The beds without cleavage between those with cleavage do not weigh quite so
heavily on me as on you. You remember, of course, Sedgwick's facts of
limestone, and mine of sandstone, breaking in the line of cleavage,
transversely to the planes of deposition. If you look at cleavage as I do,
as the result of chemical action or crystalline forces, super-induced in
certain places by their mechanical state of tension, then it is not
surprising that some rocks should yield more or less readily to the
crystalline forces.

I think I shall write to Prof. Forbes (538/1. Prof. D. Forbes.) of
Edinburgh, with whom I corresponded on my laminated volcanic rocks, to call
his early attention to your paper.


LETTER 539. TO D. SHARPE.
Down, October 16th [1851].

I am very much obliged to you for telling me the results of your foliaceous
tour, and I am glad you are drawing up an account for the Royal Society.
(539/1. "On the Arrangement of the Foliation and Cleavage of the Rocks of
the North of Scotland." "Phil. Trans. R. Soc." 1852, page 445, with Plates
XXIII. and XXIV.) I hope you will have a good illustration or map of the
waving line of junction of the slate and schist with uniformly directed
cleavage and foliation. It strikes me as crucial. I remember longing for
an opportunity to observe this point. All that I say is that when slate
and the metamorphic schists occur in the same neighbourhood, the cleavage
and foliation are uniform: of this I have seen many cases, but I have
never observed slate overlying mica-slate. I have, however, observed many
cases of glossy clay-slate included within mica-schist and gneiss. All
your other observations on the order, etc., seem very interesting. From