Falmouth | May 7/57.
My dear Tyndall,
I have been thinking of writing to you for some time, but one thing or another has prevented it. Your kind notes1 were forwarded to me here,2 and I only got them yesterday on my arrival. Since leaving home I have been able to make out several new facts respecting slaty cleavage, which I will describe to you when I am in London, which will I hope be about the 20th inst. One that is interesting as explaining the connexion of two well known facts, is this. You know that in some cases, when a bed of sandy rock has been enclosed between more yielding material, it has been much contorted, in the way shown in my fig. which you copied in your paper.3 The other is that in other cases we have no contortion, but the cleavage in the firm grained beds lies more in the plane of stratification than in the sandy, thus [image here] Now I find that these are related thus,4 so that when the general line of the beds is nearly perpendicular to the cleavage i:e: in the line of the pressure, since one yielded more than the other it became contorted, whereas where the line of pressure was inclined at an angle to the beds, those that could yield most, yielded this extra in the plane of stratification. Hence their cleavage lies more nearly in it than in the more unyielding, coarser beds, as I have described in my papers,5 and then no such contortions, as in the other case, are produced. I find that many of the so called clay slate rocks of Cornwall6 are more correctly very fine grained mica schist,7 and that their cleavage is not like that of the Welsh slates, but simply what I have described to you as ‘crumple joints.’ There has been very little or no compression of the rocks—at least nothing like what occurs in Wales—they are merely broken up by more or less close jointing, as if disturbed after having been converted into fine grained mica schist, when they could not yield as plastic substances.
Thanking you most sincerely for all the trouble you have taken on my behalf, and looking forward to the pleasure of seeing you soon
I remain | yours very truly | Henry Clifton Sorby.
RI MS JT/1/TYP/4/1352
LT Typescript Only
Your kind notes: letters missing.
here: Sorby was pursusing geological fieldwork in Cornwall. See letter 1363.
my fig. which you copied in your paper: Sorby’s figure appears in H. C. Sorby, ‘On Slaty Cleavage, as exhibited in the Devonian Limestones of Devonshire’, Phil. Mag., 11 (1856), pp. 20–37, on p. 34. Tyndall’s similar figure is in J. Tyndall, ‘Comparative View of the Cleavage of Crystals and Slate Rocks’, Phil. Mag., 12 (1856), pp. 35–48, on p. 41.
are related thus: at this point in the typescript LT noted there was a diagram in the original letter, but the original letter is missing.
as I have described in my papers: see H. C. Sorby, ‘On Slaty Cleavage, as exhibited in the Devonian Limestones of Devonshire’; and H. C. Sorby, ‘On the Theory of the Origin of Slaty Cleavage’, Phil. Trans., 12 (1856), pp. 127–9.
Cornwall: see letter 1363, n. 4.
mica schist: see letter 1344, n. 3.
Please cite as “Tyndall1392,” in Ɛpsilon: The John Tyndall Collection accessed on