80 G. F. BECKEB — FINITE STRAIN IN HOCKS. 



would cleave along the bedding exactly like slate which docs not accord 

 with observation* 



It appears to me, therefore, that no theory of slaty cleavage will be 

 satisfactory which does not apply to the case of homogeneous matter. 



Analysis of Experiments. — Slaty cleavage has been produced artificially 

 in several different ways. Plastic substances compressed between rigid 

 masses exhibit such cleavage; so too do plastic masses extruded through 

 small openings ; poor qualities of iron or brass when drawn to wire often 

 show thin splinters, indicating the presence of cleavage; metals, pastry 

 and clay rolled out into sheets show similar fissility, and. as Professor E. 

 Reyer has pointed out, the bruise produced on soft rocks by a slanting 

 blow with a pick exhibits a like structure. r 



These cases seem very different, but they must have common features, 

 unless, indeed, slaty cleavage is due to essentially diverse causes. Most 

 of the mechanical operations indicated are very complicated, hut their 

 common features may be reduced to simple terms by considering a very 

 small cubical portion of the mass before distortion and inquiring how it 

 is affected by strain. 



Figure 18. — Origin of Cleavage in Wire. 



If one end of a wire is filed to a flat surface perpendicular to its axis. 

 and the wire is then drawn through two or three successive holes of a 

 draw-plate so that the flat end is the last to come through, it will he 

 found that this end has become concave. If one considers a small cube 

 in the undistorted wire, not on the axis, it is clear that this cube will be 

 converted into an oblique parallelopiped, as is illustrated in the foregoing 

 diagram, showing the wire in section. 



The concentric layers of the wire move upon one another much like 

 the joints of a telescope. The little cube is elongated in the direction of 

 the axis, its height is diminished, and its right angles in the plane of the 

 axis are converted into acute or obtuse ones. It is clear that the sphere 

 which might he inscribed in the small cube has been distorted to an 

 ellipsoid, the major axis of winch becomes more and more nearly hori- 

 zontal as the strain increases. The strain is thus a rotational one. and. 

 according to the theory of strain set forth in this paper, a cleavage should 

 be developed nearly in the direction of the axis. 



*See p. 74. 



f Theoretische Geologie, 1888, p. ."iTT. 



