314 FRAGMENTS OF SCIENCE. 



paratively coarse gritty material something like what 

 you may suppose the contorted bed to be composed of. 

 ~Now in crossing these gritty portions, the cleavage 

 turns, as if tending to cross the bedding at another 

 angle. "When the pressure began to act, the intermedi- 

 ate 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, thi& 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 con- 

 stituent 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 squeezing the mass finds that the tendency of the 

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



