CHAP. XII. 



Rio Negro. 



37* 



town, I found, low down in the sandstone, a bed, a few 

 inches in thickness, of a white, friable, harsh-feeling 

 sediment, which adheres to the tongue, is of easy fusi- 

 bility, and of little specific gravity ; examined under 

 the microscope, it is seen to be pumiceous tuff, formed 

 of broken transparent crystals. In the cliffs south of 

 the river there is, also, a thin layer of nearly similar 

 nature, but finer grained, and not so white ; it might 

 easily have been mistaken for a calcareous tuff, but it 

 contains no lime : this substance precisely resembles a 

 most widely extended and thick formation in southern 

 Patagonia, hereafter to be described, and which is re- 

 markable for being partially formed of Infusoria. These 

 beds, conjointly with the conglomerate of pumice, are 

 interesting, as showing the nature of the volcanic action 

 in the Cordillera during this old tertiary period. 



In a bed at the base of the southern cliffs, M. d'Or- 

 bigny found two extinct fresh-water shells, namely, a 

 Unio and Chilina. This bed rested on one with bones 

 of an extinct rodent, namely, the Megamys Patagoni- 

 ensis ; and this again on another with extinct marine 

 shells. The species found by M. d'Orbigny in different 

 parts of this formation consist of : — 



1. Ostrea' Patagonica, d'Orbig. 



' Voyage Pal.' (also at St. 

 Fe, and whole coast of Pata- 

 gonia). 



2. Ostrea Ferrarisi, do. 



3. Ostrea Alvarezii, d'Orbig. 



'Voyage Pal.' (also St. Fe 

 and S. Josef). 



4. Pecten Patagoniensis, do. 



5. Venus Munsterii, do. (also St. 



Fe). 



6. Area Bonplandiana, do. (do.) 



According to M. d'Orbigny, the sandstone extends 

 westward along the coast as far as Port S. Antonio, and 

 up the Rio Negro far into the interior: northward I 

 traced it to the southern side of the Rio Colorado, where 

 it forms a low denuded plain. This formation, though 

 contemporaneous with that of the rest of Patagonia, ia 

 17 



