Dear Sir

I have duly received your specimens, and have had several sections made of the fragment of the Pampas deposit, with one or two of the Chilian tufa for comparison— I fear that the result will not be very satisfactory to you, as not affording the evidence you desire.¹

Notwithstanding the very close conformity in aspect between the Pampas specimen and the Chilian tuff,—or, rather, that portion of the latter which connects together the fragments of shell,—their minute structure is very different. The substance of the latter appears to be composed of very minute fragments of Shell, or of some other organic structures; it possesses a considerable degree of transparency in thin sections; and is destitute of any kind of mineral structure. The former, on the other hand, possesses more of the Oolitic structure; being composed of minute rounded grains, the spaces between which are filled up with amorphous matter. But it differs from the true Oolite in this,—that the sections of these granules do not present any of that concentric arrangement (precisely resembling that of Calculi) which the true Oolite exhibits; and that the spaces between them are not filled up with transparent crystalline matter, but with a peculiarly opake substance— Hence I should infer that the materials of the deposit had been subjected to long-continued attrition; and not by a process of alternate solution and deposit, such as appears to have taken place in the true Oolite— I cannot find in the Pampas deposit the least vestige of any organic structure, except some very minute fragments, which seem to be vestiges of Sponges or Alcyonia,—judging by the spicula they contain. If this sh^d be a point of any interest to you, I will send the sections to M^r Bowerbank for his examination,—that being a structure with which he is especially familiar.— I may add, that the Pampas deposit is of much closer texture than the Chilian tuff, and much more difficult to rub down. Its opacity is such, that it is only when brought to an extreme tenuity, that it begins to be translucent.

As I saw that the specimens were ticketed, I did not like to knock them to pieces for the purpose of trying different parts; but if you can allow me to do this, I will make further investigations

I am, dear Sir, | very sincerely yours | W B Carpenter Cha^s Darwin Esq.