ON CRYSTALLINE AND SLATY CLEAVAGE 325 



subside; they will arrange themselves in the manner indi- 

 cated, 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 Dar- 

 wen 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 "slatestone." But you will discern, without a 

 word from me, that this cleavage is not a crystalline 

 cleavage any more than that of a hayrick is. It is 

 molar, not molecular. 



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

 ined, and it has been agreed among geologists not to call 

 such splitting as this cleavage at all, but to restrict the 

 term to a phenomenon of a totally different character. 



Those who have visited the slate quarries of Cumber- 

 land and North Wales will have witnessed the phenome- 

 non to which I refer. We have long drawn our supply 

 of roofing-slates from such quarries; schoolboys ciphered 

 on these slates, they were used for tombstones in church- 

 yards, and for billiard-tables in the metropolis; but not 

 until a comparatively late period did men begin to in- 

 quire how their wonderful structure was produced. What 

 is the agency which enables us to split Honister Crag, or 

 the cliffs of Snowdon, into laminae from crown to base? 

 This question is at the present moment one of the great 

 difficulties of geologists, and occupies their attention per- 

 haps more than any other. You may wonder at this. 



