Geological Observations on South America · Charles Darwin
PORT S. JULIAN.
1. Ostrea Patagonica, d’Orbigny, “Voyage, Pal.” (also at St. Fe, and whole coast of Patagonia). 2. Pecten Paranensis, d’Orbigny, “Voyage, Pal.” (St. Fe, S. Josef, Port Desire). 3. Pecten centralis, G.B. Sowerby (also at Port Desire and S. Cruz). 4. Pecten geminatus, G.B. Sowerby. 5. Terebratula Patagonica, G.B. Sowerby (also S. Josef). 6. Struthiolaria ornata, G.B. Sowerby (also S. Cruz). 7. Fusus Patagonicus, G.B. Sowerby. 8. Fusus Noachinus, G.B. Sowerby. 9. Scalaria rugulosa, G.B. Sowerby. 10. Turritella ambulacrum, G.B. Sowerby (also S. Cruz). 11. Pyrula, cast of, like P. ventricosa of Sowerby, Tank Cat. 12. Balanus varians, G.B. Sowerby. 13. Scutella, differing from the species from Nuevo Gulf. At the head of the inner harbour of Port S. Julian, the fossiliferous mass is not displayed, and the sea-cliffs from the water’s edge to a height of between one and two hundred feet are formed of the white pumiceous mudstone, which here includes innumerable, far-extended, sometimes horizontal, sometimes inclined or vertical laminae of transparent gypsum, often about an inch in thickness. Further inland, with the exception of the superficial gravel, the whole thickness of the truncated hills, which represent a formerly continuous plain 950 feet in height, appears to be formed of this white mudstone: here and there, however, at various heights, thin earthy layers, containing the great oyster, Pecten Paranensis and Turritella ambulacrum, are interstratified; thus showing that the whole mass belongs to the same epoch. I nowhere found even a fragment of a shell actually in the white deposit, and only a single cast of a Turritella. Out of the eighteen microscopic organisms discovered by Ehrenberg in the specimens from this place, ten are common to the same deposit at Port Desire. I may add that specimens of this white mudstone, with the same identical characters were brought me from two points,—one twenty miles north of S. Julian, where a wide gravel-capped plain, 350 feet in height, is thus composed; and the other forty miles south of S. Julian, where, on the old charts, the cliffs are marked as “Chalk Hills.”
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