gneiss - hand/display specimen of a Proterozoic granite gneiss
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. This gneiss was collected from San Antonio Canyon in the San Gabriel Mountains, California and is cut flat on one side.
A gneiss can also be derived from sedimentary rocks and is often named for a mineral constituent of that gneiss, a biotite gneiss, for example.
George Barrow, who mapped rocks in the Scottish Highlands during the late 19th century, 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 in areas of active mountain building, subduction and volcanic arcs.
The specimen with a black pencil and coin in the photo is cut flat on both sides, with the "down" side a larger flat surface than the one shown.
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