336 FRAGMENTS OF SCIENCE 



draws them into fibres. Here is a mass acted upon by 

 dilute sulphuric acid, which exhibits in a striking manner 

 this fibrous structure. The experiment was made by my 

 friend Dr. Percy, without any reference to the question 

 of cleavage. 



Break a piece of ordinary iron and you have a granu- 

 lar fracture; beat the iron, you elongate these granules, 

 and finally render the mass fibrous. Here are pieces of 

 rails along which the wheels of locomotives have slidden; 

 the granules have yielded and become plates. They ex- 

 foliate or come off in leaves; all these effects belong, I 

 believe, to the great class of phenomena of which slaty 

 cleavage forms the most prominent example. 1 



We have now reached the termination of our task. 

 You have witnessed the phenomena of crystallization, and 

 have had placed before you the facts which are found 

 associated with the cleavage of slate rocks. Such facts, 

 as expressed by Helmholtz, are so many telescopes to our 

 spiritual vision, by which we can see backward through 

 the night of antiquity, and discern the forces which have 

 been in operation upon the earth's surface 



Ere the lion roared, 

 Or the eagle soared. 



From evidence of the most independent and trust- 

 worthy character, we come to the conclusion that these 

 slaty masses have been subjected to enormous pressure, 

 and by the sure method of experiment we have shown 

 and this is the only really new point which has been 



1 For some further observations on this subject, by Mr. Sorby and myself 

 see "Philosophical Magazine" for August, 1856. 



