96 SIERRA DE UMANGO 
C) THE PERMIAN OROGENETIC PHASE. 
Dynamical influences in the Sierra de Umango schist area. 
According to the investigations by Stappenbeck (1910) and the 
latter more detailed ones by Keidel (1914), the eastern bordering 
zone of the middle part of the Argentine Cordillera is tectonically 
composed of a pretriassic or permian folded structure, capped in a i 
great unconformity with the triassic red beds. These old folds are, — 
as Keidel has stated, of an „alpine“ character with ’nappes de i 
charriage’ etc., indicating the existence at that time of a tangential - 
orogenetic pressure from the west, possessing a much stronger cha- 
racter than later on during the tertiary diastrophism (see farther). 
This ancient folding zone, corresponding to the so called „Precor- | 
dillera“, extends from the vicinity of San Rafael in the south to the 
Sierra de Umango area in the north. The northern end of the fol- 
ding zone is formed by the „Precordillera of Guandacol“, recently ° 
investigated and mapped out by Rassmuss (1916). This part of i 
the chain rises to the west abruptly up from the Guandacol valley i 
floor. Northward it does not attain the Umango area, but is sepa- E 
rated from it by tertiary dislocations. Therefore the Umango crystal- — 
line rock complex does not come into contact with the Precordille- — 
ran structure. At the western border of the Umango area, where it 
is topographically connected with the Cordillera, no Precordilleran | 
structure appears, as seen above. 
But it is evident, that the Umango area has at least in some degree 
played the part of a „Foreland“ to the Precordilleran chain. It re- 
mains to consider to what extent the tangential movements in the 
Guandacol Precordillera have influenced the Umango block. 
In the Cerro Villa Union, situated opposite the Guandacol Precor- 
dillera, the Gondwana strata, as has already been described above, 
are disturbed only by the tertiary movements. Also in other parts 
of the area, where the lower Gondwana still remains in position 
preserved from the erosion, there are same circumstances. 
Consequently the pressure phenomena characterized on the fore- 
