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From these observations follows the inference that cleavage develops 

 in a plane normal to the pressure. The grounds for this inference are 

 discussed at some length and the author's opinion is finally stated 

 (page 639) : 



"I therefore conclude from analysis, from experiments upon viscous 

 and plastic bodies, from observations in the field, and from studies 

 with the microscope, that I am justified in the statement that the 

 secondary structure of a rock which is deformed by plastic flow develops 

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

 true cleavage." 



From the observed parallelism to the inferred normal relation to 

 pressure is a step in hypothesis. To this extent the conclusion falls 

 short of demonstration, but it is in accord with the opinions of 

 eminent students on cleavage, and is supported by mathematical 

 analysts, with one marked exception. The exception is Mr. Geo. F. 

 Becker. 



It has been maintained by Becker that cleavage always develops in 

 two shearing planes at angles to the pressure. His method of arriving 

 at this conclusion may best be indicated by a quotation from his 

 article :* 



" In the following pages the attempt will be made to develop all the 

 manifestations of uniform or homogeneous finite strain in rock-masses 

 regarded as isotropic, exhibiting viscosity and capable of flow, which 

 can be elucidated without assuming a law connecting stress and strain. 

 For this purpose finite strain must first be discussed by itself ; then it 

 must be considered just how far the relations of stresses are capable of 

 coordination with those of strain. The influence of viscosity and 

 solid flow must next be shown. Readers willing to assume that these 

 subjects have been logically treated will probably skip them and pro- 

 ceed to the geological applications which follow. Finally, the results 

 will be compared with actually observed phenomena and with the 

 experiments which several investigators have made on slaty structure." 



The method followed by Van Hise is that of induction from the 

 facts, whereas Becker pursues that of deduction from general principles. 

 Van Hise does not deny the validity of Becker's reasoning nor its general 

 application. He describes the development of fissility and jointing in 

 the zone of fracture as in accordance with Becker's view. But he con- 

 finite Homogeneous Strain, Flow, and Rupture of Rocks, by G. F. Becker, 

 Geol. Soc. Am., Vol. LV, 1891, p. 16. 



