Dear Sir

Absence from home has prevented me answering your letter of the 14^th 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