312 Palmer — jSFotes on the Andes of N. W. Argentina. 



In order to make more clear the relations of the de- 

 tailed discussion of the Calchaqui Valley, there will be dis- 

 cussed briefly in Part I, the general structure of the whole 

 group of mountain ranges and interior basins of northwestern 

 Argentina and the adjacent portiou of Chile. The section 

 (fig. 2) is a diagrammatic representation of the structures en- 

 countered along the route. It is intended to show the various 

 structural belts constituting the southern part of the Central 

 Andes. The details of structure shown on the section are not 

 precisely those found on the line of the section, but are rather 

 generalized. It should be noted that the scale is so small that 

 accurate graphic representation is impossible, and consequently 

 the type of structure is suggested, rather than the actual forms 

 reproduced. The section represents conditions along a line 

 600 kilometers (370 miles) long, drawn from some indefinite 

 point on the Argentina Pampas through Saltato Calama, Chile, 

 in the direction, K. 60° W. In no case did the route depart 

 more than 40 kilometers (25 miles) from the line of the 

 section. 



The section discussed in the present article runs from Lat. 

 25° S. at the eastern end, to Lat. 22° 30' S. at its western end. 

 The western end joins up with the section made by G. Courty,* 

 on the line of the Antofagasta-Bolivia Railway. At the eastern 

 end the section connects up with the work of Steinmann, 

 Bodenbender, and Stelzner. These men worked mostly in the 

 Cordoba region about 800 kilometers south of Salta, though 

 reconnaissance trips were made north, and Steinmann in 1883 

 visited the Atacama desert. 



The route of the present expedition had not been covered 

 by geologists before, though it has been an important trade 

 route for several centuries. Yon Tschudi, between 1857 and 

 1859, travelled over part of the route, but made no geological 

 observations. 



Previous writers on other parts of the Central Andes have 

 described three geologic zones which are characteristic of the 

 region. The first zone is that of Silurian and Devonian 

 "slates," cut by granites; it forms the Eastern Cordillera. 

 The second is a single or repeated zone of Mesozoic sediments ; 

 and the third is a zone of " diorites " of post-Cretaceous age, 

 forming the Western or Maritime Cordillera. The presence 

 of these three zones is the habit of the Central Andes. 



Part I. A Section from the Pampas of Argentina, near Salta, 

 across the Cordillera to Calama, Chile. 



The Pampas of Argentina constitute a great, prairie-like 

 plain, sloping from the Andean foothills with very gentle 



*' 'Explorations Geologiques dans TAmerique du Sud," by G. Courtyof the 

 Crequi-Montfort expedition of 1903. 



