ON CRYSTALLINE AND SLA TT CLEA VAGE. 237 



selves, men were not found to seek an explanation of slate- 

 cleavage involving a less hardy assumption. 



The first step in an inquiry of this kind is to seek facts. 

 This has been done, and the labors of Daniel Sharpe (the 

 late president of the Geological Society, who, to the loss of 

 science and the sorrow of all who knew him, has so sud- 

 denly been taken away from us), Mr. Henry Clifton Sorby, 

 and others, have furnished us with a body of facts 

 associated with slaty cleavage, and having a most important 

 bearing upon the question. 



Fossil shells are found in these slate-rocks. I have 

 here several specimens of such shells in the actual rock, 

 and occupying various positions in regard to the cleavage 

 planes. They are squeezed, distorted, and crushed; in 

 all cases the distortion leads to the inference that the 

 rock which contains these shells has been subjected to 

 enormous pressure in a direction at right angles to the 

 planes of cleavage. The shells are all flattened and spread 

 out in these planes. Compare this fossil trilobite of normal 

 proportions with these others which have suffered distor- 

 tion. Some have lain across, some along, and some oblique 

 to the cleavage of the slate in which they are found; but in 

 all cases the distortion is such as required for its production 

 a compressing force acting at right angles to the planes of 

 cleavage. As the trilobites lay in the mud, the jaws of a 

 gigantic vise appear to have closed upon them and squeezed 

 them into the shapes you see. 



We sometimes find a thin layer of coarse gritty material, 

 between two layers of finer rock, through which and across 

 the gritty layer pass the planes of lamination. The coarse 

 layer is found bent by the pressure into sinuosities like a 

 contorted ribbon. Mr. Sorby has described a striking case 

 of this kind. This crumpling can be experimentally imi- 

 tated; the amount of compression might, moreover, be 

 roughly estimated by supposing the contorted bed to be 

 stretched out, its length measured and compared with 

 the shorter distance into which it has been squeezed. We 

 find in this way that the yielding of the mass has been 

 considerable. 



Let me now direct your attention to another proof of 

 pressure; you see the varying colors which indicate the 

 bedding on this mass of slate. The dark portion is gritty, 

 being composed of comparatively coarse particles, which, 



