308 FKAGMENTS OF SCIENCE. 



from Halifax, and other masses from the 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 district 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 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 Cum- 

 berland and North Wales will have witnessed the phe- 

 nomenon 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 tomb- 

 stones in churchyards, and for billiard-tables in the 

 metropolis; but not until a comparatively late period 

 did men begin to enquire how their wonderful struc- 

 ture was 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 geolo- 

 gists, 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 

 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 al- 

 most vertical position.' But this was a mistake, and 

 indeed here lies the grand difficulty of the problem. 



