Removal of about 30 μm of material from a polished surface by ion thiining reveals microactivities in granite, diabase or gabbro as they exist in the interior of the material. A string of long, thin low aspect ratio cavities often follows grain boundaries in granites; aspect ratio of cavities ranges down to 10 −3 . In all rock types studied, individual low aspect ratio cavities rarely exceed about a tenth the grain size contrary to theoretical predictions. The strings of cavities could be partially healed early fractures. Nearly equant cavities up to several μm in size are very abundant in sodic plagioclase, less common in quartz and potash feldspar, and irregularly distributed along grain boundaries. They could be the sites of fluids remaining after crystallization, traces of original porosity, or the remains of healed cracks. Mechanical or thermal stress introduces sharpened brittle cracks and other more subtle damage near the ends of longer cavities. Accurate mapping of crack-like cavities in most unstressed rocks will require ion-thinned samples and the SEM.