82 



G. F. BECKER FINITE STRAIN IN ROCKS. 



edges, so that the cake assumes the form of an ordinary American cheese. 

 Cakes compressed to one-quarter of their thickness were very greatly 

 distorted in this sense, as shown in figure 20, c. When cooled and struck 

 sharply on the edge with a hammer, they showed slaty cleavage. The 

 character of the distortion of a small included cube follows from the dis- 

 tortion of the mass, and, as appears from the following diagram, it is a 

 distortion similar to that which takes place when a cube is subjected to 

 inclined pressure, as illustrated in figure 6, B, page 5V). 



The reason for the bulging edges is at once seen to be the frictional 

 resistance between the glass plates and the escaping wax. This resist- 

 ance, combined with the vertical pressure, gives resultant forces, marked 

 r r in the figure, which are not vertical but lie on conical surfaces about 

 the central vertical axis. When this friction is obviated by the use of a 

 lubricant, so that a nearly uniform distribution of pressure is obtained, 

 there is no tendency to relative horizontal motion among the layers, and 

 in a dozen or more trials with lubricators I failed to find any trace of 

 horizontal cleavage. A tendency to cleave is sensible in these cases, but 

 it coincides with the planes of maximum tangential strain as nearly as 

 the imperfection of the surfaces enabled me to judge. 



Figure 20. — Developmt ni of Cleavage by direct Pressure. 



Thus it appears to me that Professor Tyndall's brilliant experiment 

 has been misinterpreted. He produced slaty cleavage not by a pressure 

 uniformly distributed and vertical to the cleavage planes, hut by a sys- 

 tem of forces inclined to the cleavage planes. 



The effect of rolling metal, clay, or pastry is similar to that of direct 

 pressure combined with lateral friction. A cake of plastic material is 

 reduced to a sheet with bulging edges like figure 20, c, and an infinitesi- 

 mal cubical portion of the mass is distorted as in the other cases. 



I am aware of no other ways in which slaty cleavage has been pro- 

 duced artificially. In all of those discussed the distortion attending 

 development of the cleavage is substantially the same. The elementary 

 cube is deformed as it would be by a force inclined to one face of the 

 cube when the opposite face rests upon an inflexible support. In some 

 cases there is lateral constraint ; in others there is none. The splinters 

 on rolled metal and pastry seem to show that the cleavage developed is 

 not quite parallel to the surface of the mass. 



