My dear M^r Horner

I daresay I am going to trouble you quite uselessly, but as you seem, (or give the impression) in your Nile Paper² to quite believe Russegger’s statement about the baked upper sandstone deposit,³ I thought I would just state that from what I have seen myself at the C. of Good Hope & Australia⁴ (& from what I know has been observed in India & the Malayan archipelago) I cannot avoid a total disbelief of the whole account. But please let me premise that if Russegger shows by his observations that he is a mineralogist as well as a geologist,—if in his baked rocks he has measured any crystals of feldspar or augite &c or even tryed the rock under the blowpipe, or examined the grains of quartz & seen their edges under the microscope fused I withdraw with humble apologies to you & him, my doubts. But if he has not, I would not in my own case quote his observations as in the least trustworthy.

In the countries above specified I have seen great areas coated with from 1 or 2 to (I think) 10 or 12 feet of dark brown, & black & angularly vesicular rock, having a most curious vitrified appearance, but which certainly was owing to a curious modification of the oxides of iron contained in it. Certainly there had been no heat from below, & certainly the whole deposit is simply superficial, & alluvial: I cannot explain its origin. To show its appearance, a most intelligent young officer in the Beagle, who had read Lyell & had seen plenty of real lavas, came rushing to me to say that he had found a most splendid stream of recent lava in the mountains near False Bay:⁵ I knew what it w^d. turn out, & so it did.—

I had the greatest doubt whether I w^d trouble you with this; but I felt sure you would forgive me, & w^d. perhaps like to hear of the possibility of a very great blunder being quoted as worthy of consideration in a memoir which will have value for the next, no one can tell, how many centuries.

I will be still more presumptuous, & say that I think a little shortening in the purely geological introduction would not injure the Paper.— As you will be surprised at all this, I may as well say that I have had to Report on your paper; but have of course made none of these remarks in the Report.⁶ Indeed I wish I had not just said that the geological introduction w^d not be injured by being shortened, for it only just occurred to me as a passing suggestion.— In 50 years, the Geology will probably be all passè, whereas the whole rest of the Paper will be permanent: is not this putting new wine in old Bottles?⁷

How very curious your account (p. 58)⁸ of the non-lamination of the mud: it interests me, as I feel sure that the great upheaved Pampean mud deposit⁹ was formed by slow deposition of mud in an estuary, & yet it is not in the least laminated; & rarely, when splendid sections are exhibited, the slightest variations in bands of colour can be most obscurely be distinguished: Al. d’orbigny has argued from this want of lamination that the whole vast Pampean deposit was thrown down all at once by a debacle.¹⁰

Will you forgive my presumption & believe | My dear M^r Horner | Your’s very sincerely & humbly | Ch. Darwin

at p. 25 of your M.S. w^d. it not be well to insert some one word instead of “Thus” to show that, you do not suppose the difference of the rise of the Nile at Rhoda & Rosetta is a measure of the amount of evaporation, as the width of the Nile & current are not the same at the two places.—¹¹