308 FRAGMENTS OF SCIENCE. 



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 

 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 Cumber- 

 land and North Wales will have witnessed the pheno- 

 menon 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 churchyards, and for billiard -tables in the metropolis; 

 but not until a comparatively late period did men begin 

 to enquire how their wonderful structure 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 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 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 

 difficulty of the problem. The planes of cleavage stand 

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



