SLATES. 309 



The planes of cleavage stand in most cases at a high 

 angle to the bedding. Thanks to Sir Roderick Mur- 

 chison, 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 question 

 of stratification; what then is its cause? 



In an able and elaborate essay published in 1835, 

 Prof. Sedgwick proposed the theory that cleavage is 

 due to the action of crystalline or polar forces subse- 

 quent to the consolidation of the rock. ' We may af- 

 firm/ he says, ' that no retreat of the parts, no contrac- 

 tion of dimensions in passing to a solid state, can ex- 

 plain 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 crystal- 

 line cleavage, passing alike through all the strata/ * 

 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 whole or in part.f The boldness of the theory, in- 



* Transactions of the Geological Society, ser. ii. vol. iii. p. 477. 



t In a letter to Sir Charles Lyell, dated from the Cape of Good 

 Hope February 20, 1836, Sir John Herschel writes as follows : 

 ' If rocks have been so heated as to allow of a commencement of 

 crystallisation, that is to say, if they have been heated to a point 

 at which the particles can begin to move amongst themselves, or 

 at least on their own axes, some general law must then determine 

 the position in which these particles will rest on cooling. Proba- 

 bly that position will have some relation to the direction in which 

 the heat escapes. Now when all or a majority of particles of the 

 same nature have a general tendency to one position, that must 

 of course determine a cleavage plane.' 



