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PHYSIOGRAPHY OF THE AREA. 
Sierra de Umango is the general name of a mountain group, 
situated in the deep corner between the eastern border of the Cor- 
dillera in the west, the southern edge of the Puna de Atacama in 
the north and the lofty, meridionally streching ridge of the Sierra 
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(and Nevado) de Famatina in the east. It lies in the western part 
of the province of La Rioja, in the extremely dry, so called suban- 
_ dean zone of the central western Argentina. The closer position of the 
_ area is defined by the coordinates 28° 50’—29° 50’ S Lat. and 68° 
| -00’—69° 00’ W Long. from Greenwich. The geological study in this 
| region comprises, however, more than the topographical unity of 
the Sierra de Umango and extends sometimes to the foothills of 
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the surrounding mountains. The approximate limits of the surveyed 
area are thence: in the west: the eastern border of the Cordilleran 
block, or nearer said the „Precordillera“, marked by the steep 
western wall of the tectonical valley Guandacol — Tambillos — 
_ Guandacolinos, and further to the north by the western border of the 
Bolson de Jaguel; in the north by the southern edge of the Puna 
de Atacama; in the east by the western slope of the Sierra (Nevado) 
de Famatina; in the south the southern limit of the alluvial plain 
of Pagancillo and the water gap of the little river Vinchina, above 
its confluence with the Rio Bermejo. (See the accompanying map!) 
Regarding the orography, two elements may be distinguished: 
1) the alpine mountains of the Sierra de Umango in proper sense, 
the Cerro Cacho and the Cerro Villa Union, all comprising the old 
_ erystalline rock ground, and 2) the surrounding foothills with the 
