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



sent throughout,* ^s well as cleavage which is not parallel to 

 the mica plates of phvllites. Fracture cleavage, he says, is 

 present abundantly in the rocks of the lithosphere.f It is per- 

 haps desirable for me to comment on his paper as succinctly 

 as possible. 



Mr. Leith does not make the theory he supports entirely clear 

 to me. He states, and repeats in his summary,^ that the 

 parallel arrangement of component minerals in slates showing- 

 flow cleavage is developed by recrystallization "mainly in planes 

 normal to the greatest pressure." By greatest pressure, I under- 

 stand him to mean resultant stress. Other passages, however, 

 seem to contradict these. In one of them§ he says that the 

 final position of cleavage " may or may not be inclined to the 

 greater stress depending upon the nature of the strain." Again 

 on another pagef he admits that pure or irrotational strains are 

 of rare occurrence in rock masses and concludes from the nature 

 of rotational strain that " the final position of cleavage is usually 

 inclined to the axis of greatest stress. ^[ He maintains, however, 

 that even in such cases cleavage is always tending to develop 

 normal to the greatest principal stress. 



In dealing with the strains accompanying the development of 

 cleavage, Mr. Leith is more definite and states§ that " wherever 

 the directions of shortening of a rock mass can be determined 

 with certainty, any flow cleavage which may be present is normal 

 to the greatest total shortening which the rock has undergone." 



place and both sets are parallel to the axis of rotation. They make with the 

 greatest axis of the strain ellipsoid angles given by 



+ ±B 



tan o) 



(ABC)* 



A being the greatest axis, B the least and C the axis of rotation. The planes 

 of maximum slide contain the circular sections of the ellipsoid only in a 

 limiting case. During the progress of strain these mathematical planes sweep 

 through wedges of the mass, but the two sets of planes sweep at dif- 

 ferent rates, one set having a relative angular velocity from, say, 20 

 to an infinite number of times as great as the other. On the planes which 

 sweep rapidly, viscosity reinforces rigidity, there is no time for considerable 

 flow to take place, and, unless actual rupture occurs, so that joints form, the 

 effect will be small. On the other set of planes viscosity is small, the mass 

 has time to yield by flow, cohesion is weakened, and cleavage results. In a 

 word, the theory is that slaty cleavage is due to solid flow attendant upon 

 rotational strains. So much of the energy of the system as is not poten- 

 tialized is dissipated on the plans of maximum slide, and this may or may 

 not lead to the alteration of mineral constituents, e. g., the transformation 

 of feldspar into biotite. It is discussed in Finite Homogeneous Strain, Flow 

 and Bupture of Rocks, Bull. Geol. Soc. Amer. , vol. iv, 1893, p. 13, and in 

 Experiments on Schistosity and Slaty Cleavage, U. S. Geol. Surv., Bull. 

 241, 1904. 



* Idem, p. 127. \ Idem, p. 134. 



X Op. cit., p. 118. §Idem, p. 138. 



1 Idem, p. 113. ^ Idem, p. 106. 



