298 Prof. Tyndall, o/i the Comparative View of [Ju!ie 6, 



iiess are thus piled up, which by preserving the alternations of 

 sand and mica tell the tale of their formation. I do not wish you 

 to accept this without proof. Take the sand and mica, mix them 

 together in water, and allow them to subside, they will arrange 

 themselves in the manner I have indicated ; and by repeating the 

 process you can actually build up a sandstone mass whicii shall be 

 the exact counterpart of that presented by nature, as I have done 

 in this glass jar. Now this structure cleaves with readiness along 

 the planes in which the particles of mica are strewn. Here is a 

 mass of such a rock sent to me from Halifax : here are other 

 masses from the quarries of Over Darwen, in Lancashire.* With 

 a hammer and chisel you see I can cleave them into flags ; in- 

 deed these flags are made use of for roofing purposes in the dis- 

 tricts 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 not an arrangement produced by molecular forces ; 

 indeed it would be just as reasonable to suppose that on this jar of 

 sand and mica the particles arranged themselves into layers by the 

 forces of crystallization, instead of by the simple force of gravity, 

 as to imagine that such a cleavage as this could be the product of 

 crystallization. 



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

 has been agreed among geologists not to call such splitting as this 

 cleavage at all, but to restrict the term to a class of phaenomena 

 which I shall now proceed to consider. 



Those who have visited the slate quarries of Cumberland and 

 North Wales will have witnessed the phaenomena 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 churchyards, and for billiard-tables in the metropolis ; 

 but not until a comparatively late period did men begin to inquire 

 how their wonderful structure was produced. What is the agency 

 which enables us to split Honister Crag, or the cliflTs of Snowdon, 

 into laminae from crown to base ? This question is at the present 

 moment one of the greatest difficulties of geologists, and occupies 

 their attention perhaps more than any other. You may wonder at 

 this. Looking into the quarry of Penrhyn, you may be disposed to 

 explain the question, as I heard it explained two years ago. 

 " These planes of cleavage," said a friend who stood beside me on 

 the quarry's edge, "are the planes of stratification which have 

 been lifted by some convulsion into an almost vertical position.'* 

 But this was a great mistake, and indeed here lies the grand 

 difficulty of the problem. These planes of cleavage stand in most 

 cases at a high angle to the bedding. Thanks to Sir Roderick 



* For the specimens from Halifax I have to thank Mr. Richard Carter, and 

 for those from Darwen I am indebted to Mr. J. Singteton. 



