CHARACTERISTICS OF SLATE. 77 



schistose rocks, in which crystals have certainly undergone distortions 

 involving fluxion phenomena. 



In slate quarries there are usually " steps " produced by the presence 

 in the slate of strata differing in lithological character from the hulk of 

 the rock. The cleavage is deflected by these strata, and when they are 

 sharply defined the deflection is also sharp. When the cleavage is at 

 right angles to the stratification the deflection is nearly or quite imper- 

 ceptible, and it seems to be maximum when the angle approaches 45°. 

 The illustration on the opposite page is taken from Mr Alfred Barker's 

 admirable memoir on slaty cleavage* 



All of the above phenomena must of course be accounted for in any 

 satisfactory theory of slaty cleavage. 



Theories of slaty Cleavage. — The earlier geologists naturally associated 

 slaty cleavage and mineral cleavage, and ascribed both to the same or 

 similar causes. Professor John Phillips was the first to offer a mechani- 

 cal explanation.f In doing so he was prudently indefinite. He de- 

 scribed the distortion as a " creeping movement among the particles of 

 the rock, the effect of which was to roll them forward in a direction 

 always uniform over the same tract of country." This language has been 

 interpreted as equivalent to the hypothesis of a simple " shearing motion," 

 but it will by no means bear this limited construction. Phillips had in 

 mind a rotational strain and a fluxional structure, but his paper contains 

 nothing to indicate the absence of forces acting perpendicularly to the 

 cleavage planes. He neither denies nor asserts the cooperation of such 

 forces. He also says nothing to indicate that his theory was applicable 

 only to heterogeneous matter, and it is fair to conclude that he supposed 

 that slate might be produced from homogeneous substances. 



Mr D. Sharpe explained the structure as due to the contraction of rock 

 in the line of pressure and a partially compensating elongation at right 

 angles to it. This strain is one of two dimensions, and consists of a 

 simple shear (not a shearing motion) with a cubical compression. The 

 fissility produced he referred to the fact that a fracture perpendicular to 

 the direction of pressure would run along the flattest faces of the compo- 

 nent grains and meet the smallest number of them. This explanation 

 implies that the mass is heterogeneous, and that the adhesion between 

 the component particles is smaller than the cohesion within the particles.]; 



Dr H. C. Sorby, to whom geology owes so great a debt for the introduc- 

 tion of the microscope as an instrument of lithological research, natur- 



* Brit. Assoc. Ad. Sci., 1885, p. 813. Mr Harker's paper contains very full citations of the literature 

 of slate, and the reader who cares to pursue the subject is advised to consult it. No attempt is 

 made in the present paper to give a full bibliography. 



f Brit. Assoc. Rep., 1843, p. 61. 



JQ. Jour Geol. Soc, vol. 5, 1849, p. 128. 



