Boundary in South America. 51 



5. The extension of the San Jorge-Formation is 

 limited to Patagonia, just as is the subsequent Patagonian 

 Formation. The ancient mass in the floor of the Sierras 

 of Buenos Aires ("Archhelenis") formed the northern 

 limit of these two epicontinental seas. It was not until 

 the time of the Parana-Formation (Pliocene) that this 

 mass broke down, opening the way for the Parana-Sea, 

 that invaded the mouth of the La Plata River and 

 advanced far to the north into the region of the Parana 

 and Paraguay Rivers. 



6. In the paleontologic sense, the San Jorge is of Ter- 

 tiary character being the forerunner of the Patagonian 

 Formation. Its fossils, however, exhibit a number of 

 similarities to the Pelecypoda and Gastropoda of the 

 Upper Senonian of South Patagonia. It can be proved 

 that all these faunas sprang from the same South 

 Atlantic-Antarctic center, representing a continuous evo- 

 lution of the original stock. Other affinities of the San 

 Jorge are slight. 



7. The age of the San Jorge is Upper Paleocene or 

 Lower Eocene. It may be contemporaneous with the 

 Libyan Formation in Africa, the London Clay in Europe 

 and the Kirthar Group in India. 



8. Above the marine deposits of the San Jorge can 

 be observed a peneplain of regional character. Upon 

 this surface have been laid down the sediments with the 

 earliest Patagonian mammal-fauna (Notostylops, Astra- 

 ponotus and Pyrotherium Beds of Ameghino or Casa- 

 mayor and Deseado of Loomis). The age of these 

 deposits is fixed as Oligocene, in accordance with the 

 assumptions of many paleontologists. The subsequent 

 Patagonian Formation retains its place in the Lower 

 Miocene. 



9. The association of dinosaurs and mammals in 

 Patagonia can be interpreted only by assuming that 

 some branches of dinosaurs (Theropoda) still persisted 

 in the Tertiary. The dinosaurs mainly occur in the 

 upper part of the Variegated Sandstones (Pehuenche 

 Beds) of Cretaceous age. The Pehuenche Beds and the 

 mammal-bearing horizons are clearly separated by the 

 interwedging of marine San Jorge and the peneplain 

 cutting into it ; but in those regions where the San Jorge 

 fails, the hiatus between Pehuenche and Casamayor or 

 Deseado requires closer attention. 



