Van Rise — Metamorphism of Rocks and Rock Flowage. 83 



The reverse chemical action, that taking place with the absorp- 

 tion of heat, occurs at great depth under mass static conditions. 



The foregoing principles of alterations under molecular 

 dynamic conditions are applied to the devitrification of glass 

 and to the recrystallization of minerals. 



Some of the more distinctive features of molecular dynamic 

 action are the growth of large individuals and preservation or 

 emphasis of previous textures and structures. 



31ass Dynamic Action and Accompanying Molecular Dynamic 



Action. 



It has already been stated that in connection with mass 

 dynamic action, molecular dynamic action invariably occurs. 

 The kind and amount of resultant metamorphism varies greatly, 

 depending upon depth, upon the particular kind of defor- 

 mation, and upon other factors. It has been shown in 

 another place that, depending upon depth, there are three im- 

 portant zones of deformation of which we have definite knowl- 

 edge : (1) An upper zone of fracture, (2) an intermediate zone 

 of fracture and flowage, and (3) a lower zone of flowage. 



Zone of fracture. — In the zone of fracture deformation is 

 accomplished by considerable movements along surfaces or 

 zones, with little or no movements between these planes or 

 zones. Such fractures are faults, joints, iissility, bedding part- 

 ings, and the spaces of antoclastic rocks. The rocks are 

 broken by these fractures into great regular masses, blocks, or 

 leaves or into the irregular fragments of a dynamic breccia. 

 Into these openings water readily enters to assist in the modi- 

 fications. The movements between the individual mineral 

 particles are largely confined to thin layers along the walls of 

 the openings, and the conditions may be here those of important 

 interior deformation, but for the masses of rock between the 

 fractures the conditions are those of molecular dynamic action 

 already described, and the changes are correspondingly slow. 

 The rapid changes are confined to the material adjacent to the 

 openings. From the places of entrance waters may permeate 

 the adjacent rocks to a greater or less distance, and conse- 

 quently molecular dynamic metamorphism may occur to a much 

 greater extent than it would be were it not for the fracturing. 

 The alterations of the thin layers of material adjacent to the 

 openings are by interior movements, which are in all respects 

 like those of kneading described under the zone of flowage. 



It follows from the foregoing that the deformation 

 accomplished by widely spaced fractures does not result in the 

 obliteration of the original textures and structures, except 

 adjacent to the fractures. The rocks are merely jointed, 



