314 FRAGMENTS OF SCIENCE. 



as if tending to cross the bedding at another angle, 

 When the pressure began to act, the intermediate bed, 

 which is not entirely unyielding, suffered longitudinal 

 pressure; as it bent, the pressure became gradually 

 more transverse, and the direction of its cleavage is 

 exactly such as you would infer from an action of this 

 kind it is neither quite across the bed, nor yet in the 

 same direction as the cleavage of the slate above and 

 below it, but intermediate between both. Supposing 

 the cleavage to be at right angles to the pressure, this 

 is the direction which it ought to take across these 

 more unyielding strata. 



Thus we have established the concurrence of the 

 phenomena of cleavage and pressure that they accom- 

 pany each other ; but the question still remains, Is the 

 pressure sufficient to account for the cleavage ? A 

 single geologist, as far as I am aware, answers boldly in 

 the affirmative. This geologist is Sorby, who has attacked 

 the question in the true spirit of a physical investigator. 

 Call to mind the cleavage of the flags of Halifax and 

 Over Darwen, 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 powder, and on squeez- 

 ing 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 



