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Letter 1016

Darwin, C. R. to Sharpe, Daniel

[1 Nov 1846]

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    Discusses foliation and cleavage. Comments on dip of cleavage laminae in mountains. Mentions views of Sedgwick and Studer. Suggests reading C. L. von Buch [Travels through Norway and Lapland (1813)] "as an amusement". Praises views of William Hopkins. Suggests reading paper by H. D. Rogers ["On cleavage of slate-strata", Edinburgh New Philos. J. 41 (1846): 422–3)]. Comments on the paper.

Transcription

Down Farnborough Kent

Sunday

My dear Sir

I have been much interested with your letter & am delighted that you have thought my few remarks, worth attention. My observations on foliation are more deserving confidence than those on cleavage; for during my first year in clay-slate countries I was quite unaware of there being any marked difference between cleavage & stratification; I well remember my astonishment at coming to the conclusion that they were totally different actions, & my delight at subsequently reading Sedgwicks views; hence at that time I was only just getting out of a mist. With respect to cleavage-laminæ dipping inwards, on mountain-flanks I have certainly often observed it, so often that I thought myself justified in propounding it as usual; I might perhaps have been some degree prejudiced by Von Buch's remarks, for which in those days I had a somewhat greater deference than I now have.— The mount at M. Video (p. 146 of my Book) is certainly an instance of the cleavage laminæ of an hornblendic schist dipping inwards on both sides; for I examined this hill carefully with compass in hand & note book. I entirely admit, however, that a conclusion drawn from striking a rough balance in one's mind, is worth nothing, compared with the evidence drawn from one continuous line of section. I read Studer's Papers carefully & drew the conclusion stated, from it; but I may very likely be in an error. I only state that I have “frequently” seen cleavage-laminæ dipping inwards on mountain-sides; that I cannot give up, but I daresay a general extension of the rule, (as might justly be inferred from the manner of my statement) would be quite erroneous.

Von Buch's statement is in his Travels in Norway; I have unfortunately lost the reference & it is a high crime, I confess, ever to refer to an opinion, without a precise reference. If you never read these Travels, they might be worth skimming, chiefly as an amusement; & if you like, & will send me a line by the Gen. Post on Monday or Tuesday, I will either send it up with Hopkins on Wednesday, or bring it myself to Geolog. Soc. I am very glad you are going to read Hopkins; his views appear to me eminently worth well comprehending: false views & language appear to me to be almost universally held by geologists on the formation of fissures, dikes & mountain-chains. If you would have the patience, I shd be glad if you wd read in my “Volcanic Islands” from page 65, or even 54 to 72, viz on the lamination of volcanic rocks: I may add that I sent the series of specimens there described to Prof. Forbes of Edinburgh, & he thought they bore out my views.

There is a short extract from Prof. Rogers' in the last Eding. New Phil. Journ. well worth your attention on the cleavage of the Appalachian Chain, & which seems far more uniform in direction of dip, than in any case, which I have met with: the Rogers' doctrine of the ridges being thrown up by great waves I believe is monstrous; but the manner in which the ridges have been thrown over (as if by a lateral force acting on one side on a higher level than on the other) very curious, & he now states that the cleavage is <parallel> to the axis-plane 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.

Believe me Yours very sincerely | C. Darwin

    Footnotes Add

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    f1 1016.f1
    See South America, pp. 162–8. In a paper read to the Geological Society on 2 December 1846, Sharpe referred to CD's views (see Sharpe 1847, p. 105). During the Beagle voyage, and afterwards while he was working on the geology of South America, CD summarized his observations on cleavage and stratification, see DAR 41.
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    f2 1016.f2
    Sedgwick 1835, especially pp. 469–75.
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    f3 1016.f3
    Buch 1813. A copy is in the Darwin Library–CUL.
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    f4 1016.f4
    Studer 1842. In South America (p. 164 n.) CD refers to only this paper by Bernhard Studer. The source is erroneously given as volume twenty-three instead of volume thirty-three of the Edinburgh New Philosophical Journal (April–October 1842).
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    f5 1016.f5
    CD's discussion of inwardly dipping cleavage planes, in which he cited Buch 1813, p. 169, and Studer 1842, is in South America, p. 164. CD's statement there is slightly different: ‘On the flanks of the mountains both in Tierra del Fuego and in other countries, I have observed that the cleavage-planes frequently dip at a high angle inwards’.
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    f6 1016.f6
    Sharpe 1847, p. 100, cites three publications by William Hopkins: Hopkins 1836, 1838, and 1845–56. It is probably Hopkins 1838 that is referred to here.
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    f7 1016.f7
    See letters to J. D. Forbes, 11 October [1844] and 13 [November 1844].
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    f8 1016.f8
    H. D. Rogers 1846.
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    f9 1016.f9
    The word ‘parallel’ has been added in pencil over a repair to the manuscript. It is not in CD's hand.
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    f10 1016.f10
    Described in Sharpe 1847, pp. 75–87. CD cited Sharpe on this point in his article on geology (Collected papers 1: 237) for the Admiralty Manual of scientific enquiry (Herschel ed. 1849): Fossil shells have been found by Mr. Sharpe in slaty rocks, which have had their shapes greatly altered, and all in the same direction; here then we have a guide to judge of the amount and direction of the mechanical displacement which the surrounding slate-rocks have undergone.
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