Crystals and Slate Rocks. 



41 



sion that the mass has been pressed together at right angles 

 to the planes of cleavage. This 

 action can be experimentally 

 imitated, and I have here a piece 

 of clay in which this is done 

 and the same result produced 

 on a small scale. The amount 

 of compression, indeed, might 

 be roughly estimated by sup- 

 posing this contorted bed mn to 

 be stretched out, its length mea- 

 sured and compared with the 

 distance c d; we find in this way 

 that the yielding of the mass 

 has been considerable. 



Let me now direct your atten- 

 tion to another proof of pres- 

 sure; you see the varying colours 

 which indicate the bedding on 

 this mass of slate. The dark 

 portion, as I have stated, is gritty, 

 and composed of comparatively 

 coarse particles, which, owing to 

 their size, shape and gravity, sink 

 first and constitute the bottom 

 of each layer. Gradually, from 

 bottom to top the coarseness di- 

 minishes, and near the upper surface of each layer we have a mass 

 of comparatively fine clean mud. Sometimes this fine mud forms 

 distinct layers in a mass of slate-rock, and it is the mud thus 

 consolidated from which are derived the German razor-stones, 

 so much prized for the sharpening of surgical instruments. I 

 have here an example of such a stone ; when a bed is thin, the 

 clean white mud is permitted to rest, as in this case, 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 sometimes 

 in layers is, as might be expected, often rolled up into nodular 

 masses, carried forward, and deposited by the rivers from which 

 the slate-mud has subsided. Here, indeed, are such nodules 

 enclosed in sandstone. Everybody who has ciphered upon 

 a school-slate must remember the whitish-green spots which 

 sometimes, dotted the surface of the slate; he will remember 

 how his slate-pencil usually slid over such spots as if they were 

 greasy ; now these spots are composed of the finer mud, and 

 they could not, on accoimt of their fineness, bite the pencil like 



