Dear Sir

I am exceedingly obliged to you for all the trouble, which you have so very kindly taken for me; I hope soon to receive my plates &c from D^r. Dieffenbach’s publisher.

Your account of the Atlantic dust is most interesting; I will add every particular, which I can about the direction of the wind, but I must say, that as the months, during which it falls on the African side of the Atlantic, are the same with those of the Harmattan, & as the first beginning of the falling of the dust has been observed in several cases & has always began with the wind between NE and SE, I cannot doubt that it comes from Africa, though it may have originally travelled from S. America. Could the Gulf-stream, which is said to sweep round as far as the Bay of Biscay, formerly (or still now) have brought S. American minute organisms, & thrown them up on the African continent?

I know well how fully your time is occupied, but if could afford time to send me one line, telling me whether the little packet of dust, which I myself collected contains infusoria I sh^d be much obliged. Also, whether you have, or intend publishing the substance of the letter to me, that I may refer to it & copy correctly in my notice (which shall be read before the Geological Society) the names of the Infusoria?

I am grateful to you for your remarks on the white Patagonian stone; I had come, from several reasons to the same conclusion, with you, on its primarily volcanic origin. Unfortunately you do not tell me which of the specimens of the white stone, contains infusoria;² I believe I sent several with their localities. The formation is a great one; it is associated with much sulphate of lime; it is of the consistence of our chalk, perhaps a little softer, & has an immense extension. At Port St Julian it cannot be much less than 800 feet in thickness; it extends continuously for 200 geographical miles (& probably is of great breadth) & I believe is of much greater extension, for I have specimens from the northern parts of Patagonia & layers having exactly the same external characters at the Rio Negro, which gives an extension in a N. & S. line, of at least 550 miles. Should you be led from your own curiosity to make any further examination, would you kindly inform me of the result.

I have specimens of great beds, from the upper parts of the late-Secondary, or Cretaceous-epoch, formations of the Cordillera of Chile, which from their appearance I suspect abound or are formed of infusoria; & sh^d. you wish for such, I shall be delighted to send them.

Believe me | dear Sir, Your’s sincerely obliged | C. Darwin

P.S. If you could spare time this Spring to examine the mud of the Pampas, to see, whether it contains fresh-water or salt-water infusoria, it would be the most important assistance to my work and kindness;³ I pledge myself I would ask you to look at nothing else.

I forget whether I told you that the Fuegian white paint is collected in fresh-water brooks: how beautifully do your microscopical researches reveal the origin of things!