Granitic gneiss;
Austrian Alps
In this sample, a coarse-grained gneiss formed from a
granite has been cut by a vein of fine-grained white
granite, intruded along a straight-sided crack. The granite
from which the gneiss was formed must have been a
porphyritic rock with large feldspar crystals
(compare the porphyritic granite).
As the rock was deformed, these
have become rounded and slightly flattened, and the matrix
has developed a banding and an alignment of minerals. Both
the matrix of the gneiss and the vein of granite consist of
white feldspar, grey quartz, and dark mica.