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.” SANTA CRUZ. The gravel-capped cliffs at the mouth of the river are 355 feet in height: the lower part, to a thickness of fifty or sixty feet, consists of a more or less hardened, darkish, muddy, or argillaceous sandstone (like the lowest bed of Port Desire), containing very many shells, some silicified and some converted into yellow calcareous spar. The great oyster is here numerous in layers; the Trigonocelia and Turritella are also very numerous: it is remarkable that the Pecten Paranensis, so common in all other parts of the coast, is here absent: the shells consist of:— 1. Ostrea Patagonica, d’Orbigny; “Voyage Pal.” (also at St. Fe and whole coast of Patagonia). 2. Pecten centralis, G.B. Sowerby (also P. Desire and S. Julian). 3. Venus meridionalis of G.B. Sowerby. 4. Crassatella Lyellii, G.B. Sowerby. 5. Cardium puelchum, G.B. Sowerby. 6. Cardita Patagonica, G.B. Sowerby. 7. Mactra rugata, G.B. Sowerby. 8. Mactra Darwinii, G.B. Sowerby. 9. Cucullaea alta, G.B. Sowerby (also P. Desire). 10. Trigonocelia insolita, G.B. Sowerby. 11. Nucula (?) glabra, G.B. Sowerby. 12. Crepidula gregaria, G.B. Sowerby. 13. Voluta alta, G.B. Sowerby. 14. Trochus collaris, G.B. Sowerby. 15. Natica solida (?), G.B. Sowerby 16. Struthiolaria ornata, G.B. Sowerby (also P. Desire). 17. Turritella ambulacrum, G.B. Sowerby (also P. S. Julian). Imperfect fragments of the genera Byssoarca, Artemis, and Fusus.