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Letter 781

Darwin, C. R. to Forbes, J. D.

11 Oct [1844]

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    Discusses a specimen of Mexican obsidian with an unusual laminated structure.


Down near Bromley Kent

Oct. 11.


I venture to take the liberty of addressing you, knowing how much you are interested on the subject of your discovery on the zoned structure of glacier-ice. I have a specimen (from Mr Stokes' Collection) of Mexican obsidian, which judging from your description, must resemble to a considerable degree, the zoned ice. It is zoned with quite straight, parallel lines, like an agate, & these zones as far as I can see under the microscope, appear entirely due to the greater or lesser number of excessively minute flattened air-cavities. I cannot avoid suspecting that in this case, & in many others, in which lavas of the trachytic series (generally of very imperfect fluidity) are laminated in a very singular manner, that this structure is due to the stretching of the mass or stream during its movement, as in the ice-streams of glaciers.

It has occurred to me, that you possibly might like to see the specimen of obsidian & some curious, most finely, laminated obsidians & trachytic rocks, which I collected at Ascension island. You would not of course, I presume, think it worth the expence of carriage to have the specimens sent to Edinburgh; but shd you at any time come to London, I should be proud, if you so like, to send them for your inspection. If the subject of the lamination of volcanic rocks should interest you, I would venture to ask you to refer to p. 65–72 of my small volume, “Geolog: Obser: on Volcanic Isld” which would be in the Public Library. I there throw out the idea, that the structure in question may, perhaps, be explained by your views on the zoned structure of glacier-ice: the layers of less tension, I may add, being in the case of the Ascension Obsidian-rocks, rendered apparent chiefly by the crystalline & concretionary action superinduced in them, instead of as in your ice, by the congelation of water.

I hope you will excuse, should you feel no interest in this subject, the liberty I take in writing to you, & I beg to remain, your obedient servant | Charles Darwin

    Footnotes Add

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    f1 781.f1
    Forbes argued that the zoned structure of glacier ice was produced by tension resulting from gravitational force acting on the viscous ice. See letter to Adolf von Morlot, 9 August [1844], n. 4.
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    f2 781.f2
    The specimen from Charles Stokes's collection is discussed in Volcanic islands, pp. 67, 69–70.
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    f3 781.f3
    See Volcanic islands, pp. 70–1.
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    f4 781.f4
    The Public Library of the University of Edinburgh, i.e., the collection belonging to the whole University and open to all its members.
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    f5 781.f5
    Forbes had already made a comparison between glacier ice and lava, although coming to different conclusions from CD (J. D. Forbes 1844). See letter to J. D. Forbes, 13 [November 1844].
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    f6 781.f6
    Forbes published parts of this letter with a paragraph from another letter from CD (see letter to J. D. Forbes, [November? 1844]) in the Proceedings of the Royal Society of Edinburgh 2 (1845): 17–18.
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