Crystals and Slate Rocks. 45 



and the freshest air of our hills has a bit of poison in it. In like 

 manner there is no such thing in nature as a body of perfectly 

 homogeneous structure. I break this clay which seems so inti- 

 mately mixed, and find that the fracture presents to my eyes 

 innumerable surfaces along which it has given way, and it has 

 yielded along these surfaces because in them the cohesion of the 

 mass is less than elsewhere. I break this marble, and even this 

 wax, and observe the same result : look at the mud at the bottom 

 of a dried pond; look to some of the ungravelled walks in 

 Kensington Gardens 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 compara- 

 tively 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 converted 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 



* It is scarcely necessary to say that if the mass were squeezed equally in 

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

 to yield in a lateral direction. 



