ure 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 vapour within the slate has
caused it to swell, and the mechanical structure it reveals is
precisely that of a biscuit. During these enquiries 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 structural 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 perfectly blend together. The
process of rolling 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 granular fracture; heat
the iron, you elongate these granules, and finally render the mass
fibrous. Here are pieces of rails along which the wheels of
locomotives have slid-den; 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. [Footnote: For some further
observations on this subject by Mr. Sorby and myself, see
Philosophical Magazine for August, 1856.]

We have now reached the termination of our task. You have witnessed
the phenomena of crysta