1856.] the Cleavage of Crystals and Slate Rocks. 299 



Murchison, who has kindly permitted me the use of specimens 

 from the Museum of Practical Geology (and here I may be per- 

 mitted to express my acknowledgments to the distinguished staff of 

 that noble establishment, who, instead of considering me an in- 

 truder, have welcomed me as a brother), I am able to place the 

 proof of this before you. Here is a mass of slate in which the 

 planes of bedding are distinctly marked ; here are the planes 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 produced ? " 



In an able and elaborate essay on this subject in 1 835, Professor 

 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 phaenomena. 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 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 part.f The magnificence of the theory, indeed, has, 

 in some cases, caused speculation to run riot, and we have books 

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

 geologic magnetism, which rather astonish those who know some- 

 thing about the subject. According to the theory referred to, 

 miles and miles of the districts of North Wales and Cumberland, 

 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 



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



t In a letter from Sir Charles Lyell, dated fr(»m 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 ten- 

 dency to one position, that most of course determine a cleavage plane." 



