ON OR 7STA LLINE AND SLA TT OLE A VA GE. 235 
quarries of Over Darwen 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 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 1 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, into laminae 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 
explanation I heard given two years ago. " These p] 
explanation I heard given two years ago. " These ph 
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 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, 
