This week I have been fortunate to have met Flumgummery and today met Sallymair.   Before meeting Sally I went to the museum which I never tire of visiting as there is so much to see. In the geology section there is a large slab of schist which comes from the shores of Loch Monar in the northern Highlands of Scotland.  Many geologists have been inspired to visit the area to study the way in which rocks have folded and metamorphosed over time due to the crumpling and folding of rocks as continents collide.  More information is here and a notice states
Sheared schist
This rock originally had a different composition.  Whilst buried several kilometres underground, the minerals recrystalised into dark mica-rich layers and light coloured quartz- and feldspar-rich layers.
Tectonic forces caused the rock to fold in several directions and in some cases multiple times.  Some layers fold more easily than others.  The pressure forced gaps to emerge in some of the darker stiffer layers.  Quartz-rich fluids flowed into the gaps, eventually crystallising forming a series of white slashes.