242 #&4 GMENTS V 8UTENCK. 
Further, a mass of dried mud is full of cavities and fis- 
sures. If you break dried pipe-clay you see them in great 
numbers, and there are multitudes of them so small that 
you cannot see them. A flattening of these cavities must 
take place in squeezed mud, and this must to some extent 
facilitate the cleavage of the mass in the direction 
indicated. 
Although the time at my disposal has not permitted me 
duly to develop these thoughts, yet for the last twelve 
months the subject has presented itself to me almost daily 
under one aspect or another. I have never eaten a biscuit 
during this period without remarking the cleavage 
developed by the rolling-pin. You have only to break a 
biscuit across, an-d to look at the fracture, to see the 
laminated structure. We have here the means of pushing 
the analogy further. I invite you to compare the struc- 
ture of the slate, which was subjected to a high tempera- 
ture 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 volition 
of the pastrycook has entered into its formation. It has 
been his aim to preserve a series of surfaces of struc- 
tural 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 
prevent 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 
