324 FRAGMENTS OF SCIENCE 



ent directions: heavy spar presents an example of tliis 

 kind of cleavage. 



Turn we now to the consideration of some other phe- 

 nomena to which the term cleavage may be applied. 

 Beech, deal, and other woods cleave with facility along 

 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 re- 

 gard 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 pro- 

 duce 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 me- 

 chanical 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, or an 

 estuary with water which holds such powder in suspen- 

 sion: how will it sink? The rounded grains of sand will 

 reach the bottom first, because they encounter least re- 

 sistance, the mica afterward, 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 



