406 R. A. DALY METAMOKPHlSxM AND ITS PHASES 



morphism affords a useful starting-point in the problem of the Precani- 

 brian crystalline schists. 



The live arguments outlined are of unequal strength, but their cumu- 

 lative power is great. In any case it seems eminently wise to provide 

 load metamorphism a place in a general classification of rock clianges, if 

 that classification is to meet the needs of geologists who have to deal with 

 the Precambrian formations. 



DEFINITION OF ''DYNAMO-STATIC METAMORPHISM'' 



A special combination of static and dynamic conditions is worthy of 

 recognition. A rock formation which has become covered hy a iliich 

 overtJirust mass may itself not he crushed or othenuise deformed and yet 

 may he recrystallized because of the new load on it. If so, the tempera- 

 ture being necessarily high and the pressure on the recrystallizing rocks 

 being vertical, the process is an example of load metamorphism, wliile the 

 special inciting cause is dynamic. To distinguish, such a case, the name 

 "dynamo-static metamorphism,'^ symbolizing a third principal subdivision 

 of regional metamorphism, may be employed. 



DEFINITION OF CONTACT METAMORPHISM 



Contact metamorphism comprises all metamorphic clianges due to con- 

 tact ivith or proximity ta any body of eruptive (igneous) rode, the iieir 

 crystaUizations not being definitely directed by dead-weight stress. Tbis 

 definition, adapted from Geikie (1903, page 766), is that generally fol- 

 lowed by geologists. 



Certain authors have tried to restrict the term to mean the effects of 

 mere heating by eruptive magma. Thus de Lapparent (1893, page 1I:0'3) 

 used "metamorphisme peripherique" to symbolize the metamorphism in- 

 duced by gases and liquids emanating from magma and included it with 

 ''metamorphisme de contact" in a dichotomous division of "metamor- 

 phisme d'influence." Haug (1907, page 176) appears to agree in tins 

 usage. Yon Wolff (1914, page 240) makes ''contact metamorpb ism in 

 the narrower sense'' a synon3'm for this purely thermal contact action. 



On the other hand, Boeke (1915, page 384) reverses the definition and 

 regards "Kontakt-Metamorphose" as that due to the recrystallizing in- 

 fluence of magmatic fluids on the invaded rocks, while his 'Thermo- 

 metamorphose" is that induced when high temperature plays the chief 

 role at igneous contacts. 



Inasmuch as it is, in many instances, impossible to distinguish tbe 

 effects of mere heating from those of gaseous emanation, most geologists 

 have been right in refusing to use either principle as a criterion for con- 



