Gr. F. Becker — Current Theories of Slaty Cleavage. 13 



est shortening which the rock has undergone." Certainty 

 seems to me a strange word to apply to conclusions from such 

 evidence, Not one of the methods can be depended upon to 

 determine the position of the strain ellipsoid within many 

 degrees, excepting the untried expedient of seeking the undis- 

 torted sections of a fossil. 



Mr. Leith discusses at some length the superposition of 

 fracture cleavage on flow cleavage. He interprets many 

 observed cases as indicating that after flow cleavage has been 

 developed, a fracture cleavage has been superimposed upon it, 

 of course under radically different conditions of stress. He 

 borrows from a report by Mr. Dale* an illustration of such a 

 case taken from a thin section, and I shall follow his example 



, 6 



in figure 6. When a double structure is developed in rocks 

 the hypothesis that each is due to a separate cause always seems 

 to me very dubious. If parting, or weakness (cleavage), is 

 developed in one direction, a subsequent fortuitously oriented 

 force intense enough to produce rupture at all is almost certain 

 to lead to movements on the old surfaces, and, if this will not 

 suffice to relieve strain, something like granulation usually 

 ensues. Hence I am led to believe that the divisions shown in 

 Mr. Dale's plate have a common origin and were simultaneously 

 developed. Mr. Leith concludes that the diagonal fractures in 

 the cut are developed on my theory. But if I apply my theory 

 in extenso to this case, the exact orientation of the strain ellip- 

 soid is at once determined. The major axis should bisect the 

 acute angle between cleavage and the cracks, and therefore 

 stand at an angle of 19° to each. The position of the minor 

 axis follows. Supposing the volume of the rock constant, the 

 minor axis will be proportional to the tangent of 19° or 0"314 

 =B. Hence at once the linear compression in the direction of 



*U. S. Geol. Surv., 19th Am. Eep., pt. iii, pi. 28, p. 208. From a slide 

 of a Cambrian roofing slate near West Paulet, Vermont. The black spots in 

 the lower part of the cut are pyrite. 



