Dr. Tyndall on Slaty Cleavage. 135 



To Mr. Williams of Basingstoke I have been recently indebted 

 for some very interesting specimens of iron plates, peeled off by the 

 compressing and sliding action of railway trains. The specimens 

 exhibit the laminated structure in a very striking degree. '' Prac- 

 tically/^ says my intelligent correspondent, "it is a constant 

 fault in rails : the men who are always handling and taking them 

 up say that it is worst near the stations, where the break acts ; 

 but sometimes they find one here and there upon the main road 

 — not very often .'^ One of the specimens sent to me was of this 

 latter class, and it had probably been submitted to the rolling of 

 the trains for a period of eighteen years. In addition to the 

 pressure, which of itself causes the granules of iron gradually to 

 spread out, and thus tends to destroy their natural cohesion, 

 when the wheel slides it has a tendency to displace the upper 

 surface of the rail in the direction of motion. Suppose the rail 

 to be divided into distinct strata, each of these being held to its 

 neighbours by the cohesion of the iron. Whenever a train passes, 

 a slight displacement occurs, and every such displacement tends 

 to rupture the cohesion of the particles which hold each pair of 

 strata together. The act is repeated, perhaps thousands of times, 

 until finally the rail yields at the place where, owing to crystal- 

 line or mechanical structure, the force which holds the strata 

 together is weakest. When slate mud is compressed, its differ- 

 ent layers cannot be expected to move laterally with exactly the 

 same velocity ; they slide over each other, and this action, as in 

 the case of the iron, must also tend to weaken the cohesion of 

 the layers and to facilitate the cleavage of the mass. 



With regard to Mr. Sorby^s concluding paragraph, I confess 

 that my sole object was to account for the " splitting '' of slate- 

 rock. The facts observed by Phillips arid Sharpe, and added to 

 in so important a degree by Mr. Sorby's researches, lead to a 

 conclusion, first I believe announced by Sharpe, and assented to 

 by all who have observed the facts, that the masses have been sub- 

 jected to pressure in a direction at right angles to the planes of 

 cleavage ; and the sole question to be decided was, whether this 

 pressure was sufficient to produce the cleavage. Both Mr. Sorby 

 and I say yes, — but on different grounds ; which of us is right 

 (perhaps each of us has a little reason on his side) others must 

 determine, and to them I now resign the consideration of the 

 question. 



