42 Prof. Tyndall on the Cleavage of 



the surrounding gritty portions of the slate. Here is a beauti- 

 ful example of the spots : you observe them on the cleavage sur- 

 face in broad patches ; but if this mass has been compressed at 

 right angles to the planes 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 pressure, 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 Lon- 

 don, 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 Mmes. 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. „ „ ... ^03 



Mean . . 5-99 



Greenish Slate. 



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



2. „ „ ... 302 



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 enor- 

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

 if inca])able of modification. All that I deem important is the fact that 



