332 FRAGMENTS OF SCIENCE 



This geologist is Sorby, who has attacked tlie question 

 in the true spirit of a physical investigator. Call to 

 mind the cleavage of the flags of Halifax and Over Dar- 

 wen, which is caused by the interposition of layers of 

 mica between the gritty strata. Mr. Sorby finds plates 

 of mica to be also a constituent of slate-rock. He asks 

 himself, what will be the effect of pressure upon a mass 

 containing such plates confusedly mixed up in it ? It will 

 be, he argues, and he argues rightly, to place the plates 

 with their flat surfaces more or less perpendicular to the 

 direction in which the pressure is exerted. He takes 

 scales of the oxide of iron, mixes them with a fine pow- 

 der, and on squeezing the mass finds that the tendency 

 of the scales is to set themselves at right angles to the 

 line of pressure. Along the planes of weakness produced 

 by the scales the mass cleaves. 



By tests of a different character from those applied by 

 Mr. Sorby, it might be shown how true his conclusion 

 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 production 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 self- same 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 lamina of sur- 

 passing tenuity, and proves at a single stroke that press- 



