PREFACE. 
Engaged as geologist of the Direction of Mines, Section of Geology, 
Buenos Aires, during the years 1914—17, I had to explore the geolo- 
gical structure. of a mountain complex, situated in the western part of the 
province of La Rioja, near the border of the Cordillera and named La 
Sierra de Umango. | started from Buenos Aires in beginning of may 
1915 and returned in august the same year. Simultaneously with me a 
topographer from the Section of Topography, Direction of Mines, was 
committed with the mapping of the region. His map could, however, be 
used only at a time after my return to Buenos Aires, and later on I had 
no occasion to start a new expedition to this region with its very com- 
plicated geology for bringing the observations into closer accordance with 
the topographical map. 
The geological sketch map, covering the reduced copy of the above 
named topographical sheet, accompanying this paper, expresses of course 
only the main geological features. In order to illustrate the intricate 
relations of the rock members of the area (mingled rocks, veins, banded 
‘schists, etc.) a map of much greater scale would be necessary. The petro- 
graphical descriptions below may, however, give a picture of the litholo- 
gical relations, and that will be the chief purpose of the present paper. 
The task of exploration of the Sierra de Umango area was undertaken on 
proposal of Dr Hans (Juan) Keidel, Director of the Section of Geology, 
Buenos Aires, in order to obtain a closer knowledge on the first hand of 
the relations between the structural element of the ,Sierras pampeanas“ 
to the east and the permian and andean (tertiary) structures to the west. 
That this matter of study really was to be found in the Sierra de Umango 
area, seemed probable already from an examination of the geological map 
of Ludwig Brackebusch (Gotha, 1891). 
The geological mapping of the Sierra de Umango area should also 
be a continuation of the field works carried out in the near lying province 
-of Catamarca by the geologists of the Direction of Mines, Dr Walther 
