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



ally attacked the question from a microscopical standpoint. He found 

 that the mica of the slates was largely concordant with the cleavage and 

 referred the fissility to the effect of direct pressure in deflecting mica 

 scales toward a direction at right angles to the line of pressure* 



This theory is supplementary to that of Mr Sharpe, and it is to the 

 united effect of the flattening and deflection that slaty cleavage is now 

 usually ascribed. 



Mr A. Laugel assumed on the authority of Sharpe that the strain con- 

 sists of a simple shear. He pointed out the fact that in a simple shear 

 in a homogeneous mass the planes of least resistance (or, as I have called 

 them, of maximum tangential strain) stand at an angle with the axes 

 of the shear dependent upon the deformation. In the notation of this 

 paper f he reached the result tan 2 w == BjA. He gave no proof of this, 

 however, and did not explain how the double cleavage implied in this 

 equation of the second degree could I >e reduced to the simple cleavage 

 of slate. X In my opinion he was on the right path to a sufficient expla- 

 nation, but he certainty did not achieve it. 



Professor John Tyndall's famous experiments on slaty cleavage in 

 wax in a direction perpendicular to the pressure were published in 1856.§ 

 He dissented from Sorby's theoiy, regarding his wax as homogeneous, 

 and finding that the intermixture of scales rather interfered with than 

 promoted cleavage. Dr Sorby replied to Tyndall, citing experiments of 

 his own on clay mixed with mica scales and pointing out that wax con- 

 tains prismatic crystals ; so that, in his opinion, the wax must be consid- 

 ered as composed of elongated elements capable of re-arrangement by 

 pressure, according to his theory. || 



Mr Daubree found that clay without mica scales when extruded 

 through a small opening assumes a schistose structure, the lamination 

 being close in proportion as the material is more finely divided.^] He 

 also obtained evidence of schistose structure in flint glass, softened by 

 heat and forced through an opening. In this case at least there could 

 be no question that the resultant structure was independent of hetero- 

 geneous particles. 



Dr Sorby made an addition to his theory of slaty cleavage in 1880. 

 In his original theory it was assumed that the mica before compression 

 was distributed through the mass without any order. As a matter of fact, 

 the mica scales in shale are, for the most part, parallel to the bedding. 



*Ed. New Phil. Mas;., vol. 55, 185:;, p. 1::;. 



fSee formula (9), p. 33. 



tComptes Rendus, vol. 40, 1855, p. 978. 



gPhil. Mag., vol. 12, 1856, p. 37. 



|| Phil. Mag., vol. 12, 185G, p. 127. 



|G6ol. Exp., 1879, p. 413. 



