Pihtipudas mica schist.

The schists are metamorphic rocks of highly variable structure and mineral composition. Those most commonly used for building purposes are mica schists, phyllites and quartzite. The mica schists and phyllites are composed of scaly micas (biotite and sericite) with small interstitial quartz grains and are dark grey in colour. Their mineral grains are less than 5 mm in diameter, those of phyllite being the smaller of the two.