ia and Peru to the work of
Ignacio Domeyko on the geology of Chili. Several papers by this author
were published in the "Annales des Mines" between 1840 and 1869, also in
the "Comptes Rendus" of 1861, 1864, etc.) 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, would know. What you say
about the plications being steepest in the central and 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, and have been
brought into view by an enormous superincumbent mass having been
denuded. If a large and deep box were filled with layers of damp paper
or clay, and a blunt wedge was slowly driven up from beneath, would not
the layers above it and on both sides become greatly convoluted, whilst
those towards the top would 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 (page 232) which would be
worth your looking at, if you have ever felt perplexed, as I often was,
about the sources of volcanic rocks in mountain chains. You have stirred
up old memories, and at the risk of being a bore I should like to call
your attention to another point which formerly perplexed me much--viz.
the presence of basaltic dikes in most great granitic areas. I cannot
but think the explanation given at page 123 of my "Volcanic Islands" is
the true one. (495/5. On page 123 of the "Geological Observations on the
Volcanic Islands visited during the Voyage of H.M.S. 'Beagle,'" 1844,
Darwin quotes several instances of greenstone and basaltic dikes
intersecting granitic and allied metamorphic rocks. He suggests that
these dikes "have been 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.")

LETTER 496. TO VICTOR CARUS. Down, March 21st, 1876.

The very kind expressions in your letter have gratified me deeply.

I quite forget what I said about my geological works, but the papers
referred to in your letter are