SLATES. 391 



them on the cleavage surface in broad round patches. But 

 turn the slate edgeways and the section of each nodule is 

 seen to be a sharp oval, with its longer axis parallel to the 

 cleavage. This instructive fact has been adduced by Mr. 

 Sorby. I have made excursions to the quarries of Wales 

 and Cumberland, and to many of the slate-yards of London, 

 and found the fact general. Thus we elevate a common 

 experience of our boyhood into evidence of the highest 

 significance as regards a most important geological problem. 

 From the magnetic deportment of these slates, I was led to 

 infer that these spots contain a less amount of iron than the 

 surrounding dark slate. An analysis was made for me by 

 Mr. Hambly in the laboratory of Dr. Percy at the School of 

 Mines, with the following result : 



Analysis op Slate. 

 Dark Slate, two Analyses. 



1. Percentage of iron 5.85 



2. " " 6.13 



Mean 5.99 



1 1 It itish-ffrcoi Slate. 



1. Percentage of iron 3.24 



2. " " 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 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 unhappy trilobites 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 



