44 2 Tierra del Fuego . paet n. 



Tlie clay-slate is generally fissile, sometimes siliceous 

 or ferruginous, with veins of quartz and calcareous 

 spar ; it often assumes, especially on the loftier moun- 

 tains, 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 muclstone or into grauwacke, in- 

 cluding 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 con- 

 sist of: — 



1. Ancyloceras simplex, d'Orbig. • Pal. Franc' (PI. V. f. 2) Mount Tarn. 



2. Fusus (in imperfect state) do. 



3. Xatica do. do. 



4. Pentacrimus do. do. 



5. Lucina excentrica, G-. B. Sowerby (PL V. fig. 21), Port Famine. 



6. Venus (in imperfect state) do. 



7. Tnrbinolia \ do. do. 



8. Hamites elatior, G. B. Sowerby, do, 



M. d'Orbigny states l that MM. Hombron and 

 Grange found in this neighbourhood an Ancvloceras, 

 perhaps A. simple:':, an Ammonite, a Plicatula and 

 Modiola. 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 earlv 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 de- 

 scription is given by Mr. Sowerby in the Appendix, and 

 which is remarkable from its large size, has not been 

 1 ' Voyage, Part, Geolog.' p. 242, 



