36 Windhansen — The Cretaceous- Tertiary 



ancient Patagonian platform. From San Antonio to the 

 region south of Gualichu, this line may be perceived 

 orographically by a remarkable difference of level in the 

 Rio Negro-valley, if it is compared with the elevation, 

 that is situated a little south from here. While in the 

 Bajo de Gualichu deposits of the Roca Beds and Pata- 

 gonian Formation are filling the depression, southward 

 from here at a much higher level, granites, slates, diabase 

 and quartz porphyry indicate the ancient Patagonian 

 platform. 83 In the direct northwest prolongation of this 

 line is met near Roca the alleged breaking down of the 

 Variegated Sandstones, and there this fault is to be 

 regarded as a posthumous one above the ancient Pata- 

 gonian land-mass hidden in the underground. But this 

 fault does not end at Roca; from there it goes over to 

 the northern side of the valley crossing it obliquely. At 

 the north of Cipolletti as far as Tratayen, near the great 

 bend of the Rio Neuquen, this tectonic line determines 

 the western bank of the Rio Neuquen. In the region of 

 Tratayen the existence of this line is recorded by special 

 erosion phenomena, which have been caused originally 

 by tectonic events. 84 Probably this line does not end 

 here, but goes on to the north, contributing not far from 

 here to the formation of a great depression around the 

 so-called Ojo de Agua. 85 



Everywhere observations indicate that the Neuquen 

 line is the west or southwest margin of the Roca-trans- 

 gression. To the west or southwest of this line deposits 

 of the Roca Beds are not found. 86 Thus, from San 



83 This fault-scarp is orographically indicated by the so-called Subida 

 del Indio. 



84 Windhausen, Contribution al eonocimiento geologico de los Territorios 

 del Eio Negro etc., plate IV, fig. 3. 



85 The Topographic Section of the Argentine Geological Survey has 

 proved that there are very remarkable depressions in the Territory of 

 Neuquen. For instance the bottom of the Ojo de Agua (20 miles NW. of 

 the Anelo at the great bend of the Eio Neuquen) is 225 meters above the 

 sea, while the bottom of the valley of the river near the Anelo is 400 meters 

 above the sea-level. Likewise the writer has observed very deep depres- 

 sions surrounding the Sierra Auca Mahuida. All these phenomena cannot 

 be considered as merely physiographic features, but must be assigned to 

 tectonic movements connected with ancient structural elements in the 

 underground. 



80 The strata that unconformably overlap the Upper Jurassic and Creta- 

 ceous sediments at the eastern wing of the Cerro Lotena, have been corre- 

 lated by the writer with the Boca-Beds (Contribution al eonocimiento etc., 

 p. 11 and plate III, fig. 1). That could be considered as inconsistent with 

 the above statement. But Keidel, who studied a more extensive zone to 

 the east of the Cerro Lotena, assigns these strata to the base of the Varie- 

 gated Sandstones. This interpretation is probably the correct one. 



