DEFINITION OF META MORPHISM 385 



formation of one rock to another is strongly marked, as in gneiss from 

 granite or mica schist from clay shale. Thongh the mechanical eifects 

 of pressure may he conspicuous, metamorphism is ah\'ays characterized 

 hy chemical clianges in the component minerals ; the composition of tlie 

 rock itself may remain constant/^ He gives a definition of weathenng 

 as including ^^the changes of rocks near the surface due to the decom- 

 posing and oxidizing action of percolating waters al)ove the pei-manent 

 water level." The zone of weathering lias a depth "determined hy the 

 level of the ground Avater or by the depth to wliich fi-ee oxygen can pene- 

 trate in large quantities." 



Tornquist, in a recent textbook (1913, page 18), distinguishes as (a) 

 "regionalmetamorph" those metamorphic rocks which have originated in 

 their present form under a heavy rock cover; as (6) "kontaktmetamorph" 

 those which owe their present character to the influence of igneous 

 magma; and as (c) "dynamometamorph" those developed under orogenic 

 pressure. The separation of (a) and (c) is worthy of note. Tornquist 

 makes two astonishing suggestions. He proposes to call the change from 

 clay to shale "Fossilisierung" ; the change from shale to ci'ay slate "Ve- 

 riinderung," and the change from clay slate to phyllite or mica schist 

 "Metamorphose." 



According to Boeke (1915, page 381), metamorphism represents the 

 sum of the effects of high temperature, or high pressure, or both, so acting 

 on a solid ("fertig gebildete") rock that its constituents are no longer in 

 physico-chemical equilibrium. He admits three kinds: (a) dynamic 

 metamorphism, witli pressure playing the principal role: (b) thermo- 

 metamorphism, with temperature in the principal role: and (c) contact 

 metamorphism, which in his vicAv implies the entrance of foreign sub- 

 stances, derived from invading magma, and further implies metasomatic 

 interchange. Boeke makes no explicit statement as to the relative sig- 

 nificance of either load or static metamorphism. He seems to exclude 

 weathering and ordinary cementation from the domain of metamorphism. 



Pirsson (1915, page 315) defines metamorphism as "a general tci-ni 

 for all those changes by which the original characters of rocks are more 

 or less completely altered, in that their component kinds of minerals and 

 textures are transformed into other minerals or textures, or both." ITe 

 considers weathering effects as, "strictly speaking," metamorphic; but, 

 like de Lapparent, he felt no need of using "metamorphic" in systematic 

 chapters dealing with the work of the atmosphere and the production of 

 soils. Like Schuchert, his collaborator, and like nearly all other writers, 

 Pirsson excludes the rcgolith, as well as gravels, sands, shales, etcetera, 

 from tlie class of metamorphic rocks, Ihus implying a failure of the 



