100 SIERRA DE UMANGO 
Many dark concordant lava sheets are met with in the Tambillos | 
valley in the variegeted continental sediments in different grade 
dislocated and occupying the whole valley floor. A melaphyre lava in — 
the vicinity of the Puesto Tambillos shows microscopically a great 
abundance of ferric oxyde. This matter forms an opaque mesosta- 
sis between the feldspar laths. Cavities are scarce. 
On both sides of the entrance to the Quebrada Arroyo La Totora 
southward off the Cerro Cordobés (opposite the Cerro Avalao) a 
series of red sandstone layers, dipping southward and reposing 
directly on the crystalline basement, contain in their lower part 
concordant sheets of an aphanitic melaphyre partly of amygdaloide 
development. The structure being a typical ophitic one shows 
a network of fine plagioclase laths, slightly albite twinned. The 
small-granulated augite is yellowish-grey; sometimes it is developed 
in larger, idiomorphic individuals. The interstices are filled with 
glass. Ore dust is quite abundant, partly associated with a gree- 
nish-yellowish serpentine and thence evidently an alteration product 
of olivine. 
To the north of Nacimientos on the right hand of the way to Tam- 
billos rocks of red sandstone, dipping southward and forming a 
westward continuation of the mantle at the entrance to the Quebrada 
La Totora, contain a concordant basic sheet of a somewhat different 
aspect. The rock is dark brown and porphyritic with a dense me- 
sostasis. The distinct phenocrysts of plagioclase are pale greenish 
and isometric. The structure is closer andesitic than melaphyric. 
The feldspar phenocrysts show the zonar polygones well developed 
with a maximum extinction angle (on cuts perpendicular to M and - 
P) of + 25°, i. e. Ab,‚An,,. The groundmass consists of small feld- 
spar laths and minute ore grains, cemented by intensely pigmented 
glass. The numerous cavities of the groundmass are filled with 
calcite or viridite in radiating aggregates. Isolated grains (relics of 
augite) do occur too. 
Near to the Pirca post in the above described small dislocation — 
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