310 FKAGMENTS OF SCIENCE. 



the action of polar forces and geologic magnetism, whicfc 

 rather astonish those who know something about the 

 subject. According 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. Tii the course of ages the 

 mixture 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 inadmissible, 

 stretch of analogies, this hypothesis has done good 

 service. Right or wrong, a thoughtfully uttered theory 

 has a dynamic power which operates against intellectual 

 stagnation ; and even by provoking opposition is event- 

 ually 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 question. 



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

 here several specimens of such shells in the actual rock, 

 and occupying various positions in regard to the cleavage 

 planes. They are squeezed, distorted, and crushed ; in 

 all cases the distortion leads to the inference that the 

 rock which contains these shells has been subjected to 



