24 | SIERRA DE UMANGO 
All these intercalating basic layers are, as stated above, very 
strongly metamorphosed (corresponding to the „metabasites“ of 
Hackman). The metamorphism was either of purely plutonic cha- 
racter, or a result of dynamic influence during later diastrophic 
periods. The original petrographical character is therefore very diffi- 
cult or impossible to approve, regarding the circumstance too, that 
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chemical analyses are still lacking. It seems probable, that the 
original geological appearance of these rocks are for the most part 
basic intrusions or sills of diabasic or gabbroid kind, from a time 
anterior to the orogenetic movements and granite injections. 
In order to carry, on a deductive petrographical study of these 
rocks it is necessary to go to the neighbouring regions, where there 
are better conditions for such a study. Observations in this respect 
are, however, still lacking. The occurrence of cambrosilurian fossili- 
ferous slates, slightly metamorphosed, in the Nevado de Famatina 
seems to offer a favourable possibility to discover tracks of the 
primary structures in the basic sills. 
The secondary structure of the „metabasites“ of the Umango area 
is partly of granoblastic habit, due to a complete recrystallisation 
under abyssal conditions, partly of a crystalline schistosity, caused 
by a stress influence under contactmetamorphic conditions. In 
some parts are found also purely kataclastic types. 
The amphibolites have undergone a complete recrystallisation and 
developed a granoblastic structure, especially in the vicinity of peg- 
matitic veins of larger size, where pneumatolytic action has favoured 
the former. The types may be described going from east to west. 
The above named amphibolite layers in the Cerro Asperecito, 
situated in the bottom of the Vinchina valley, near to Villa Castelli, 
are intimately penetrated with small granitic veins and also larger 
apophyses, and therefore they ought to be completely recrystallized, 
but a later pression (apparently tertiary movements) has caused a 
kataclasis. The principal minerals are amphibole and plagioclase, 
the first named generally developed in individuals of greater size than 
