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.
Down Farnborough Kent
Absence from home has prevented me answering your letter of the 14
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.— 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.—
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.— I think I sent specimens from St. Fe, 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
- f1 985.f1The date is based on an endorsement ‘1846 (W. C. W.)’ by Williamson.
- f2 985.f2CD recorded payment for a trip to London in his Account Book (Down House MS) in an entry dated 17 June 1846.
- f3 985.f3Geological 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.
- f4 985.f4Williamson found no infusoria in any of the specimens, a finding confirmed by William Benjamin Carpenter (Williamson 1848, pp. 66, 128 ‘addenda’).
- f5 985.f5See letter to C. G. Ehrenberg, 23 March .
- f6 985.f6Williamson'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.).
- f7 985.f7Santa 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).