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Darwin Correspondence Project

From Henry Stephen Fox1   31 October 1833

Rio de Janeiro

October 31st: 1833

Dear Sir

I have just learnt that the Packet Cockatrice is likely to find the Beagle still either at, or near to, Montevideo.— My object in troubling you with the present letter, is to urge you, in case you have not yet done so, to visit the little Island of Flores,2 on which is the Lighthouse. You will find it a geological curiosity, and worthy I think of your inspection.— The whole island is a formation of Greenstone, different from any thing that occurs along the neighbouring continent, but similar to that (as far as I can recollect) of which I saw specimens, brought from Cape Horn, by Captain FitzRoy on his last voyage.— The formation is the more remarkable, as the island of Flores, though apparently out at sea as you go to it from Montevideo, is in fact considerably within the general line of coast.— You will find the finest ledges and cliffs of the Greenstone along the southeastern or seaward side of the island, much waterworn, and exceedingly hard and difficult to break.— All this history will however be quite superfluous, if you have already visited the place.— The island of Lobos I was not able to visit.— I found the islands of Martin-Garcia, the Dos Hermanas, and some other small islands in the middle of the channel of the Uruguay, above its confluence with the Paraná, to be small low knolls of granite, though surrounded, on both sides of the river, by interminable alluvial plains, pierced I believe by no rock whatever.—

I have taken the liberty of sending to you, by Mr: Rees of the Cockatrice, several specimens of rocks from Porto Alegre.3 The main fundamental rock of that Country is granite, of remarkably white colour, from containing a superabundance of quartz. I found, lying over the granite, large formations, composing clusters of considerable hills, of the rock (specimen no. 1)—perhaps you will have the goodness to tell me what it is. I also found large quantities, in the same neighbourhood, of the volcanic porphyry (no. 2), not exactly in situ, but forming lines of disjointed masses, closely piled together, along the ridges of several hills. This rock very much resembles many of the ancient volcanic rocks of the north of Italy.— I again found large quantities of greenstone, not in situ, but in loose blocks, both in the island of St. Catherine’s, and in the island of St. Sebastian; but none on the coast of the continent, opposite these islands.— In one place only, on the west side of the island of St. Sebastian, I found veins of greenstone, of a texture almost as close as basalt, in the granite.— The main rock along the whole length of coast, from Rio Grande up to this place, is granite.— I have thought you might like to hear these few particulars of the geology of the south of Brazil. The Province of Rio Grande would I think afford great interest to a geologist.—

I have not yet unpacked the greater part of my specimens; but if you should like to be furnished with any more specimens of the rocks of Porto Alegre, or of the greenstones above mentioned, I shall have great pleasure in sending them to you, with such statement of their site, &c, as my very slight knowledge may enable me to give.

I presume your head quarters, for the next cold season, will be Valparaiso. Perhaps you would in return oblige me, by sending me a specimen or two of the common rocks (not the rare minerals) of the coasts of the straits of Magellan, and of Chile, which you will visit.— When I had the pleasure of seeing you at Buenos Ayres, you were in expectation of discovering a volcanic formation in the neighbourhood of Bahia Blanca.—

I beg you to have the goodness to make my compliments to Captain FitzRoy, and to believe me | Your faithful and | obedient humble servant | H. S. Fox

CD annotations

Verso last page: ‘November 1832’


Fox had arrived at Rio de Janeiro as British Minister Plenipotentiary to Brazil in August 1833.
CD preserved this letter among his geological notes (DAR 39.1: 1–4) for possible use in writing up the results of the voyage, but no mention is made in his later works of this or any of the other islands to which Fox refers. For CD’s description of the geology of the Montevideo area see South America, pp. 145–7.
The specimens are listed in DAR 39.1: 75.


South America: Geological observations on South America. Being the third part of the geology of the voyage of the Beagle, under the command of Capt. FitzRoy RN, during the years 1832 to 1836. By Charles Darwin. London: Smith, Elder & Co. 1846.


Urges CD to visit Flores Island near Montevideo if he has not already done so. Describes formation of greenstone and the granite formations on small islands in the Uruguay channel.

Sends specimens from Pôrto Alegre [Brazil] for identification by CD.

Letter details

Letter no.
Henry Stephen Fox
Charles Robert Darwin
Sent from
Rio de Janeiro
Source of text
DAR 39.1: 1–4
Physical description
ALS 7pp †

Please cite as

Darwin Correspondence Project, “Letter no. 226,” accessed on 8 June 2023,

Also published in The Correspondence of Charles Darwin, vol. 1