ON OR TSTA LLTNtt AND SLA TY CLEA VAGE. 34! 
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 Lave 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 Mm in his effort to obtain a granular powder. 
