372 Tertiary Formations. paet n. 



quite different in mineralogical composition, being con- 

 nected with it only by the one thin white layer : this 

 difference may be reasonably attributed to the sediment 

 brought down in ancient times by the Rio Negro ; by 

 which agency, also, we can understand the presence of 

 the fresh- water shells, and of the bones of land animals. 

 Judging from the identity of four of the above shells, 

 this formation is contemporaneous (as remarked by 

 M. d'Orbigny) with that under the Pampean deposit in 

 Entre Eios and in Banda Oriental. The gravel capping 

 the sandstone plain, with its calcareous cement and 

 nodules of gypsum, is probably, from the reasons given 

 in the eighth chapter, contemporaneous with the upper- 

 most beds of the Pampean formation on the upper plain 

 north of the Colorado. 



San Josef. — My examination here was very short : 

 the cliffs are about 100 feet high ; the lower third 

 consists of yellowish-brown, soft, slightly calcareous, 

 muddy sandstone, parts of which when struck emit a 

 fetid smell. In this bed' the great Ostrea Patagonica, 

 often marked with dendritic manganese and small coral- 

 lines, were extraordinarily numerous. I found here the 

 following shells : — 



1. Ostrea Patagonica, d'Orbig. ' Voyage Pal.' (also at St. Fe and 



whole coast of Patagonia). 



2. Ostrea Alvarezii, d'Orbig. ' V. Pal.' (also St. Fe and E. Negro.) 



3. Pecten Paranensis, d'Orbig. 'V. Pal.' and PI. III. f. 30 of this 



work (also St. Fe, S. Julian, and Port Desire). 



4. Pecten Darwinians, d'Orbig. « V. Pal.' and PL III. f . 28 and 29 



(also St. Fe.) 



5. Pecten actinodes, G. B. Sowerby, PI. III. f . 33. 



6. Terebratula Patagonica, G. B. Sowerby, PL II. 1 26 and 27 



(also S. Julian). 



7. Casts of a Turrit ella. 



The four first of these species occur at St. Fe in 

 Entre Bios, and the two first in the sandstone of the 

 Rio Negro. Above this fossiliferous mass, there is a 



