1856.} the Cleavage of Crystals and Slate Rocks. 305 



dens on drying after rain, — they are cracked and split, and other 

 circumstances being equal, they crack and split where the cohesion 

 of the mass is least. Take then a mass of partially consolidated 

 mud. Assuredly such a mass is divided and subdivided by surfaces 

 along which the cohesion is comparatively small. Penetrate the 

 mass, and you will see it composed of numberless irregular nodules 

 bounded by surfaces of weak cohesion. Figure to your mind's eye 

 such a mass subjected to pressure, — the mass yields and spreads out 

 in the direction of least resistance ;* the little nodules become con- 

 verted into laminae, separated from each other by surfaces of weak 

 cohesion, and the infallible result will be that such a mass will cleave 

 at right angles to the line in which the pressure is exerted. 



Further, a mass of dried mud is full of cavities and fissures. 

 If you break dried pipe-clay you see them in great numbers, and 

 there are multitudes of them so small that you cannot see them. I 

 have here a piece of glass in which a bubble was enclosed ; by the 

 compression of the glass the bubble is flattened, and the sides of 

 the bubble approach each other so closely as to exhibit the colours 

 of thin plates. A similar flattening of the cavities must take place 

 in squeezed mud, and this must materially facilitate the cleavage of 

 the mass in the direction already indicated. 



Although the time at my disposal has not permitted me to 

 develope this thought as far as I could wish, yet for the last twelve 

 months the subject has presented itself to me almost daily under 

 one aspect or another. I have never eaten a biscuit during this 

 period in which an intellectual joy has not been superadded to 

 the more sensual pleasure ; for I have remarked in all such cases 

 cleavage developed in the mass by the rolling-pin of the pastrycook 

 or confectioner. I have only to break these cakes, and to look at 

 the fracture, to see the laminated structure of the mass. Nay, I 

 have the means of pushing the analogy further ; I have here some 

 slate which was subjected to a high temperature during the confla- 

 gration of Mr. Scott Russell's premises. I invite you to compare 

 this structure with that of a biscuit : air or vapour within the mass 

 has caused it to swell, and the mechanical structure it reveals is 

 precisely that of a biscuit. I have gone a little into the mysteries 

 of baking while conducting my inquiries on this subject, and have 

 received much instruction from a lady friend in the manufacture of 

 puff-paste. Here is some such paste baked in this house under my 

 own superintendence. The cleavage of our hills is accidental 

 cleavage, but this is cleavage with intention. The volition of the 

 pastrycook has entered into the formation of the mass, and it has 

 been his aim to preserve a series of surfaces of structural weakness, 



* It is scarcely necessary to say that if the mass -v^ere squeezed equally in 

 all directions no laminated structure could be produced ; it must have room to 

 yield in a lateral direction. 



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