CLASSTFICATTON OF METAMORPmn PKO<'ESS]']S 'VM^ 



to provide for ilic description and dit^eiiysioii of metajiiorpliisiu in the 

 future. 



All luetainorpliism is due to the aetivit}' of solutions. Hence tlie fac- 

 tors to be used in distinguishing ternary and still lower classes of nieta- 

 morphic processes may well he of physico-chemical nature. If these finer 

 subdivisions can be so made, the more purely geoh:)gical factors appearing 

 in the primary and secondary categories, a full genetic scheme is |)()ssi])]e. 



The march oT crystallization in a rock depends on temperature, i)res- 

 sure, the presence oi^ licpiid and gaseous solvents, and the chemical com- 

 position of the rock as a whole; for none of these factors are the ([uan- 

 titative data sufficient to allow its rigorous application to the present 

 problem, and it is hopeless to expect an adequate collection of the data 

 for generations to come. Tlie vocabulary of metamorphism is poverty- 

 stricken for a very good reason. It lacks in names for the respective sub- 

 divisions of dynamic, static, contact, or load contact metamorphism, if 

 made on the basis of any one of the four physico-chemical factors; nor 

 is there at present any apparent need of spinning the web of classification 

 so ela])orate]y. N'evertheless, the content of either static metamorphism 

 or dynamic metamorphism is so huge that their further subdivision is 

 already advisable. 



In the present state of knowledge temperature may be assumed as the 

 most appropriate factor for the ternary, genetic subdivision. The static 

 metamorphism of rocks situated at comparatively small depths takes 

 place at low iemperaiure and is possible only in tlie presence of ivater or 

 ofJier fliiiels willi loir freezing-points. Cementation or lithif action, wdien 

 dependent on new crystallizations, is metamorphism of this sort. It may 

 be designated as Injdronietaniorpliism (see TIarker, 1889, page 15; Mer- 

 rill, 1897, page IGl; Lindgren, 1905, page 124), if the name be under- 

 stood as applying to changes in rocks not subjected to orogenic stress 

 during the metamorphism. This use of the word is arbitrar}^, inasmuch 

 as certain hydrous formations have been partially recrystallized during 

 their deformation yet Avithout the development of high temperature. No 

 unequivocal synonym has been found in print. To supply one, the ex- 

 pression ''stato-liydral metamorpliisin'' has been coined. 



The corresponding alteration under dynamic conditions, hut at low 

 temperature, may be called ''dynanio-hydral metamorphism/' A simpler 

 name might be ''slaty inetamorphisnt,'^ since certain slates have been so 

 developed. However, other slates ]ia\e been formed at temperatures that 

 can not be called low; hence this adaptation is not wholly satisfactory. 



The analogous term "stato-tliermat mefamorplrisni." has been coined to 

 mean regional allnralion, under sialic condiHons and at high temperature. 



