Comments on manuscript [? "On slaty cleavage", J. Geol. Soc. Lond. 5 (1849): 111–29]. Discusses phenomenon of cleavage. Will write to J. D. Forbes about DS’s paper.
Down Farnborough Kent
I am very much obliged for the M.S.f1 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, & that you omitted them in your paper from the observations not being finished.f2
I am strongly tempted to suspect that the cleavage planes will be proved by you to have slided a little over each other, & to have been planes of incipient tearing to use Forbe’s expression in ice: it will in that case be beautifully analogical with my laminated lavas & these in composition are intimately connected with the metamorphic schists.—f3
The beds without cleavage between those with cleavage, do not weigh quite so heavily on me, as on you— you remember, of course, Sedgwick facts of limestonef4 & mine of sandstone, breaking in the line of cleavage, transversely to the planes of deposition.f5 If you look at cleavage, as I do, as the result of chemical action or crystalline forces, superinduced in certain planes by their mechanical state of tension, then it is not surprising that some rocks shd yield more or less readily to the crystalline forces.f6
I think I shall write to Prof. Forbes of Edinburgh, with whom I corresponded on my laminate volcanic rock, to call his early attention to your paper.f7
Sincerely Yours | C. Darwin