Boundary in South America. 17 



istic and lithologic differences separate the Luisaen from 

 the Roca-Salamanca-Beds, and that evidence cannot be 

 found for contemporaneity or equivalence of these for- 

 mations. 42 Since, as mentioned above, no ammonites 

 occur at Roca, the most important evidence for all earlier 

 opinions as to the age of Roca has disappeared. 43 Fur- 

 thermore, it will be shown in this paper that equivalent 

 deposits of the Roca-Salamanca-Beds also exist in South- 

 ern Patagonia, but these deposits are obviously separated 

 from the Luisaen by a faunistic and stratigraphic break. 



Consequently, the denomination "San Jorge-Forma- 

 tion" as applied in this paper, is not entirely identical 

 with what Wilekens understands by it. I wish to apply 

 this name to the marine deposits of the Roca-Salamanca- 

 Beds and related formations, the age of which will be 

 discussed further on, but special attention may be called 

 to the fact that the strata with Lahillia luisa Wilck. are 

 excluded from my definition. 



To the hitherto known exposures of these deposits will 

 be added in this paper some new data about the Roca- 

 Beds in the region of the Rio Grande and around the 

 Cerro Pay en, and further in the Bajo de Gualichii, north- 

 west of San Antonio at the mouth of the Rio Negro. 

 For information about these localities, I am gratefully 

 indebted to my colleagues, Dr. Groeber and Dr. Wich- 

 mann, who kindly allowed me to take advantage of still 

 unpublished observations. Furthermore, some new 

 observations I made on a trip to Roca in September, 

 1916, complete the evidence brought out in my previous 

 publications and bring the discussion of the Roca ques- 

 tion up to the present. 



In attempting a critical examination of the various 

 sections from a stratigraphical point of view and accord- 

 ing to their geographical distribution, we may start in 

 the north, in the region of Canada Colorada, a small vil- 

 lage situated in the northern part of the Province of 

 Mendoza, at the foot of the Cordillera. Bodenbender 44 



42 v. Ihering has called the Luisaen a formation of more archaic character 

 and he always considered it as older than Roca-Salamanea. 



43 Windhausen, Einige Ergebnisse zweier Reisen in den Territorien Rio 

 Negro und Neuquen, p. 344. 



44 Sobre el terreno jurasico y cretaceo de los Andes Argentinos entre el 

 Rio Diamante y el Rio Limay. Boletin Academia Nac. de Ciencias en Cor- 

 doba, 12, 1892. — Sobre el carbon y asfalto carbonizado de la Provincia de 

 Mendoza, ibid., 13, 1893. 



Am. Jour. Sci. — Fourth Series, Vol. XLV, No. 265.— January, 1918. 

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