CRYSTALLINE BASEMENT 17 
On the transverse fracture surfaces of the very quartzitic schists 
may be observed too quite a distinct glassy habit. These rocks 
are, as shown later on, strongly recrystallized. The mica marks the 
very bedding planes of the rock (Pl. I, fig. 1), as becomes evident from 
the fact, that these schists partly carry interstratifications of sericitic 
schists. With regard to the relatively considerable amount of mica 
in the mass of the schists, it is quite probable, that the schist in 
its actual metamorphic habit represents an original psammitic rock 
of the arcose type, where the feldspar has contributed to the forma- 
tion of mica. The mica is in most cases a biotite, oriented in 
subparallel direction between the quartz grains. Other varieties of 
he schists contain also sericite, whose relative quantity rises occa- 
sionally forming sericitic schists. Garnet of reddish colour also 
occurs, sometimes quite abundantly, with rounded contours and of 
- granoblastic habit. The whole microstructure indicates a strong 
 recrystallisation, undergone by the rock. 
_ The mica schists occupy, as mentioned, principally the broader 
zones, relatively free from granitic intrusion, but very intensely pe- 
netrated with concordant sheets of amphibolite. The most important 
of these zones runs over the Cerro Villa Union and over the Sierra 
de Umango ridge with a general NNW-trend.! Another zone with 
‚heavy amphibolitic sheets occurs in the Vinchina valley ground, 
exposed on the surface in the Asperecito hills close to Villa Castelli. 
The ordinary habit of these schists resembles the alpine ,,schistes- 
lustrées“ with silverglittering planes carrying abundantly quartz-lenses 
and garnet aggregations. The schistosity planes are uneven, generally 
tippled. The amphibolitic layers are strongly concordant in the 
rock structure. The mica is generally represented by muscovite, but 
in many parts biotite also occurs, attaining the same relative quan- 
tity as muscovite. The last case results in a type, corresponding 
to a two-mica-schist. Another variety is a sericitic schist of pale 
greenish colour and with intensely lustering planes. It is garnet- 
Serowsee fig. 2, pag. 7. 
