To Ernst Dieffenbach¹   9 February [1847]

My dear Sir

I have taken a long time to thank you for your long & most interesting letter of the 5^th of last month, but I wished to read it again, before writing, & I lent it to M^r. Lyell who returned it only a few days since.— I fe⁠⟨⁠lt⁠⟩⁠ sure that you would excuse my sending it to Lyell; he was deeply interested in your clear exhibition of Prof. Bunsen’s views & facts on Iceland,² though they were “sour grapes” to him, as he had just printed off in a new & seventh Edition of the Principles³ all the volcanic chapters. He would, otherwise, have much liked to have quoted Bunsen’s views.—

I have never before heard of Pelagonite⁴ which you refer to as a hydrated tuff; I cannot but suspect it is the same, as the rock which I have described in some detail in my Volcanic Island volume⁵ (p 99, 100) at the Galapagos, which I thought new: I was induced to think that it was produced by the action of water on particles of Scoriæ. If Prof. Bunsen compares his Geolog. observations with those of others, I wish you would point out this page to him. In several parts of the Cordillera (⁠⟨⁠s⁠⟩⁠ee index to my S. American volume⁶ ) I found very thin & uneven layers of black pitchstone, often almost composed of angular concretionary masses, & which appeared to me to have certainly been in origin of a sedimentary nature; this has always appeared to me strange, seeing how clearly igneous, pitchstone generally is. What a grand mass of observations Bunsen seems to have made; I am glad he is entering on Amygdaloids; I believe I saw cases in the Uspallata range,⁷ where lavas flowing over tuffs had produced amygdaloids, which at the time suprised me much, as I had been accustomed to connect amygdaloids exclusively with true molten rock.

I am much obliged by your informing me that Von Dechen had not received his copy; I have made enquiries, & am assured that it went through a safe channel, viz Williams & Norgate & I hope it will yet arrive: I trust that you have received your copy. You will have heard that a translation of Tschudi’s travels has appeared; I have sent for it & expect to enjoy it much.—⁸ I presume you see the Geolog. Journ of our Society, allow me to call your attention to what appears to me an important step in the right direction on the curious subject of cleavage; it is by M^r Sharpe in the number for this month.⁹ I have for the present given up Geology, & am hard at work at pure Zoology & am dissecting various genera of Cirripedia, & am extremely interested in the subject. I always, however, keep on reading & observing on my favourite work on Variation or on Species, & shall in a year’s time or so, commence & get my notes in order;¹⁰ should any important paper appear on this subject in any German periodical, I sh^d be greatly indebted for information of it.— Allow me to thank you again cordially for the very great trouble which you have so kindly taken to inform me of Prof. Bunsen’s views.—

with my best wishes, believe me, my dear Sir | Yours very sincerely | C. Darwin