Palmer — Notes on the Andes of N. W. Argentina. 309 



Art. XXYI. — Geological Notes on the Andes of Northivestern 

 Argentina; by Harold S. Palmer. 



Part I — A Section Across the Cordilleras. 

 Part II — The Calchaqui Graben. 



(A paper based on observations made while assisting Professor Isaiah 

 Bowman on a geographical expedition to the Central Andes in 1913, under 

 the auspices of the American Geographical Society of New York.) 



Introductory. 



The Central Andes, namely that part of the Andes between 

 the so called " Knot of Yilcanota " in southern Peru and the 

 minor chains of northwestern Argentina, constitute a mountain 

 region distinct from the Peruvian and Colombian Andes on 

 the north, and from the Andes of Chile to the south. They 

 are characterized by the presence of a steep, faulted eastern 

 scarp, cut by torrential streams and covered with a heavy 

 forest growth, and further by a flexed and faulted western 

 scarp, which, as shown by its elevated beaches, has been 

 modified by wave erosion. The chief feature, however, which 

 distinguishes the Central Andes from the rest of the Andean 

 Cordillera, is the north-south line of high interior basins, 

 flanked on the east by mountain ranges inclosing structural 

 valleys which drain south and east to the broad Argentine 

 Pampas. These border ranges are in turn fronted by the so- 

 called pre-Cordillera, including such extra-marginal ranges as 

 the Serrania de San Antonio, the Serrania de Lumbrera, and 

 others near Cordoba and Tucuman. The stratigraphy exhibited 

 in the walls of the through valleys of the eastern border ranges 

 and the entire Andean Cordillera is characterized by sediments, 

 chiefly sandstones and shales, of great thickness, in places as 

 much as two miles. Practically all of this great sedimentary 

 series shows features characteristic of deposition on land or in 

 shallow water, and is to be regarded therefore as of terrestrial 

 origin. There is, however, a thin basal portion with marine 

 fossils, which are probably of Lower Jurassic age. It is with 

 the basal portion that this article will largely deal. A recon- 

 naissance trip from Salta, Argentina, to Calama, Chile, gave an 

 opportunity to study the entire sedimentary series in which it 

 occurs, in the Calchaqui Valley. Several structure sections 

 were also made and typical rock and fossil specimens collected. 

 The time for study was necessarily limited by the conditions 

 of travel, but the results seem to be of interest on account of 

 the geographical and structural relations of the sedimentary 

 strata to the Paleozoic schists and slates, to the Tertiary volcanic 



