The so-called beds in the metamorphic schists, so unlike common cleavage laminae, seems the best, or at least one argument against such a suspicion. Yet I think it is a point deserving your notice. Have you thought at all over Rogers' Law, as he reiterates it, of cleavage being parallel to his axes-planes of elevation?
If you know beforehand, will you tell me when your paper is read, for the chance of my being able to attend? I very seldom leave home, as I find perfect quietude suits my health best.
(PLATE: CHARLES DARWIN, Cir. 1854. Maull & Fox, photo. Walker & Cockerell, ph. sc.)
LETTER 540. TO C. LYELL. Down, January 10th, 1855.
I received your letter yesterday, but was unable to answer it, as I had to go out at once on business of importance. I am very glad that you are reconsidering the subject of foliation; I have just read over what I have written on the subject, and admire it very much, and abide by it all. (540/1. "Geological Observations on South America," Chapter VI., 1846.) You will not readily believe how closely I attended to the subject, and in how many and wide areas I verified my remarks. I see I have put pretty strongly the mechanical view of origin; but I might even then, but was afraid, have put my belief stronger. Unfortunately I have not D. Sharpe's paper here to look over, but I think his chief points [are] (1) the foliation forming great symmetrical curves, and (2) the proof from effects of form of shell (540/2. This refers to the distortion of shells in cleaved rocks.) of the mechanical action in cleaved rocks. The great curvature would be, I think, a grand discovery of Sharpe's, but I confess there is some want of minuteness in the statement of Sharpe which makes me wish to see his facts confirmed. That the foliation and cleavage are parts of curves I am quite prepared, from what I have seen, to believe; but the simplicity and grandeur of Sharpe's curves rather stagger me. I feel deeply convinced that when (and I and Sharpe have seen several most striking and obvious examples) great neighbouring or alternating regions of true metamorphic schists and clay-slate have their foliations and cleavage parallel, there is no way of escaping the conclusion, that the layers of pure quartz, feldspar, mica, chlorite, etc., etc., are due not to original deposition, but to segregation; and this is I consider the point which I have established. This is very odd, but I suspect that great metamorphic areas are generally derived from the metamorphosis of clay-slate, and not from alternating layers of ordinary sedimentary matter. I think you have exactly put the chief difficulty in its strongest light--viz. what would be the result of pure or nearly pure layers of very different mineralogical composition being metamorphosed? I believe even such might be converted into an ordinary varying mass of metamorphic schists. I am certain of the correctness of my account of patches of chlorite schists enclosed in other schist, and of enormous quartzose veins of segregation being absolutely continuous and contemporaneous with the folia of quartz, and such, I think, might be the result of the folia crossing a true stratum of quartz. I think my description of the wonderful and beautiful laminated volcanic rocks at Ascension would be worth your looking at. (540/3. "Geological Observations on S. America," pages 166, 167; also "Geological Observations on the Volcanic Islands," Chapter III. (Ascension), 1844.)
LETTER 541. TO C. LYELL. Down, January 14th [1855].
We were yesterday and the day before house-hunting, so I could not answer your letter. I hope we have succeeded in a house, after infinite trouble, but am not sure, in York Place, Baker Street.
I do not doubt that I either read or heard from Sharpe about the Grampians; otherwise from my own old suspicion I should not have inserted the passage in the manual.
The laminated rocks at Ascension are described at page 54. (541/1. "Volcanic Islands," page 54. "Singular laminated beds alternating with and passing into obsidian.")
As far as my experience has gone, I should speak only of clay-slate being associated with mica-slate, for when near the metamorphic schists I have found stratification so gone that I should not dare to speak of them as overlying them.