180 Dr. Tyndall on Slaty Cleavage, 



During an accidental meeting with Mr. Woodward, the geologist, 

 in the pass of Llanberis, on Saturday the 30th of September, 

 1854, my longing for information on this head made itself mani- 

 fest, and soon after my return to London I was gratified by the 

 receipt of Mr. Daniel Sharpens papers on cleavage, sent to me 

 by the lamented author ; and of a paper by Mr. Sorby, sent to 

 me, I believe^ by Mr. Woodward. All these papers interested 

 me in a high degree. Returning to my experiments in the 

 autumn of 1865, and making use, as on former occasions, of 

 pressure as a means of modifying the magnetic force, I was gra- 

 dually led to a theory of slaty cleavage different, as I conceived, 

 from that proposed by Mr. Sorby. I had previously seen at 

 Glasgow a specimen of the cleavage produced by Mr. Sorby, by 

 means, as he considered, of oxide of iron scales ; but that which 

 I had succeeded in producing, without such scales^ being incom- 

 pai-ably finer, I was naturally led to doubt the influence which 

 he assigned to them. 



The foregoing remarks, however, represent me as labouring 

 under a most serious misapprehension of Mr. Sorby^s opinions ; 

 my statement of his views " is so entirely different from what is 

 really the case, that it would not be proper to allow it to pass 

 uncorrected.^^ Nobody can regret this misapprehension more 

 than I do ; and all I intend to do at present is to show, by quo- 

 tations from the paper to which I have referred, the basis of my 

 interpretation of Mr. Sorby's views. Whether these quotations 

 will justify my interpretation in the eyes of the i-eader I know 

 not ; but I have at least the satisfaction of knowing that I by 

 no means stand alone in the meaning which I have attached to 

 Mr» Sorby's language. 



The paper which contains Mr. Sorby's theory of the cleavage 

 of slate rocks is to be found in the Edinburgh New Philosophical 

 Journal, vol. Iv. p. 137. At the commencement the writer states 

 as follows : — 



P. 137. — **The examination of thin sections of slate rocks with 

 high powers, and a comparison with those of similar mineral com- 

 position not possessing cleavage, have led me to form a theory to 

 account for their difference of structure materially different from any 

 yet propounded, and which, in my opinion, not only does so most 

 satisfactorily, but also explains perfectly every fact that I am ac- 

 quainted with connected with the subject.** 



What this theory is the following extracts will explain : — 



P. 142. — *• Usually, besides wic«, there is found in good roofing- 

 slate, like that at Penrhyn, a certain proportion of decomposed 

 felspar, a few minute grains of quartz sand, and granules of phos- 

 phate of iron. The red tint is produced by the presence of very 

 numerous minute crystals of peroxide of iron, and the dark by those 



