Rock 365 : Day 151 : Pseudotachylite

Day 151 : The dark, fine-grained rock in the centre of the image is
pseudotachylite. Literally a ‘false’ tachylite (basaltic glass), a
pseudotachylite is formed when frictional heat from earthquake
faulting melts the host rock which then chills instantly forming a
glass. The host rock here is a biotite acid gneiss from the quarry at
the west end of Lock Tollie, Gairlock, Scotland. Keele collection.