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Darwin Correspondence Project

To William Crawford Williamson   23 June [1846]1

Down Farnborough Kent

June 23d.

Dear Sir

Absence from home has prevented me answering your letter of the 14th more promptly.—2 Unfortunately I did not keep any list of the specimens, which I sent you, & cannot charge my memory whence they all came, & therefore am not able very fully to answer your queries.3

I could perceive no evidence of any particular chemical action in the Tertiary strata of S. America, certainly there has been no metamorphic action. The fossils are almost invariably calcareous & well preserved; but I do not think the strata, of which specimens were sent you, were those which contained fossil shells: I did not know your object & picked out specimens, which appeared to my eye most likely to contain infusoria.—4 The white pumiceous mudstone from Patagonia contains no shells.— Most of such strata, as I sent you, have, I believe, resulted primarily in volcanic action, that is either from erupted ashes, or triturated volcanic rocks.—5

I am much surprised at what you say about the abundance of the siliceous matter; my impression was a different one.— You refer to a ‘Tufaceous Layer’ & speak of each fragment being of siliceous matter: is this Tufaceous layer, marked R. Negro? if so, I think, you will find that the whole is easily fusible & therefore cannot be silica; but I do not know what microscopical test you have to distinguish glassy feldspar & silica.—6 I think I sent specimens from St. Fe,7 I have no reason to suppose that they have originated in volcanic action; & the beds are associated with others abounding with calcareous fossil shells.— In many of the Tertiary formations of S. America, the fossils occur in sandstone concretions, which have been formed by the aggregation of calcareous matter.—

I wish it was in my power to give you more satisfactory information.

Believe me dear Sir | your’s faithfully | C. Darwin


The date is based on an endorsement ‘1846 (W. C. W.)’ by Williamson.
CD recorded payment for a trip to London in his Account Book (Down House MS) in an entry dated 17 June 1846.
Geological specimens from the Tertiary strata of Patagonia. They were sent to Williamson by CD in connection with the rewriting and enlargement of a paper read by Williamson on 4 November 1845, but not published at that time. The final revised version of the paper included comments made by CD and references to his South American specimens (Williamson 1848, pp. 66–7). A separate, advance printing of Williamson’s paper dated 1847 is in the Darwin Libary–Down House.
Williamson found no infusoria in any of the specimens, a finding confirmed by William Benjamin Carpenter (Williamson 1848, pp. 66, 128 ‘addenda’).
Williamson’s original observation was corrected in the published version: ‘This specimen appears to contain neither Polythalamia nor siliceous organisms’ (Williamson 1848, p. 67). He noted that the specimen from Rio Negro was glassy feldspar (Williamson 1848, p. 67 n.).
Santa Fé Bajada. CD discussed the fossil deposits in Journal of researches 2d ed., pp. 129–30. Williamson described both the limestone and the calcareous marl of the Pampas, noting that they had been altered by some agency after their deposition (Williamson 1848, pp. 94, 109).


Journal of researches 2d ed.: Journal of researches into the natural history and geology of the countries visited during the voyage of HMS Beagle round the world, under the command of Capt. FitzRoy RN. 2d edition, corrected, with additions. By Charles Darwin. London: John Murray. 1845.


Does not remember where specimens came from. CD picked fossils most likely to contain Infusoria. Discusses composition of Tertiary strata of South America from which they came. Questions WCW’s statement that they contained siliceous matter.

Letter details

Letter no.
Charles Robert Darwin
William Crawford Williamson
Sent from
Source of text
Missouri Botanical Garden Library
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 985,” accessed on 31 May 2020,

Also published in The Correspondence of Charles Darwin, vol. 3