78 SIERRA DE UMANGO 
mass. A stratification may be observed in the conglomerate sho- 
wing, that the layers have turned vertically. To the west, that is 
upward in the profile, the conglomerate changes into a graywacke © 
like the rock from Agua Blanquita. All the pebbles in the conglo- | 
merate are composed of crystalline rocks. Probably the conglome- 
rate has a glacifluvial origin. Farther to the west no continuation : 
of the profile is seen, because this whole outcrop lies as an isola- ° 
ted sunk remnant in the crystalline basement. Some engl. miles - 
to the west there runs the eastern cuesta of the Cerros de San An- | 
tonio, composed of strata of the higher continental series, but without 
connection with the graywacke at Carrizal. 
The above-mentioned cuesta of the Cerros de San Antonio shows 
at the base, composed of the same mica schists as at Carrizal, a 
graywacke rock like the former (well visible in a locality to the 
NW of Carrizal). Here, however, the graywacke has been extre- 
EI Färna Zn ben Lee ee NERE 
mely crushed by the tertiary movements. This rock is succeeded \ 
upward in the bluff by reddish and brown coloured sandstones — 
with gently westward dipping layers. These are not torn as the 
graywacke, a circumstance which may be explained only by way of 
assuming a special tectonical phenomenon (see later on). 
In the other locality, where the bottom layers occur, at the Puesto 
Altillo, situated on the eastern border of the intermontane zone in 
the southern part of the Cerro Potrero Viejo, no graywacke nor 
conglomerate occurs, but a yellowish sandstone of lower Gondwana 
character. The sandstone lies immediately over the crystalline ba- 
sement, separated from it only by a thin sheet of a conglomeratic 
psephite. The sandstone contains regular interstratifications of a 
clayish slate. The thickness of this series is quite considerable, 
probably many hundred meters. The whole complex is strongly 
folded as consequence of the tertiary movements (in connection with the 
formation of the intermontane zone). The best exposures are visible 
not in the very zone, but aside from it, on the eastern slope of the 
southern part of the Cerro. On the very border, marked by a fault, 
