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76 The upper ledge of hard vesicular tuff as colpodon-age fossils extent only into the upper part of this, and the large parastrapo-there occurs in its lower part definitely above the lava flows. This vesicular tuff contains local unconformities at more than one level, and in places original oblique bedding makes at least local disconformity self-evident. This probably defines the boundary between Colpodonen + Pyrot., although taking the barranca as a whole one would have no reason aside from the fossils to consider this as more important than other levels of local unconformity. Colpodonense: Colpodon age fossils begin to appear in the upper part of the cornice series of hard vesicular tuffs etc. and extend in considerable abundance through a maximum thickness of about 70'. Above the fossiliferous part and conformable unit is a maximum of about 150' of tuffs which contain only a few bone fragments + in which we found no identifiable specimens. 77 Dec. 3-6, 1930- EXCURSION UP THE R�O SENGUER At the Vuelta del Senguer there is only Chubutiano, according to recent work by Cabrera + Y.P.F. geologists and contrary to Windhausen + V. Huene (pers. com.) who state that there is Triassic. The Sierra San Bernardo ends on the n. side of the river here, but its folds, completely peneplaned, continue under the flat level of the Valle Hermoso on the S. side. From here due to a bad road + reported absence of mammal beds we made a long detour over pampas without exposures and rejoined the river at La Confluencia. We then followed the west bank of the river as far as the settlement of the Ensanche de la Colonia Sarmiento, where we crossed the river and followed the east bank down for about 3 leagues, then returned to the direct Ensanche-Sarmiento road and went back to Sarmiento. The beds encountered would not repay museum work and we therefore did not spend the time necessary to take detailed profiles. The general conditions, from our rapid survey, appear to be as follows: On the south side of the Rio Mayo Valdez at the Confluencia the marine upper Patagoniano is exposed. Following the west bank northward, one first crosses a lava flow which appears to lie beneath