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Structure," by Leander Miller Hoskins, who treats the subject by- 

 mathematical analysis. 



In 1878 Heim published his discussion of " Umformung durch 

 Bruch and Umformung ohne Bruch." 1 It was necessary less than 

 twenty years ago that he should present an elaborate argument to show 

 that without the action of any softening agent rocks might be deformed 

 without fracture under appropriate mechanical conditions. He had 

 seen and understood qualitatively the facts of folded strata. Van 

 Hise carries the interpretation to quantitative expression in terms of 

 pressure per square inch dependent on depth beneath the earth's sur- 

 face. Following Heim, and going further than he did, Van Hise for- 

 mulates three principles : 



(1) Rocks under less weight than their ultimate strength when rap- 

 idly deformed are in the zone of fracture. (2) Since the boundary 

 between the zone of fracture and the zone of flowage is at a different 

 depth for two rocks of unequal strength, and for the same rock under 

 different conditions of stress, there is a zone of combined fracture 

 and flowage. (3) Rocks buried to such depth that the weight of the 

 superincumbent strata exceeds their ultimate strength are in the zone 

 of plasticity and flowage. 



Under the assumption that rocks are porous, and that, therefore, 

 interstitial spaces at a depth are sustained by a column of water extend- 

 ing to the surface, the author calculates, from formulas furnished by 

 Hoskins, that no cavity can exist permanently in even the strongest 

 rock at depths of 10,000 meters or more below the surface. As the 

 development of cavities is a feature of fracturing, deformation with 

 fracture is not possible below that depth. That zone in which defor- 

 mation occurs without fracture is designated by Van Hise the zone of 

 flowage. 2 



Since rocks vary in strength the depth of the zone of flowage 

 beneath the surface is variable, and where rocks of unequal strengths 

 are interbedded the harder may be in the zone of fracture, the softer in 

 the zone of flowage. It follows that there is a broad zone of fracture 



1 Mechanismus der Gebirgsbildung, Vol. II, pp. 3-75. 



2 Since the paper reviewed was written, Van Hise has described much more fully 

 the minute phenomena of the zone of flowage (Bull. G. S. A., Vol. IX, pp. 295^312). 

 According to his recent conclusions, the deformation in this zone includes both micro- 

 scopical granulation of the mineral particles and their recrystallization. Therefore in 

 this zone he recognizes microscopic fractures and openings. 



