ON CRYSTALLINE AND SLATY CLEAVAGE 333 



ure is sufficient to produce cleavage, and that this cleav- 

 age is independent of intermixed plates or scales. I have 

 purposely mixed this wax with elongated particles, and 

 am unable to say at the present moment that the cleavage 

 is sensibly affected by their presence if anything, I should 

 say they rather impair its fineness and clearness than pro- 

 mote it. 



The finer the slate is the more perfect will be the re- 

 semblance of its cleavage to that of the wax. Compare 

 the surface of the wax with the surface of this slate from 

 Borrodale in Cumberland. You have precisely the same 

 features 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. 1 



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 

 innumerable surfaces along which it has given way, and 

 it has yielded along those 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 at 



1 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 compressed specimens in a mixture of pounded ice and 

 salt, and when thus cooled they split cleanly. 



