DEVELOPMENT OF THE DISCOIDAL HYPOTHESIS 285 



stressed solid rock, recrystallizing under unequal stresses, takes on a 

 foliated or schistose structure, witli the longer axes of the crystals or the 

 lamina? of foliation oriented in the direction of least stress.^^ Van Hise, 

 in 1896, first formulated the latter proposition in terms of rock flowage: 



"The secondarj' structure of a rock which is deformed by plastic flow de- 

 velops in the plane normal to the greatest pressure, and this structure is true 

 cleavage." 



Becker contended that secondary structure must develop in the shear- 

 ing, planes or planes of maximum tangential stress, which would stand 

 at 45 degrees or at greater angles to the line of force, as stated by him in 

 1893.39 



It is important to recognize that the controversy which ensued dealt 

 with two distinct processes, both of which affect deep-seated rocks, but 

 under different conditions. Van Hise's concept of flowage by recrys- 

 tallization involved a physico-chemical process of molecular or particular 

 solution and redeposition in response to stresses within the elastic limit. 

 Becker's "maximum tangential strain" is a mechanical effect, capable of 

 producing actual shear only when in excess of the elastic strength of the 

 rock. Since, in the present discussion, we are dealing with stresses 

 within the elastic limit, we are treating the case described by Van Hise. 

 His views, originally based on most extended and thorough research in 

 the field and laboratory, have been fully confirmed by the writers re- 

 ferred to above. When a rock recrystallizes under non-uniform pressures 

 or stress, the crystals develop in a plane at right angles to the maximum 

 pressure or in the direction of least stress. 



In the preceding paragraphs we have considered the mibalanced 

 stresses within the elastic limit, set up by erosion and deposition and 

 directed as already described, upward in the unloaded mass, downward 

 in the loaded mass, and horizontally or diagonally between them, forming 

 a circuit. Nowhere, presumably, are these stresses strictly vertical or 



88 C. R. Van Hise : Principles of pre-Cambriun geology. U. S. Geol. Survey, 16th Ann. 

 Report, 1896, p. 639. 



C. R. Van Hise : Metamorphism and rock flowage. Bull. Geol. Soc. Am., vol. 9, 

 1898, pp. 296-328. 



C. K. Leith : Rock cleavage. U. S. Geol. Survey, Bull. 239, chap, vl, 1905, pp. 107- 

 118. 



G. F, Becker : Experiments on schlstoslty and slaty cleavage. U. S. Geol. Survey, 

 Bull. 241, 1904. 



F. E. Wright : Schlstoslty by crystallization. Am. Jour, of Sci., fourth series, vol. 

 xxil, 1906, pp. 224-230. 



C. K. Leith : Structural geology, 1913, pp. 16-21. 



C. K. Leith : Metamorphic geology, 1915, p. 176. 

 » G. F. Becker: Finite homogeneous strain, flow, and rupture of rocks. Bull. Geol. 

 Soc. Am., vol. 4, 1893, p. 50. 



