28 SIERRA DE UMANGO 
2) Peridotitic rock (sill). 
In the eastern part of the group of hills called Las Ramaditas 
and situated to the east from the Cerro Villa Union, there occurs 
at the foot of a red sandstone ridge in the crystalline basement a 
black, coarse grained rock of quite a massive character and running 
as a sheet concordantly between steeply inclined quartzitic schists. 
fue lower serpentine and tert Oxyde Mia Man OLS 
Ramaditas. 
The whole complex is intruded with red granite and pegmatite dikes. 
In handspecimen the rock is of a coarse granular habit with a black, 
brilliant mineral as a dominant component. The massive aspect of 
the rock is remarkable, regarding the circumstance, that the other 
pregranitic rocks show stress influences. 
Microscopically the specimens (see fig. 4) show a peridotitic 
rock consisting of pyroxene and olivine, both colourless in the 
slides, but both also partly transformed to green-yellowish ser- 
