Letter icon
Letter 1083

Darwin, C. R. to Sharpe, Daniel

[23 Jan 1847]

    Summary Add

  • +

    Comments on manuscript [? "On slaty cleavage", J. Geol. Soc. Lond. 5 (1849): 111–29]. Discusses phenomenon of cleavage. Will write to J. D. Forbes about DS's paper.

Transcription

Down Farnborough Kent

Saturday

Dear Sharpe

I am very much obliged for the M.S. which I return. I do not quite understand from your note, whether you have struck out all on this point in your paper, I much hope not; if you have, allow me to urge on you to append a note, briefly stating the facts, & that you omitted them in your paper from the observations not being finished.

I am strongly tempted to suspect that the cleavage planes will be proved by you to have slided a little over each other, & to have been planes of incipient tearing to use Forbe's expression in ice: it will in that case be beautifully analogical with my laminated lavas & these in composition are intimately connected with the metamorphic schists.—

The beds without cleavage between those with cleavage, do not weigh quite so heavily on me, as on you— you remember, of course, Sedgwick facts of limestone & mine of sandstone, breaking in the line of cleavage, transversely to the planes of deposition. If you look at cleavage, as I do, as the result of chemical action or crystalline forces, superinduced in certain planes by their mechanical state of tension, then it is not surprising that some rocks shd yield more or less readily to the crystalline forces.

I think I shall write to Prof. Forbes of Edinburgh, with whom I corresponded on my laminate volcanic rock, to call his early attention to your paper.

Sincerely Yours | C. Darwin

    Footnotes Add

  • +
    f1 1083.f1
    Later published as Sharpe 1847. See letter to Daniel Sharpe, [19 January 1847], n. 4.
  • +
    f2 1083.f2
    CD may be referring to a section in Sharpe's paper describing the ‘crystalline matter which is frequently found between the planes of cleavage’ (Sharpe 1847, p. 99), a phenomenon which CD believed important in showing that the cleavage planes had gaped open after the initial pressure was relieved (South America, pp. 152, 160, 163). Sharpe appended a note indicating that had he known of this phenomenon, he would have sought for similar instances in Caernarvonshire.
  • +
    f3 1083.f3
    See Volcanic islands, pp. 65–72. CD had been struck by James David Forbes's explanation that the laminated and fissured appearance of glaciers resulted from viscous stretching as they flowed (J. D. Forbes 1842). He suggested that the lamination of some volcanic rocks could be explained similarly.
  • +
    f4 1083.f4
    Sedgwick 1835.
  • +
    f5 1083.f5
    Described in CD's paper ‘On the geology of the Falkland Islands’ (Collected papers 1: 206).
  • +
    f6 1083.f6
    Sharpe did not think that cleavage was related to crystallisation (see Sharpe 1849, p. 116).
  • +
    f7 1083.f7
    No such letter has been located. For CD's previous correspondence with James David Forbes see Correspondence vol. 3, letters to J. D. Forbes, 11 October [1844], [November? 1844], and 13 [November 1844].
Maximized view Print letter