24 



1 1 "indh a men — Th e Cretaceo us- Tertiary 



Liniay rest directly upon the Pehuenche Beds with dino- 

 saurs. This fact explains why a non-geologist might 

 consider these sediments of quite different age as a uni- 

 form formation. Further, the lithologic character and 

 color of the Chichinales Beds depends a great deal on 

 their base, and consequently in regions where they rest 

 upon variegated sediments, they bear a great similarity 

 to their base in color and material. 



Summarizing our present knowledge, we may establish 

 the following table of formations for the region to the 

 south and north of the village of General Roca: 



Bowlder or shingle formation of the Tehu- 

 elche Beds ("Rodados Patagonicos"). 



Sandy clays, tuffs and tuff -bearing sand-"^ 



stones with Colpodon, Hegetotherium and Chichinales 



turtles ; probably in a lower level also I Beds 



with Pyrotherium etc. Intercalations of 

 sandstones with opalized trunks of trees 

 (palms). 



Unconformity 

 Peneplain. 



Dolomitic limestone, marls and limestones"^ 

 with marine invertebrates (Ostrea Ame- 

 ghinoi etc.). Gypsum-bearing clays with 

 Gryphaea burckhardti and fish-remains. 

 Near-shore deposits of fine sand and clay 

 at the south bank of the Rio Negro. 



(terrestrial). 



Marine Roca 



Beds 

 ( transgression ) . 



Unconformity. \ 



Crossbedded sandstones. Sandstones of | Pehuenche Beds 

 brown color with brackish mollusca. Keu- )- (lacustrian and 

 per-like deposits with dinosaurs (Diplo- j brackish deposits). 

 docidce). Fossil wood. J 



Variegated Sandstones of the Upper Cre- I Continental 

 taceous (Areniscas Abigarradas). > deposits. 



Advancing south or southeast, the next locality where 

 deposits of the San Jorge have been found is in the 

 depression of the Bajo de Gualichu, a few miles to the 

 south of the Isle Choele-Choel, formed by two branches 

 of the Rio Negro. Ameghino 59 has already reported the 

 appearance of some Roca fossils here and Wichmann, 



M Les formations sedimentaires, etc., p. 53. 



