Crystals and Slate Rocks, 3§ 



planer of cleavage, and you see that one of them makes a large 

 angle with the other. The cleavage of slates is therefore not 

 a question of stratification, and the problem which we have 

 now to consider is, " By what cause has this cleavage been pro- 

 duced ?'' 



In an able and elaborate essay on this subject in 1835, Prof. 

 Sedgwick proposed the theory that cleavage is produced by the 

 action of crystalline or polar forces after the mass has been con- 

 solidated. " We may affirm,'^ he says, " that no retreat of the 

 parts, no contraction of dimensions in passing to a solid state 

 can explain such phsenomena. They appear to me only resol- 

 vable 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 rearranged whole mountain masses, producing a 

 beautiful crystalline cleavage, passing alike through all the 

 strata*/^ The utterance of such a man struck deep, as was 

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

 there are few who do not entertain this view either in whole or 

 in partt- The magnificence of the theory, indeed, has, in some 

 cases, caused speculation to run riot, and we have books pub- 

 lished, aye and largely sold, on the action of polar forces and 

 geologic magnetism, which rather astonish those who know 

 something about the subject. According to the theory referred 

 to, miles and miles of the districts of North Wales and Cumber- 

 land, comprising huge mountain masses, are neither more nor less 

 than the parts of a gigantic crystal. These masses of slate were 

 originally fine mud ; this mud is composed of the broken and 

 abraded particles of older rocks. It contains silica, alumina, 

 iron, potash, soda, and mica mixed in sensible masses mechani- 

 cally together. In the course of ages the mass became consoli- 

 dated, and the theory before us assumes that afterwards a pro- 

 cess of crystallization rearranged the particles and developed in 

 the mass a single plane of crystalline cleavage. With reference 

 to this hypothesis, I will only say that it is a bold stretch of ana- 

 logies ; but still it has done good service : it has drawn atten- 



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



t In a letter from 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 crystallization, 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." 



