SLATES. 307 



the fibre, and this cleavage is most perfect when the 

 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 sensi- 

 ble 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, de- 

 posits its duplex layer day after day, and finally masses 

 of immense thickness are piled up, which by preserv- 

 ing 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 pre- 

 sented 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 



