1. Ostrea Patagonica, d’Orbigny, “Voyage, Pal.” (also at St. Fé, and whole coast of Patagonia).
  2. Ostrea Ferrarisi, d’Orbigny, “Voyage, Pal.”
  3. Ostrea Alvarezii, d’Orbigny, “Voyage, Pal.” (also at St. Fé, and S. Josef).
  4. Pecten Patagoniensis, d’Orbigny, “Voyage, Pal.”
  5. Venus Munsterii, d’Orbigny, “Voyage, Pal.” (also at St. Fé).
  6. Arca Bonplandiana, d’Orbigny, “Voyage, Pal.” (also at St. Fé).

According to M. d’Orbigny, the sandstone extends westward along the coast as far as Port S. Antonio, and up the R. Negro far into the interior: northward I traced it to the southern side of the Rio Colorado, where it forms a low denuded plain. This formation, though contemporaneous with that of the rest of Patagonia, is quite different in mineralogical composition, being connected with it only by the one thin white layer: this difference may be reasonably attributed to the sediment brought down in ancient times by the Rio Negro; by which agency, also, we can understand the presence of the fresh-water shells, and of the bones of land animals. Judging from the identity of four of the above shells, this formation is contemporaneous (as remarked by M. d’Orbigny) with that under the Pampean deposit in Entre Rios and in Banda Oriental. The gravel capping the sandstone plain, with its calcareous cement and nodules of gypsum, is probably, from the reasons given in the First Chapter, contemporaneous with the uppermost beds of the Pampean formation on the upper plain north of the Colorado.

San Josef.—My examination here was very short: the cliffs are about a hundred feet high; the lower third consists of yellowish-brown, soft, slightly calcareous, muddy sandstone, parts of which when struck emit a fetid smell. In this bed the great Ostræa Patagonica, often marked with dendritic manganese and small coral-lines, were extraordinarily numerous. I found here the following shells:—

  1. Ostrea Patagonica, d’Orbigny, “Voyage, Pal.” (also at St. Fé and whole coast of Patagonia).
  2. Ostrea Alvarezii, d’Orbigny, “Voyage, Pal.” (also at St. Fé and R. Negro).
  3. Pecten Paranensis, d’Orbigny, “Voyage, Pal.” (also at St. Fé, S. Julian, and Port Desire).
  4. Pecten Darwinianus, d’Orbigny, “Voyage, Pal.” (also at St. Fé).
  5. Pecten actinodes, G. B. Sowerby.
  6. Terebratula Patagonica, G. B. Sowerby (also S. Julian).
  7. Casts of a Turritella.

The four first of these species occur at St. Fé in Entre Rios, and the two first in the sandstone of the Rio Negro. Above this fossiliferous mass, there is a stratum of very fine-grained, pale brown mudstone, including numerous laminæ of selenite. All the strata appear horizontal, but when followed by the eye for a long distance, they are seen to have a small easterly dip. On the surface we have the porphyritic gravel, and on it sand with recent shells.

Nuevo Gulf.—From specimens and notes given me by Lieutenant Stokes, it appears that the lower bed consists of soft muddy sandstone, like that of S. Josef, with many imperfect shells, including the Pecten Paranensis, d’Orbigny, casts of a Turritella and Scutella. On this there are two strata of the pale brown mudstone, also like that of S. Josef, separated by a darker-coloured, more argillaceous variety, including the Ostrea Patagonica. Professor Ehrenberg has examined this mudstone for me: he finds in it three already known microscopic organisms, enveloped in a fine-grained pumiceous tuff, which I shall have immediately to describe in detail. Specimens brought to me from the uppermost bed, north of the Rio Chupat, consist of this same substance, but of a whiter colour.

Tertiary strata, such as here described, appear to extend along the whole coast between Rio Chupat and Port Desire, except where interrupted by the underlying claystone porphyry, and by some metamorphic rocks; these hard rocks, I may add, are found at intervals over a space of about five degrees of latitude, from Point Union to a point between Port S. Julian and S. Cruz, and will be described in the ensuing chapter. Many gigantic specimens of the Ostrea Patagonica were collected in the Gulf of St. George.

Port Desire.—A good section of the lowest fossiliferous mass, about forty feet in thickness, resting on claystone porphyry, is exhibited a few miles south of the harbour. The shells sufficiently perfect to be recognised consist of:—

  1. Ostrea Patagonica, d’Orbigny, (also at St. Fé, and whole coast of Patagonia).
  2. Pecten Paranensis, d’Orbigny, “Voyage, Pal.” (also at St. Fé, S. Josef, S. Julian).
  3. Pecten centralis, G. B. Sowerby (also at S. Julian and S. Cruz).
  4. Cucullæa alta, G. B. Sowerby (also at S. Cruz).
  5. Nucula ornata, G. B. Sowerby.
  6. Turritella Patagonica, G. B. Sowerby.

The fossiliferous strata, when not denuded, are conformably covered by a considerable thickness of the fine-grained pumiceous mudstone, divided into two masses: the lower half is very fine-grained, slightly unctuous, and so compact as to break with a semi-conchoidal fracture, though yielding to the nail; it includes laminæ of selenite: the upper half precisely resembles the one layer at the Rio Negro, and with the exception of being whiter, the upper beds at San Josef and Nuevo Gulf. In neither mass is there any trace to the naked eye of organic forms. Taking the entire deposit, it is generally quite white, or yellowish, or feebly tinted with green; it is either almost friable under the finger, or as hard as chalk; it is of easy fusibility, of little specific gravity, is not harsh to the touch, adheres to the tongue, and when breathed on exhales a strong aluminous odour; it sometimes contains a very little calcareous matter, and traces (besides the included laminæ) of gypsum. Under the microscope, according to Professor Ehrenberg,[[2]] it consists of minute, triturated, cellular, glassy fragments of pumice, with some broken crystals. In the minute glassy fragments, Professor Ehrenberg recognises organic structures, which have been affected by volcanic heat: in the specimens from this place, and from Port S. Julian, he finds sixteen Polygastrica and twelve Phytolitharia. Of these organisms, seven are new forms, the others being previously known: all are of marine, and chiefly of oceanic, origin. This deposit to the naked eye resembles the crust which often appears on weathered surfaces of feldspathic rocks; it likewise resembles those beds of earthy feldspathic matter, sometimes interstratified with porphyritic rocks, as is the case in this very district with the underlying purple claystone porphyry. From examining specimens under a common microscope, and comparing them with other specimens undoubtedly of volcanic origin, I had come to the same conclusion with Professor Ehrenberg, namely, that this great deposit, in its first origin, is of volcanic nature.