SLATES. 311 



and occupying various positions in regard to the cleav- 

 age 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 distortion. 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 

 vice 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 imitated; 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 dis- 

 tance into which it has been squeezed. We find in this 

 way that the yielding of the mass has been consider- 

 able. 



Let me now direct your attention to another proof 

 of pressure; you see the varying colours which indicate 

 the bedding on this mass of slate. The dark portion is 

 gritty, being composed of comparatively coarse par- 

 ticles, which, owing to their size, shape and gravity, 

 sink first and constitute the bottom of each layer. 



