My dear Henslow

This is the last letter, which I shall ever write to you from the shores of America.— and for this reason I send it.— In a few days time the Beagle will sail for the Galapagos Is^ds.— I look forward with joy & interest to this, both as being somewhat nearer to England, & for the sake of having a good look at an active Volcano.— Although we have seen Lava in abundance, I have never yet beheld the Crater.— I sent by H.M.S. Conway two large boxes of Specimens. The Conway sailed the latter end of June.— With them were letters for you.— Since that time I have travelled by land from Valparaiso to Copiapò & seen something more of the Cordilleras.— Some of my Geological views have been subsequently to the last letter altered.— I believe the upper mass of strata are not so very modern as I supposed.— This last journey has explained to me much of the ancient history of the Cordilleras.— I feel sure they formerly consisted of a chain of Volcanoes from which enormous streams of Lava were poured forth at the bottom of the sea.— These alternate with sedimentary beds to a vast thickness: at a subsequent period these Volcanoes must have formed Islands, from which have been produced strata several thousand feet thick of coarse Conglomerate.— These Islands were covered with fine trees; in the Conglomerate I found one 15 feet in circumference, perfectly silicified to the very centre.— The alternations of compact crystalline rocks (I cannot doubt subaqueous Lavas) & sedimentary beds, now upheaved, fractured & indurated form the main range of the Andes. The formation was produced at the time, when Ammonites, several Terebratulæ, Gryphites, Oysters, Pectens, Mytili &c &c lived.—³

In the central parts of Chili, the structure of the lower beds are rendered very obscure by the Metamorphic action, which has rendered even the coarsest Conglomerates, porphyritic.— The Cordilleras of the Andes so worthy of admiration from the grandeur of their dimensions, to rise in dignity when it is considered that since the period of Ammonites, they have formed a marked feature in the Geography of the Globe.— The geology of these Mountains pleased me in one respect; when reading Lyell, it had always struck me that if the crust of the world goes on changing in a Circle, there ought to be somewhere found formations which having the age of the great Europæan secondary beds, should possess the structure of Tertiary rocks, or those formed amidst Islands & in limited Basins. Now the alternations of Lava & coarse sediment, which form the upper parts of the Andes, correspond exactly to what would accumulate under such circumstances. In consequence of this I can only very roughly separate into three divisions the varying strata (perhaps 8000 ft thick) which compose these mountains. I am afraid you will tell me to learn my A.B.C.—to know quartz from Feldspar—before I indulge in such speculations.— I lately got hold of ⁠⟨⁠    ⁠⟩⁠ report on M. Dessalines D’Orbigny’s labors in S. America. I experienced rather a debasing degree of vexation to find he has described the geology of the Pampas, & that I have had some hard riding for nothing; it was however gratifying that my conclusions are the same, as far as I can collect, with his results.—⁴ It is also capital, that the whole of Bolivia will be described. I hope to be able to connect his Geology of that country, with mine of Chili.— After leaving Copiapò, we touched at Iquique. I visited, but do not quite understand the position of the Nitrate of Soda beds.— Here in Peru, from the state of Anarchy, I can make no expedition.—

I hear from Home, that my Brother is going to send me a box with Books & a letter from you.— It is very unfortunate that I cannot receive this before we reach Sydney, even if it ever gets safely so far.— I shall not have another opportunity for many months of again writing to you.— Will you have the charity to send me one more letter (as soon as this reaches you) directed to the C. of Good Hope Your letters besides affording me the greatest delight, always give me a fresh stimulus for exertion. Excuse this Geologico-prosy letter & Farewell till you hear from me at Sydney & see me in the Autumn of 1836. Believe me, dear Henslow, Yours affectionately obliged | Charles Darwin