386 D. MACKINTOSH ON THE ORIGIN OP SLICKENSIDES. 



25. On the Origin of Slickensides, with Remarks on Specimens 

 from the Cambrian, Silurian, Carboniferous, and Triasstc 

 Formations. By D. Mackintosh, Esq., F.G.S. (Read Feb- 

 ruary 24, 1875.) 



[Abstract.] 



"While admitting that the sides of many fractures have been coated 

 with foreign substances, the author's observations led him to believe 

 that true slickensides, or polished and smoothly striated surfaces, 

 have been produced by the movement of one face of rock against 

 another, accompanied by partial fusion *. In the case of many of 

 the specimens, the surface had apparently been metamorphosed by 

 heat resulting from pressure, as the whitened and hardened coating 

 graduated imperceptibly, into the ordinary structure of the rock. In 

 some cases sandstone had apparently been converted into quartzitc. 

 He ventured to bring " the theory of metamorphism by heat in the 

 case of many slickensided surfaces " before the Society, in expecta- 

 tion that it might lead to further investigation. 



Discussion. 



Mr. Sorby said that while some facts seem to indicate the action 

 of heat, there are many reasons which apparently show that the 

 operation of that agent is unlikely. He believed that many actions 

 and causes which we do not at present . understand have been at 

 work in producing slickensides. The subject deserved careful 

 study, 



Mr. Forbes could not agree with the author of this paper that the 

 phenomena of slickensides were due to a partial fusion of the sides, 

 since he had observed some of the very finest examples of slicken- 

 sides on pyrites in the great deposits of Rio Tinto ; and as this 

 mineral was in a perfectly unaltered condition, it is evident that no 

 heat reaching to any thing like its fusing-point could have been 

 developed ; nor could this be the case in limestones or in hydrated 

 rocks, both of which would be altered in chemical composition if 

 strongly heated. It would also be the same with the slickensides 

 seen in the cinnabar from Borneo and Almaden, which mineral sub- 

 limes if exposed to even a comparatively gentle heat. 



Prof. Tennant said that at Matlock galena showed the same 

 appearances. 



* Since this paper was read and discussed, the author has seen reason for 

 restricting the term " partial fusion " to certain cases. He has lately found that 

 Mr. John Aitken, F.G-.S., of Bacup, advocated metamorphism amounting to 

 vitrification by heat, as the cause of slickensides in 1873 (Proc. Manchester 

 Geol. Soc. vol. xii.). and that Professor Marsh, of Yale College, U. S., in 18G7, 

 regarded slickensides as "friction-marks," "and the accompanying coating as 

 "merely a portion of the rock finely divided and compressed" (Proc. Am. 

 Association of Science). — D. M., June 1875. 



