304 PTfof. Tyndall, on the Comparative View of [June 6, 



even if these plates of mica were wholly absent the cleavage of 

 slate rocks would be much the same as it is at present. 



I will not dwell here upon minor facts, — I will not urge that 

 the perfection of the cleavage bears no relation to the quantity of 

 mica present ; but I will come at once to a case which to my mind 

 completely upsets the notion that such plates are a necessary element 

 in the production of cleavage. 



Here is a mass of pure white wax : there are no mica particles 

 here ; there are no scales of iron, or anything analogous mixed 

 up with the mass. Here is the self-same substance submitted to 

 pressure. I would invite the attention of the eminent geologists 

 whom I see before me to the structure of this mass. No slate ever 

 exhibited so clean a cleavage ; it splits into laminae of surpassing 

 tenuity, and proves at a single stroke that pressure is sufficient to 

 produce cleavage, and that this cleavage is independent of the 

 intermixed plates of mica assumed in Mr. Sorby's theory. 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 promote it. 



The finer the slate the more perfect will be the resemblance 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 by the 

 cleavage of the mass : I entertain the conviction that if any close 

 observer compares these two effects, he will 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. 



Nature is everywhere imperfect ! The eye is not perfectly 

 achromatic, the colours of the rose and tulip are not pure colours, 

 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 in- 

 numerable 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 Gar- 



* I have iisually softened the wax by wanning it, kneaded it with the 

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

 ordinary summer temperature the 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 beautifully. 



