190 A TKEATISE ON METAMORPAISM. 



a depth. Since there is no theoretical way accurately to evaluate these 

 factors and thus to calculate the maximum depth of the upper part of the 

 zone of no wage, one can judge of its real depth only by observation in 

 mountain areas where deep-seated rocks have been deformed when buried 

 under an approximately determinable thickness of rocks and afterwards 

 have been brought to the surface by denudation. From observation data 

 I suspect the maximum depth calculated is much too great, perhaps twice 

 too great even for the strongest rocks; and for the weaker rocks it is certain 

 that the alterations characteristic of the zone of flowage occur at depths but 

 a fraction of 10,000 or 12,000 meters. 



The boundary between the zone of fracture and the zone of flowage is 

 approximately the same as the boundary between the upper and lower 

 physical-chemical zones if indeed it is not identical with it. We may 

 therefore say that the upper physical-chemical zone, the zone of katamor- 

 phism, and the zone of fracture are synonymous terms, as are also the 

 lower physical-chemical zone, the zone of anamorphism, and the zone of 

 flowage. The reasons for the correspondence of the zone of fracture with 

 the zone of katamorphism, and of the zone of flowage with the zone of 

 anamorphism, are clear. 



To the bottom of the zone of fracture the rocks are strong enough to 

 support themselves, hence there is not pressure in all direction greater than 

 the strength of the rocks, and openings may exist, The reactions may 

 therefore take advantage of these spaces and fill them, thus expanding the 

 volume of the rocks without lifting them or doing the mechanical work of 

 rupturing them. The openings which may thus be utilized vary from those 

 of supercapillary size, such as bedding partings, fault and joint openings, to 

 subcapillary openings between the individual grains. In order to thus fill 

 these spaces, no large amount of work must be done against pressure by the 

 chemical agents, but in proportion as the spaces are filled it is more and 

 more difficult for the reactions to occur requiring expansion of volume, as 

 an increased amount of work must be done against pressure. 



However, below the bottom of the zone of fracture, in the zone of 

 flowage, the pressure in all. directions is greater than the strength of the 

 rocks. If supercapillary spaces be supposed to be present they would be 

 closed by flow, unless this were prevented by occluded water or some other 

 liquid or a gas which could not escape. If a reaction here occurs which 



