t is molar, not molecular.

This, so far as I am aware of, has never been imagined, and it has
been agreed among geologists not to call such splitting as this
cleavage at all, but to restrict the term to a phenomenon of a totally
different character.

Those who have visited the slate quarries of Cumberland and North
Wales will have witnessed the phenomenon to which I refer. We have
long drawn our supply of roofing-slates from such quarries;
school-boys ciphered on these slates, they were used for tombstones in
churchyards, and for billiard-tables in the metropolis; but not until
a comparatively late period did men begin to enquire how their
wonderful structure was produced. What is the agency which enables us
to split Honister Crag, or the cliffs of Snowdon, into laminae from
crown to base? This question is at the present moment one of the
great difficulties of geologists, and occupies their attention perhaps
more than any other. You may wonder at this. Looking into the quarry
of Penrhyn, you may be disposed to offer the explanation I heard given
two years ago. 'These planes of cleavage,' said a friend who stood
beside me on the quarry's edge, 'are the planes of stratification
which have been lifted by some convulsion into an almost vertical
position.' But this was a mistake, and indeed here lies the grand
difficulty of the problem. The planes of cleavage stand in most cases
at a high angle to the bedding. Thanks to Sir Roderick Murchison, I
am able to place the proof of this before you. Here is a specimen of
slate in which both the planes of cleavage and of bedding are
distinctly marked, one of them making a large angle with the other.
This is common. The cleavage of slates then is not a question of
stratification; what then is its cause?

In an able and elaborate essay published in 1835, Prof. Sedgwick
proposed the theory that cleavage is due to the action of crystalline
or polar forces subsequent to the consolidation of the rock. 'We may
affirm,' he says, 'that no retreat of the parts, no contraction of
dimensions in passing to a solid state, can explain such phenomena.
They appear to me only resolvable on the supposition that crystalline
or polar forces acted upon the whole mass simultaneously in one
direction and with adequate force.' And again, in another place:
'Crystalline forces have re-arranged whole mountain masses, producing
a beautiful crystalline cleavage, passing alike through all the
stra