CRYSTALLINE BASEMENT | 39 
rock shows a remarkable inhomogeneity due to the appearance of 
“biotite spots of irregular size and distribution. Somewhere the 
biotite concentrates to greater streaks or Schlieren. These are, 
as: was stated above, often subparallel, evidently by flow du- 
ting the consolidation. Microscopically the rock is quite quartz- 
bearing. According to measurements of the angle of extinction 
i in cuts perpendicular to M and P of the principal mineral, a white 
_ plagioclase feldspar, it has the composition Ab,,An,,. The chief 
3 mafic component is, as has been stated, a biotite with a strong 
BETEN | 
absorption. This mineral is frequently penetrated with poicilitic 
_ grains of quartz, with common areal optical orientation. The border 
zone between the feldspar and the quartz develops a myrmekitic 
intergrowth. The structure of the rock is, however, in gene- 
tal allotriomorphic. Only small scattered apatite individuals show 
: idiomorphic forms. The quartz is of jagged contours, probably 
due to recrystallisation, while traces of pressure are not visible. 
- Amphibole occurs to a reduced quantity as insignificant individuals. 
The biotite is partly transformed to an opaque mass. Iron ore 
also occurs in minute spots. 
Aside from the basic concentrations (biotite bearing masses) in 
this rock there are numerous acid Schlieren, apparently real diffe- 
rentiation products of the syntectic magma. These acid varieties are 
of a pale red colour and have no precise limits towards the main 
rock. The shape of the Schlieren is irregular and may be kept 
apart from the crosscutting pegmatite dikes of the last eruption 
phase of the batholith, described below. 
To the west, towards the Umango schist body, the syntectic, 
| quartzbearing diorite shows a marked irruptive contact, well visible 
in the Asperecito hills to the west of Villa Castelli. 
A dark gneissic rock, occurring as quite a long zone in the 
abrupt western slope of the Cerro La Vatea, is very probably of 
syntectic origin too. In handspecimen the rock seems to be more a 
basic granite with a marked parallel structure, visible on the rock- 
