392 FRAGMENTS OF SCIENCE. 



subjected to enormous pressure in a direction at right angles 

 to the planes of cleavage. 



In reference to Mr. Sorby's contorted bed, I have said 

 that by supposing it to be stretched out and its length 

 measured, it Avould give us an idea of the amount of yield- 

 ing of the mass above and below the bed. Such a measure- 

 ment, however, would not give the exact amount of yielding. 

 I hold in my hand a specimen of slate with its bedding 

 marked upon it ; the lower portions of each layer being 

 composed of a comparatively coarse gritty material some- 

 thing 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 an- 

 other angle. When the pressure began to act, the inter- 

 mediate bed, which is not entirely unyielding, suffered 

 longitudinal pressure ; as it bent, the pressure became grad- 

 ually more lateral, and the direction of its cleavage is ex- 

 actly 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 inter- 

 mediate 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 phe- 

 nomena of cleavage and pressure — that they accompany 

 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 Darwcn, 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 



