Boundary in South America. 39 



are entirely lacking in the borings of Argerich. The 

 strata of the Patagonian Formation in these borings, 

 between 630 and 1800 feet, go over into red and white 

 clays and then into loess-like sediments, that have been 

 correlated by Stappenbeck and Wichmann 92 with the 

 Cretaceous. But these relations as well as the origin of 

 the artesian water in this region have not yet been per- 

 fectly cleared up. In case the above interpretation 

 should be right, it would be necessary to suppose, that in 

 the depression situated at the foot of the continental 

 mass ("Archhelenis"), there existed a zone of a con- 

 siderably strong accumulation. Consequently this zone 

 could not be reached by the Roca-Sea, and the subsequent 

 transgression of the Patagonian Formation resulted 

 from a later depression. 



Summarizing, we may assert that the San Jorge- 

 transgression in its northern half was represented by a 

 branch, which in the region of the present mouth of the 

 Ri© Negro entered into a graben-like depression along a 

 SE.-NW. strike. This depression was limited towards 

 the southwest by the great fault bordering the ancient 

 Patagonian mass, and towards the north and northwest 

 by old structures, partly belonging to the Pampine 

 Sierras, partly to the Precordilleran- Sierras of Buenos 

 Aires element. The prolongation of this latter element 

 towards the east formed a barrier, which may be 

 regarded as the northern limit of the South Atlantic 

 basin and which at the same time confined the extension 

 of the Roca-Sea towards the north. 



It is next in order to set forth the limits of the San 

 Jorge-Sea to the south across the Patagonian Continent. 



The most westerly exposure of the fossil-bearing San 

 Jorge sediments is in the Valle de las Plumas of the 

 region of the Rio Chubut. We see deposits of an almost 

 continuous chain extending from the lower valley of the 

 Rio Chubut and its junction with the Rio Chico to Lake 

 Colhue Huapi (or Colhuapi). There, "un peu a l'Ouest 

 de Rio Chico," Ameghino 113 also placed the ancient shore 

 of the San Jorge-Sea. Similarly, Stappenbeck 94 is 

 inclined to extend the ancient San Jorge shore-line across 

 the region of Lake Musters ; while Ameghino 's sections 



92 Personal communication. 



8S Les formations sedimentaires, pp. 93, 98. 



94 Personal communication. 



