22 SIERRA DE UMANGO 
The old rocks to the east of the valley of Fiambalå are again 
divided further to the east by granite from an extensive, precambrian 
terrane in the Sierra del Aconquija mountain complex, studied by 
Rassmuss. This last named precambrian shows to the east a 
marked unconformity against a still older gneissic complex, regarded 
by Rassmuss (1916) as archaean. 
The great Famatina—Fiambalä intrusive body of lower palaeozoic 
age seems to form a kind of interformational batholith between the 
Umango structure to the west and the precambrian gneisses to the 
east. One may assume, that the Famatina intrusion was cutting off 
the connections between the lower palaeozoic structure and its base- 
ment, so that an unconformity between these two elements nay 
probably never be found here. 
b) THE BASIC INTRUSIVE (AND EFFUSIVE?) ROCKS (METAVOLCANICS). 
The very abundant intercalations of basic schistose rocks between 
the doubtless sedimentary schists are characteristic members of the 
„pregranitic“ schist complex of the Umango area too. These basic 
rocks are partly ordinary amphibolites, according to the definition 
given by Rosenbusch, partly of different aspect and almost diifi- 
cult to be classified, because of the intense kataclasis undergone by 
the rocks. But it may be assumed, that the greater part of these dark 
schists represents igneous (partly gabbroid) injections into the sedi- 
mentary schist masses. These injections appeared already at a time, 
when the sediments still lay in their original position, i. e., before 
the first palaeozoic diastrophic movements and before the intrusion 
of the Famatina batholith took place. The basic rocks are therefore 
„pregranitic“ too. 
The original relations between the sedimentary schists and the 
basic sills have been destroyed at different times to a considerable 
extent. The thin layers and lenses of amphibolite in the limestone 
masses are certainly in most cases only the remnants of thicker 
volcanic sheets, afterwards, during the folding movements, extremely 
