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 w^d 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 w^d no doubt find a reference;³ but I think the sections w^d 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 w^d 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, w^d not the layers above it & on both sides become greatly convoluted, whilst those towards the top w^d 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 w^d 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 sh^d like to call y^r 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