one side on a higher
level than on the other) is very curious, and he now states that the
cleavage is parallel to the axis-planes of these thrown-over ridges.
Your case of the limestone beds to my mind is the greatest difficulty
on any mechanical doctrine; though I did not expect ever to find
actual displacement, as seems to be proved by your shell evidence. I am
extremely glad you have taken up this most interesting subject in such
a philosophical spirit; I have no doubt you will do much in it; Sedgwick
let a fine opportunity slip away. I hope you will get out another
section like that in your letter; these are the real things wanted.

LETTER 538. TO D. SHARPE. Down, [January 1847].

I am very much obliged for the MS., which I return. I do not quite
understand from your note whether you have struck out all on this point
in your paper: I much hope not; if you have, allow me to urge on you to
append a note, briefly stating the facts, and that you omitted them in
your paper from the observations not being finished.

I am strongly tempted to suspect that the cleavage planes will be proved
by you to have slided a little over each other, and to have been planes
of incipient tearing, to use Forbes' expression in ice; it will in that
case be beautifully analogical with my laminated lavas, and these in
composition are intimately connected with the metamorphic schists.

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.)