12 G. F. Becker — Current Theories of Slaty Cleavage. 



fractures to the resultant cleavage shown in the cut by broken 

 lines. In fact, if my theory is correct, some further information 

 might possibly be obtained from such pebbles as to the oper- 

 ations to which the mass has been subjected. It may be observed 

 in the diagram that the line of the centers of inertia of the 

 several fragments nearly coincides with the direction of cleavage, 

 but does not do so absolutely. The difference is so slight that 

 it might be attributed to bad drawing, but this is not the case. 

 The difference is only two-thirds of one degree, and the sig- 

 nificance of this difference is this : The lines of the center of 

 inertia coincide with the direction of the first fibers to undergo 

 maximum tangential strain, whereas the cleavage represents 

 the final direction of maximum slide. Now, as has been men- 

 tioned above, there is another set of planes of maximum 

 tangential strain which, in this particular case, has wandered 

 through a wedge of the mass bounded hj planes at 28° from 

 one another ; thus the particles lying between the direction of 

 the lines of the center of inertia and the cleavage have been 

 subjected to maximum tangential strain more than forty times 

 as long as the particles in the other direction, and it is to this 

 difference that I attribute the development of the cleavage. 

 If instead of being absolutely brittle the enclosed cube yielded 

 plastically to a minute extent before rupture, the fractured mass 

 would show a trace of cleavability in the direction of the centers 

 of inertia of the slices. If rupture were to take place simul- 

 taneously on both sets of planes of maximum slide, double 

 displacements resembling those shown in my paper on Simul- 

 taneous Joints,"- figure 9, would probably occur. Fig. 5 is 



borrowed from Mr. 

 Diller's paper on the 

 Taylorsville region of 

 California,t figure 4, 

 and show T s a sliced 

 quartzite pebble from 

 a schist. A sliced rock 

 pebble is more in- 

 structive than a 

 cleaved feldspar be- 

 cause the cleavage of 

 the mineral will mod- 

 ify the angle of slicing. 

 Mr. Leith presents no further argument from observation 

 for his contention that " wherever the directions of shortening 

 and elongation of a rock mass can be determined with certainty, 

 any flow cleavage which may be present is normal to the great- 



* Washington Academy Sci., vol. vii, p. 274, 1905. 



f IT. S. Geol. Surv. Bull, on Taylorsville Region, not yet in print. 



sw 



