le surfaces along which it
has given way, and it has yielded along those surfaces because in them
the cohesion of the mass is less than elsewhere. I break this marble,
and even this wax, and observe the same result; look at the mud at the
bottom of a dried pond; look at some of the ungravelled walks in
Kensington Gardens on drying after rain,--they are cracked and split,
and other circumstances being equal, they crack and split where the
cohesion is a minimum. Take then a mass of partially consolidated
mud. Such a mass is divided and subdivided by interior surfaces along
which the cohesion is comparatively small. Penetrate the mass in
idea, and you will see it composed of numberless irregular polyhedra
bounded by surfaces of weak cohesion. Imagine such a mass subjected
to pressure,--it yields and spreads out in the direction of least
resistance; the little polyhedra become converted into laminae,
separated from each other by surfaces of weak cohesion, and the
infallible result will be a tendency to cleave at right angles to the
line of pressure. [Footnote: It is scarcely necessary to say that if the
mass were squeezed equally in all directions no laminated structure
could be produced; it must have room to yield in a lateral direction.
Mr. Warren De la Rue informs me that he once wished to obtain
white-lead in a fine granular state, and to accomplish this he first
compressed it. The mould was conical, and permitted the lead to
spread out a little laterally. The lamination was as perfect as that
of slate, and it quite defeated him in his effort to obtain a granular
powder.]

Further, a mass of dried mud is full of cavities and fissures. 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 develope
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, and 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 struct