310 FKAGMENTS OF SCIENCE. 



deed, 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. Ac- 

 cording to this theory whole districts of North "Wales 

 and Cumberland, mountains 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 mix- 

 ture became consolidated, and the theory before us 

 assumes that a process of crystallisation afterwards re- 

 arranged the particles and developed in it a single 

 plane of cleavage. Though a bold, and I think inad- 

 missible, stretch of analogies, this hypothesis has done 

 good service. Eight or wrong, a thoughtfully uttered 

 theory has a dynamic power which operates against in- 

 tellectual stagnation; and even by provoking opposi- 

 tion is eventually of service to the cause of truth. It 

 would, however, have been remarkable if, among the 

 ranks of geologists themselves, men were not found 

 to seek an explanation of slate-cleavage involving a less 

 hardy assumption. 



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

 facts. This has been done, and the labours of Daniel 

 Sharpe (the late President of the Geological Society, 

 who, to the loss of science and the sorrow of all who 

 knew him, has so suddenly been taken away from us), 

 Mr. Henry Clifton Sorby, and others, have furnished 

 us with a body of facts associated with slaty cleavage, 

 and having a most important bearing upon the ques- 

 tion. 



Fossil shells are found in these slate-rocks. I have 

 here several specimens of such shells in the actual rock, 



