ON CRYSTALLINE AND SLATY CLEA VAGE. 241 



rod ale in Cumberland. You have precisely the same fea- 

 tures in both; you see flakes clinging to the surfaces of 

 each, which have been partially torn away in cleaving. 

 Let any close observer compare these two effects, he will, 

 I am persuaded, be led to the conclusion that they are the 

 product of a common cause.* 



But you will ask me how, according to my view, does 

 pressure produce this remarkable result? This may be 

 stated in a very few words. 



There is no such thing in nature as a body of perfectly 

 homogeneous structure. I break this clay which seems so 

 uniform, and find that the fracture presents to my eyes in- 

 numerable surfaces along which it has given way, and it 

 has yielded along those surfaces because in them the cohe- 

 sion of the mass is less than elsewhere. I break this mar- 

 ble, and even this wax, and observe the same result; look 

 at the mud at the bottom of a dried pond; look at some of 

 the ungraveled walks in Kensington Gardens on drying 

 after rain they are cracked and split, and other circum- 

 stances being equal, they crack and split where the cohesion 

 is a minimum. Take then a mass of partially consol- 

 idated mud. Such a mass is divided and subdivided by 

 interior surfaces along which the cohesion is comparatively 

 small. Penetrate the mass in idea, and you will see it com- 

 posed of numberless irregular polyhedra bounded by surfaces 

 of weak cohesion. Imagine such a mass subjected to pres- 

 sure it yields and spreads out in the direction of least 

 resistance;! the little polyhedra become converted into 

 lamina?, separated from each other by surfaces of weak 

 cohesion, and the infallible result will be a tendency to 

 cleave at right angles to the line of pressure. 



* I have usually softened the wax by warming it, kneaded it with 

 the fingers, and pressed it between thick plates of glass previously 

 wetted. At the ordinary summer temperature the pressed wax is 

 soft, and tears rather than cleaves; on this account I cool my com- 

 pressed specimens in a mixture of pounded ice and salt, and when 

 thus cooled they split cleanly. 



f 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. Mr. Warren De la 

 Rue informs me that he once wished to obtain white-lead in a fine 

 granular state, and to accomplish this he first compressed it. The 

 mold was conical, and permitted the lead to spread out a little later- 

 ally. The lamination was as perfect as that of slate, and it quite 

 defeated him in his effort to obtain a granular powder. 



