398 R. A. DALY METAMORPIIISM AKD ITS PHASES 



Geikie (1903, page 805) speaks of the ^'statical pliase" of regional meta- 

 morphism as that connected with '^enormous pressure leading to little or 

 no molecular crushing, but with some shearing movement in the rock." 

 He remarks that it "does not produce such striking results as the . . . 

 dynamical phase." 



Van Hise (1904, page 47) wi'ote : "Metamorphism by molecular move- 

 ment has generally been called static metamorphism." 



Eies and Watson (1915, page 204) note simply that static metamor- 

 phism refers to "quiescent conditions." 



Here, again, there is no consensus of opinion. Judd specified that "the 

 rocks do not yield to the pressures [exerted by thick covers]." Geikie 

 assumes such yielding, for there is "some shearing movement in the rock," 

 though he makes the cryptic remark that the pressure leads to ^'little or 

 no molecular crushing." Judd stressed the work of fluid solutions. 

 Dana stressed the influence of the earth's internal heat. Van Hise re- 

 gards molecular movement as the essential feature. 



Solutions, heat, molecular movement, and some yielding to pressure 

 are necessary characteristics of all kinds of metamorphism. The residual 

 condition, which may be taken as a workable criterion, is the absence of 

 deformation of the orogenic type. Accordingly, as already implied, static 

 metamorphism may be defined as iliat phase of regional meiamorpMsni 

 ivkicli is not induced ty orogenic deformation. 



Contact metamorj^hism is usnally, in a literal sense, also "static," inas- 

 iniich as alterations l)y magmatic heat and gases are not conditioned by 

 crustal deformation. The proposed restriction of meaning for "static" 

 is therefore arbitrai-y, but no published name other than "static meta- 

 morphism" so well expresses the required negative of "dynamic metamor- 

 pliism," just delimited. As a couple the two definitions are logical, and 

 the corresponding terms can directly tell what the geologist most needs 

 to know concei'ning the principal condition for the regional alteration of 

 rocks. 



I'll A^ EH 01' HTATJC AND DYNAMIC METAMORPHISM 



Supported to a considerable extent hy traditional usage, one may thus 

 divide all metamorphic processes into two primary classes, symbolized by 

 the expressions '^regional metamorphism" and "local metamorphism"; 

 ajid also divide regional metamorphic processes into secondary classes, 

 sym])olized by "static metamorphism" and "dynamic metamorphism." 

 Further logical subdivision is not so well guided by the principle of long 

 usage and is inti-insica.Uy more difficult. N'evertheless, more intensive 

 terms are urgently needed to portray the existing state of knowledge and 



