SLATES. 363 



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 trilo- 

 bites 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 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 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. 

 Gradually, from bottom to top the coarseness diminishes, 

 and near the upper surface we have a layer of exceed- 

 ingly fine grain. It is the fine mud thus consolidated 

 from which are derived the German razor-stones, so 

 much prized for the sharpening of surgical instruments. 



