ON CR YSTALLINE AND .SLA TT GLEA VAGE. 239 



Whitish Green Slate. 



1. Percentage of iron 3.24 



2. " " 312 



Mean . . 3.18 



According to these analyses the quantity of iron in the 

 dark slate immediately adjacent to the greenish spot is 

 nearly double the quantity contained in the spot itself. 

 This is about the proportion which the magnetic experi- 

 ments suggested. 



Let me now remind you that the facts brought before 

 you are typical each is the representative of a class. We 

 have seen shells crushed, the trilobites squeezed, beds con- 

 torted, 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 sub- 

 jected 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 yield- 

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

 ment, however, would not give the exact amount of yield- 

 ing. I hold in my hand a specimen of slate with its bed- 

 ding marked upon it; the lower portions of each layer being 

 composed of a comparatively coarse gritty material some- 

 thing like what you may suppose tlie 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 inter- 

 mediate bed, which is not entirely unyielding, suffered 

 longitudinal pressure; as it bent, the pressure became 

 gradually more transverse, and the direction of its cleav- 

 age 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 cleav- 

 age to be at right angles to the pressure, this is the 

 direction which it ought to take across these more unyield- 

 ing strata. 



Thus we have established the concurrence of the phenom- 

 ena of cleavage and pressure that they accompany each 

 other; but the question still remains, Is the pressure sufn> 



