ZONES OF METAMORPHISM, FRACTURE, AND FLOW AGE. 187 



is understood and its chemical and physical significance appreciated, one 

 sees how partial, how overlapping, and how variable are the criteria upon 

 which are based such classifications as thermo-metamorphism, hydro- 

 metamorphism, dynamic metamorphism, contact metamorphism, regional 

 metamorphism, etc. 



RELATIONS OF ZONES OF KATAMORPHISM AND ANAMORPHISM TO 

 ZONES OF FRACTURE AND FLOWAGE. 



Ill connection with metamorphism in the zones of katamorphism and 

 anamorphism it may be recalled that the outer part of the surface of the 

 earth may be divided into two zones upon a different basis. 8 It has been 

 shown from the structural point of view that we may divide the rocks of 

 which we have knowledge into an upper zone of rock fracture and a lower 

 zone of rock flowage. Where the rocks are subjected to deformation while 

 in the upper zone they mainly undergo mass fractures called bedding part- 

 ings, faults, joints, fissility, etc. The deformation is accomplished but to a 

 slight extent by fracturing of the individual particles and by differential 

 movement between them. In the zone of fracture ojjeiiings other than 

 those formed by deformation may exist. In the sedimentary rocks are 

 openings between the grains. In the lavas there may be gas openings. 

 So far as the mass-mechanical forces are concerned openings of all of these 

 different classes may persist indefinitely. In the lower zone — that of rock 

 flowage — if openings could be supposed to be produced in any way, the 

 pressure is so great that the rock flows and fills them nearly completely, 

 except that water solutions and gases are to a small extent included in 

 minute cavities. Rock flowage will be subsequently shown to be mainly 

 accomplished by innumerable fractures of the mineral particles, by the 

 recrystallization of the mineral particles, or by a combination of the two 

 processes in any proportion. Mass fractures play but a subordinate part. 



ITPPER EIMIT OF ZONE OF FLOWAGE. 



The depth at which deformation by flowage occurs depends upon many 

 factors, of which the character of the rocks, the temperature, the water 

 content, and the speed of deformation are of consequence. The more 



« Van Hise, C. E., Principles of North American pre-Carnbrian geology; with an appendix on 

 flow and fracture of rocks as related to structure, by L. M. Hoskins: Sixteenth Ann. Rept. U. S. Geol. 

 Survey, pt. 1, 1896, p. 589. 



