SLATES. 313 



ANALYSIS OP SLATE. 



Dark Slate, two analyses. 



1. Percentage of iron ...... 6'85 



Mean . . 6-99 



WhitisJi Green Slate. 



1. Percentage of iron 3-24 



2. n n m ...... 3-12 



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 mag- 

 netic experiments 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 trilo- 

 bites squeezed, beds 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 com- 

 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, 



