SLATES. 315 



by Mr. Sorby, it might be shown how true his con- 

 clusion is that the effect of pressure on elongated 

 particles, or plates, will be such as he describes it. 

 But while the scales must be regarded as a true cause, 

 I should not ascribe to them a large share in the pro- 

 duction of the cleavage. I believe that even if the 

 plates of mica were wholly absent, the cleavage of 

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



Here is a mass of pure white wax : it contains no 

 mica particles, no scales of iron, or anything analogous 

 to them. Here is the selfsame substance submitted to 

 pressure. I would invite the attention of the eminent 

 geologists now before me to the structure of this wax. 

 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 inter- 

 mixed 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 

 resemblance of its cleavage to that of the wax. Com- 

 pare the surface of the wax with the surface of this 

 slate from Borrodale in Cumberland. You have pre- 

 cisely 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 



1 I have usually softened the wax by warming it, kneaded it 

 with the fingers, and pressed it between thick plates of glass pre- 



