CLASSIFICATION OF METAMORPIIIC PROCESSES 395 



special claims for their definition — claims so strong that its retention in 

 a working classification seems highly expedient. 



The correlative term ''local metamorphism' is preferable to ^'contact 

 metamorphism/'' for a reason to be more fully seen in a following section 

 on "load-contact metamorphism/^ Many Precambrian terranes have 

 been metamorphosed by a combination of causes involving both igneous 

 intrusion and widely spread, truly regional conditions of recrystallization, 

 namely, those of load metamorphism. One can not say whether the neces- 

 sarily local, igneous-rock influence or that of load cooperating with gen- 

 eral earth heat is the more important. It seems best, therefore, to group 

 both load metamoi'phism and load-contact metamorphism under the one 

 head of local metamorphism, which is thus defined as metamorphism 

 genetically connected with tlie eruption of magma. 



Like "regional metamorphism," the term "local metamorphism'^ may 

 yield its place when origins have become sufficiently ascertained. Until 

 that distant day each will continue to serve a most useful purpose as one 

 member in the grand, dichotomous division of all metamorphic processes. 



DEFINITION OF DYNAMIC METAMORPHISM 



Eosenbusch's definition of dynamo-metamorphism has been given above. 

 He added (1910, page 73) the following statements: "That it effects im- 

 mediate changes in the structure of the rock concerned — through stress, 

 crushing, displacement, stretching, cleaving — can not be doubted ; whether 

 it directly causes chemical alteration is not fully proved, but probable. 

 In any case it facilitates the access of transforming agents and extraor- 

 dinarily increases the amount of surface on which those agents may act. 

 Thus in dynamo-metamorphism we have displacement in the rock and 

 the development of a new structure" (translated). 



Barker (1889, page 16) supplements Eosenbusch's definition with the 

 theoretical view that the term should imply conditions of low temperature 

 and high pressure. 



De Lapparent (1893, page 140(5) understood dynamic metamorpliism 

 as resulting "from the mechanical actions (French, 'actions') to which 

 the solid rocks are subjected during the building of mountains." On 

 page 1573 he remarked: "The orogenic action does not seem to be limited 

 to the production of inecbanical effects. It appears to have been also a 

 potent cause of metamor]:>bisin." On ])agc 012 a formal definition is 

 given as follows: Dynamic metamorpliism is "'tlie sum of the changes 

 which orogenic movements luive occasioned, cither in compressing and 

 dislocating the minerals or in facilitating the circulation of hot waters, 

 capable of reacting on the mineral species existing in the rock." De 



