80 SIERRA DE UMANGO 
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The coal is mingled with sandy matter. The yellowish sandstone © 
at Altillo contains on the bedding planes abundantly Cruziana- 
like figures. a 
The pale yellowish sandstone and the clayish interstratifications — 
do occur only along the eastern border of the whole intermontane — 
zone. The principal mass of the filling sediments are younger 
sandstones of various colours (cinnabar-red, and also white, chocolate- 
brown, pale brown). The stratigraphical succession of all these — 
layers is quite difficult to state, because the tertiary movements have 
resulted in a number of strong folds and faults (see details under 
the tertiary tectonics). The elder members are, as seen, the pale 
yellowish sandstone and clayish slate with the coal seams. The 
crom-red sandstone of triassic(?) age, succeeding these beds in the 
slope of the Cerro Villa Union, seems to be lacking here, while 
some apparently younger brown beds develop a great thickness. Up- 
ward these gradually change into pale brown sandstone beds. These 
sandstones have a distinct continental littoral character with sun- 
cracks and ripplemarks on the bedding planes. The upper, paler 
variety contains some thinner conglomeratic layers. This sandstone 
occupies the western border of the zone and is overthrust from the 
west by the great Sierra de Umango fault block. On the very gli- 
ding plane there are, however, some small remnants of a crom-red 
sandstone, possibly the last remnants of the triassic (?) sandstone. 
The writer is inclined to consider the cinnabar-red sandstone to the 
east of the brown one as a younger one (cretaceous?). 
An excellent cut through the whole breadth of the zone may be 
studied in a river gorge between the Jaguel bolson and the Vin- 
china valley. Here the ,intermontane zone“ lies outside the moun- 
tain blocks and plays the part of a ridge or a dividing crest-line 
between the two depressions. Its eastern flanc consists of a more 
than hundred meters high bluff of some sinuous trench and can. 
be followed from the northern end of the Cerro Potrero Viejo NNE- 
ward into the Valle Hermoso. The strata forming this bluff are 
