86 



G. F. BECKER — FINITE STRAIN IN ROCKS. 



Conclusion as to Sidle. — The fact that slaty structure occurs not only in 

 argillaceous rocks hut, though less frequently, in limestone, grit beds, 

 granite and basic eruptives, while it has been artificially produced in 

 wax, clay, metals, dough and glass, throws much doubt on the hypothesis 

 that slaty cleavage is due to re-arrangement under pressure of embedded 

 flakes and grains of matter. This doubt seems confirmed by the fact 

 that although the component grains of many undisturbed shales and 

 sandstones are so arranged that their largest sections lie parallel to the 

 planes of bedding, such rocks do not show any cleavage closely resem- 

 bling that of slate. Hence a satisfactory explanation must apply to 

 homogeneous matter. 



Examination of the experimental methods of producing slaty structure 

 shows that in all cases the distortion of a small portion of the mass is 

 rotational, and is such as would be produced upon a cube resting on a 

 rigid support and affected by an inclined force, with or without the co- 

 operation of lateral forces in the plane of support. 



=Z=^ 



Figure 22. — Deflection of Cleavage by Grit. 



The theory of finite strain in viscous plastic masses shows that rota- 

 tional strains of this description should be accompanied by the develop- 

 ment of a cleavage. The grain of a mass thus distorted should have an 

 absolutely constant direction parallel to the plane of support and per- 

 pendicular to the line of force. Elongation should, in general, take place 

 in the direction of the grain, and contraction at right angles to the grain 

 on the cleavage plane. When, however, the force makes no angle with 

 the plane of support there should be no distortion in the plane of cleav- 

 age. There should also in all cases be a second direction of elongation 

 perpendicular to the grain and at a considerable angle to the cleavage 

 plane. 



This theory explains at least most of the characteristics of slate, in- 

 cluding that of steps. The second elongation just mentioned certainly 

 exists in some case-*, but I have not data enough to assert its universality. 

 The practical difficulties of fully determining the position of the strain 

 ellipsoid from a fossil are such that the omission of other observers to 



