ON CRYSTALLINE AND SLA TT OLE A VAOE. 235 



quarries of Over Darwen in Lancashire, are here before 

 you. With a hammer and chisel I can cleave them into 

 Sags; 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 crystal- 

 line 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 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 

 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; schoolboys ciphered on these 

 slates, they were used for tombstones in churchyards, and 

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

 paratively late period did men begin to inquire how their 

 wonderful structure is produced. What is the agency 

 which enables us to split Honister Crag, or the cliffs of 

 Snowdon, intolaminaB 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. Yon may wonder at this. Looking into the 

 quarry of Penrhyn, you may be disposed to offer the 

 explanation I heard given two years ago. " These planes 

 of cleavnge," 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 posi- 

 tion." But this was a mistake, and indeed here lies the 

 grand difficulty of the problem. The planes of cleavage 

 stand in most cases at a high angle to the bedding. Thanks 

 to Sir Roderick 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 common. The cleavage of slates then is not a ques- 

 tion 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, 



