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Letter 993

Darwin, C. R. to Horner, Leonard

[17 Aug – 7 Sept 1846]

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    Discusses proposed survey of Glen Roy. Mentions Glen Roy theories of Agassiz and William Buckland. Includes a memorandum calling for a careful survey of the parallel roads of Glen Roy. Mentions M. A. Bravais ["On the lines of ancient level of the sea in Finmark", Q. J. Geol. Soc. Lond. 1 (1845): 534].

Transcription

Down Farnborough Kent

Monday

My dear Mr Horner

In following your suggestion in drawing out something about Glen Roy for the Geolog. Committee, I have been completely puzzled how to do it.— I have written down what I shd say, if I had to meet the head of the Survey & wished to persuade him to undertake the task, but as I have written it, it is too long, ill expressed, seems as if it came from nobody & was going to nobody, & therefore I send it to you in despair, & beg you to turn the subject in your mind.—

I feel a conviction if it goes through the Geolog. Part of Ordnance Survey, it will be swamped, & as it is a case for more accurate measurements, it might, I think without offence, go to the head of the real Surveyors.

If Agassiz or Buckland are on the Committee, they will sneer at whole thing & declare the beaches are those of a glacier lake,—than which I am sure I cd convince you, that there never was a more futile theory.

I look forward to Southampton with much interest & hope to hear tomorrow, that the lodgings are secured to us.—

You cannot think how thoroughily I enjoyed our geological talks & the pleasure of seeing Mrs Horner & yourself here.

Ever your obliged | C. Darwin

[Enclosure]

The Parallel Roads of Glen Roy in Scotland have been the object of repeated examination, but they have never hitherto been levelled with sufficient accuracy. Sir T. Lauder Dick procured the assistance of an engineer for this purpose, but owing to the want of a true ground-plan, it was impossible to ascertain their exact curvature, which, as far as could be estimated, appeared equal to that of the surface of the sea. Considering how very rarely the sea has left narrow & well-defined marks of its action at any considerable height on the land, & more especially considering the remarkable observations by M. Bravais on the ancient sea-beaches of Scandinavia, shewing that they are not strictly parallel to each other & that the movement has been greater nearer the mountains than on the coast,—it appears highly desirable, that the roads of Glen Roy should be examined with the utmost care during the execution of the Ordnance Survey of Scotland. The best instruments & the most accurate measurements being necessary for this end, almost precludes the hope of its being ever undertaken by private individuals; but by the means at the disposal of the Ordnance, measurements would be easily made even more accurate than those of M. Bravais.

It would be desirable to take two lines of the greatest possible length in the district, and at nearly right angles to each other, and to level from the beach at one extremity to that at the other, so that it might be ascertained, whether the curvature does exactly correspond with that of the globe, or if not, what is the direction of the line of greatest elevation. Much attention would be requisite in fixing on either the upper or lower edge of the ancient beaches, as the standard of measurement, & in rendering this line conspicuous.— The heights of the three roads, one above the other & above the level of the sea, ought to be accurately ascertained. Mr Darwin observed one short beach-line North of Glen Roy & he has indicated on the authority of Sir David Brewster, others in the valley of the Spey; if these could be accurately connected, by careful measurements of their absolute heights or by levelling, with those of Glen Roy, it would make a most valuable addition to our knowledge on this subject. Although the observations here specified would probably be laborious, yet considering how rarely such evidence is afforded in any quarter of the world, it cannot be doubted that one of the most important problems in geology,—namely the exact manner in which the crust of the earth rises in mass,—would be much elucidated, & a great service done to Geological Science.

    Footnotes Add

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    f1 993.f1
    The earlier date is based on CD's reference to the Horners' visit to Down following his return from Shrewsbury on 9 August (see letter to J. D. Hooker, [3 September 1846]). The closing date is the last Monday before the British Association meeting in Southampton.
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    f2 993.f2
    A reference to the sectional committee for geology (section C) of the British Association. Leonard Horner was president of the section at the Southampton meeting.
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    f3 993.f3
    Thomas Frederick Colby.
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    f4 993.f4
    CD's memorandum was acted upon at the British Association meeting in Southampton in September 1846. The Association recommended that ‘the so-called parallel roads of Glen Roy and the adjoining country be accurately surveyed, with the view of determining whether they are truly parallel and horizontal, the intervening distances, and their elevations above the present Sea-level’ (Report of the 16th meeting of the British Association for the Advancement of Science held at Southampton in 1846, p. xix). The survey was undertaken and the results were published in 1874 (see ML 2: 174 n.). By that time, however, the problem of the origin of the roads had been settled by other evidence in favour of the glacial lake theory (see Rudwick 1974, pp. 147–53).
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    f5 993.f5
    Rudwick 1974 describes the debate over the origin of the roads. In the Autobiography, p. 84, CD recalls his explanation as a ‘great failure’ .
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    f6 993.f6
    The original enclosure has not been found; the text printed here is taken from a copy in DAR 145.
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    f7 993.f7
    Dick 1823.
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    f8 993.f8
    Bravais 1845.
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