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Letter 840

Darwin, C. R. to Chairman, Committee of Papers, Royal Society of London

16 Mar [1852]

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    Referee's report on paper by Daniel Sharpe ["On foliation and cleavage", Philos. Trans. R. Soc. Lond. 142 (1852): 445–62].

Transcription

Down Kent

March. 16th

Sir

In my opinion Mr Sharpe's paper is eligible for publication in the Philosophical Transactions. Its object is to show that the laminæ in the metamorphic schists follow the same laws with the cleavage planes in clay-slates. The subject has been little attended to; & I do not believe that it has hitherto been attempted to show on a map the range, as well as dip, of the foliation (whatever its theoretical origin may have been) over an extensive district.— I am on the other hand bound to state that owing to the obscurity of the structure in many parts, and to the presence (as admitted by the author) of many minor curvatures, I think further evidence is necessary, before the existence of the several great arches extending across Scotland can be accepted as at all certain. I should have hesitated in advising the publication of this paper, had not the places, in which a vertical & fan-like arrangement of the folia occur, been separately marked on the map; for such facts seem liable to little error:— Moreover, statements by Macculloch & other Scotch authors are used by Mr Sharpe, & these support his general inferences. If we consider in the author's sections, simply the fan-like layers, & neglect as doubtful, the broad intermediate districts with convoluted & less steeply inclined layers, we may, I think, see evidence of arrangement, distinct from ordinary stratification. I may here just allude to the observations by Humboldt & others, showing the uniformity of strike in the metamorphic schists in one given direction over surprisingly large regions in other parts of the world,—as for instance in New S. Wales, where the strike serves as a compass to people travelling through the woods,— I allude to this, as offering some proof that the subject is not of limited interest.—

With regard to Mr Sharpe's map, I must express a strong opinion (though fully admitting that the author in most cases is the best judge) that it would have been very much better, if not a single line had been inserted without some distinct authority: on account of the rather small scale, it might perhaps have been necessary to have extended the lines a little beyond the points observed, but some definite limit ought, in my opinion, to have been fixed on & stated. As the map now stands, it tells at once the authors views, but it is not possible to discover which lines, & how far along each line, are the result of actual observation, & how much has been conjecturally added. The author's attention might be called to one other point,—namely to his use of the term “arches or lines of elevation”, as applied to his arches of foliation; considering that the very object of the memoir is to show that the folia are not merely upturned layers of deposition in a metamorphosed condition, it appears unfortunate that this term, which at once calls before the mind beds folded by ordinary mechanical forces, should have been adopted, when axes of foliation or some other term might have easily been found, which would not have been confounded with ordinary lines of elevation, & yet would not have precluded the idea of the foliation having resulted indirectly from mechanical action,—as seems to be the Author's view.—

Notwithstanding the foregoing objections, from the novelty &, as it appears to me, importance of the subject, I am of decided opinion that Mr Sharpe's memoir ought to be published in the Transactions.

Apologising for the length of this Report | I remain, Sir, Your obedient servant | Charles Darwin
To the Chairman of | the Committee of Papers.

P.S. | The author ought to state whether his bearings have been corrected for variation; & a scale of colours should appear on the map, together with an explanation of the several lines used.

    Footnotes Add

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    f1 840.f1
    Sharpe 1852, which Daniel Sharpe had told CD he was writing for the Royal Society (letter to Daniel Sharpe, 16 October [1851]). The paper had been read at meetings of the Royal Society on 12 and 19 February 1852 (Abstracts of the papers communicated to the Royal Society of London 6 (1850–4): 152).
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    f2 840.f2
    John MacCulloch, physician and geologist, had been commissioned in 1826 to prepare a geological map of Scotland.
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    f3 840.f3
    CD cited Gardner 1840, Schomburgk 1842, and Humboldt 1814–29 in South America, p. 141. No location is given for Alexander von Humboldt's observation, but see Humboldt 1814–29, 4: 384–5, and 5: 394–8. CD's copy of this work is in the Darwin Library–CUL.
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    f4 840.f4
    In his explanation of the map (Sharpe 1852, pp. 460–1), Sharpe stated that: ‘The strike or direction of the foliation and cleavage across the surface is indicated by black lines … The dotted lines show the direction supposed to be followed, in unexamined districts, by the lines just explained. All the lines are laid down on the Map with more continuity and regularity than really exist; this error can only be fully corrected by a minute examination of the whole country.’
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    f5 840.f5
    The term ‘arches’ appears in the published version, but ‘lines of elevation’ has been omitted. Theoretical reconstructions are illustrated on Plate XXIII.
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    f6 840.f6
    Another report recommending that Sharpe's paper be published, dated 4 March 1852, was written by Adam Sedgwick. Sedgwick observed: ‘If the cleavage planes of the unequivocably stratified slates be parallel to the foliations of the crystalline slates, it seems to follow, of absolute necessity, that the foliations must themselves be due, not to planes of original & mechanical deposition, but to a subsequent crystalline action. This is the fundamental point of the paper; & the conclusion agrees with the views published by Mr Darwin’ (Royal Society (RR2: 224)).
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    f7 840.f7
    In his explanation of the map, Sharpe included a note detailing how the variation in the measurements of strike was corrected. He also explained the various lines used on the map. The map itself bears a key relating colour to rock type and this is also spelled out in the explanatory text (Sharpe 1852, pp. 460–1).
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