ON CRYSTALLINE AND SLATY CLEAVAGE 327 



whole or in part. 1 The boldness of the theory, indeed, 

 has, in some cases, caused speculation to run riot, and we 

 have books published on the action of polar forces and 

 geologic magnetism, which rather astonish those who know 

 something about the subject. According to this theory 

 whole districts of North Wales and Cumberland, moun- 

 tains included, are neither more nor less than the parts of 

 a gigantic crystal. These masses of slate were originally 

 fine mud, composed of the broken and abraded particles 

 of older rocks. They contain silica, alumina, potash, soda, 

 and mica mixed mechanically together. In the course of 

 ages the mixture became consolidated, and the theory be- 

 fore us assumes that a process of crystallization afterward 

 rearranged the particles and developed in it a single plane 

 of cleavage. Though a bold, and I think inadmissable, 

 stretch of analogies, this hypothesis has done good ser- 

 vice. Bight or wrong, a thoughtfully uttered theory has 

 a dynamic power which operates against intellectual stag- 

 nation; and even by provoking opposition is eventually 

 of service to the cause of truth. It would, however, have 

 been remarkable if, among the ranks of geologists them- 

 selves, men were not found to seek an explanation of 

 slate -cleavage involving a less hardy assumption. 



The first step in an inquiry of this kind is to seek 



1 In a letter to Sir Charles Lyell, dated from the Cape of Good Hope, Febru- 

 ary 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 among 

 themselves, or at least on their own axes, some general law must then deter- 

 mine 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." 



