ON CRYSTALLINE AND SLATY CLEAVAGE 329 



moreover, be roughly estimated by supposing the con- 

 torted 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, 

 owing to their size, shape and gravity, sink first and con- 

 stitute the bottom of each layer. Gradually, from bottom 

 to top the coarseness diminishes, and near the tipper sur- 

 face we have a layer of exceedingly 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. When a bed is thin, the fine- 

 grain slate is permitted to rest upon a slab of the coarse 

 slate in contact with it; when the fine bed is thick, it is 

 cut into slices which are cemented to pieces of ordinary 

 slate, and thus rendered stronger. The mud thus depos- 

 ited is, as might be expected, often rolled up into nodular 

 masses, carried forward, and deposited among coarser ma- 

 terial by the rivers from which the slate-mud has sub- 

 sided. Here are such nodules enclosed in sandstone. 

 Everybody, moreover, who has ciphered upon a school- 

 slate must remember the whitish-green spots which some- 

 times dotted the surface of the slate, and over which the 

 pencil usually slid as if the spots were greasy. Now these 

 spots are composed of the finer mud, and they could not, 

 on account of their fineness, bite the pencil like the sur- 

 rounding gritty portions of the slate. Here is a beautiful 

 example of these spots: you observe them, on the cleav- 



