ON CRYSTALLINE AND SLA TT CLEA VAGE. 243 



perfectly blend together. The process of rolling draws 

 them into fibers. 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 granular 

 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 exfoliate 

 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.* 



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 trustworthy 

 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 brought before you how 

 the pressure is sufficient to produce the cleavage. Expand- 

 ing our field of view, we find the selfsame law, whose foot- 

 steps we trace amid the crags of Wales and Cumberland, 

 extending into the domain of the pastrycook and iron- 

 founder; nay, a wheel cannot roll over the half-dried mud 

 of our streets without revealing to us more or less of the 

 features of this la\v. Let me say, in conclusion, that the 

 spirit in which this problem has been attacked by geologists, 

 indicates the dawning of a new day for their science. The 

 great intellects who have labored at geology, and who have 

 raised it to its present pitch of grandeur, were compelled to 



*For some further observations on this subject by Mr. Sorby and 

 myself, see Philosophical Magazine for August, 1856. 



