6 G. F. Becker — Cum nt Theories of Slaty Cleavage. 



Iniiigure 2, 1 have drawn out the equipotentials for a rota- 

 tional strain which is identical with that illustrated in figures 

 3 and 4. The hyperbolas are the traces on the plane of 

 the diagram of the hyperboloids of revolution whose equation 

 is stated above, when the axes of co-ordinates are the prin- 

 cipal axes of the strain ellipsoid. The two asymptotes are 

 the traces of a two-sheeted cone of revolution, so that in 

 the whole sj^stem of surfaces there is not a plane area. It is 

 upon these surfaces of double curvature that mica should be 

 deposited at the final moment of strain were it true that this 

 mineral crystallizes perpendicularly to resultant stress. If such 



crystallization occurred during the whole progress of stress, the 

 mica would be found not only on one system of hyperboloids, 

 but upon innumerable intersecting systems of hyperboloicls. 



I am not aware of any lithological phenomena of a character 

 corresponding to such equipotentials. Nothing more unlike 

 the structure of a slate belt can be imagined, and I conclude 

 that the hypothesis under discussion is wholly without founda- 

 tion." The fallacy, of course, consists in confusing the forces 

 acting on the exterior of the mass with the resultant of these 

 and the internal forces. It is this resultant which is actually 

 exerted on any small group of molecules within the body. 



The lines of flow may be considered as representing the ab- 

 solute motion of particles of the mass. There is also a relative 

 motion of the elements of mass which is rectilinear and takes 

 place along the planes of maximum tangential strain. It is to 

 impairment of cohesion caused by this relative motion that I 

 suppose cleavage and jointing due. 



Although the equipotentials are as far as possible from being 

 plane surfaces, yet the mica scales in phyllites are arranged in 



*Mr. F. E. Wright has published a preliminary note on some experiments 

 which would seem to offer some support to the supposition that crystalliza- 

 tion in glass is determined by external forces. In these experiments, how- 

 ever, insufficient care was taken to ensm - e uniformity of temperature, and 

 when they were repeated with more precaution the results were practically 

 negative. This Journal, vol. xxii, p. 224, 1906. 



