An orbicular phase of the San Marcos gabbro occurs near the contact with Triassic (?) schist. Orbicules average several inches in diameter, have distinct radial structure and numerous concentric zones. Most orbicules havee gabbroic cores, others lack nuclei and some surround schist fragments. Orbicule fonnation appears to be aided by (1) abundant mineralizers, (2) fluctuations of magma temperature and (3) presence of xenoliths.