Fig. 18. Microcline (black) contains tiny islands of plagioclase, most of which cannot be seen in the computer image but which are in optical parallel continuity with the albite-twinned plagioclase crystal (light gray and tan) that surrounds the microcline on three sides. The fourth (left) side against quartz (light gray, white) is a straight boundary that is interpreted to be the edge of the former euhedral plagioclase crystal that once filled this space, and, in that case, the microcline is pseudomorphic after the plagioclase. The plagioclase adjacent to the microcline is myrmekitic, but the quartz vermicules are so tiny that they are difficult to see.