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features in both: you see flakes clinging to the surfaces of
each, which have been partially torn away in cleaving.
Let any close observer compare these two effects, he will,
I am persuaded, be led to the conclusion that they are the
product of a common cause}l

But you will ask me how, according to my View, does
pressure produce this remarkable result. This may be
stated in a very few words:

There is no such thing in Nature as a body of perfectly
homogeneous structure. I break this clay which seems so
uniform, and find that the fracture presents to my eyes in-
numerable surfaces . along which it has, given way, and it
has yielded along those surfaces because in themthe cohe-
sion of the mass is less than elsewhere. I break this mar-
ble, and even this wax, and observe the same result; look
at the mud at the bottom of a dried pond; look to some of
the ungravelled Walks in Kensington Gardens on drying af-
ter rain—they are'cracked and split, and, other circumstances
being equal, they crack and split where the cohesion is least.
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-Fit yields and
spreads out in the direction of least resistance ; 2 the little

1 I have usually softened the wax by warming it, kneaded it with the
fingers, and pressed it between thick plates of glass previously wetted.
At the ordinary summer temperature the pressed Wax is soft, and tears
rather than cleaves ; on this account, I cool my compressed specimens in
a mixture of pounded ice and salt, and when thus cooled they split beau-
tifully. ’

9 It is scarcely necessary to say that, if the mass were squeezed equal-
ly 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

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