(JLAh^SinCATlON OF METAxMOUl'llKJ I'KOCESSES 397 



different, set controls regional metamorphism in the absence of strong 

 crnstal movements. Tliese two conceptions, rnling in the geological 

 profession, together snggest the definition of dynamic metamorphism as 

 metamorpliism wldch is induced in rocJcs because of their deformation, 

 the crustal movement being of the orogenic type. 



The proposed definition lias tlie advantage of not being too intensive 

 for general acceptance. It does not state the physical chemistry involved. 

 It does not presuppose an increase of temperature, however general such 

 increase may be during mountain-building. It does not presuppose an 

 enforced, special circulation of water or other fluids; nor does it by any 

 means cover all instances where the changing rocks have undergone crusli- 

 ing or mere increase of pressure. The one essential, and perhaps tlie only 

 unassaila])le criterion, for dynamic metamorphism is its genetic relation 

 to orogenic movement, the transfer of large masses of the earth's crust. 

 According to the suggested definition, the mental picture called up by tbe 

 use of the term is a geological picture, as it should be. The single genetic 

 condition emphasized is in ultimate control of the reactions leading to 

 chemical equilibrium in the rocks; but it is also the supreme fact for tlie 

 general geologist who is studying the given region or is reading tlie re- 

 ports of others about tliat region. So restricted, the name "dynamic 

 metamorphism" may 1)e saved for science and serve as a perfect counter- 

 part of tliat otlier most usefid name, ^'static metamorphism." 



DEFINITION OF STATIC METAMOnrillSM 



Judd (1889, pages 243-246) introduced the expression ^^statical meta- 

 morphism" to designate the rock clianges resulting from '^'^chemical and 

 crystallizing processes which certainly go on at great depths, and under 

 enorinous pressures, even when the rock-masses do not yield to the pres- 

 sures and thus become subjected to the movements which result in dy- 

 namo-metamorphic action. Such changes, I'csulting from pressures tliat 

 do not affect movements in the ]'0ck-masses, may be appropriately called 

 '^statical metamorphism.' . . . The most potent agency by which 

 change is effected consists in the penetration of the wdiole mass of the 

 rock by various liquid or gaseous solvents. It is for the whole group of 

 such changes — of which ^schillerization' is a conspicuous example — that 

 I propose to employ the teim stalical metamorphism." He noted that 

 statical metamorphism may either precede or follow dyuamic metamor- 

 phism, and tliat tlie latter is much less iuipoi'tant tban was gejierally 

 thought at the time of his writing. 



Dana (1805, page 440) described '^^tatical metamorphism" as that 

 "dependent on heat of a statical source — the earth's mass and the vapors 

 about it." 



