38 Prof. Tyndall on the Cleavage of 



Halifax : here are other masses from the quarries of Over Dar- 

 wen in Lancashire*. With a hammer and chisel you see I can 

 cleave them into flags ; indeed these flags are made use of 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 

 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 gra- 

 vity, 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 phae- 

 nomena which I shall now proceed to consider. 



Those who have visited the slate quarries of Cumberland and 

 North Wales will have witnessed the phsenomena 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 pro- 

 duced. 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 greatest diffi- 

 culties 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 ques- 

 tion 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 Murchison, 

 .who has kindly permitted me the use of specimens from the 

 Museum of Practical Geology (and here I may be permitted to 

 express my acknowledgements to the distinguished staff of that 

 noble establishment, who, instead of considering me an in- 

 truder, have welcomed me as a brother), I am able to place the 

 proof of this before you. Here is a mass of slate in which 

 the planes of bedding are distinctly marked; here are the 



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

 and for those from Darwen I am indebted to Mr. J. Singleton. 



