Boundary in South America. 37 



Antonio on the Atlantic coast as far as the region north- 

 ward from the great bend of the Rio Neuquen, the shore- 

 line of the Roca-Sea coincides with the tectonic line at the 

 northern border of the ancient Patagonian plaform. 81 



In this way, is fixed the southern shore of a branch of 

 this sea. We will now examine the conditions found to 

 the north and northwest and also determine that 

 boundary. 



At the east of the Sierra Pintada the line of the Rio 

 Salado represents an old boundary, such as has already 

 been pointed out several times in this paper. It must 

 be regarded as a graben-like depression between the Pre- 

 cordillera-Sierra Pintada and the more ancient mass of 

 the Pampine Sierras, the most western portion of which, 

 the Sierra of San Luis and adjacent parts, is here ended 

 by longitudinal as well as transverse faults. 



The strata with Hidrobia, Corbicula, Cyrena etc. in 

 the depression of the Alto Pencoso (southwest of the 

 Sierra of San Luis) as well as deposits of the same char- 

 acter in the intermountain troughs between the Sierras 

 of Famatina and Velasco have likely been connected with 

 the San Jorge-transgression by small and shallow 

 branches. ss The depression of the Rio Salado may have 

 represented the eastern limit of the sea during this 

 period. A little more southward, in the region where 

 Rio Atuel and Rio Salado take the name of Rio Chadi 

 Leufii, lies the base-leveled root of an ancient range built 

 up by gneiss, crystalline slates, granites, amphibolites 

 and continental sandstones. In the underground of the 



87 Accordingly, in the central parts of Neuquen, where the Patagonian 

 mesetas begin to emerge as a new element, the northern portion of the 

 Patagonian landmass may be still regarded as an essential part of the 

 underground. Outcrops of this old mass in the form of gneisses, granites, 

 etc., are known in this region from the following localities: near Piedra 

 Pintada (Both), to the north of the Cerro Lotena (Keidel) and to the 

 south of the Eio Limay (Eovereto, Studi di Geomorfologia Argentina. La 

 Valle del Eio Negro. Boll. Soc. Geol. Ital., vol. 31, p. 181, Rome, 1912). 

 This mass resisted in the period of Andine folding against the regional 

 forces of folding, producing in this way a deviation of folds and, in some 

 cases, a certain disturbance of normal tectonic conditions. This is a simple 

 explanation of the departing strike in several parts of Neuquen (Cerro 

 Lotena, Sierra Vaca Muerta, Covunco etc.) while the peculiar parallelism 

 of certain chief hydrographie lines in this region is apparently connected 

 with this phenomenon. 



88 Gerth, Constitution geologica, hidrogeologia y minerales de aplicacion 

 de la Provincia de San Luis, Anales Ministerio de Agricultura, Sece. 

 Geologia etc., 10, No. 2, Buenos Aires, 1914. — See also the paper of the 

 ■same author: Die Pampinen Sierren Centralargentiniens, Geolog. Eund- 

 schau, 4, pp. 577 etc., 1913. 



