SLATES. 309 



to Sir Koderick 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 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 subsequent to 

 the consolidation of the rock. 'We may affirm,' he says, 

 'that no retreat of the parts, no contraction of dimen- 

 sions 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 : ' Crys- 

 talline 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 whole or in part.* The 

 boldness of the theory, indeed, has, in some cases, caused 

 speculation to run riot, and we have books published on 



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



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 crys- 

 tallisation, 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. Probably 

 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.' 



