ON CRYSTALLINE AND SLATY CLEAVAGE 335 



developed by the rolling-pin. You have only to break a 

 biscuit across, and to look at the fracture, to see the lam- 

 inated structure. We have here the means of pushing the 

 analogy further. I invite you to compare the structure 

 of this slate, which was subjected to a high temperature 

 during the conflagration of Mr. Scott -Russell's premises, 

 with that of a biscuit. Air or vapor within the slate has 

 caused it to swell, and the mechanical structure it reveals 

 is precisely that of a biscuit. During these inquiries I 

 have received much instruction in the manufacture of 

 puff-paste. Here is some such paste baked under my own 

 superintendence. The cleavage of our hills is accidental 

 cleavage, but this is cleavage with intention. The voli- 

 tion of the pastry-cook has entered into its formation. It 

 has been his aim to preserve a series of surfaces of struct- 

 ural weakness, along which the dough divides into layers. 

 Puff -paste in preparation must not be handled too much; 

 it ought, moreover, to be rolled on a cold slab, to pre- 

 vent the butter from melting, and diffusing itself, thus 

 rendering the paste more homogeneous and less liable to 

 split. Puff-paste is, then, simply an exaggerated case of 

 slaty cleavage. 



The principle here enunciated is so simple as to be 

 almost trivial; nevertheless, it embraces not only the cases 

 mentioned, but, if time permitted, it might be shown you 

 that the principle has a much wider range of application. 

 When iron is taken from the puddling furnace it is more 

 or less spongy, an aggregate in fact of small nodules: it 

 is at a welding heat, and at this temperature is submitted 

 to the process of rolling. Bright, smooth bars are the 

 result. But notwithstanding the high heat the nodules 

 do not perfectly blend together. The process of rolling 



