386 FRAGMENTS OF SCIENCE. 



the one case it is the molecules arranging themselves ac 

 cording to organic laws which produce a cleavable struct 

 ure, in the other case the easy separation in one direction 

 is due to the mechanical arrangement of the coarse sensible 

 masses of the stalks of hay. 



This sand-stone rock was once a powder, more or less 

 coarse, held in mechanical suspension by water. The pow 

 der 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 w T ill it sink ? The rounded grains of sand 

 w r ill 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 tnica, and mix them 

 together in water, and allow them to subside ; they will ar 

 range 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 Over Darwen in Lancashire, are here before you. With 

 a hammer and chisel I can cleave them into flags ; indeed, 

 these flags are employed for roofing purposes in the districts 

 from which the specimens have come, and receive the name 

 of &quot; slate-stone.&quot; But you will discern without a word 

 from me, that this cleavage is not a crystalline cleavage 

 any more than that of a hay-rick. It is molar, not molec 

 ular. 



This, so far as I am aware of, has never been imagined, 



