412 R. A. DALY METAMORPIIISM AIsD ITS PHASES 



cation. In many instances the Prccambrian specialist has gone into the 

 field with too much reliance on single principles affecting rock alteration. 

 He has overemphasized dynamic action, or the efficiency of vadose waters, 

 or the purely thermal effects of burial, or the power of hot gases rising 

 from the earth's interior. Preconceptions have thus too often prevented 

 workers from observing critical facts in the field. Some of these geol- 

 ogists have preached false doctrines, not because they were too theoretical, 

 l)ut l)ecause they were too little theoretical and did not apply thoroughly 

 the principle of multiple hypotheses. Scarcely one fundamental modern 

 idea on metamorphism was not foreshadowed, by writers in the heroic age 

 of geology. Field men stumble, teachers are puzzled, and students are 

 worried because the geological profession has not insisted on the maxi- 

 mum possible completeness of a systematic, rigorous classification of 

 principles suggested long ago. Though but a report of progress and thus 

 unfinished, such a classification serves as a means of expression and, yet 

 more valuably, as a stimulus to further correct observation in nature. 



The writer does not, of course, pretend to have formed definitions or 

 classification which will satisfy geologists in general. He merely offers 

 a scheme for criticism and. then improvement. It may be 'pointed out 

 that all names used in classification are either now represented in the 

 leading languages of Europe or are capable of ready translation. The 

 proposed scheme thus follows a peremptory rule in building a scientific 

 system and specially invites international cooperation for its bettering. 



Additional descriptive Terms 



Since 1833 many words, other than those embodied in the present classi- 

 fication and yet denoting aspects of metamorphism, have been coined or 

 adapted. Some have permanent value as aids to the description of meta- 

 morphic rocks, when the causes of their alteration are only partly known. 



If one wislies to emphasize pressure as a leading physico-chemical con- 

 dition in a given case, the term pressure mefam^orphism might be used. 

 If so, the context should clearly indicate that its employment is due to a 

 lack of knowledge as to the source of the pressure itself, whether primarily 

 an incident of mountain-building or the effect of simple burial. If the 

 clioice between these alternatives is possible, then ^'dynamic metamor- 

 phism" or "load metamorphism,'^ as the case may be, should be preferred. 



The development of schistose structure in a rock is a stress phenom- 

 enon. That change might ])e called stress metwmorphism., if the observer 

 has not the data for assigning it to either static or dynamic metamorphism 

 and yet wishes to contrast the type of recrystallization witli that yielding 

 a massive rock, such as common marble. 



