Fig. 9. Right side shows interior replacement of albite-twinned plagioclase crystals (light gray) by microcline (black) in a discontinuous vein of rectangular islands. On left side, microcline (black, white; grid-twinning) replaces plagioclase (speckled tan). Biotite (brown); quartz (tan). From sample of Popple Hill gneiss west of Emeryville.