326 FRAGMENTS OF SCIENCE 



Looking into the quarry of Penrhyn, you may be dis- 

 posed 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 strati- 

 fication 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 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 mak- 

 ing a large angle with the other. This is common. 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 passing to a solid state, can explain such phenomena. 

 They appear to me only resolvable on the supposition 

 that crystalline or polar forces acted upon the whole mass 

 simultaneously in one direction and with adequate force." 

 And again, in another place: "Crystalline forces have re- 

 arranged whole mountain masses, producing a beautiful 

 crystalline cleavage, passing alike through all the strata." 1 

 The utterance of such a man struck deep, as it ought to 

 do, into the minds of geologists, and at the present day 

 there are few who do not entertain this view either in 



1 "Transactions of the Geological Societ7," aer. ii. vol. iii. p. 477. 



