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DETRITIC FORMAT IONS 83 
fed by following the way from Zapallar to Guandacol along the 
crosscutting Quebrada de la Troya, reminding much of the above 
_ named gorge of the Rio Jaguel in the northern ridge. 
The strata in this profile dip under several angles. The river 
_ upward, i. e. from the eastern mouth of the Quebrada, where the 
great bluff runs, there are first the brown, slightly disturbed sand- 
stone layers. They dip gently westward, and at their base red 
layers are visible, exactly as at Vinchina. Along the river gorge 
upward one observes, that the dipping-angle of the brown layers 
gradually increases and that the position finally becomes vertical. 
_ Further upward the strata are again less steeply inclined and simul- 
_ taneously they change their lithological aspect and become softer 
and more pale-coloured. The general strike is NE—NNE. 
The westward bending branch of the zone is principally composed 
of brown sandstones with a substratum of crom-red sandstone. 
This last named is doubtless the lowermost horizon here, because 
it lies in a sedimentary contact directly upon the crystalline base- 
ment, as seen on the southern slope of the Cerro Cordobés. 
Upward in the profile the brown sandstone passes into a pale 
Drown one exactly as in the last described profile. These pale- 
coloured strata are outcropping on the inner border of the Naci- 
miento’s depression. 
Summarizing the composition of this remarkable structural feature 
in the Umango area, called the ,intermontane zone“, may be stated, 
that this part of the continental sediment cover, strongly pressed 
during the tertiary diastrophism and downfolded between basement- 
_ blocks, shows quite a uniform composition: chocolate-brown and 
pale-brown sandstones dominate, the former occupying the eastern, 
the later the western part of the zone. Transitions exist between 
these two members. A crom-red sandstone occurs as small rem- 
nants in thinner zones or as basal layers of the series. Also a 
cinnabar-red and a white sandstone are seen in one locality. In 
the northern part of the zone, where this has a considerable breadth, 
