From Henry Clifton Sorby   Sep. 1/56

Broomfield | Sheffield, Sep. 1/56.

My dear Sir,

I was very sorry you were not at Cheltenham,¹ for I wished particularly to see you, and took with me a number of specimens and microscope objects on purpose to show to you. However as that could not be, I must content myself with thus writing. In the first place I must beg to thank you for the plumbago which I got all safe, and also the copy of your paper.²

I fear we have been calling by the term ‘slaty cleavage’ different things. In my paper on the microscopical structure of mica-schist, read at Cheltenham,³ I described a structure which is seen to the greatest perfection in that of the Loch Lomond district.⁴ For this I proposed the term ‘crumple or contortion joints.’ If the rock was of such a nature as not to yield as clay, but was a metamorphosed schist, and was compressed laterally, sharp contortions of small size are produced thus

If the force was not exactly evenly along the beds there is a lateral movement of parts with or without actual small breaks thus –

In some cases these joints are ½ inch apart, but in others only 1/100, and then it appears very much like cleavage. However I have not called it so, because you cannot cleave between joints – you can only fracture along them. It is not an ultimate structure, as in genuine solid slates, but only finite, like joints; though it may be on a very small scale. This structure explains the double cleavage of such slate as that at Shap,⁵ in which one is of the ordinary ultimate kind, and the other is like that just described, in which the joints are often only 1/4000 of an inch apart. None of the beds nearly coincide with such a structure, there occurs exactly what you suggest in your last paper. The various parts do not move over one another with the same velocity but slip over each other and produce what I have always classed with [word illeg] sided joints, running nearly in the plane of cleavage. Such indeed to a small extent exist in very many slates, and must materially influence its splitting; but in my paper I have not alluded to this, since my object was to explain that ultimate structure which is found in the slates and which I have defined to myself as genuine slaty cleavage, as distinct from close jointing. Now you will see from this that if by pressure of any substance you gave rise to a close system of joints, I should demur to its being called cleavage; though it might indeed cause its material to cleave up into very thin laminæ. I may be scarcely correct in your opinion in doing this, but the microscopical structure in the two cases is so different, that some difference in times is absolutely requisite. You will understand me better by this drawing,

where we have for instance close joints at (a a) and you could not cleave the material in the intermediate space, whilst there is a structural arrangement of the ultimate particles in quite a different direction as you see drawn, along which you could cleave.

Thinking if you did not thus understand my views respecting what I would define as cleavage and close jointing, we might be led in our publications to call by the same term things that are different, I thought it best to write thus to you: for it would give me much pain if anything should occur to make us squabble about mere words. I wish you could contrive to come and see me some time or other, and inspect my collection to illustrate chemical and physical geology. I do not know whether you ever go into the country to lecture; but, if you would, I think I could arrange for you to deliver a course before our Sheffield Society.⁶ If you would will you inform me.

Trusting you are well and enjoyed your trip

I remain | Yours very truly | Henry Clifton Sorby.

RI MS JT/1/4/1345–6.

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