Chapter 5 - Page 2
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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 Ostraea 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. Fe and whole coast of Patagonia). 2. Ostrea Alvarezii, d'Orbigny, "Voyage, Pal." (also at St. Fe and R. Negro). 3. Pecten Paranensis, d'Orbigny, "Voyage, Pal." (also at St. Fe, S. Julian, and Port Desire). 4. Pecten Darwinianus, d'Orbigny, "Voyage, Pal." (also at St. Fe). 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. Fe 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 laminae 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
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