302 Prof, Tyndall, on the Comparative View of [June 6, 



of cleavage, ought we to expect the same marks when we look at 

 the edge of the slab ? The nodules will be flattened by such pres- 

 sure, and we ought to see evidence of this flattening when we turn 

 the slate edgeways. Here it is. The section of a nodule is a 

 sharp ellipse with its major axis parallel to the cleavage. There 

 are other examples of the same nature on the table ; I have made 

 excursions to the quarries of Wales and Cumberland, and to many 

 of the slate-yards of London, but the same fact invariably appears, 

 and thus we elevate a common experience of our boyhood into 

 evidence of the highest significance as regards one of the most 

 important problems of geology. In examining the magnetism of 

 these slates, I was led to infer that these spots would contain a less 

 amount of iron than the surrounding dark slate. The analysis was 

 made for me by Mr. Hambly, in the laboratory of Dr. Percy, at the 

 School of Mines. The result which is stated in this Table, justifies 

 the conclusion to which I have referred. 



Analysis of Slate. 

 Purple Slate, two analyses. 



1. Per-centage of iron . . . 5*85 



2. „ „ ... 6-13 



Mean . . 5*99 



Greenish Slate. 



1 . Per-centage of iron . . . 3 * 24 



2. „ „ ... 3-12 



Mean . . 3*18 



The quantity of iron in the dark slate immediately adjacent to the 

 greenish spot is, according to these analyses, nearly double of the 

 quantity contained in the spot itself. This is about the proportion 

 which the magnetic experiments suggested. 



Let me now remind you that the facts which I have brought 

 before you are typical facts — 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 subjected to enormous pressure 

 in a direction at right angles to the planes of cleavage.* 



* While to my mind the evidence in proof of pressure seems perfectly 

 irresistible, I by no means assert that the manner in which I have stated it 

 is incapable of modification. All that I deem important is tlie fact that 

 pressure has been exerted ; and provided this remain firm, the fate of any 

 minor portion of the evidence by which it is here established is of- compara- 

 tively little moment. 



