2 H. E. Gregory — A Geologic Reconnaissance 



Page 



The Paehatucsa Formation 59 



The Huayllabamba Formation 61 



The Hauyllabamba Formation north of Rio Huatanay CI 



The Huayllabamba Formation south of Rio Huatanay 66 



The Mollo-Orcco conglomerate member . 66 



Origin and age 71 



The Yucay Formation 71 



The Bambanusa Formation 74 



Sedimentary rocks of Lower Cuzco Valley 77 



The San Sebastian Formation .1 78 



Origin and age 85 



Pleistocene and recent formations 85 



IV. The Igneous Rocks 86 



Introduction.. 86 



Description of areas 87 



Ttica-Ttica Pass 88 



Sacsahuaman and El Rodadero 88 



Huaccoto 91 



Ichchu-Orcco 93 



Atasccasa 94 



Chchiraura 95 



Oropesa _ 96 



Sierra-Bella 96 



Rumiccolca 98 



Tinta 100 



INTRODUCTION 



Scope of the paper. — A journey from Mollendo to Lake 

 Titicaca, continued over La Raya Pass and along Urubamba 

 River, is sufficient to indicate the wide range of problems 

 awaiting solution in southern Peru. The Cuzco Valley, in 

 particular, appeals to the imagination of the scientific man. 

 In several respects it is an unworked field. Its unusually rich 

 archeologic material has been inadequately discussed. A study 

 of its present and ancient inhabitants with reference to physical 

 environment has not been undertaken; and its interesting 

 geologic features have received but incidental mention. 2 The 

 Cuzco Valley is a key area for the interpretation of the history, 

 geography, and geology of the central Andes; it fairly bristles 

 with scientific problems. 



It was the writer's good fortune to spend the months of 

 September, October, and November, 1912, in southern Peru. 

 Seven weeks were devoted to geologic field work in the region 

 adjoining Cuzco. The particular object of the expedition was 

 a study of the gravel deposits from which human remains 



2 The pamphlet prepared by Dueiias (Aspecto minero del Departamento 

 del Cuzco, Cuerp. Ing. de Min. del Peru, Bol. 54, Lima, 1907) marks 

 the beginning of modern geologic studies in the Department of Cuzco. 

 The bulletin contains valuable economic and petrographic material. 



