SLATES. 387 



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 Cumberland 

 and North Wales will have witnessed the phenomenon to 

 which I refer. We have long drawn our supply of roofing- 

 slates from such quarries ; school-boys ciphered on these 

 slates, they were used for tombstones in church-yards, and 

 for billiard-tables in the metropolis ; but not until a com- 

 paratively 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 cliffs of 

 Snowdon, into lamina? from crown to base ? This question 

 is at the present moment one of the great 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 offer the ex- 

 planation I heard given 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 mistake, and indeed here lies the grand diffi- 

 culty of the problem. The planes of cleavage stand in most 

 cases at a high angle to the bedding. Thanks to Sir Roder- 

 ick Murchison, I am able to place the proof of this before 

 you. Here is a specimen of slate in which both the planes 

 of cleavage and of bedding are distinctly marked, one of 

 them making a large angle with the other. This is com- 

 mon. The cleavage of slates, then, is not a question of 

 stratification ; what, then, is its cause ? 



In an able and elaborate essay published in 1835, Pro- 

 fessor Sedgwick proposed the theory that cleavage is due 

 to the action of crystalline or polar forces subsequent to the 

 consolidation of the rock. " We may affirm," he says, " that 

 no retreat of the parts, no contraction of dimensions in pass- 



