44 Prof. Tyndall on the Cleavage of 



I could show you by tests of a totally different character from 

 those applied by Mr. Sorby, how true his conclusion is, that the 

 effect of pressure on elongated particles, or plates, will be such as he 

 describes it. Nevertheless, while knowing this fact, and admiring 

 the ability with which Mr. Sorby has treated this question, I 

 cannot accept his explanation of slate-cleavage. I believe that 

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

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



1 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 neces- 

 sary 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 geolo- 

 gists whom I see before me to the sti*ucture 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 inde- 

 pendent of the intermixed plates of mica assumed in Mr. Sorby's 

 theory. I have purposely mixed this wax with elongated par- 

 ticles, 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.thisremarliiibl&r^ult. This may be stated in a very few 

 words... ^,,.j» y(^ ,-^t*.rif ,')iii r? 



Nature is everywhere imperfect ! The eye is not perfectly 

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



♦ I have usually 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 sialt, and wlicn thus cooled they spUt beautifully. 



