chap. xi. St. Fd Bajada. 335 



parts the great Ostrea Patagonica l chiefly abounds. 

 In the upper part, the limestone alternates with layers 

 of fine white sand. The shells included in these beds 

 have been named for me by M. d'Orbigny ; they consist 

 of,— 



L 5 



1. Ostrea Patagonica, d'Orbig. 



' Voyage,' Part. Pal. 



2. „ Alvarezii, do. 



3. Pecten Paranensis, do. (and 



PL IV. of this work, fig. 30). 



4. ,, Darwinianus, do. and PI. 



IV. of this work, figs. 28, 29. 



5. Venus Munsterii, d'Orbig. 



' Voyage,' Pal. 



6. Area Bonplandiana, do. 

 7 Cardium Platense, do. 



8. Tellina, probably nov. spec, 

 but too imperfect for de- 

 scription. 



These species are all extinct : the six first were 

 found by M. d'Orbigny and myself in the formations of 

 the Rio Negro, S. Josef, and other parts of Patagonia ; 

 and therefore, as first observed by M. d'Orbigny, these 

 beds certainly belong to the great Patagonian forma- 

 tion, which will be described in the ensuing chapter, 

 and which we shall see must be considered as a very 

 ancient tertiary one. North of the Bajada, M. d'Orbigny 

 found, in beds which he considers as lying beneath the 

 strata here described, remains of a Toxodon, which he 

 has named as a distinct species from the T. Platensis 

 of the Pampean formation. Much silicified wood is 

 found on the banks of the Parana (and likewise on the 

 Uruguay), and I was informed that they come out of 

 these lower beds; four specimens collected by myself 

 are dicotyledonous. 



The upper half of the cliff, to a thickness of about 

 thirty feet, consists of Pampean mud, of which the 

 lower part is pale coloured, and the upper part of a 

 brighter red, with some irregular layers of an arenaceous 

 variety of tosca, and a few small concretions of the 

 ordinary kind.. Close above the marine limestone, 



1 Capt. Sulivan R.N., has given me a specimen of this shell, 

 which he found in the cliffs at Point Cerrito, between twenty and 

 thirty miles above the Bajada. 



