Crystals and Slate Rocks. 37 



the resulting solid being a rhomboid. In each of these cases the 

 mass cleaves with equal facility in all three directions. For the 

 sake of completeness I may say that many substances cleave with 

 unequal facility in different directions, and the heavy spar I hold 

 in my hand presents an example of this kind of cleavage. 



Turn we now to the consideration of some other phsenomena 

 to which the term cleavage may be applied. This piece of beech- 

 wood cleaves with facility parallel to the fibre, and if our ex- 

 periments were fine enough we should discover that the cleavage 

 is most perfect when the edge of the axe is laid across the rings 

 which mark the growth of the tree. The fibres of the wood lie 

 side by side, and a comparatively small force is sufficient to 

 separate them. If you look at this mass of hay severed from a 

 rick, you will see a sort of cleavage developed in it also ; the 

 stalks lie in parallel planes, and only a small force is required 

 to separate them laterally. But we cannot regard the cleavage of 

 the tree as the same in character as the cleavage of the hayrick. 

 In the one case it is the atoms arranging themselves according to 

 organic laws which produce a cleavable structure, in the other 

 case the easy separation in a certain direction is due to the me- 

 chanical arrangement of the coarse sensible masses of the stalks 

 of hay. 



In like manner I find that this piece of sandstone cleaves 

 parallel to the planes of bedding. This rock was once a powder, 

 more or less coarse, held in mechanical suspension by water. 

 The powder was composed of two distinct parts, fine grains of 

 sand and small plates of mica. Imagine a wide strand covered 

 by a tide which holds such powder in suspension* : how will it 

 sink ? The rounded grains of sand will reach the bottom first, 

 the mica afterwards, and when the tide recedes we have the little 

 plates shining like spangles upon the surface of the sand. Each 

 successive tide brings its charge of mixed powder, deposits its 

 duplex layer day after day, and finally masses of immense thick- 

 ness are thus piled up, which by preserving the alternations of 

 sand and mica tell the tale of their formation. I do not wish 

 you to accept this without proof. Take the sand and mica, mix 

 them together in water, and allow them to subside, they will 

 arrange themselves in the manner I have indicated; and by 

 repeating the process you can actually build up a sandstone mass 

 which shall be the exact counterpart of that presented by nature, 

 as I have done in this glass jar. Now this structure cleaves 

 with readiness along the planes in which the particles of mica 

 are strewn. Here is a mass of such a rock sent to me from 



* I merely use this as an illustration ; the deposition may have really 

 been due to sediment carried down by rivers. But the action must have 

 been periodic, and the powder duplex. 



