376 R. A. DALY METAMORPIIISAr AND ITS PHASES 



use of words therefore reiorescnts a principal duty of every scientific in- 

 vestigator. Experience shows how hard it is, in a fluid science like 

 geology, to hold technical words to constant, universally accepted defini- 

 tions. The causes of this unrest are many : progress in the discovery of 

 facts, progress in coordination, progTess in interpretation or theory, and, 

 one must add, the varying subjectivity, if not carelessness, of writers. 



Few geological topics are as far-reaching and profoundly important as 

 rock metamorphism. Few have liad, and are liaving, such increase of 

 content, both empirical and theoretical. Invented eighty years ago, the 

 words '^'metamorphic" and "metamorphism" are almost as old as scientific 

 geology. Unnumbered facts and theoretical ideas have been clustered 

 under these captions. How far liave their original definitions borne the 

 strain of new discoveries? What changes in their definition have been 

 proposed since the issuing of the first edition of Ly ell's "Principles of 

 Geology" ? Is any definition of metamorphism acceptable to the geolog- 

 ical profession as a whole ? To what extent should theoretical explanation 

 enter into its definition and into that of each of its phases? Is a sys- 

 tematic classification of metamorphic processes possible? In view of 

 much uncertainty as to conditions affecting the development of crystalline 

 schists, is an attempt to fonn such a classification at the present time 

 advisable ? 



These are the questions to l)e discussed in the foil oaring pages. Tlie 

 writer^s form of statement has been made clearer as tbe result of debates 

 Avith his colleagues. Professors Gi-aton, Pal ache, and AVarren, and espe- 

 cially because of analysis of the original manuscript by Mr. A. S. R. 

 Wilson, candidate for the doctor's degree at Harvard University. 



Definition of METAMorvPi-TiSM 



FORMER USE OF THE TERM 



Early in its history, though not at the beginning, "metamorphism" was 

 used in two difi^erent senses, and corresponding definitions are still recog- 

 nized by some authors. "Metamorjohism in the broader meaning" ap- 

 proximates more nearly to tbe literal etymology, denoting simply rock 

 alteration. "Metamorphism in the narrower meaning" lias led to several 

 definitions, all of which, however, disregard literal etymology and exclude 

 rock weathering, or rock cementation, or concretionary action, or all of 

 these examples of alteratioii, from tbe field of metamorphism. 



The range of the definitions is illustrated in the ensuing historical 

 sketch, which, brief as it is, suffices to show tlie need of a universal lan- 

 guage in dealing with niiiltitudes of facts and principles vital to geology. 



