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Letter 590

Darwin, C. R. to Lyell, Charles

[21 Feb – 4 Apr 1841]

    Summary Add

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    Answers a number of queries from Lyell concerning geography and geology of Chiloé Island and its relationship to the Cordilleras.

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    Asks about "perched rocks" on Jura and notes their relevance to Louis Agassiz's theory. Discusses Agassiz's view on Jura.

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    Mentions seeing Robert Brown.

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    Notes R. I. Murchison's discovery of shells in central England.

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    Weakness of negative evidence.

Transcription

My dear Lyell

I will write down my answers as I read Chiloe is 100 miles long. (I believe I said 90 in Journal )— Channel between Chiloe & Cordillera, about 25 miles wide, with some isld— Soundings were carried across only in one place in line of islets where greatest depth was 95 fathoms— Some highest points in Chiloe were estimated at 3000 ft—general heigt much less In mentioning blocks on Chiloe put granite first, because I know more certainly that syenite came from Cordillera so that your sentence will be more accurate.

If you choose you may add that the blocks are strewed on shores of islets & in narrow creeks on coast of Chiloe, where there must formerly have been channels, which after hypothetical elevation would correspond with those of Agassiz in interior valleys of Jura— I give these facts in paper which I hope to send very soon to Soc.

Chiloe consists of stratified shingle & boulder formation & horizontal strata of tertiary sandstone & volcanic grit—with two volcanic regions & West coast of Mica Slate

There are no fossils with Chiloe boulder formation—only on them—

Are you sure there are perched rocks on Jura ? My impression (I am not up to looking so I return Agassiz) there are not—that the surface is rather uniform.— The perched rocks if on pinnacles would be to my mind fearful argument for Agassiz's sheet of ice.—

Do not you think it worth stating that Agassiz himself seems to consider the angularity of Jura fragments a difficulty on ordinary moraine or glacier action—

If I were you, I think, I would not give hypothetical union of the two kinds of action, but would leave the alternative cases, as you have put them—though I daresay your view is very probable.

Yours most truly | C. Darwin

Excuse this untidy note: my talk with R. Brown after that with you has knocked me up a little

    Footnotes Add

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    f1 590.f1
    According to his ‘Journal’ (Correspondence vol. 2, Appendix II), CD was writing ‘On the distribution of the erratic boulders and on the contemporaneous unstratified deposits of South America’ (Collected papers 1: 145–63) between these dates. The erratics of Chiloé, mentioned here, are discussed on pp. 160–1.
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    f2 590.f2
    Journal of researches, p. 333.
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    f3 590.f3
    A comment on the manuscript of C. Lyell 1841. Lyell was attempting to draw a parallel between erratic blocks of Andean origin, which CD found on Chiloé, and Jura erratics that had come from the Alps, suggesting that the Jura erratics had been floated across a narrow intervening sea on icebergs. Since some of the blocks on Chiloé were granite and others syenite, Lyell concluded that they had come from different Andean sites (C. Lyell 1841, 1: 250–3).
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    f4 590.f4
    For Agassiz's view of the Jura erratics see letter to Charles Lyell, [12 March 1841], n. 5.
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    f5 590.f5
    On the verso of this sheet, CD added and then deleted: ‘I have brought my mind to neglect all negative evidence, especially absence of shells— Who would have anticipated Murchison's few [interl] shells in centre of England’. This was a reference to shells discovered by Murchison in drift deposits ‘in various parts of Shropshire, Cheshire and Staffordshire, likewise between Dudley and the Severn’, which were identified as living species. Murchison argued that the deposits had been laid down when these areas were covered by an arm of the sea (Murchison 1839, p. 532; also Murchison 1838). If the Chiloé erratic blocks were marine depositions, geologists would expect to find fossils among them. Lyell faced a similar problem in trying to explain the erratics of the Jura in Switzerland (C. Lyell 1841, 1: 252–3). CD also cited Murchison's discovery in accounting for the absence of fossil shells at Glen Roy (Collected papers 1: 112–14).
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    f6 590.f6
    A note (in an unidentified hand) opposite CD's question reads: ‘Vide p. 268 Agassiz [1840] on perched rocks of the Alps in contradistinction to those of Jura??’. Perched blocks could hardly have been transported by icebergs.
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    f7 590.f7
    See letter to Charles Lyell, [12 March 1841], n. 6.
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    f8 590.f8
    That is, glaciers and icebergs.
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