234 FRA GMKN TS Ob 1 SVIENCK. 



Iceland spar 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 frith 

 equal facility in all three directions. For the sake of com- 

 pleteness I may say that many crystals cleave with unequal 

 facility in different directions: heavy spar presents an 

 example of this kind of cleavage. 



Turn \ve no\v to the consideration of some other phenom- 

 ena to which the term cleavage may be applied. Beech, 

 deal, and other woods cleave with facility along the fiber, 

 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 hori- 

 zontal 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 accord- 

 ing 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 mechan- 

 ical 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 suspension: how 

 will it sink? The rounded grains of sand will reach the 

 bottom first, because they encounter least resistance, 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 subside; they will arrange them- 

 selves in the manner indicated, and by repeating the proc- 

 ess 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 



