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Letter 595

Darwin, C. R. to Lyell, Charles

[12? Mar 1841]

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    Discusses at length Louis Agassiz's book [Études sur les glaciers (1840)] and Agassiz's explanation of moraines. Defends his own theory of the importance of floating ice. Relates glacier theory to his own interpretation of Glen Roy.

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    Mentions a paper he is writing on South American boulders and till [Collected papers 1: 145–63].

Transcription

[12 Upper Gower Street]

My dear Lyell

Your extract has set me puzzling me very much & as I find I am better at present for not going out, you must let me unload my mind on paper.— I thought everything so beautifully clear about Glaciers, but now your case & Agassiz's statement about the cavities in the rock formed by cascades in the glaciers shows me, I don't understand their structure at all— I wish out of pure curiosity I could make it out.—

If the glacier travelled on, (& it certainly does travel on,) & the water kept cutting back, over the edge of the ice there would be a great slit in front of ye cascade— if the water did not cut back the whole hollow & cascade, as you say, must travel on; & do you suppose the next season it falls down into some new crevice higher up?— In any case how in the name of Heaven can it make hollow in solid rock, which surely must be work of many years?— I must point out, another fact which Agassiz does not, as it appears to me leave very clear: he says all the blocks on surface of the glaciers are angular, & those in moraines rounded, yet he says the medial moraines whence the surface blocks come & are a part are only two lateral moraines united.— Does he refer to terminal moraines, alone when he says fragments in moraines are rounded?—

What a capital book Agassiz's is— in all the early part I gave up entirely the Jura blocks & was heartily ashamed of my appendix (& am so still of the manner in which I presumptuously speak of Agassiz) but it seems by his own confession that ordinary glaciers could not have transported the blocks there & if an hypothesis is to be introduced, the sea is much simpler.— floating ice seems to me to account for every thing as well as, and somethings better, than the solid glaciers.— The hollows, however, formed by the ice-cascades, appear to me strongest hostile fact,—though certainly, as you said, one sees hollow round cavities, on present rock-beaches.—

I am glad to observe, that Agassiz does not pretend that direction of scratches is hostile to floating ice.— By the way how do you & Buckland account for the “tails of diluvium” in Scotland? I thought in my appendix this made out strongest argument for rocks having been scratched by floating ice?—

Some facts about boulders in Chiloe, will I think in a very small degree elucidate some parts of Jura case— What a grand new feature all this ice-work in Geology— how old Hutton would have stared!—

I ought to be ashamed of myself for scribbling on so— Talking of shame!, I have sent a copy of my Journal with very humble note to Agassiz—as an apology for tone I used, though I say I daresay he has never seen my appendix, or would care at all about it—

I did not suppose my note about Glen Roy could have been of any use to you.— I merely scribbled what came uppermost.— I made one great oversight, as you would perceive.— I forgot, that glacier-theory, if glacier most gradually disappeared from mouth of Spean-valley, would account for buttresses of shingle below lowest shelf— The difficulty, I put about ice -barrier of middle Glen-Roy shelf keeping so long at exactly same level does certainly appear to me insuperable— I wish you had in your mind's eye the quantity of solid rock removed on this beach.

What a wonderful fact this break down of old Niagara is— how it disturbs all the calculations about lengths of time before river would have reached lakes.

I hope Mrs Lyell will read this to you, then I shall trust for forgiveness for having scribbled so much— I should have sent back Agassiz sooner, but my servant has been very unwell— Emma is going on pretty well.—

My paper on S. American boulders & “till”, which latter deposit is perfectly characterized in Tierra del Fuego is progressing rapidly—

Farewell | Ever yours | C. Darwin
Friday

I much like the term post-pliocene & will use it in my present paper several times.—

P.S. I should have thought that the most obvious objection to marine-beach theory for Glen Roy, would be the limited extension of the shelves— Though, certainly this is not a valid one, after the existence of an intermediate one, only half a mile in length & no where else appearing even in the valley of Glen Roy itself, has been shown to exist.

    Footnotes Add

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    f1 595.f1
    The letter is dated with reference to letter to Louis Agassiz, 1 March [1841], and letter to Charles Lyell, [9 March 1841], which preceded it; 12 March 1841 was the Friday following 9 March.
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    f2 595.f2
    Evidently Lyell's description of the manner in which falling water erodes the ledge of Niagara Falls (C. Lyell 1840, 1: 341–3). See also n. 11, below.
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    f3 595.f3
    Agassiz had commented that waterfalls that keep their place for some time eventually create cavities in the rock below the crevices through which they fall (L. Agassiz 1840, pp. 199–200). Yet Lyell's account led CD to doubt whether such falls could in fact remain in one place for long.
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    f4 595.f4
    L. Agassiz 1840, p. 100.
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    f5 595.f5
    Agassiz had argued that the Alps were raised in a great catastrophic action, breaking through the ice sheet that covered all of northern Europe, and sending large rocks sliding down the inclined ice across the present valley of Switzerland to rest on the site of the Jura. These were deposited on the Jura as the earth warmed and the ice melted. In CD's view the present valley of Switzerland was formerly submerged and icebergs, broken off from Alpine glaciers, carried the blocks. See L. Agassiz 1838a and 1838b; Journal of researches, pp. 617–25; and L. Agassiz 1840, pp. 304–6, 315–18. For Agassiz's glacial theory and its reception in Britain, see Davies 1969, pp. 273–94, and Rudwick 1969.
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    f6 595.f6
    Agassiz asserted that blocks pushed by ordinary glaciers would be much rounder and smaller than the angular boulders that appear in the Jura (L. Agassiz 1838a, 1838b, and 1840, pp. 299–300).
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    f7 595.f7
    The ‘tails’ consist of erratic boulders imbedded in clay and marked by parallel striations. To CD the striations indicated that the boulders had not been rolled along by a glacier, but were held fast by floating ice (Journal of researches, pp. 622–5).
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    f8 595.f8
    James Hutton would not, in fact, have stared. Though he was known for emphasising volcanic effects, Hutton was also aware of glaciers and even suggested them as a possible means of transport for erratic boulders (Hutton 1795, 2: 218 and Playfair 1802, pp. 388–9).
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    f9 595.f9
    See letter to Louis Agassiz, 1 March [1841].
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    f10 595.f10
    See letter to Charles Lyell, [9 March 1841].
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    f11 595.f11
    Presumably a reference to estimates by Robert Bakewell and others concerning the time required to excavate the ravine of the Niagara below the falls (see C. Lyell 1840, 1: 343–5).
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    f12 595.f12
    ‘On the distribution of the erratic boulders and on the contemporaneous unstratified deposits of South America’, Collected papers 1: 145–63. The paper was completed on 4 April 1841 (‘Journal’; Correspondence vol. 2, Appendix II) and read on 4 May 1841.
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