SLATES. 359 



edge of the axe is laid across the rings which mark the 

 growth of the tree. If you look at this bundle of hay 

 severed from a rick, you will see a sort of cleavage in it 

 also ; the stalks lie in horizontal 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 that of the hayrick. In the one case it is 

 the molecules arranging themselves according to organic 

 laws which produce a cleavable structure, in the other 

 case the easy separation in one direction is due to the 

 mechanical arrangement of the coarse sensible stalks 

 of hay. 



This sandstone rock was once a powder held in 

 mechanical suspension by water. The powder was com- 

 posed of two distinct parts, fine grains of sand and 

 small plates of mica. Imagine a wide strand covered 

 by a tide, or an estuary with water which holds such 

 powder in suspension : how will it sink ? The rounded 

 grains of sand will reach the bottom first, because they 

 encounter least resistance, 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 thickness are piled up, which by 

 preserving the alternations of sand and mica tell the 

 tale of their formation. Take the sand and mica, mix 

 them together in water, and allow them to subside ; 

 they will arrange themselves in the manner indicated, 

 and by repeating the process you can actually build 

 up a mass which shall be the exact counterpart of that 

 presented by nature. Now this structure cleaves with 

 readiness along the planes in which the particles of 

 mica are strewn. Specimens of such a rock sent to me 

 from Halifax, and other masses from the quarries of 



