gneiss - Proterozoic granitic gneiss from the San Gabriel Mountains, California - hand specimen

This Proterozoic granite gneiss, an orthogneiss, was derived from granitic rocks that were deeply buried and metamorphosed. Pressure and heat caused the minerals to recrystallize and align in bands. Gneiss contains less mica than schist and does not split along mica layers. It shouldn't be a hard sell to convince students that this rock is metamorphic. An orthogneiss is deprived from igneous rocks. When derived from sedimentary rocks, the gneiss is a paragneiss. A gneiss is often named for a mineral constituent of that gneiss, a biotite gneiss, for example. George Barrow, 1853-1932, mapped rocks in the Scottish Highlands for the British Geological Survey during the late 19th century, and showed there was a metamorphic progression starting with shale as a parent rock. The series begins with slate, then phyllite, schist and gneiss. Students should be able to recognize examples. The Barrovian metamorphic progression is often encountered where temperature and pressure increase gradually i