CLASSIFICATION OF METAMORPHIC PROCESSES 407 



tact metamorphism, and^ expressly or tacitly, liave used tlie name witli 

 the broad meaning given above. Their view is reflected in tlie definitions 

 given in the geological mannals of Dana, Geikie, Tornqiiist, Pirsson and 

 Sehuchert, Cleland, and many others. The term is thereby made essen- 

 tially synonymous with the French "metamorphisme d^inflnence'^ and 

 with the older names '^abnormal metamorphism^^ and "accidental incta- 

 morphism.'^ 



Barrell (1907, page 116) separates contact metamorphism from contact 

 metasomatism. The former is described as "taking place witliont addi- 

 tion of materials and resnlting in a crystallization of the wall rocks.'^ 

 The changes are "those of volnme and not of mass." Contact meta- 

 somatism "indicates a mass change in the composition of the rock other 

 than the elimination of gases involved in simple metaniorphism. The 

 action takes place through magmatic emanations." These usages conflict 

 with the definition of metamorphism here proposed and also with defini- 

 tions of contact metamorphism by the majority of writers (for example, 

 Grubenmann, 1910, page 70), who regard magmatically controlled meta- 

 somatism as a trne metamorphic phenomenon. 



V. M. Goldschmidt (1911, page 119) describes a mere recrystalliza- 

 tion of the coimtry rock, without the addition of material to it from tlie 

 magma, as "normal contact metamorphism," and describes alteration of 

 the country rock through such addition of material as "pneumatolytic 

 contact metamorphism." Von Wolff (1914, page 240) adopts the latter 

 name with Goldschmidt's definition. 



Bunsen's (1851, page 241) "pneuma4olytic" referred to sublimations 

 from truly volcanic masses. Brogger (1890, page 213) enlarges its mean- 

 ing so as to take in all the metamorphic changes due to magmatic gases 

 in general. Barrell (1907, page 117) suggests that "contact metasoma- 

 tism may be divided into pneumatolitic (sic) and hydrothermal meta- 

 somatism, according to whether the magmatic emanations are above or 

 below 365° C. and 200 atmospheres pressure — the critical temperature 

 and pressure of water." In principle Irving (1911, page 298) follows 

 this usage. It raises the question whether the field of contact metamor- 

 phism might be divided into three parts: thermal-contact metamor- 

 phism, covering rock changes due to njere heating; hydrothermal-contact 

 metamorphism, co^ering rock changes involving magmatic fluid emana- 

 tions at temperatures less than 365° C; and pneumatolytic-contact meta- 

 morphism, invohing gases at temperatures above 365° C. This query 

 seems best answered in tlie- negative. First, because present knowledge 

 does not permit tlie distinction in practice on the basis of the given tem- 

 perature; secondly, because the exclusion of many reactions, controlled 



