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