404 Rev. 0. Fisher — 0)i Cleavage and Distortion. 



of a slate as revealed by the microscope. The nnequiaxed particles 

 referred to by Dr. Sorby would also be rearranged, as already 

 explained, mostly in the said plane, and would assist in promoting 

 cleavage parallel to that plane. Thus we should conclude that Dr. 

 Sorby and Prof. Tyndall have both asserted vercB causae concerned in 

 the production of this structure. 



Dr. Sorby has remarked that, '" Many of the finer-grained slates 

 used for roofing contain minute rounded grains of mica, seldom so 

 much as to o^th of an inch in diameter, and usually much less, which 

 are of nearly the same thickness as width, and not merely flakes. 

 When these are cut through in the thin sections used for micro- 

 scopical examination, they are seen to be composed of many laminae. 

 When the line [plane] of lamination — that is, of the crystalline 

 cleavage of the mica — coincides with the cleavage of the slate, then 

 these rounded grains retain their form unaltered." In this position, 

 according to our view, the grains would be elongated, but not broken 

 up in any way, because every lamina would be similarly afi'ected in 

 the direction of lamination. Perhaps this may satisfy the above 

 observation, because the original form cannot be known. " If the 

 lamination is perpendicular to the cleavage, the rounded form still 

 remains, but the laminae are generally not straight, being irregularly 

 bent in just such a manner as if they had been compressed in the 

 direction perpendicular to the cleavage of the slate." In fact they 

 ai-e in the case of the minor axis of our ellipse, which had originally 

 the length of the radius of the circle. " Those, however, which lie 

 with their lamination at intermediate angles, as, for instance, 30° or 

 40° to the cleavage of the slate, do not retain their original form, 

 but are broken up and extended out in the plane of their lamina- 

 tion." ^ These particles I should suppose to have had their planes 

 of lamination nearly in the direction of the shear. Being planes of 

 little friction in the crystal, it has become deformed by minute 

 faulting along them, instead of by viscous shearing. In the instance 

 referred to by Dr. Sorby, it would therefore appear that the plane of 

 shear was inclined to that of cleavage at between 30° and 40°. Dr. 

 Geikie gives an illustration '•* showing one of these faulted grains, 

 and it accords perfectly well with the above explanation. 



The diagram, representing the traces of the several planes in 

 ordinary positions upon a vertical plane perpendicular to the strike, 

 will render the relation of the various angles more intelligible. If 

 a be the angle which the side of either rhombus in Fig. 2 makes 

 with its base, then cot a will be the measure of the shear. is the 

 inclination of the shear to the horizon, and is the same angle as 

 in the former Parts of this treatise. is the inclination of the 

 cleavage plane to the shear. Then the dip of the cleavage will be 

 — 0, which is an angle that can be directly observed, ^p■ is the 

 inclination of the sheared bedding to the shear. Hence ■»//• — is 



1 " In just such a manner as would occur if the dimensions of the slate had been 

 changed as previously mentioned." — " On the Origin of Slaty Cleavage," Edin. New 

 Phn. Journ. vol. Iv. p. 140, 1853. 



•-i Text-Book of Geology, p. 310. 



