This paper gives the results of a reconnaissance made by the writer across the Cordillera de Vilcanota and the Cordillera de Carabaya, both forming the Cordillera Oriental, and as far as Maldonado on the Madre de Dios River in southeastern Peru. As it is the first time that this part of Peru has been geologically investigated, it was possible to obtain interesting data on the geology, stratigraphy, and structure of this little-known part of the Andes.

The Cordillera de Carabaya is a rugged, snow-clad range. The elevation of the lowest pass, Abra de Hualla-Hualla, is 4,820 meters above sea-level. Toward the east the range drops in a short distance to an elevation of about 600 meters at Rio Nusiniscato. A complete traverse could be made across the Cordillera Oriental between Urcos and the valley of Rio Madre de Dios. The regional geology is largely represented by older, pre-Cambrian, metamorphic schists and quartzites, which are overlain by Ordovician slates. Upper Paleozoic brown-red shales and sandstones occur west of the section, at Urcos. At its eastern half, east of Quincemil, vast Tertiary plains extend. A remarkable feature of the section is the Araza gneiss underlying the Marcapata metamorphic rocks. The Araza gneiss appears o be early pre-Cambrian, probably Archean, in age. From the writer's knowledge of the South American Andes, it would seem that this is the first recognized occurrence of gneiss on the eastern slope of the Andes, since none is known to have been described from Colombia, Ecuador, northern Peru, or Bolivia.

Structurally, this part of the Cordillera Oriental appears to be block-faulted. The faulting is normal; there is no evidence of large-scale thrusting.