1856.] tJie Cleavage of Crystals and Slate Rocks. 



301 



experimentally imitated, and I 

 which this is done and the same 



cleavage. This action can be 

 have here a piece of clay in 

 result produced on a small scale. 

 The amount of compression, in- 

 deed, might be roughly estimated 

 by supposing this contorted bed 

 mn to be stretched out, its 

 length measured and compared 

 with the distance cd; we find in 

 this way that the yielding of the 

 mass has been considerable. 



Let me now direct your at- 

 tention 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 sur- 

 face 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 con- 

 solidated from which are derived 

 the German razor-stones, so much prized for the sharpening of sur- 

 gical 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 deposit- 

 ed 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 sand-stone. Everybody who has ciphered upon 

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

 times 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 account of their fineness, bite the pencil like the surrounding 

 gritty portions of the slate. Here is a beautiful example of the 

 spots : you observe them on the cleavage surface in broad patches ; 

 but if this mass has been compressed at right angles to the planes 



