Boundary in South America. 21 



thickness and constitute a very important physiographic 

 element. They are exposed on a long tract always along 

 the heights of the river bank. Following the example of 

 Doering, 50 Wichmann proposed for these sediments the 

 denomination "Capas de Chichinales" (Chichinales 

 Beds). 51 In the Rio Negro-valley they begin near Chi- 

 chinales (about 40 miles east of Roca) and, crossing the 

 junction of Rio Neuquen and Rio Limay, they spread as 

 far as the lower course of the Rio Limay. 52 



These Chichinales Beds rest to the north of Roca upon 

 the eroded surface of the marine Roca Beds, whereas 

 to the south of the village they lie on a base which offers 

 high interest in a stratigraphic as well as tectonic sense. 

 These relations can be studied in the surroundings of the 

 two ferryboats (balsas) opposite to the village (see 

 fig. 2). 



Close upon the bank of the Rio Negro the sharp slope 

 of the Variegated Sandstones (marked A A in fig. 2) 

 obviously has the character of a fault-scarp, and about 

 two miles southward this tectonic line disappears under 

 the terrace of the Chichinales Beds (marked Ch). East- 

 ward from this line, typical marine sediments appear and 

 occupy the place of the Continental Variegated Sand- 

 stones. They consist in soft clayey, sandy and gypsum- 

 bearing sediments with the character of Bad Lands. 

 With regard to their lithologic appearance, these sedi- 

 ments can be compared only with the "Capas del 

 Jaguel" in the region of the Sierra Auca Mahuida 

 (marked R in the figure). There can be no doubt that 

 these strata on the south bank of the Rio Negro also 

 belong to the Roca Beds, and their character as deposits 

 of sand and mud indicates clearly their near-shore origin. 

 I do not hesitate to hold the fault-scarp that represents 

 the eastern limit of the Variegated Sandstones in this 

 region as the ancient shore-line of the Roca-Sea. 



Consequently, the following stratigraphic, tectonic and 

 physiographic elements appear on the south bank of the 



50 Expedicion al Eio Negro, vol. 3, Geologia, Buenos Aires, 1882. 



51 Contribucion a la geologia de la region entre el Eio Negro y Valeheta, 

 Anales del Ministerio de Agriculture, Buenos Aires. In press. 



52 In the sections published by Both (Bev. Museo La Plata, 9, pp. 149, 

 169, etc., plate I) these sediments have been compared with the "Arenisca 

 del Bio Negro" (Bio Negro-Sandstone) that in the lower course of the 

 Bio Negro is widely spread and is correlated with the marine Entre Bios- 

 Formation (Parana Beds). Both's erroneous interpretation appeared in 

 my previous papers as I too believed that the Bio Negro-Standstone existed 

 in this region. But this is not so. 



