GG G. F. BECKER — FINITE STRAIN IN ROCKS. 



one parallelogram of the figures must receive a certain amount of relief, 

 and if this is not entirely accomplished by flow it must be completed by 

 rupture; but a rupture at each end of the parallelogram would relieve 

 the strain without the help of flow. Thus it appears most logical to 

 suppose that in such cases short " close joints " will be distributed through 

 the plastically deformed mass, excessively minute variations in the resist- 

 ance of the material determining their precise disposition. 



The set of planes corresponding to the long side of the parallelogram 

 cannot behave in the same way as those already discussed. This set 

 sweeps through the mass so rapidly that there is no time for flow of con- 

 siderable amount to take place. Hence, if they receive expression at all, 

 it must be as sharply cut fissures or as u master joints." 



Theory of Slaty Cleavage. — In considering what properties would he ex- 

 hibited by a plastic, viscous rock which had been rigidly supported and 

 subjected to a pressure inclined at a moderate angle to the plane of sup- 

 port, it is difficult to see how the mass would differ from true slate. The 

 relative tangential motion along the set of planes which eventually makes 

 an angle « d- ^ with the plane of support would inevitably manifest itself 

 as a cleavage, alternating in some cases with close jointing. In the direc- 

 tion of 02, or perpendicular to the plane of the figures 10 and 11, this 

 cleavage would be invariable. In the direction a> -\- v the cleavage would 

 be confined to a very small angle, less than one degree in the examples 

 given above. Thus the mass would cleave very sharply along lines 

 parallel to o z, less sharply along w 4- v - Expansion would take place 

 parallel to o z, while contraction would take place in the direction w -j- v. 

 This contraction might be accompanied by a puckering of the cleavage 

 surfaces, because the cleavage planes formed at the inception of strain 

 would be still further contracted as strain progressed. The amount of 

 relative distortion in the directions o z and <o + v would vary with the 

 direction of the force and the intensity of the strain. The only case in 

 which there would be no distortion on the cleavage plane occurs when 

 the force is parallel to the fixed support. All of these peculiarities of 

 this strain are characteristic of slate, and they seem to cover all of the 

 principal properties of that much-debated rock. I shall return to the 

 comparison of properties in a later portion of this paper. 



Influence of Shock. — Although the preceding discussion shows how 

 sheets of rock may be produced by the action of orogenic forces, I am 

 not satisfied that all fractures are produced in this way. There seem, in 

 fact, to be instances in which the spacing of more or less nearly rectangu- 

 lar fissure systeaisis closer than can lie accounted for on the assumption 

 that the Assuring is of minimum amount. 



