eruptive bodies). 



I. Contact metamorphism (magmatic influence in control). 

 II. Load-contact metamorphi.wi (combination of load and magmatic 

 influences ) . 



ATTEMPT AT AN ALTERNATIVE CLASSIFICATION 



With this scheme in mind, the ground for the exclusion of pure vola- 

 tilization from the list of metamorphic processes may again be profitably 

 considered. The question is whether it is expedient to regard as tech- 

 nically metamorphic rock changes that are typified by the conversion of 

 soft coals into anthracite and by the mere expulsion of water from buried 

 mud. In order to discuss this enlargement of the conception of meta- 

 morphism, a special name for changes through pure volatilization is de- 

 sirable. The expression "alembic metamorphism'' is suggested for the 

 purpose. 



Pure distillation takes place under the conditions of static, or dynajnic, 

 or contact metamorphism. In a similarly tentative way, let these phases 

 of pure volatilization be called, respectively, "stato-alembic,'^ "dynamo- 

 alembic," and "contact-alembic." To fit into the classification so far 

 given, "stato-alembic metamorphism" must be rigorously distinguished 

 from "sta,to-hydral metamorphism"; "dynamo-alembic metamorphism" 

 from "dynamo-hydral metamorphism," and "contact-alembic metamor- 

 phism" from the purely thermal phase of contact metamorphism as the 

 cause of new crystallizations. I^one of the three distiiiistions seems pos- 

 sible in practice, i^or has there been better success in attempting a 

 workable dichotomous division of regional metamorphism and then con- 

 tact metamorphism, each pair of subdivisions consisting of a class of rock 

 changes induced by pure volatilization and a class induced by other 

 causes, with or without volatilization. 



In short, the inclusion of pure distillation in metamorphism seems 

 inevitably to lead to excessive com])li cation and to the abandonment of 

 the effort to give strict definition to such established terms as "dynamic 

 metamorphism," "static metamorphism," "load metamorphism," "local 

 metamorphism," and "contact metamorphism." It is simpler to make 

 new crystallization the criterion for metamorphism and to describe rock 

 changes through pure volatilization by some such expression as ''alembic 

 (French, "alambic;" German, "Alembik") transformation." 



FAVORED CLASSIFICATION IN ACTUAL PRACTICE 



As far as possible, terms already in use have been preferred in building 

 the classification. The violence done to existing definitions of the adjec- 

 tives "regional,'' "dynamic," "static," and '^contact" generally consists in 



