Dear and highly honoured Sir

I do not know how to thank you enough for your last most valuable letter.¹ I am particularly obliged for your examination of the Pampas mud, which I fear you undertook entirely for my sake. Your results have quite confirmed my view of its origin.² Considering the number & strangeness of the mammifers embedded in it (more different from our present quadrupeds, than are the Eocene fossil mammifers of Europe) the history of the Pampæan deposit is really interesting. I presume you did not examine the infusoria sufficiently to know whether the species are extinct or recent; as that would throw light on its age, which, as well as the manner of its formation, is disputed by the French Geologists.³ In case you should ever further investigate the subject, I hope you will kindly inform me of the result; & I will endeavour to send you a few more & good samples. The Bahia Blanca is the least interesting part of the Pampæan Formation.

—. In the many volcanic districts, which I visited, I found obsidian only at Ascension; I will send some chips, though probably you will already have got them from this well-known island.—

As you seem interested with the great, chalk-like, Patagonian tufaceous formation, I will send soon to you through the Chevalier Bunsen,⁴ a set of specimens from every station which I visited; & I will label them outside, so as to give you little trouble by sending you duplicates of those already sent. Santa Cruz is the most southern spot, where I collected specimens, though I saw the formation still further S.: Rio Negro is the most northern part where it occurs, & there only in thin beds in sandstone. The gypsum occurs in vast quantity in plates or veins, running in every direction, especially at P. St Julians: some gypsum is disseminated throughout the mass.— My specimens are small, but I will send you half of my largest, which I think will be an inch square as you desire. I shall publish a small volume on the geology of S. America this ensuing winter & how greatly will your Researches add to its value!

I have looked through all the accounts given by those on board vessels, where the Atlantic dust has fallen; & I find the wind has generally been between NE & East, & sometimes SE.—and it has always fallen during the same four months, when the harmattan blows from NE & ENE.—⁵ I will detail all the facts in my little paper, which I will immediately send to the Geological Soc.— The dust to which I allude, which fell on the Princess Louise, was not during Meyen’s voyage, but subsequently:⁶ I doubt whether Meyen’s dust was our infusorial dust, for it fell during a different month from any other case, & (as far as I can judge) further off the African coast.— I am sorry to find that I have misled, though in so unimportant point, as hardly to be worth your correcting. The packets marked by you IA & IB are same: I had so little expectation of the the first packet, which I sent, interesting you, that I did not describe it fully & gave only the approximate latitude & did not specify that it was not collected by myself. That collected by myself, fell on our ship, when 10 miles NW of St. Jago, (& is marked in your list ‘St. Jago’) as I stated in the M.S. which I sent you. But these inaccuracies are quite unimportant.

Believe me, Sir, with many thanks | Yours very faithfully & obliged | C. Darwin.