380 FRAGMENTS OF SCIENCE 



age surface, in broad round patches. But turn the slate 

 edgewise 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 Cum- 

 berland, and to many of the slate yards of London, and 

 found the fact general. Thus we elevate a common ex- 

 perience of our boyhood into evidence of the highest sig- 

 nificance 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 



Whitish Green 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 trilobites squeezed, beds 



