1856.] the Cleavage of Crystals and Slate Rocks. 297 



with peculiar facility in one direction. Here again I have a mass 

 of rock-salt : I lay my knife upon it, and with a blow cleave it in 

 this direction ; but I find on further examining this substance 

 that it cleaves in more directions than one. Laying my knife at 

 right angles to its former position, the crystal cleaves again ; and 

 finally placing the knife at right angles to the two former positions, 

 the mass cleaves again. Thus rock-salt cleaves in three direc- 

 tions, and the resulting solid is this perfect cube, which may be 

 broken up into any number of smaller cubes. Here is a mass of 

 Iceland spar, which also cleaves in three directions, not at right 

 angles, but oblique to each other, 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 phaenomena 

 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 suflficient 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- 



♦ 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. 



