390 I ll AC MK NTS OF SCIENCE. 



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 

 roughtly 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 consider 

 able. 



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 upper sur 

 face we have a layer of exceedingly fine mud. It is the 

 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 white mud is 

 permitted to rest upon a slab of the coarser slate in contact 

 with it : when the bed is thick it is cut into slices, which 

 are cemented to pieces of ordinary slate, and thus rendered 

 stronger. The mud thus deposited is, as might be ex 

 pected, often rolled up into nodular masses, carried for 

 ward, and deposited among coarser material by the rivers 

 from which the slate-mud has subsided. Here are such 

 nodules enclosed in sandstone. Everybody, moreover, who 

 has ciphered upon a school-slate must remember the whitish- 

 green spots which sometimes 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 surrounding gritty portions of the slate. 

 Here is a beautiful example of these spots : you observe 



