400 K. A. DALY .METAMUUrJliSM AiS'D ITS PHASES 



Its simpler s^aionyni is Milcli's 'load iitelantorpliism." "Load metamor- 

 pliism" directly connotes vertical stress, but jnst as truly also liigli tem- 

 perature. iS^ew crystallization controlled by dead weight can not take 

 2ilace except at deep levels, wlierc the rocks feel strongly tlie internal heat 

 of the earth. This type of alteration has been perhaps the most nnpor- 

 tant of all ; yet recent writers, over-enthusiastic about dynamic metamor- 

 phism, have strangely overlooked it or else left it without due emphasis. 

 Hence a following section of this paper is specially devoted to load met- 

 amorphism. 



The fourth mcmljer of the ternary series may bear the name ''dynamo- 

 ihcnnal mciamorplvisui." This is regional metamorphism under dynaniic 

 condiiions and at liiyh iciaperaturc. A simple synonym is hard to find, 

 but Gosselet^s (1883, page 202) term ''friction metamorpltism" might be 

 revived for tlie purpose. It sliould, however, be used symbolically, with 

 proper guarding, for friction is clearly not the sole cause for the high 

 temperature so often operative in purely dynamic metamorphism. 



The suggested ternary subdivision is imperfect. It depends on a dis- 

 tinction between "low temperature" and "high temperature" — one that 

 can not yet be made, in practice, on a quantitative basis. Nevertheless, 

 there is some advantage in so enlarging the vocabulary of metamorphism 

 that temperature control may be, at least approximately, indicated in 

 accounts of recrystallized rockd. 



DEFINITION AND SANCTION OF ''LOAD METAMORPHISM" 



Geology owes to Milch (1894, page 121) an important paper, in which 

 two kinds of regiojial metamorphism are described and named. The first 

 is called "Dislocationsmetamorphismus,*' Lossen's old name for what 

 most geologists call dynamic metamorphism, originating in pressure di- 

 rected tangentially with respect to the earth's curved surface. The second 

 is called "Belastungsmetamorphismus," with the exact English transla- 

 tion, "load metamorphism." This tyi^e originates in "verticale Belas- 

 tung." Milcii holds that load metamorphism "is represented in the de- 

 velopment of every rock; it changes CAcry rock which is not in 2)rocess of 

 destruction by Aveathering agents." 



Milch points out that dynamic and local metamorphism result in very 

 similar or identical kinds of mineralogical composites, because each phase 

 may entail the same physico-chemical conditions underground. Through 

 load metamorphism he explains the ul)iquity of the "Grundgebirge" and 

 the Aery common parallelism between original stratification planes and 

 schistosity planes in metamor])liosed sediments. He believes also that in 

 some instances load metamorpliisni can induce planes of schistosity cross- 



