The clay-slate is generally fissile, sometimes siliceous or ferruginous, with veins of quartz and calcareous spar; it often assumes, especially on the loftier mountains, an altered feldspathic character, passing into feldspathic porphyry: occasionally it is associated with breccia and grauwacke. At Good Success Bay, there is a little intercalated black crystalline limestone. At Port Famine much of the clay-slate is calcareous, and passes either into a mudstone or into grauwacke, including odd-shaped concretions of dark argillaceous limestone. Here alone, on the shore a few miles north of Port Famine, and on the summit of Mount Tarn (2,600 feet high), I found organic remains; they consist of:— 1. Ancyloceras simplex, d’Orbigny “Pal Franc” Mount Tarn. 2. Fusus (in imperfect state), d’Orbigny “Pal Franc” Mount Tarn. 3. Natica, d’Orbigny “Pal Franc” Mount Tarn. 4. Pentacrimus, d’Orbigny “Pal Franc” Mount Tarn. 5. Lucina excentrica, G.B. Sowerby, Port Famine. 6. Venus (in imperfect state), G.B. Sowerby, Port Famine. 7. Turbinolia (?), G.B. Sowerby, Port Famine. 8. Hamites elatior, G.B. Sowerby, Port Famine. M. d’Orbigny states that MM. Hombron and Grange found in this neighbourhood an Ancyloceras, perhaps A. simplex, an Ammonite, a Plicatula and Modiola. (“Voyage” Part Geolog. page 242.) M. d’Orbigny believes from the general character of these fossils, and from the Ancyloceras being identical (as far as its imperfect condition allows of comparison) with the A. simplex of Europe, that the formation belongs to an early stage of the Cretaceous system. Professor E. Forbes, judging only from my specimens, concurs in the probability of this conclusion. The Hamites elatior of the above list, of which a description has been given by Mr. Sowerby, and which is remarkable from its large size, has not been seen either by M. d’Orbigny or Professor E. Forbes, as, since my return to England, the specimens have been lost. The great clay-slate formation of Tierra del Fuego being cretaceous, is certainly a very interesting fact,—whether we consider the appearance of the country, which, without the evidence afforded by the fossils, would form the analogy of most known districts, probably have been considered as belonging to the Palaeozoic series,—or whether we view it as showing that the age of this terminal portion of the great axis of South America, is the same (as will hereafter be seen) with the Cordillera of Chile and Peru.