ON CRYSTALLINE AND SLATY CLEAVAGE 331 



contorted, nodules of greenish marl flattened; and all 

 these sources of independent testimony point to one and 

 the same conclusion, namely, that slate-rocks have been 

 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 would give us an idea of the amount of 

 yielding of the mass above and below the bed. Such a 

 measurement, 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 ma- 

 terial something like what you may suppose the contorted 

 bed to be composed of. Now, in crossing these gritty por- 

 tions, the cleavage turns, as if tending to cross the bed 

 ding 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 be- 

 came gradually more transverse, and the direction of its 

 cleavage is exactly such as you would infer from an ac- 

 tion 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. Sup- 

 posing 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. 



