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Letter from Charles Robert Darwin to Charles Lyell 20 May 1869
Down. | Kent. S.E.
May 20 1869
My dear Lyell
I have been much pleased to hear that you have been looking at my S. American book, which I thought was as completely dead & gone as any pre-Cambrian fossil.1 You are right in supposing that my memory about American geology has grown very hazy. I remember however a paper on the Cordillera by D. Forbes, with splendid sections, which I saw in M.S., but whether referred to me or lent to me, I cannot remember. This wd be well worth your looking to, as I think he both supports & criticizes my views.2 In Ormerod’s index to the Journal, which I do not possess, you wd no doubt find a reference;3 but I think the sections wd be worth borrowing from Forbes.
Domeyko has published in the Comptes Rendus papers on Chili, but not, as far as I can remember, on the structure of the mountains; Forbes however wd know.4 What you say about the plications being steepest in the central & generally highest part of the range is conclusive to my mind that there has been the chief axis of disturbance.
The lateral thrusting has always appeared to me fearfully perplexing. I remember formerly thinking that all lateral flexures probably occurred deep beneath the surface, & have been brought into view by an enormous superincumbent mass having been denuded. If a large & deep box were filled with layers of damp paper or clay, & a blunt wedge was slowly driven up from beneath, wd not the layers above it & on both sides become greatly convoluted, whilst those towards the top wd be only slightly arched?
When I spoke of the Andes being comparatively recent, I suppose that I referred to the absence of the older formations. In looking to my volume, which I have not done for many years, I came upon a passage (p. 232) which wd be worth your looking at if you have ever felt perplexed, as I often was, about the sources of volcanic rocks in mountain chains.5
You have stirred up old memories, & at the risk of being a bore, I shd like to call yr attention to another point which formerly perplexed me much, viz. the presence of basaltic dykes in most great granitic areas; I cannot but think the explanation given at p. 123 of my Volcanic I’s is the true one.6
My dear Lyell | Yours affectionately | C. Darwin
2.4 chief] interl in CD’s hand
6.1 My … affectionately] in CD’s hand
1
Lyell’s discussion of South America may have been in the now missing section of his letter of 5 May 1869.
2
CD refers to ‘On the geology of Bolivia and southern Peru’ by David Forbes (Forbes 1860). For Forbes’s references to CD, see Forbes 1860, pp. 10, 11, and 29.
3
George Wareing Ormerod’s revised Classified index to the Transactions, Proceedings, and Quarterly Journal of the Geological Society of London was published in February 1870. The first edition appeared in 1858 (Woodward 1907, pp. 205–6).
4
Ignacy Domeyko published ‘Mémoire sur la constitution géologique de Chile’ in Annales des Mines (Domeyko 1846).
5
CD had written that it was difficult to trace the streams of porphyries to their eruptive sources and conjectured that it was because some original points of eruption tended to become points of injection (South America, p. 232).
6
CD hypothesised that the dikes were formed by ‘fissures penetrating into partially cooled rocks of the granitic and metamorphic series, and by their more fluid parts, consisting chiefly of hornblende, oozing out, and being sucked into such fissures’ (Volcanic islands, p. 123).